electricity

capacitance maths

C=q/V W=1/2CV*2 units cm/mm must be metres relative permittivity must be multiplied by e diameter or radius if its n amount of plates then it is only n-1 capacitors

farad

a conductor has a capacitance of 1 farad if 1 coulomb on it raises its potential by 1 volt 1 farad is one coulomb per volt

depletion layer

a depletion layer is a region at the junction which has no charge carriers as the free electrons have combined with positive holes. this region has a high resistance

what property of a diode makes it useful in a rectifier circuit

a diode allows current to flow in only one direction

to charge an insulator by contact [friction]

-a plastic rod is rubbed with a cloth -electrons leave the rod and transfer to the cloth -as the rod has lost electrons it is now positively charged -as the cloth has gained electrons it is now negatively charged the positive charge on the plastic rod remain in the rod. your hand does not act as the earth if the rod was made of metal then your hand would earth of the charge. the charge would not stay on the rod

discharging a capacitor

-close the switch and a current flows in the circuit due to the charge on the capacitor -the discharge is non linear -while charged capacitors store energy their use as an energy source is limited -discharge time is very short, often just a few seconds -the output is no linear -the electric field of the charged capacitor would adversely affect surrounding electrical components

charging a capacitor

-close the switch and current flows as charge from the battery builds up on the capacitor -when the capacitor is fully charged the current stops flowing and the voltage across the plates is the same as the voltage of the battery -the build up of charge is non linear

explain how a pure semiconductor can be converted into a p type a n type

-doping with group 3 element [boron] gives a p type semi conductor -doping with group 5 element [phosphorus] gives a n type semiconductor

to measure the resistivity of a wire describe the procedure used in measuring the length of the sample wire describe the steps involved in finding the average diameter of the wire the experiment was repeated on a warmer day. what effect did this have on the measurements -explain why several readings of the micrometer were taken -what measurement would be the most accurate

-make sure the wire is straight and stretched [taut] now measure the distance with a metre stick between the points on the wire for which the resistance was measured -check the micrometer for a zero error place the wire between the grips pf the micrometer tighten the grip on the wire using the slip control and read the main scale and the vernier scale repeat this procedure at several points along the wire calculate the average value for the diameter -a higher temperature may lead to an increase in diameter and an increase in radius -the micrometer measures the diameter and the diameter could be non uniform. several readings allow for an average to be calculated which improves the accuracy of the experiment - a measurement that has more significant figures or is an average of multiple reading is the most accurate. you can improve resistance accuracy by using a more sensitive ohmmeter

what happens to the resistance of a wire if its temperature falls below zero as its length is increased if its diameter is increased

-resistance decreases -resistance increases -resistance decreases

demonstration of force between charges

-suspend a positively charged rod using an insulator type thread -bringing a positively charged rod near the suspended one results in repulsion of the rods -bringing a negatively charged rod near the suspended one results in attraction of the rods lie charges repel and opposite charges attract

capacitors and alternating current

-the bulb lights in the circuit shown above -electrons from the a.c supply continually pass through the bulb as they flow to the capacitor and back from the capacitor -the capacitor is being charged and discharged continuously -the larger the capacitance of the capacitor the brighter the light from the bulb -the higher the frequency of the alternating current supply the brighter the light from the bulb

capacitors and direct currents

-the bulb will not light in the circuit shown above -electrons from the battery flow to the capacitor and pass through the bulb only once. -a once off passage of electrons will not make the bulb light -we say that capacitors do not allow d.c to flow

metre bridge

the sliding contact is moved along the uniform resistance wire until the galvanometer reads zero [the bridge is balanced] you can now use Rx x L2 = Ry x L1 if you know the resistor Rx then you can calculate the value of Ry ADVANTAGE gives an accurate value of resistance DISADVANTAGE large in physical size, expensive

gold leaf electroscope

-this device is used to tell if an object is charged or neutral. it can also be used to tell whether an object is positively or negatively charged -a metal cap is attached to a metal rod which passes through an opening in the top of the box. the box is made of metal or if it is made of wood is should have a metal band inside -the metal rod must be insulated from the box -the box is usually earthed -gold leaf[paper] is connected to the metal rod inside the box. this gold leaf moves away [diverges] from the metal rod if they both have the same charge. the size of the divergence is a measure of the amount of charge i.e more charge gives more divergence of the golf leaf

demonstration experiment to show that a charged capacitor stores energy

-when the switch is closed the capacitor is charged up from the battery -you now open the switch and replace the battery with a bulb -you now close the switch and the bulb lights briefly - the conclusion is that the charged capacitor stored energy

kilowatt hour

1000 watts by 36000 seconds = 3600000 joulesyh

mandatory experiment; to investigate the variation of current with voltage for a semiconductor diode

DATA; FORWARD BIASED move the sliding contact of the potential divider [variable resistor] down until the voltmeter reads zero record this voltmeter reading and the corresponding reading on the milliammeter move the sliding contact upwards until the voltmeter reads 0.1V. record this voltmeter reading and the corresponding milliammeter reading repeat this procedure for 0.2V, 0.3V, 0.4 V, 0.5 V, 0.6V and 0.7V DATA; REVERSE BIASED move the sliding contact of the potential divider[variable resistor] down until the voltmeter reads zero record this voltmeter reading reading and the corresponding reading on the microammeter. move the sliding contact upwards until the voltmeter reads 1V. record this voltmeter reading and the corresponding microammeter reading repeat this procedure 2V,3V,4V, 5V,6V GRAPH plot a graph of current on the y axis and voltage on the x axis ACCURACY ensure good electrical contacts as poor contactss causes variations in resistance check the voltmeter and the milliammeter [microammeter] for a zero error and correct if necessary in forward bias mode the resistor limits the current through the diode and prevents it from overheating in reverse bias mode the voltmeter is connected across the diode and the microammeter. this ensures that only the current through the diode is measured with microammeter

heat

H=VIt by ohms law H=I2Rt unit is the joule

transmission at very high voltage [EHT]

In the power station voltage is x 1000 [step up transformer] in the power station current is divided by 1000 [to obey the conservation of energy] heat generated in transmission is divided by 1000000 [joules law heat change is d.p to current squared] heat losses to the air are reduced by a factor of 1000000. this means a huge reduction of energy losses and a huge saving in money terms

current/ voltage graphs for different materials

METALS if the temperature is constant the current is proportional to the voltage ohms law is obeyed the charge carriers are electrons FILAMENT BULB if the voltage across the bulb is increased then the current is also increased but not in a linearly proportional manner ohms law is not obeyed the charge carriers are electrons IONIC SOLUTION/ ACTIVE ELECTRODES the electrodes react chemically with the electrolyte current is proportional the the voltage ohms law is obeyed the charge carriers are positive and negative ions IONIC SOLUTION/ INERT ELECTRODES the electrodes do not react chemically with an electrolyte current will not flow until the voltage reaches a certain value. after this value the current increases as the voltage increases teh current is not proportional to the voltage and ohms law is not obeyed the charge carriers are positive and negative ions GASES a discharge tube contains a gas at a low pressure. some of the gas molecules split into ions due to cosmic radiation or background radioactivity a voltage is applied across the electrodes at the ends of the tube and positive ions move towards the negative electrode and negative ions move towards the positive electrode as you increase the voltage the current will increase. as the voltage increases more and more of the ions move to the electrodes after a certain voltage the current levels off. at this stage all the ions are moving to the electrodes at higher voltages, ions are moving to the electrodes at greater speeds and as ions collide with molecules of the gas more ions are made. therefore at these higher voltages the current increases again. ohms law is not obeyed charge carriers are positive ions, negative ions and electrons VACUUM as a vacuum has no charge carrier no current can flow however electrons may be emitted from the negative electrode and the move towards the positive. as the voltage increases so does the current i.e. more and more of the electrons move to the positive electrode at a certain voltage the current levels off. at this stage all the electrons emitted at the negative electrode are moving to the positive electrode ohms law is not obeyed charge carriers are electrons SEMICONDUCTOR as the voltage increases so does the current but they are not linearly proportional as the current rises so does the temperature of the semiconductor and this results in an increase in the number of charge carriers ohms law is not obeyed charge carriers are electrons and positive holes

power

P=VI by ohms law P=I2R unit is the watt

LDR

a light dependant resistor is used to control the switching on and off of street lights

lightning conductor

a lighting conductor is a thick copper strip fixed to an outside wall reaching above the highest part of a building. it is connected to the earth the top of the copper strip is a sharp spike a negatively charged cloud overhead will repel electrons from the sharp spike to the earth and therefore induce a positive charge on the sharp spike because the spike is sharp there is a high concentration of positive charge on the spike situation1 a huge number of electrons from the cloud are attracted to the positive spike and travel through the air. normally the air is an insulator but here it is forced to conduct this is observed as a bolt of lightning from the cloud to the sharp spike the electrons flow through the thick copper strip to earth there is no damage done to the building situation 2 there is a high concentration of positive charge on the sharp spike ionises the surrounding air ionised air is a good conductor electrons leave the cloud travel through the ionised air to the spike and then travel through the copper strip to earth no lighting is observed. the charge 'leaks' off the cloud the charge cloud has been neutralised and no damage is done to the building

demonstration experiment to show that total charge resides on the outside of a conductor

a metal can is placed on the top of a van de graaff generator as it being charged up. charge accumulates on the sphere on the generator and on the outside of the can let a proof plane touch the inside of the metal can. transfer the proof plane to the cap of a gold leaf electroscope. the gold leaf does not diverge as there is no charge on the inside of the metal can let a proof plane touch the outside of the metal can. transfer the proof plane to the cap of a gold leaf electroscope. the gold leaf will diverge as charge reside on the outside of the metal can

proof plane

a metal disc is attached to an insulator handle when it touches a charged body it takes some charge from the body the greater the concentration of charge on the body the more charge the proof plane takes

mandatory experiment; to investigate the variation of the resistance of a metallic conductor with temperature

a metallic conductor [wire] is wrapped around an insulator to make it compact size the metallic conductor is placed in a test tube of oil [glycerol] and sits in a water bath the temperature of the metallic conductor is the same as the temperature of the oil data when the water bath contains melting ice measure and record the temperature of the metallic conductor [o degrees celsius] with the thermometer and the resistance of the metallic conductor with the ohmmeter as the water bath is heated record several corresponding values of temperature and resistance for the metallic conductor graph plot a graph of resistance on the y axis and temperature on the x axis accuracy heat the water bath slowly. this allows time for the metallic conductor and the oil to have the same temperature check and correct the ohmmeter for an zero error ensure good electrical contacts between the metallic conductor and the ohmmeter as poor contacts affects the resistance stirring the oil ensures that the oil and metallic conductor have a uniform temperature we use oil instead of water in the test tube as oil is a better conductor of heat and water tends to have impurities which conduct electricity

semiconductor

a semiconductor is a material which a conductivity or resistivity between that of conductor and an insulator

p-n junction

a single crystal of silicon is p type doped on the left and n type doped on the right some free electrons diffuse across the junction from the n type region to the p type region dome positive holes diffuse across the junction from the p type region to the n type region near the junction the n type region losses electrons and becomes positively charged near the junction the p type region gains electrons and becomes negatively charged eventually these charged regions are sufficient to prevent further diffusion of charges across the junction. a potential barrier has been set up the potential barrier is called the junction voltage. it is 0.6V for silicon and 0.2V for germanium there are no charge carriers on either side of the junction and this area is called the depletion layer

demonstration experiment to show the magnetic effect of electricity

a small plotting compass lying on a table will have its needle point along the magnetic north/south line a long piece of wire is placed over the plotting compass along the north/ south line when a current flows in this wire the needle of the plotting compass will rotate 90 degrees and lie perpendicular to the long wire the needle of the plotting compass is influenced by the magnetic field of the current in the wire conclusion is that a current has a magnetic effect a practical example of this would be an electrical motor

reverse bias p-n junction

a voltage is applied across the p-n junction as shown above. negative terminal to the p type and positive terminal to the n type this external voltage reinforces the potential barrier the width of the depletion layer is increased the resistance of the p-n junction is very high. the p-n junction does not conduct

forward bias p-n junction

a voltage is applied across the p-n junction as shown above. positive terminal to the p type and negative terminal to the n type the width of the depletion layer is greatly reduced a significant current will flow if the external voltage exceeds the junction voltage which is 0.6V for silicon the resistance of the p-n junction is very low. the p-n junction is now a conductor

direction and current

actual current electrons gradually flow from the negative terminal to the positive terminal of the battery. this is the actual current conventional current the early scientists thought the current was the flow of positive charge from the positive terminal to the negative terminal of a battery. we now know this is not correct. however we still use their idea today. the flow of positive charge is called conventional current

what is one advantage and disadvantage of a wheatstone bridge over a metre bridge

advantage accuracy disadvantage expensive and difficult to balance the bridge

point discharge [point action]

air molecules near the sharp point of a charged object are ionised. the air molecules split into positive and negative ions these negative ions are attracted to the large concentration of positive ions at the sharp point. the negative ions combine with some of the charge on the sharp point the positive ions are repelled by the very large concentration of positive ions at the sharp point . this movement of the positive ions away from the sharp point gives rise to an electrostatic wind

some electrical appliances are supplied with two pin plugs, why is an earth wire not required in these devices

all the external parts of these electrical appliances are made of an insulator type material

demonstration experiment to show the heating effect of electricity

allow a current flow for a few minutes in the circuit shown above the thermometer will show a rise in temperature conclusion is that current has a heating effect a practical example would be using an electrical kettle to boil water

demonstration experiment to show the chemical effect of electricity

allow a current to flow for a few minutes in the circuit shown above the copper anode will lose mass and the copper cathode will gain mass conclusion is that a current caused a chemical reaction, known as electrolysis a practical example of this would be electroplating a less expensive metal with silver recharging your mobile phone battery is also an example of the chemical effect of electricity

explain why replacing a fuse with a piece of aluminium foil is dangerous

aluminium can carry a large current which could cause overheating in the wires which would lead to a fire

electric fields in everyday life

an aircraft in flight may become charged due to a friction between the air and the body of the aircraft. this build up of a charge could lead to sparks when the aircraft lands and is being refuelled. tyres made of conducting materials are used to remove the charge from the body of the body of the aircraft upon landing an electrostatic precipitator is used to remove dust particles from the chimney of a power station. the dust particles become charged as they pass through a charged wire grid. the charged dust particles are then attracted to oppositely charged metal plates further up the chimney

earth

an earth connection is only for appliance with metal on their outer surface the earth connection provides a good conducting path from the outer metal surface to the earth if the live wire was accidentally come in contact with the outer metal surface the current flows to earth rather than through a person who may be in contact with the appliance the earth connection protects the person from electric shock

electric field

as electric field is the region around a charged object in which its electric forces act

Variable potential divider

as the sliding contact moves upwards the voltage between A and B increases to a maximum value of 6V as the sliding contact moves downwards the voltage between A and B decreases to a minimum of 0V the variable potential divider allows you to vary the voltage from a 6V battery

as the temperature of the room increases, explain why the resistance of the thermistor decreases the potential at A decreases

as the temperature increases the number of electrons and holes [charge carriers] increases in the thermistor. more charge carriers mean better conduction and therefore less resistance if the resistance of the thermistor decreases then the resistance of the parallel arrangement decreases also. a smaller resistance means a smaller potential difference across the resistors. thus the potential at A is bigger than before

to charge a gold leaf by induction

bringing a positively charged rod near the cap of the electroscope. the gold leaf rises. the leaf region is positively charged. the cap has a negative charge touch the cap with your finger i.e earth the cap. the leaf falls as the charge in that region is neutralised. electrons come from the earth to the leaf region to neutralise the positive charges remove your finger[earth connection] from the cap. then remove the positively charged rod. the leaf rises again as the negative charge from the cap spreads over the leaf region and the cap the gold leaf electroscope has been charged by induction

resistance and ohms law

by moving the sliding contact the voltage across a conducting material was varied the values of the current in the ammeter and voltage in the voltmeter were noted ohm noticed that the voltage was directly proportional to the current if the temperature of the conducting material was constant V is directly proportional to I voltage is also called potential difference the unit of resistance is the ohm resistance is the voltage across a conductor divided by the current flowing through it

Wheatstone Bridge

by trial and error change the value of the resistors in the circuit shown above until the galvanometer reads zero [the bridge is balanced] you can now use r1/r2= r3/r4 if you know the value of three of the resistors and the fourth is unknown then it can be calculated using the equation above ADVANTAGE compact, portable, gives an accurate value of resistance DISADVANTAGE expensive, full explanation of principle of operation not on course

uses of capacitors

capacitor can be used to tune a radio from one station to another. turning the tuning control knob changes the common are of the tuning capacitor the flash on a camera can use a capacitor to store charge and the release this charge very quickly to give a very intense burst of light capacitors can be used to block the flow of direct current in circuits while allowing ac to flow a device called a rectifier can change a.c to d.c. the d.c obtained can have a variable voltage. a smoothing capacitor can be used to make the voltage constant

capacitors

capacitors consists of two of two parallel metal plates separator by an insulator which is called a dielectric capacitors are used to store electric charge capacitors come in different shapes and sizes but are essentially parallel plates

what is the difference between a battery and a capacitor

capacitors store electrical energy [charge] while batteries store chemical energy capacitors discharge faster than batteries batteries give a constant current while capacitors do not

charge distribution

charge resides on the outer surface of the object. the charge is within a layer which is a few atoms from the surface [skin depth] the surface charge density [charge per metre squared] is greatest at the sharp corners of an object

name an instrument to measure the diameter of the wire and describe how it is used

check the micrometer for a zero error place the wire between the grips of the micrometer tighten the grip on the wire using the slip control and read the main scale and the vernier scale repeat this procedure at several points along the wire calculate an average value for the diameter

demonstration experiment for definition of ampere

close the switch to allow current flow in the foil strips the foil strips exert a force on each other and they move away from each other each conductor exerted a force on the other

use of charges residing outside

co axial cable electrostatic shielding faraday cage [to protect people or equipment enclose them in a hollow conductor]

domestic circuit

colour code live wire, brown insulation neutral wire, blue insulation earth wire, green/ yellow insulation ring circuit in the ring circuit the current takes two paths from the fuse to the socket the current along each path is half the current the appliance will use the heat generated in the wire is reduced by a factor of four [joules law] the reduced risk of the wire overheating is a definite safety factor fuse a fuse is a protective device that cuts off the flow of current if the current is about to exceed a certain value a fuse limits the current that can flow in a circuit a fuse prevents a wire from overheating and therefore reduces the risk of a for starting a fuse does not protect a person from being electrocuted

what test would you use to determine that the bridge is balanced

connect a galvanometer across points A and C the bridge is balanced when no current flows in the galvanometer

how might a diode be protected from having too large a current flowing through it when it is connected across a battery

connecting a resistor in series with the diode will protect it

why is a lightning conductor made of copper

copper is a good electrical conductor

current

current is the flow of charge examples -passage of electrons along a copper wire -motion of ions between the electrodes in a solution - motion of positive holes and electrons in a semiconductor current is also viewed as the charge per unit time passing a point I=Q/t its unit is the ampere this formula is not in the formula book

current and voltage

current is the number of electrons flowing in the wire. its unit is the ampere voltage is the ability of electrons to do a task. the unit is the volts

mandatory experiment to measure the resistivity of a wire

data measure and record the diameter of the wire at several points along the wire. use a micrometer to measure the diameter. calculate the average diameter measure and record a length of wire and measure and record the resistance of this length of wire using an ohmmeter calculations use the resistivity formula accuracy check the micrometer for a zero error and adjust the diameter values if necessary use the slip control to tighten the grip on the wire to avoid squeezing the wire check the ohmmeter for a zero error and adjust the resistance values if necessary avoid the error of parallax when measuring the length of wire with the metre stick avoid small values of length as measuring small values leads to greater percentage errors take several values of diameter as the diameter of the wire may be non uniform. use the average values of the diameter in your calculations graph you might decide to record several values of length and resistance. you could then plot a graph of resistance on the y axis and length on the x axis slope of graph is R/l therefore p=RA/l which means slope by area

mandatory experiment; to investigate the variation of current with voltage for a copper sulfate solution with copper electrodes

data move sliding contact of the potential divider[variable resistor] down until the voltmeter reads zero record this voltmeter reading and the corresponding reading on the milliammeter move the siding contact upwards until the voltmeter reads 1V. record this voltmeter reading and the corresponding milliammeter reading repeat this procedure for 2V, 3V,4V,5V,6V graph plot a graph of current on the y axis and voltage on the x axis the graph is a straight line through the origin showing that current and voltage are directly proportional for a copper sulfate solution with copper electrode accuracy ensure good electrical contacts as poor contacts causes variations in resistance check the voltmeter and the milliammeter for a zero error and correct if necessary if inert electrodes were used in this experiment the graph would not pass through the origin. the graph like below would be obtained . inert electrodes dont react chemically with the copper sulfate whereas the copper electrodes would

mandatory experiment: to investigate the variation of current with voltage for a metallic conductor[wire]

data move the sliding contact of the potential divider[variable resistor] down until the voltmeter reads zero record this voltmeter reading and the corresponding reading on the milliammeter move the sliding contact upwards until the voltmeters 1V. record this voltmeter reading and the corresponding milliammeter reading repeat this procedure for 2V, 3V,4V,5V,6V graph plot a graph of current on the y axis and voltage on the x axis the graph is a straight line through the origin showing that current and voltage are directly proportional for a metallic conductor [provided the temperature remains constant] accuracy avoid using large currents as this causes the metallic conductor to heat up. such temperature changes will result in a non linear graph ensure good electrical contacts as poor contacts causes variations in resistance check the voltmeter and the milliammeter for a zero error and correct if necessary

give a use of a semiconductor diode

diodes are used as rectifiers [to change ac to dc]

demonstration experiment to show the factors that capacitance of a capacitor depends on; distance, common area, permittivity

distance -move the plates closer together keeping e and A constant - the multimeter shows an increase in capacitance - in conclusion a smaller distance gives a larger capacitance, C d.p 1/d2 common area -slide the plates apart keeping e and d constant - the multimeter shows a decrease in capacitance - in conclusion a smaller common area gives a smaller capacitance C d.p A permittivity -place a piece of wax between the plates keeping A and d constant. the permittivity of wax is greater than the permittivity of air -the multimeter shows an increase in capacitance - in conclusion a larger permittivity gives a greater capacitance C d.p e

doping

doping is the addition quantity of a chemical impurity to a semiconductor crystal to increase conductivity

properties of electric field lines

electric field lines start on a positive charge and end on a negative charge electric lines do not intersect each other electric field lines are perpendicular to the surface of a charged conductor there are no electric field lines inside an empty hollow charged conductor a metal like aluminium can be used to alter an electric field. this is the basis of electrostatic shielding

what happens at the boundary of the two adjacent layers

electrons and holes diffuse access the junction a depletion layer is set up at the junction a junction voltage is created

what does it mean to be positive negative or neutral by convention

electrons are said to be negative and protons are said to be positive and negative are just adjectives to tell the difference between electrons and positive . this is agreed internationally neutral an object is neutral if the atoms in the object have the same number of electrons as protons an object is positively charged if some of its atoms have more protons than electrons. this would be caused by a loss of electrons. positive ion an object is negatively charged if some of its atoms have more electrons than protons this would be caused by a gain of electrons

extrinsic conduction [doping]

extrinsic conduction is when the addition of a controlled quantity of an impurity increases conduction n type doping a controlled quantity of a group V elements such as Phosphorous is added to the silicon Phosphorous have five outer electrons. four of the outer electrons are involved in bonding and the fifth is available to act as a conductor adding phosphorous to silicon makes it a conductor due to heat in the surrounding air some of the bonds will be broken making electron hole pairs available to conduct also however the fifth outer electron of the phosphorous is the majority charge carrier since the majority charge carrier is the negative electron this is called n type doping p type doping a controlled quantity of a group three element such as boron is added to the silicon boron has three outer electrons. this results in a positive hole. this positive hole is available to act as a conductor adding boron to the silicon makes it a conductor due to heat in the surrounding air some of the bonds will be broken making electron hole pairs available to conduct also however the positive hole due to boron is the majority charge carrier since the majority charge carrier is the positive hole is called p type doping

unit of a capacitance

farad

give two differences between the gravitational force and electrostatic force between two electrons

gravitational force is attractive and electrostatic force is repulsive gravitational force is much smaller than the electrostatic force

describe how the temperature was varied

heat from the bunsen burner heated the water which heated the oil and the thermistor this varied the temperature of the thermistor

bonding

in a house metal water pipes, metal radiators, metal hot water cylinders etc are connected to earth by a live wire if a live wire was accidentally touch any of these metal objects the current current would flow to earth rather than through a person who may be in contact with them this ideas of bonding is to protect a person from electrocution

forward bias and reverse bias graphs

in forward bias little current flows until the junction voltage is exceeded. then a significant current flows in reverse bias a minisclue current [almost no current] flows

why was a fixed mass of water used throughout the experiment of joules law

in this experiment we are trying to establish a relationship between temperature rise and current [two variables] however temperature rise also depends on the mass of water i.e the mass of the water could be a variable in this experiment to establish a relationship between two variables all other variables must remain constant

how do you increase the capacitance

increase, area, decrease distance, electroplate dielectric on

intrinsic conduction

intrinsic conduction is when a semiconductor material starts to conduct due to heating the material silicon is an insulator at very low temperatures at very low temperatures at the outer electrons of the silicon atoms are involved in bonding the atoms together. electrons are not available to act as conductors. almost no conduction takes place. the silicon is essentially an insulator at room temperature the silicon atoms vibrate more vigorously. some of the bonds are broken. there are now free electrons and positive holes available to act as conductors the silicon is now a conductor the higher the temperature the greater the conduction in intrinsic conduction the number of free electrons and positive holes are equal and conduction is due to both a light dependant resistor LDR is a semiconductor material that responds to light instead of heat. as the intensity of the light falling on the LDR increases the level of conduction increases. a brighter light means less resistance or more conductivity

give two precautions that should be taken when measuring the length of the wire

keep wire straight and stretched [taut] when measuring length avoid error of parallax when measuring the length with the metre stick

how would the graph for the ionic solution differ if its concentration were reduced

less concentration results in more electrical resistance due to a reduction in charge carriers. this would give a smaller current for a given voltage

the faraday cage

michael farday once sat in a wire cage charged to a very high voltage in front of a line audience. no harm came to him. this was to demonstrate that there is no electric field inside an empty hollow charged conductor

mandatory experiment: to investigate the variation of current with voltage for a filament bulb

move the sliding contact of the potential divider [variable resistor] down until the voltmeter reads zero record this voltmeter reading and corresponding reading on the milliammeter move the sliding contact upwards until the voltmeter reads 1V. Record this voltmeter reading and the corresponding milliammeter reading repeat this procedure 2V,3V,4V,5V,6V,7V graph plot a graph of current on the y axis and voltage on the x axis accuracy avoid using very large currents as this may cause the bulb to overheat and blow ensure good electrical contacts as poor contacts causes variations in resistance check the voltmeter and the milliammeter for a zero error and correct if necessary note since the graph in non linear ohms law is not obeyed as the current increases the temperature of the filament increases. this causes increased thermal vibrations of the atoms in the filament. this ,makes it more difficult for the electrons to flow through the filament and therefore the resistance of the filament increases the increasing resistance means that the current is not proportional to the voltage

ohmmeter

ohmmeters can be analog or digital. you can clip the connections from the ohmmeter across the ends of the resistor you want to evaluate. simply read the value on the digital display always check the ohmmeter for a zero error and allow for this in calculations ADVANTAGE compact, portable, fast DISADVANTAGE can be fragile, must check for zero error, less accurate

why is the thermistor usually immersed in oil rather than in water

oil is a better conductor of heat impurities in the water conduct electricity

demonstration of an electric field pattern in the laboratory

place two aluminium electrodes in a beaker of olive oil. the voltage across the electrodes would be of the order 2000v sprinkle semolina grains onto the oil the grains line up in such a way that they show the electric field pattern

explain why pointed surfaces should be avoided when using high voltage electrical equipment

point discharge could occur. this flow of charge through the air could lead to a spark causing a fire

under what circumstances will point discharge occur

point discharge will occur when there is a high concentration of charge[charge density] at a sharp point

apart from using insulation, give one other way of reducing heat losses in the experiment

put an insulating lid on the calorimeter do not allow the temperature of the water get too high and this will reduce heat lost to the surrounding air

what is the function of the rectifier

rectifier converts a.c to d.c

factors affecting resistance

resistance is directly proportional to the length l resistance is directly proportional to the material of the conductor [resistivity p] resistance is inversely proportional to the cross sectional area of conductor

how do you change a circuit from forward bias to reverse bias

reverse the polarity of the diode replace the milliammeter with a microammeter change the way the voltmeter is connected in the circuit e.g place it in parallel with both the diode and the microammeter

forward bias mode diode; explain why the curve suggests that the diode was a silicon diode rather than a germanium diode

since the curve rises sharply upwards at around 0,6 volts we would assume that the diode is made from silicon in case of a germanium diode the sharp rise happens at around 0.2 volt

fixed potential divider

some of the 6V is lost across resistor 1 and the rest of the voltage is lost across resistor 2 the voltage is divided in the ratio of the resistances values.

to charge a metal sphere by induction

take a conducting sphere [metal] sitting on an insulated stand bring a negatively charged rod near the sphere. charge separation happens on the sphere i.e some electrons move to the far side of the sphere leaving behind atoms lacking some of their electrons [positive charge] when the right side of the sphere is earthed the electrons that moved to the right now run to earth remove the earth connection and then remove the negative rod. some electrons from atoms on the right side of the sphere move over to the positive atoms on the left side. the result is now that some atoms all over the sphere lack electrons i.e the positive charge has been distributed all over the sphere charging by conduction only works for conductor type materials charging by contact works for conductor and insulator type materials

resistors in series

the same current I passes through all three resistors Ir= I1+I2+I3 The total potential difference V [voltage drop] between A and B is the sum of the three individual potential differences Vt =V1+V2+V3 using ohms law V=RI RtIt=R1I1+R2I2+R3I3 since all the i values are equal Rt=R1+R2+R3

Practical uses of the wheatstone bridge

temperature control.. to maintain a room at a fixed temperature fail safe device.. to prevent poisonous gas leaking from a boiler a gas boiler has a pilot light that lights all the time . a heat sensitive resistor is positioned above this flame. the wheatstone bridge is balanced if the pilot light goes out, gas can flow into the boiler without burning and this gas spread into the air in a house, poisoning the people in the house. however if the pilot light goes out then the resistance of the heat sensitive resistor rapidly changes and the wheatstone bridge is no longer balanced a current flows through the galvanometer and this current can be used to activate a switch that cuts off the supply of gas. the people in the house are now safe

Miniature Circuit Breaker (MCB)

the MCB does the same job as the fuse. it limits the current flowing in a circuit when a fuse blows it has to be removed and replaced. when the MCB trips it can be reset by the flick of a switch. this is a definite advantage of MCB has over the fuse if the current flowing in the electromagnet increases the magnet flux density is quite large. this increased magnetic flux density can be used to break the circuit by opening the switch. the current now stops flowing

ampere

the ampere is that current which when flowing in two infinitely long parallel conductors, of negligible cross sectional area, one metre apart in a vacuum, will produce a force on each other of 2x10*-7 N per one metre of each wire [5 conditions]

capacitance

the capacitance of an object is the ratio of the charge q on the object to the voltage of the object C=q/V

co - axial cable

the central wire carries the electric signal a copper wire mesh surrounds the central wire. the mesh prevents any interference from external electric fields. the mesh acts like a faraday cage

the coulomb

the coulomb is the charge that passes a point when a current of 1 amp flows for 1 second

what would be the effect on the current flowing in this diode if the terminals of the battery were reversed

the diode is now reversed biased

electric field strength

the electric field strength at a point in an electric field is the force acting on a test charge q placed at the point divided by the magnitude of the charge

coulomb's law

the force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them

why should the frame of an electroscope always be earthed

the frame is earthed to ensure there is a potential difference between the leaf region and the metal frame

how does the presence of ions in the air to be more conducting

the ions act as charge carriers

all the charge resides on the surface of a van de graff, explain why

the like charges on the dome repel each other and move to be at the maximum distance away from each other i.e on the surface

how does a full body metal foil suit protect an operator when working on a high voltage power lines

the metal foil suits blocks out external electrical fields. there is no electric field lines inside an empty hollow charged conductor

the toaster has exposed metal parts. how is the risk of electrocution minimised

the metal parts are earthed

what is the net charge on the capacitor

the net charge on the capacitor is zero, the plates have equal and opposite charge

demonstration experiment to show that charge concentrates at the sharp corner of an object

the pear shaped conductor shown above is charged by induction two similar proof planes touch the conductor A at the sharp end and B at the blunt end the proof planes are now placed on the caps of identical gold electroscope proof plane A causes a larger deflection than proof B showing that there was a greater concentration of charge at the sharper end

potential difference

the potential difference between the points A and B is the work done per unit charge to move the charge from B to A the unit for the potential difference is joule/coulomb = J/C = volt

resistors in parallel

the potential difference[voltage drop] across each individual resistors is the same as the total potential difference across the combination of the three resistors Vt=V1+V2+V3 The total current splits into three parts It=I1+I2+I3 using ohms law V=RI Vt/Rt=V1/R1+V2/R2+V3/R3 since all the V values are equal It/Rt=I1/R1+I2/R2+I3/R3

mandatory experiment to verify joules law

the rate of conversion of electrical energy to heat is proportional to current squared. power is proportional to current squared data by adjusting the rheostat select a current of 1A record the current and the temperature rise that occurs when the current flows for a time interval for three minutes by adjusting the rheostat note and record several corresponding values of current using the ammeter and temperature rise using the thermometer for a time interval of 3 minutes graph plot a graph of temperature rise on the y axis and current squared on the x axis the straight line through the origin shows that temperature rise is proportional to the current squared, joules law is verified accuracy make sure the time interval for which the current flows is fixed and the mass of the water is constant check and correct the ammeter for any zero error avoid very small values of current as they give small temperatures rises. such small temperature rises result in larger percentage errors the calorimeter should be made of an excellent insulator type material so that all the heat energy is given to the water showing that temperature rise is directly proportional to current squared is equivalent to showing that power is directly proportional to the current squared in this experiment heat generated by electricity= heat gain of water I2Rt=mc . delta theta

resistivity

the resistivity of a conductor is the resistance of the conductor of length 1m and cross sectional area 1 metre squared. the unit is the ohm metre cross sectional area is always circular resistance also depends on temperature

electromotive force emf

the symbol used is E and its unit is the volt the emf is equal to the potential difference across the terminals of a battery or generator when no current is being drawn sources of emf an electric cell[battery]: the emf depends on the chemicals used in the cell an ESB mains: the emf of the mains is 230V a thermocouple: the emf depends on the material of the two wires and the temperature difference between between the ends on the wires

explain why the current was allowed to flow for a fixed length of time in each case

the temperature rise of the water also depends on the time for which the current flows as well as the value of the current squared. therefore to investigate the relationship between temperature rise and current squared keep the time value constant. we only want two variables in this experiment i.e temperature rise and current squared

mandatory experiment; to investigate the variation of the resistance of a thermistor with temperature

the thermistor is placed in test tube of oil [glycerol] and sits in a water bath the temperature of the thermistor is the same as the temperature of the oil data when the water bath contains melting ice measure and record the temperature of the thermistor [ 0 degrees celsius] with the thermometer and the resistance of the thermistor with the ohmmeter as the water bath is heated record several corresponding values of temperature and resistance for the thermistor graph plot a graph on the y axis and temperature on the x axis accuracy heat the water bath slowly. this allows time for the thermistor and the oil to have the same temperature check and correct the ohmmeter for any zero error ensure good electrical contacts between the thermistor and the ohmmeter as poor contacts affects the resistance stirring the oil ensures that the oil and thermistor have a uniform temperature we use oil instead of water in the test tube as oil is a better conductor of heat and water tends to have impurities which conduct electricity

why are raised umbrellas and golf clubs not recommended during thunderstorms

they could act as lightning conductors. a large charge could flow through them a person in contact with them would conduct this charge to earth and be electrocuted

why was the diameter of the wire measured at several points along it length

this enables an average value of the diameter to be obtained as the diameter may be non uniform. this improves the accuracy of the experiment

no of electrons associated with that charge

total charge/ charge on 1 electron

suggest a method of reducing the energy lost in the cables

transmitting electricity at a very high voltage and a low current greatly reduces the energy lost as heat in the cables

electric field lines

types of electric fields radial field the electric field lines radiate out from the positive charge. they show the path taken by a test charge uniform field the electric field lines travel from the positively charged plate to the negatively charged plate. the electric field is only uniform in the region between the plates opposite charges the electric lines travel from the positive charge to the negative charge a test charge is a tiny imaginary object which has a positive charge an electric field line is the path along which the test charge moves

describe what happens at the boundary when the semiconductor forward biased reverse biased

when forward biased the width of the depletion layer is reduced. the diode is in conduction mode when reversed biased the width of the depletion layer is increased. the diode does not conduct

explain why the resistance of the bulb is different when it is not connected to the mains

when no connected to the mains [bulb is off] the temperature of the filament is much lower. since the resistance of the filament depends on temperature the lower temperature means a lower resistance i.e a different resistance

explain how the presence of phosphorous and boron makes the silicon a better conductor

when phosphorous and boron are added to silicon there are more free electrons and positive holes to increase conductivity

what happens when two conductors that are charged touch each other

when the conductors touch the charge is distributed evenly between them. you can calculate the charge difference before and after using coulombs law to see if there is an increase or decrease

residual current device RCD

when using an electrical appliance at home the current flowing in the live wire and the neutral wire should be the same if the live current and the neutral current are not the same the rcd will be activated and the current will be stopped immediately the response of the rcd is very fast this offers great protection to a person against electrocution