Electricity

Practical - experiment to investigate how current varies with voltage for different components:

1) Set up the circuit 2) Turn the variable resistor to its maximum value 3) Close the switch and take the redings from the ammeter and the voltmeter 4) After the value of the variable resistor again and take a new pair of readings from the meters 5) Repeat the whole process at least six times 6) Place the results in a table and draw a graph (current against voltage)

converting watts into kilowatts

1kW = 1000W

What is a LDR

A light-dependent resistor (LDR) has a resistance that changes when light is shone on it. In the dark its resistance is high but when the light is shone on it the resistance decreases. LDRs are often used in light-sensitive circuits in devices such as photographic-exposure equipment, automatic lighting controls, and burglar alarms

What is a thermistor?

A temperature dependent resistor Resistance changes in difference of temperature. Thermistors are used in temperature-sensitive circuits in devices such as fire alarms. They are also used in devices where it is important to make sure there is no change in temperature for example in freezers and computers. Hot -> resistance decreases Cold -> resistance increases

The circuit on the right contains a 12 V battery and two resistors connected in series. Calculate: a) The current in each of the resistors b) The voltage across each resistor

A) The toral resistance in the current must be 2 Ω + 4 Ω = 6 Ω B) The current in the current is therefore I = V/R 12/6 = 2 A The current in a series circuit is the same everywhere. So the current in both resistors is 2A. c) Using both V = IR - For the 2 Ω resistor = 2 A x 2 Ω = 4V - For the 4 Ω resistor = 2 A x 4 Ω = 8 V

Basic symbols of electrisity

Ampere (A) - unit for current Coulomb (C) - unit for charge Joule (J) - unit for energy Ohm (Ω) - resistance Second (s) - unit for time Volt (V) - unit for voltage Watt (W) - unit for power

What is happening to charge as it flows through a circuit

As the charges flow around a circuit the energy is transferred by the components as they pass though. for example, when current passes through a bulb, energy is transferred to the surroundings as heat and light. When a current passes though the speaker of a radio, most of the energy Is transferred as sound.

The voltage across a heater is 230 V. there is a current of 5 A though the heater for 5 mins. Calculate the amount of energy transferred during this time?

Because Q = I x t, we can also write this equation as E = I x t x v E = I x t x V = 5 x (5x60) x 230 = 345 000 J or 345 kJ`

Equation for charge

Charge, Q coulombs = current, I amps x time, t seconds Q = I x t

What are circuit breakers?

Circuit breakers - if too large a current flows in a circuit a switch automatically opens making the circuit incomplete. Once the fault in the circuit has been corrected, the switch is reset, usually be pressing the reset button. There is no need to be replaced as not a fuse.

What are the common fuses?

Common fuses include 3A, 5A, 13A. The correct fuse for a circuit is the one that allows the correct current but blows if the current is a little larger. If the correct current in a curciut is 2A than it should be a 3A fuse.

Difference between current and charge

Current - The rate of flow of charge Charge - Value for electricity flowing in a circuit We measure electric current (I) in amperes or amps (A). if there is a current of 1 A in a wire it means that 1 C of charge is passing along the wire each second

in parallel circuits why is energy conserved?

Current is conserved at a junction since no current is lost here and the current split between the branches. A junction is when a branch splits in parallel.

Examples of series and parallel circuits

Decorative lights are often wired in series. Each bulb only needs a low voltage, so even when the voltage from the mains supply is 'shared' out between them each bulb still gets enough energy to produce light. If the filament in one of the bulb breaks then all the other bulbs will go out. The lights in your home are wired in parallel. We know this because lights can be switched on and off separately, and the brightness of each light does not change when other lights are on or off. Also, if a bulb breaks or is moved, you can still use the other lights.

What is a variable resistor

Different kind of resistors are called variable resistors as it is possible to alter their resistance. A variable resistor being used to control the size of the current in a bulb. If the resistance is decreased there will be a larger current and the bulb shines brighter. If the resistance is increased, the current will be smaller, and the bulb will glow less brightly or not at all. The variable resistor is behaving in this circuit as a dimmer switch.

Where are diodes used

Diodes are often used in circuits where it is important that electrons flow only in one direction. For example, they are used in rectifier circuits that convert alternating currents into direct current. Some diodes glow when charges flow though them. They are also called light emitting diodes.

Energy equation

E = I x V x t

The voltage across a light bulb is 12 V. calculate the electrical energy transferred when 50 C of change passes though tit

E = Q x v = 50 C x 12 V = 600 J

what are the three types of wire

Earth, live and neutral wire

How does electricity travel though your house?

Electricity comes from your mains supply. This electrical energy usually enters our homes though an underground cable. The cable is connected to an electricity meter, which measures the amount of electrical energy used. From here, the cable is connected to a consumer unit or fuse box which contains fuses or circuit breakers for the various circuits in your home.

Energy transferred equation

Energy transferred, E (joules) = Charge, Q (coulombs) x voltage, V (volts) E = Q x v

What are fuses and circuit breakers used for?

Fuses and circuit breakers are safety devices which shut off the electricity in a circuit if the current of them becomes too large

Calculate the correct fuse that should be used for a 230V, 1 kW electrical hairdryer

I =p/v 1000/230 = 4.35 A The correct fuse for this hairdryer is therefore a 5 A fuse.

Resistance in series

If there are two or more resistors connected in a series circuit, their total resistance is found by simply adding the individual resistors together.

What do plugs and sockets do?

If you come into contact with mains electricity you could receive a large electric shock, which might be fatal. To prevent this, the outer layer of the plug, called the casing is made from plastic, which is a good insulator. Connections to the circuits are made via three brass pins, as the metal brass is a really good conductor of electricity.

Difference between series and parallel with bulbs

In a series circuit containing bulbs: - One switch placed anywhere in the current can turn all the bulbs on and off - If any one of the bulbs break, it causes a gap in the circuit and all of the other bulbs will 'stop working' - The energy supplied by the cell is 'shared' between all the bulbs, so the more bulbs you add to a series circuit, the less bright they all become. In a parallel circuit containing bulbs: - Switches can be placed in different parts of the circuit to switch each bulb on and off individually, or all together - If one bulb breaks, only the bulbs on that same branch of the circuit will be affected - Each branch of the circuit receives the same voltage, so if more bulbs are added to a circuit in parallel they all keep the same brightness

What is the difference between direct and alternating current

In an alternating current the current value increases and decreases and then does the same again but in the opposite direction. Eg: like in mains electricity and power stations Cells and batteries provide currents and voltages that are always in the same direction and have the same value. This is called a direct current. On a graph it would be a horizontal line Direct current (DC) is the flow of electric charge in only one direction. Alternating current (AC) is the flow of electric charge that periodically reverses direction.

Why can't electricity flow though insulators?

In insulators all the electrons are held tightly in a position and are unable to move from atom to atom. Charges are therefore unable to move though insulators.

What are fixed resistors?

In many circuits you will find components like bulbs, voltage, resistors etc. they are called fixed resistors. They are included in circuits in order to control the sizes of currents and voltages. The resistor in the current in the diagram to the right resistor so that both the current In the bulb and the voltage applied across it are correct. Without a resistor, the voltage across the bulb may cause too large a current and the bulb may 'blow' or break

What does the voltage tell us

In the external part of a circuit (outside the cell or battery) the voltage across each component tells u show much energy is it transferring. If the voltage across a component is 1 V this means that the component is transferring 1 J of energy each time 1 C of charge passes though it.

Current in a parallel circuit

Is shared between branches In a parallel circuit the currents will not be the same in different parts of the circuit. The types of components will affect the current. In a parallel circuit the number of electrons that flow into a junction each second must be equal to the number that leave each second. This means that the current entering a junction must always be equal to those that leave the junction. For example: current entering a junction is 0.6, the current that leaves the junction must be 0.2 + 0.4 = 0.6

What should applications should be connected with

Many applications have metal casing. This should be connected to the earth wire so that if the live wire becomes damaged or breaks and comes into contact with the chasing the earth wire provides a low-resistance path for the current. The current is likely to be large enough to blow the fuse and turn the circuit off. Without the earth wire anyone touching the casing of the faulty appliance would receive a severe electric shock as the current passed though them to earth.

A 230 v television takes a current of 3 A. calculate the power pf the television.

P = IV = 3 x 230 = 690 W

1) Example: Calculate the charge flowing though a wire in 5 s if the current is 3 A 1) How many electrons flow though the wire in this time

Q = It Q = 3 x 5 Q = 15 C 1 C of charge is carried by 6 x 10^18 electrons 15 C requires 15 x 6 x 10^18 electrons = 90 x 10^18 electrons So 90 million million electrons will flow along the wire in 5s.

Voltage in a series

Shared between components

why do applications have plastic insulation

Some modern appliances now use casing made from an insulator such as plastic rather than from metal. If all the electrical parts of an appliance are insulted in this way, so that they cannot be touched by the user, the appliance is said to have double insulation. Appliances that have double insulation use a two wire flex. There is no need for an earth wire.

Where should switches be placed?

Switches in mains circuits should always be placed in the live wire so that when the switch is open no energy in the form of electricity can reach an appliance. If the switch is placed in the neutral wire, energy can still enter a faulty appliance, and could possibly cause an electric shock.

Current in a series circuit

The current is the same in all parts of the current. The current is not used up. The size of the current in a series circuit depends on the voltage supplied to it, and the number and type of components in the circuit. If a second identical cell is added in a series the voltage will double and so the current will also double

What is voltage

The energy transferred per unit charge passed and it therefore follows that one volt is equal to one joule per coulomb. one volt is one joule per coulomb

Voltage: charge: current: resistance: power: joule

The energy transferred per unit charge passed Value for electricity flowing in a circuit The rate of flow of charge opposition to flow the rate at which energy is transferred A joule is a unit that measures work and energy

What is a fuse?

The fuse is a cylinder shape which contains a thin piece of wire made form a metal that has a low melting point. If there is too large current in the circuit the fuse wire becomes very hot and melts. The fuse 'blows'; the circuit is now incomplete so there is no current. This prevents from you getting shocked and reduces the possibility of an electrical fire. Once the fault causing the increase in current has been corrected, the fuse has to be replaced.

Current/voltage graph for a Resisitor:

The graph is a straight line. It has a constant slope, so the resistance of this component does not change.

Current/voltage graph for a filament bulb:

The graph is not a straight line. The resistance of this bulb changes. At higher currents and voltages the slope of the graph shows us that the resistance of the filament bulb increases - that is, as the temperature of the filament increases the current decreases. The flatter the slope, the higher the resistance

Practical - experiment to investigate how current varies with voltage for different components - RESULTS

The graph shows a straight line passing though the origin. The slope of the graph tells us about the reisistance of the wire. The steeper the slope the smaller the resistance.

What do the earth, live and neutral wire do?

The live wire provides the path along which electrical energy from the power station travels to the appliance The neutral wire completes the circuit. The earth wire usually has no current in it. It is there to protect you if an appliance develops a fault. It provides a path for current to escape without passing though the user

equation for power

The power (P) of an appliance is related to the voltage (v) across it and the current (I) flowing though it. The equation is: Power, p (watts) = current, I (amps) x voltage, v (volts) P = I x v

How to calculate the total energy transferred by an appliance:

The power of an appliance (p) tells you how much energy it transfers each second. This means that the total energy (E) transferred by an appliance is equal to its power multiplied by the length of time (seconds) the appliance is used Energy, E (joules) = power, p (watts) x time, t (seconds) E = p x t Or since P = I x v E = I x V x t

What are the different wires in a house?

The wiring in a house is designed to let current pass though it easily. As a result, the wires do not become warm when appliances are being used. We say that the wires have a low resistance. However, in some appliances, for example kettles and toasters. We want wires (more usually called heating elements) to become warm. The wires of a heating element are designed to have a high resistance so that as the current passes though them energy is transferred and the element heats up. We use this heating effect of current in many different ways in our home. Other common appliances that make use of the heating effect of electricity include kettles, dishwashers, electric cookers, washing machines, electric fires and hair dryers.

What is the difference between a series and a parallel circuit

There are those circuits where there are no branches or junctions and there is only one path the current can follow. These simple 'single loop' circuits are called series circuits. Circuits that have branches or junctions and move more than one path are called parallel circuits.

What is charge measured in

Therefore, we measure the electric charge (Q) in much bigger units called coulombs (C). one coulomb of charge is equal to the charge carried by approximately 6 x 10 ^18 electrons

What are diodes?

They are resistors that only let current flow in one direction eg:When a diode is connected in a circuit (a), the diode offers little resistance to the charges flowing though it. But if the diode is connected opposite way round, the diode has very high resistance and the rate at which the charges can flow though the diode is much less-that is the current is very small.

Current/voltage graph for a diode:

They have a high resistance when the current is in one direction and a low resistance when it is the opposite direction

When a voltage of 12 V is applied across a doorbell there is a current of 0.1 A. calculate the resistance of the doorbell

V = I x r R = V/I R = 12 / 0.1 R = 120 Ω

What is charge

Value for electricity flowing in a circuit

What do we put into a circuit to see if it is on?

We can put a small bulb or lamp in a circuit to show us if a circuit is 'turned on'. When there is a current the bulb glows.

Equation of resistance

We measure the resistance (R) of a component by comparing the size of the current in that component and the voltage applied across its ends. Voltage current and resistance are related as follows Voltage, V (volts) = current, I (amps) x resistance, R (ohms) V = I x R We measure resistance with ohms Ω

How to measure voltage

We measure voltages using a voltmeter. This is connected across ( in parallel with) the component we are investigating. A voltmeter connected across a cell or battery will measure the energy given to each coulomb of charge that passes through it. A voltmeter connected across a component will measure the electrical energy transferred when each coulomb of charge passes though it

How do we measure current in a circuit

We use an ammeter to measure the size of current. This is connected in series

What is the heating effect of electric current#

When an electric current is passed through a conductor, the conductor becomes hot after some time and produce heat. This happens due to the conversion of some electric energy passing through the conductor into heat energy. This effect of electric current is called heating effect of current.

Electric current though a bulb

When current passes though the very thin wire (filament) of a traditional light bulb it becomes very hot and glows (shines) white. The bulb is transferring electrical energy to heat and light energy.

what is a resistor

a component of an electrical circuit that has resistance and is used to control the flow of electric current

Calculate the energy transferred by a 60W bulb that is turned on for a) 20s and b) 5 mins

a) E = p x t = 60 x 20 = 1200 J or 1.2 kJ b) E = p x t = 60 x 5 x 60 = 18 000 J or 1.8 kJ

how are charges carried in a wire?

charges are carried by very small particles called electrons. Electrons flow easily though all metals. Therefore metals are good conductors of electricity. Electrons do not flow easily though plastics - they are poor conductors of electricity. A very poor conductor is known as a insulator and is often used in situations where we want to prevent the flow of charge - for example, In the casing of a plug In metals, some electrons are free to move between the atoms. Under normal circumstances this movement is random - that is, the number of electrons flowing in any one direction is roughly equal to the number flowing in the opposite direction, there is therefore no overall flow of charge.

What is an electric current?

flow of charge

what is an electric current in a metallic conductor? what carries a current

flow of charge of negatively charged electrons charges are carried by very small particles called electrons.

Current in a resistor results how can this be used?

in the electrical transfer of energy and a increase in temperature Resistance causes transfer of electrical energy to heat energy. Some components are designed to have a high resistance to make sure this happens, for example electrical heaters that have lots of resistors to ensure a high resistance so a lot of heat is produced.

a fuse has a _____ melting point

low

All components of a circuit offer

offer some resistance to the flow of charge. Some (for example, connecting wires) allows charges to pass though very easily losing very little of their energy. We describe connecting wires as having very low resistance. The flow of charge though some components is not so easy, and a large amount of energy may be used to move the charges though them. This energy is transferred, usually as heat. Components like these are said to have a high resistance.

What is voltage

pressure that pushes electricity through an electrical circuit The volt is a joule per coulomb

Voltage in parallel

same

What are the two types of electrical circuits?

series circuits and parallel circuits

What does Ohm's law state?

the current through a resistor at a constant temperature is directly proportional to the potential difference applied The current is directly proportional to the voltage and inversely proportional to the resistance.

The size of an electric current indicates

the rate at which the charge flows.

What is power? what is it measured in?

the rate at which work is done Power is measured in joules per second or watts (W) Devices that transfer lots of energy very quickly have their power rating expressed in kilowatts (kW)

What is a joule

unit of energy