HL Electromagnetic Induction + SL Electricity (units 11 + 5)
Reasons why a transformer is not 100% efficient
- Flux leakage: not all of the magnetic flux produced by the primary coil is linked with the secondary coil, resulting in a loss of energy - Joule heating: the wires in the primary and secondary coils have resistance, leading to a loss of power (P=I^2R) - Eddy currents: the loops of electrical current produce heat due to the resistance of the iron, resulting in a loss of energy. - Magnetic hysteresis: magnetizing the core requires the magnetic dipoles to change directions to line up. This requires work to be done, therefore energy is lost
What impacts the size of the force on a wire in a magnetic field?
- field strength (B) - the length of the wire in the field (L) (the number of wires x the length of wire) - the current in the wire (I) F = BILsinθ (QUESTION: when can there be a k)
How to calculate resistance and capacitance in a parallel circuit
1 / (Total resistance in a parrallel-circuit) = 1/R(1) + 1/R(2) + 1/R(3) Total capacitance in a parallel circuit = C1 + C2 + C3
What is an ampere (A) in terms of coulombs or electron flow
1 Amp = 1 Coulomb per Second 1 Amp = 6.2 × 10^-19 (elementary charge) electrons per second. 1 Coulomb = 6.2 × 10^-19 (elementary charge) electrons
How does the Motor Effect work
1) A coil of insulated wire is connected to the battery by two metal or graphite brushes. 2) The brushes press onto a split ring commutator, completing the circuit. 3) The alternating current passing through the wire causes a force to be produced (as the current is perpendicular to the magnetic field). 4) When the current changes direction due to the split-ring commutator, the direction of the force also changes.
How do transformers work?
1) Alternating current is supplied to the primary coil. 2) This creates an alternating magnetic field in the iron core, which is laminated to reduce eddy currents. 3) The constantly changing magnetic field induces an alternating emf in the secondary coil. 4) This induced emf drives an alternating current in the secondary coil. 5) The variation in emf from the primary to secondary coils is determined by the number of turns in each coil. (QUESTION: WHAT DOES LAMINATED MEAN)
How to answer Kirchoff's law questions
1) Label the diagram 2) Apply the rules at each junction and loop to work out what you do and don't know 3) Use simultaneous equations to solve the problem The arrows point in the direction of convection current (positive to negative) which is used for the calculations and is in the opposite direction of electron current (negative to positive) If something (eg I2) isn't going through a resistor, ignore it in your initial calculations An arrow pointing in the opposite direction of the others in the calculation, so is going in electron current direction from negative to positive, will be negative
Secondary Cell
A cell that has a reversible electrochemical reaction - it can be recharged
charge Definition Letter symbol Unit
A charge is a fundamental property possessed by some fundamental particles. Letter symbol: Q Unit: Coulombs (C)
solenoid
A coil of wire with a current
Galvanometer
A device that measures very small charges in a current (eg a microammeter)
Diode current and potential difference graph - why ohmic conductor or non-ohmic conductor?
A diode DOES NOT obey Ohm's law. A diode has a very high resistance in one direction, so current only flows in one direction.
Filament lamp current and potential difference graph - why ohmic conductor or non-ohmic conductor?
A filament bulb DOES NOT obey Ohm's law In a filament bulb, the resistance of the lamp increases as the temperature increases due to a higher voltage. Reversing the voltage (flipping the y axis) reverses the direction of the electric current but the shape of the curve stays the same.
Ferromagnetic definition
A material that contains permanent magnetic dipoles that can become oriented parallel to each other: Iron (steel), Cobalt, Nickel
Dielectric (definition)
A material that does not conduct electricity but can store it. They have a HIGHER PERMISSIVITY and therefore a higher capacitance. They often contain polar molecules which line up well, increasing capacitance. (There is a dielectric layer between capacitance plates) Capacitor materials are dielectric because they cannot conduct electricity but can store it.
Charge definition
A physical property that causes it to experience a force when in an electromagnetic field
What is a transformer? What is a step-up transformer? What is a step-down transformer?
A transformer: a device that transferred electrical energy from one circuit to another at a different voltage. Step-up transformer: increase the voltage, decrease the current (increase number of coils) - do so to reduce heat of wires and increase energy efficiency Step-down transformer: decrease the voltage, increase the current (decrease number of coils)
what is a voltmeter and where does it go in a series circuit
A voltmeter measures voltage / potential different and must go around the component you are measuring the potential difference of
Graphs of voltage waveform AFTER RECTIFICATION for: AC DC of half-wave rectification DC of full-wave rectification
AC is a sin wave. Half-wave rectification is only the positive halves of a sin waves. Full-wave rectification is a sin wave flipped to be entirely positive.
What direction does a compass point?
Along the field lines to the south magnetic pole. A compass points along magnetic field lines toward the north geographic pole of the Earth. Magnetically, this is actually the south pole. Therefore, a compass always points along magnetic field lines towards the south magnetic pole.
what is an ammeter and where does it go in a series circuit
An ammeter measures current and since current is the same throughout a circuit, it can go anywhere in a series circuit
Capacitor definition
An electrical component which has two metal plates separated by an insulating material
Definition of an ideal ammeter or voltmeter
An ideal ammeter should have zero resistance as it should not impact the current in the circuit. A ideal voltmeter should have infinite resistance as it should not change the amount of current going through the element between those two points. A voltmeter is in parallel to the component.
Lenz's law
An induced current always slows in a direction such that it opposes the change which produced it. ANOTHER-EQUATION-NOT-IN-DATABOOKLET:Fv = IV ε = Bvl ε = emf B = Magnitude of the magnetic field v = speed of wire l = length of wire When a magnet enters a coil of wire with no current passing through it, an instantaneous current is induced. The ammeter goes in one direction and the opposite direction when the magnet is removed (in the opposite direction)
What does the are under a line of a potential difference and charge graph show?
Area under the line = 1 /2 QV = Energy stored in capacitor
In the generator effect what happens to acceleration
As current is induced, kinetic energy is transferred into electrical energy, this decline in kinetic energy causes NEGATIVE acceleration due to conservation laws (less kinetic energy = slowing down)
why does resistance increase as temperature increases
As temperature increases, more kinetic energy leads to more frequent particle collisions, leading to more resistance
Equation using capacitance, charge, and potential difference
Capacitance = Charge / Potential Difference Farads = Coulombs / Volts C = Q / V (in data booklet)
What can capacitors do?
Capacitors can: - convert AC to DC - be charged and discharged - release a large amount of electricity in a short amount of time
Factors which increase the rate of energy transfer from capacitors Factors which decrease the rate of energy transfer
Capacitors that are further apart and have a smaller surface area = faster energy transfer Capacitors that are closer together and have a larger surface area = slower energy transfer Larger surface area also means that more charge is stored.
How can an object accelerate if the kinetic energy remains the same
Change direction
What is the relationship between flux and emf
Changing the flux (the number of magnetic field lines passing through a field) changes the emf.
Charge Capacity Definition and units
Charge capacity = the amount of charge a battery can hold. mAh (millliampere per hour) or Ah (ampere per hour)
conventional current Definition
Convectional current is in the opposite direction of electron flow - it is incorrect, but it is used for the mathematical calculations.
How to convert AC voltage to DC
Convert AC voltage to DC/root-mean-squared-voltage equivalent by dividing by AC voltage by √2
Current vs Voltage throughout a circuit
Current (the flow of electrons) is the same throughout a circuit, voltage (the amount of work done per unit charge) changes in a circuit depending on the resistance of a component.
What determines current?
Current is determined by potential difference and resistance
How can insulators be charged?
DO THIS ONE
Primary Cell / Dry Cell
Designed to be used once - more wasteful (requires 50x more energy to make than it contains)
Diode definition
Diodes allow conventional current through them in the direction of the arrow
Energy stored in a capacitor equation
E = 1/2 C V^2 = 1/2 x Capacitance x (Voltage)^2 in data booklet
How to calculate energy transferred
E = QV Energy transferred (Joules) = Charge (coulombs) x Potential Difference (voltage) NOT IN DATA BOOKLET
Calculating electric field strength in a uniform field
E = V / d E = Electric field strength V = Electromotive force d= distance between the plates NOT IN DATA BOOKLET
Why is the iron core laminated to prevent eddy currents?
Eddy current is loops of electrical current that produces heat due to the resistance of the iron resulting in a loss of energy. Laminating the iron core by electrically insulating the iron core with a coating varnish reduces these eddy currents and reduces energy loss.
EMF definition
Electromotive Force: the maximum potential difference between two points of the battery when no current flows from the source in the case of an open circuit. / the work done per unit charge in moving charge from one terminal of a cell to the other
Electromotive force of the battery equation and meaning
Electromotive force of the battery = ε = I(R+r) R = circuit resistance r= internal resistance
How does a capacitor work?
Electrons on the plate on the positive side are attracted to positive terminal of the cell, creating a potential difference and making the plate positively charged. The positive charge of the plate on the side of the cell's positive terminal attract the negatively charged electrons from the plate on the negative side of the cell. This causes electrons to move to the plate and make it negatively charged. This occurs at an equal rate, creating an equal but opposite charge. When the capacitor is disconnected from the power source, energy is released.
Electronvolt
Electronvolt (eV) is the energy gained when an electron is accelerated by the potential difference of 1 volt 1 eV = 1.6 x 10^-19 J
Why do transformers only work with AC?
Emf is generated in a magnetic field due to changing flux linkage. The magnetic field generated by DC current is stationary, therefore no emf is induced.
How to calculate the velocity or charge during motion of a charge in a field
F = qvBsinθ F = force q = charge v = velocity of charge B = magnitude of magnetic field (N/C) or (V/m)
What does faster rate of discharging indicate in relation to capacitance?
Faster discharging = higher RC value = higher capacitance (assuming resistance is constant)
Fermi Velocity definition
Fermi Velocity is the speed that free electrons are generally moving HOW TO CALCULATE
Uses of capacitors
Flash photography Defibrillators
Relationship between flux linkage and emf
Flux linkage (the total number of times that any field line crosses an area) is a cos graph whilst emf is a sin graph. When the plane is perpendicular to the field lines and parallel to the normal, emf is lowest and flux linkage is highest. When the plane is parallel to the field lines and perpendicular to the normal, emf is highest and flux linkage is lowest.
What direction does a charge go in a moving field
Force is always at right angles to velocity so the charge will move in a circle in the speed is constant 1) The force is perpendicular to an object so there is no work done and the kinetic energy remains the same. 2) As there is still a force but the kinetic energy is not changing, the force acts to accelerate the object by changing its direction (F=ma). 3) The object follows a circular path.
Full wave rectification definition
Full-wave rectification is the process where the whole of the AC wave is converted into pulsating, positive DC waves, using a diode bridge. 4 Diodes in a square, facing the output.
What does the gradient of a potential difference and charge graph show?
Gradient of the graph = 1 / Capacitance = Potential Difference / Charge
What material is normally used for a rotating commutator and why?
Graphite as it has little friction
What is the right hand grip rule?
Grip your hand with your thumb pointing in the direction of the current, the direction your fingers go is the direction of the magnetic field around the wire.
Half wave rectification definition
Half wave rectification is the process when the negative sections of the AC wave are removed by a diode, leaving only the positive half of the original current, using a diode. Power output reduced by 50%. (only one diode is used)
How does current influence the consistency of the voltage output
Higher current = less consistent voltage output / larger ripple effect
Why does the gradient of a non-ohmic resistor on a voltage (x) current (y) begin to plateau at higher voltages (not the case for all non-ohmic resistors, but for some of them eg a filament bulb)
Higher voltage for extended periods of time leads to increasing the temperature of the substance. Higher temperatures leads to more particle collisions, increasing resistance
Annotate the drift velocity equation
I = nAvq I = current n = number of charge carries per unit volume (m^3) A = Cross-sectional area of the wire v = Average drift velocity of the charge carriers q = charge of each carrier
How to calculate current
I = q / t Current (amp) = Charge (coulombs) / Change in time (s) q = It Charge (Coulombs) = Current (Amps) x Time (s) I = V / R Current (Amps) = Potential Difference (Volts) / Resistance (ohms) IN DATA BOOKLET (as I = q/t)
How to know if the interaction between two fields is attractive or repulsive
If the fields are going in the same direction there is a mutual attraction. If the fields are going in opposite directions there is a mutual repulsion.
How do ions conduct electricity and in what state?
In a molten / aqueous solution, the charged ions are free to move within the solution and can flow to create a current
What happens when the wire is polarized vs unpolarized CHECK THE RELEVANCE OF THIS
In an unpolarized wire, the dipoles are randomly oriented and there is no difference in charge / potential difference. In a polarized wire, the dipoles align and creates a potential difference.
How do you "smooth out" / make the voltage more consistent of the output of a rectifier to better resemble AC supply. How do you increase the consistency of the voltage output?
In order to "smooth out" the output of a rectifier to better resemble AC supply, a capacitor is placed in the circuit When the potential difference of the AC waveforms begins to falls, the capacitor discharges to maintain a more constant output To make a more consistent voltage output of the DC, you could use a larger capacitor or use an appliance that draws a smaller current
How does the Generator Effect work
Kinetic -> Electrical 1) The magnetic flux cuts through the wire, changing the magnetic flux of the coil. 2) When the magnetic flux through a conductor changes, a potential difference and emf is induced across it USE FLEMMINGS RIGHT HAND RULE
Kirchoff's laws
Kirchoff's first law: The sum of the currents enetering a junction is always equal to the sum of the currents out of the junction (conservation of charge). This explains why current in a series circuit is all the same and it is split between branches of a parallel circuit. ΣI = 0 (junction) (I 1 - I 2 - I 3 = 0) Kirchoff's first law: The sum of the electromotive force (emf) is in a closed circuit equal to the sum of the potential differences. This explains why voltage is split between components in a series circuit but is the same in branches of a parallel circuit. ΣV = 0 (loop) (emf - v = 0) (like the parallel and series rules learnt in igcse)
How is domestic power generated
Large solenoid with soft iron core inside to generate electricity 1) Coil of wire is driven by an eternal force 2) As the coil rotates, the magnetic flux linkage passing through the coil changes 3) By Faraday's Law, this induces an emf and causes a current to flow
What is the longer line in a cell vs the shorter line in a cell
Longer line on cell = positive terminal Shorter line on cell = negative terminal
When do you use Fleming's left hand rule vs right hand rule
MOTOR EFFECT = FLEMMINGS LEFT HAND RULE GENERATOR EFFECT = FLEMMINGS RIGHT HAND RULE
What are magnetic field lines the same as
Magnetic field lines = magnetic flux lines
Microscopic properties related to resistivity
Microscopic properties related to resistivity: - The density of the mobile electrons - Characteristics of the atomic lattice - Presence of impurities of fractures in the lattice - Internal energy of the lattice (lower internal energy = lower resistance, due to increased collisions reducing electron flow)
How to know what is the north and south pole of a solenoid
North pole aligns with the negative end of the battery South pole aligns with the positive end of the battery Counterclockwise coil at end point view = North pole (kind of makes the shape of an N) Clockwise coil at end point view = South Pole (kind of makes the shape of an S)
Which poles of the earth are poles of a bar magnet
North pole on earth = South pole of magnet (field lines going away) South pole on earth = North pole of magnet (field lines going towards)
Ohmic resistor definition vs Non-ohmic resistors definition
Ohmic resistors obey Ohm's law. Any increase in voltage creates a linear increase in the current. Ohmic reisstors always go through (0,0) on a voltage (x), current (y) graph. Non-ohmic resistors do not obey Ohm's law. Any increase in voltage creates non-linear increase in the current. The resistance of a material changes based on the current moving through it.
Ohms law
Ohms law states that the potential difference (voltage) across a component is directly proportional to the current through it - provided the temperature is constant. Only Ohmic resistors obey Ohms law
Relationships between poles
Opposite poles attract, like poles repel
Permanent magnet
Permanent magnets are made of magnetically "hard" materials (such as steel). These materials retain their magnetization once magnetized. Eg Steel
Peak power equation Average power equation
Pmax = I0V0 Maximum power = maximum current x maximum voltage Average power = 1/2 the maximum power = 1/2 x maximum current x maximum voltage
what has higher and lower potential - positive charge or negative charge, and where do they move
Positive charge = higher potential Negative charge = lower potential Particles with a positive charge (high potential) move from a place of higher electrical potential to lower electrical potential (positive to negative) Particles with a negative charge (low potential) move from a place of lower electrical potential to higher electrical potential (negative to positive)
What direction do electric field lines travel in? What direction do magnetic field lines travel in?
Positive to negative North to South
How does terminal potential difference changes over service time What happens when the current is increased
Potential difference across the terminal of a battery loses its initial value quickly, has a stable and constant value for most of its lifetime, followed by a rapid decrease to zero as the cell completely discharges. Higher current -> lower working value -> shorter service time
Difference between a variable resistor and a potentiometer's ability to change voltage
Potentiometer can change the voltage in the circuit to 0, a variable resistor cannot
What are the terms in the equation: Q = Q0 (e)^-(t/RC) Same equation for current (I) and voltage (V)
Q = charge Q0 = initial charge e = e t = time R = resistance of the resistor that the capacitor discharges through C = capacitance RC = (weird T symbol) = the time constant. Smaller time constant = the quick the exponential function will decay.
What is RC
RC is the time constant SI, it is the time it takes for the charge to reach 0.37 (e^-1) of its initial value
Rectifier definition
Rectifiers convert AC to DC (using diodes)
resistance Definition Letter symbol Unit
Resistance measures how difficult it is for electrons to move around the circuit. Letter symbol: R Unit: Ohms (Ω)
Resistance vs Resistivity definitions
Resistance measures how difficult it is for electrons to move around the circuit. Resistivity is the resistance per unit length of the material with a unit cross-section
Resistivity Definition Letter symbol Unit
Resistivity is the the resistance per unit length of the material with a unit cross-section Letter symbol: p Resistivity unit: Ω⋅m^2⋅m^-1 Ω⋅m Equation: p = RA / l
How to find the direction of magnetic field around the wire using your hands
Right hand with thumb up and fingers curled. Thumb points in direction of convectional current / the north pole and fingers curl in direction of magnetic field / wire.
Tesla
SI unit for magnetic field The magnetic field when the force of 1m of wire is carrying a current of 1 A is N
self-discharge definition What has a high rate of self-discharge?
Self-discharge is when chemical substances inside the batteries reacts even without any connections between the electrodes.
How is the voltage, charge, and resistance divided in a series circuit vs a parallel circuit? FINISH THIS
Series: Voltage is divided between the components Parallel: Voltage is not divided between components
what does the gradient of a voltage (x) current (y) graph show
Since Voltage is on the X axis, Current is on the Y axis, therefore the gradient is I / V I / V = 1/R
How do know which direction the current is going in by looking at the diagram.
Small dot = going out the paper (think of the head of an arrow coming towards you) Cross = going in the paper (think of the back feather / fetching of an arrow going away from you)
Soft magnet
Soft magnets are made from magnetically "soft" materials (such as iron) that lose their magnetization easily. They are suitable for temporary magnets such as electromagnets.
How does static electricity work?
Static electricity is the result of an imbalance between negative and positive charges in an object. Objects become charged through friction, creating static charge. One insulator becomes positively charged as the electrons leave it to go to the other insulator to make it negatively charged. Conductors discharge the static electricity as electrons are transferred through the conductors.
What is the Generator Effect? What is the Motor Effect?
The Generator Effect is inducing a current and generating emf by moving a wire in a magnetic field (kinetic -> electrical) The Motor Effect is when a current is passed through a wire in a magnetic field, causing motion (electrical -> kinetic)
permittivity definition
The ability of a substance to store electrical energy in an electric field.
Potential difference Definition Letter symbol Unit
The amount of work done per unit charge Letter Symbol: E or V Unit: Voltage (V) Potential difference = current x resistance
What does the area underneath a potential difference, service time graph give you
The area underneath a Potential Difference and Service Time graph is Charge
What does the area underneath a current, service time graph give you
The area underneath a current, service time graph is Charge Capacity Charge capacity = the amount of charge a battery can hold.
Drift velocity definition and equation meaning
The average velocity that electrons are moving due to being in an electric field (still random movement of electrons but in general it is in one direction) v = I / (nAq) v= drift velocity (m/s) I = Current (Amperes) n= charge density (C⋅m^−3) A = cross sectional area (m^2) q= charge (C) In the data booklet it is: I = nAvq
Capacitance definition
The charge stored per unit potential difference across a capacitor
Radial field
The circle that the electric field operates
How do you show a stronger force on a field diagram
The closer the lines, the stronger the force
Why does the wire vibrate during the motor effect?
The continual change in direction of the current, and therefore the force, makes the wire vibrate vertically.
power dissipated by a resistor
The energy difference between the end of the resistor per unit time. It can be assumed that all the energy goes into heating the resistor. P = (I^2)R Higher current = greater power dissipated (the heat effect of the current)
current Definition Letter symbol Unit
The flow of electrons (charge) in a given period of time. Letter symbol: I Unit: Amperes (A)
Relationship between charge, force and distance when in a vacuum or air
The force is proportional to the product of the two charges and is inversely proportional to the square of the distance between the charges. F = k ( (q1q2) / r^2 ) F = force (newton) k = coulomb constant q = charge (coulomb) r = distance between the center of the charges In data booklet
Electric field strength definition
The force per unit charge on a small positive test charge (proportional to flux density) E = F / q Electric field strength (NC^-1) = Force / Charge = N / C
What happens when something is magnetized
The magnetic dipoles align (going from North to South)
Why does the "soft" iron core mean?
The magnetic dipoles can realign easily
what is the magnitude of the force when the wire is parallel to the magnetic force (normal to the plane)
The magnitude is 0. F = BILsinθ sin180 = 0 Flux Linkage = magnitude of force, therefore when the wire is parallel to the magnetic force there is no flux linkage so the force is 0. However, emf is the rate of flux linkage change, therefore the emf is at its highest as there is the greatest rate of change.
fundamental charge
The magnitude of charge on an electron: 1.6 x10^-19
Faraday's Law
The magnitude of the induced emf is proportional to the negative rate of change of magnetic flux linkage through a conducting loop.
Why do metals conduct electricity?
The metallic lattice creates delocalised electrons (free to move and charge can flow to create a current)
How do the charges of a capacitor correlate with the charges of a battery
The negative side of the capacitor is the same as the negative / shorter side of the cell, the positive side. The positive side of the capacitor is the same as the positive / longer side of the cell, the positive side.
Why is the potential difference across a cell less than the emf
The potential difference across a cell less than the emf due to energy loss in internal resistance (emf is when no current is flowing through the cell)
Potential gradient Definition Unit
The potential gradient is the difference between two positions in a field, where objects move an area of high potential to an area of low potential The unit depends on what it is referring to, but it is typically in volts
How to calculate the size on force on a wire
The size of the force is dependent on: the size of the field, the length of the wire in the field, and the current in the wire F = BILsinθ B = field strength (N/C) or (V/m) I = current L = length of wire θ = Angle to the BETWEEN THE CURRENT AND THE MAGNETIC FIELD
What is a split-ring commutator
The split-ring commutator allows the coil of wire to continuously rotate by reversing the current in the wire. The wire touches the other side of the commutator and reverses the current in the loop of wire.
Flux linkage
The total number of times any field line crosses an area NΦ = number of times it crosses the area x magnetic field strength x flux = NBA = BA cosθN
What is a Farad
The unit of Capacitance (1 Farad = 1 Coulomb / Volt)
What is the Root Mean Square (rms) of an alternating current / emf
The value of direct current or emf that would supply the same power in each resistor
Electric potential
The work done per unit charge on a small positive test charge to move it from infinity to that point (Volts) V = W / q V = voltage W = work done (joule) q = charge (coulomb)
Internal resistance (r)
The-resistance-across the cell / power supply when the current is flowing (ohms) (symbol is a cell with a resistor)
How do ions become charged?
They contain a different number of protons as electrons
What is Fleming's left hand rule?
Thumb - Force (movement) (N) First finger - Magnetic Field (Tesla) MIddle finger - CONVENTIONAL Current (Amps) Thumb goes up and other two fingers are at right angles For thumb / force think "jog on" pointing up For middle finger / current think "currently f u"
What is Fleming's right hand rule?
Thumb : Force Pointer Finger: Field Middle Finger: CONVENTIONAL CURRENT (same as left hand rule just opposite direction - used for the generator effect)
How to calculate resistance in a series circuit
Total resistance in a series circuit = R1 + R2 + R3 1 / (total capacitance in a series circuit) = 1/c(1) + 1/c(2) + 1/c(3) (Capacitance equation is in the data booklet)
What to do if calculating the capacitance of a combined series and parallel circuit
Treat the parallel circuit as if it is only one component
What happens if you split a magnet in two
Two magnets are formed
How to calculate the relationship between charge, force and distance when NOT in a vacuum or air / free space
Use the equation F = (kqq/r^2) BUT replace the k with the k below (using the ε0 form the data booklet) Ke = 1/4πε0 Ke = coulomb constant ε0 = permissivity of freespace When they ask you what is the "permittivity of the material", solve for ε0 as the unknown and subbing the above equation in for k
Voltage equation
V = IR R = V / I Potential Difference: Voltage: V Current: Amps: I Resistance: Ohms: R IN DATA BOOKLET (as R = V/I)
Watts, Amps, Volts equation
Watt = Amp x Volts NOT IN DATA BOOKLET (?)
Why do we convert AC to DC
We convert AC to DC because semi-conductors only work with a DC current - computer microchips contain semi-conductors. Electricity is always generated as AC.
what is the unit for magnetic flux?
Weber (Wb)
What happens to current, charge and voltage when a capacitor is charged?
When a capacitor is charging, current DECREASES EXPONENTIALLY (over time) due to the accumulation of charge on one end leading to the negatively charged plate beginning to repel electrons (the same happens on the other side when discharging). When a capacitor is charging, charge increases as a positive logarithmic graph (over time) as charge increases due to the increases number of electrons, however the rate of change gradually decreases (the opposite happens when discharging). As the voltage in a capacitor increases with the same trend as charge, the voltage in the rest of the circuit decreases
Motor Effect
When a current is placed in a magnetic field it will experience a force
What happens when a battery discharges? What happens when a battery charges?
When discharging, electrons flow from negative terminal (eg lead) to positive terminal (eg lead oxide) in sulphuric acid When charging, electrons flow from positive terminal (eg lead sulfate) to negative terminal (eg lead sulphate) in water and sulphuric acid. In the circuit, the positive sides are connected and the negative sides are connected.
Why do they rotate the coil in the generator effect rather than moving it in and out
When you rotate the coil, it is always in the magnetic field so emf is always induced
Potential Difference definition
Work done per unit charge
free space defintion
a region that has no gravitational and electromagnetic fields
ohmic conductor example
copper wire
Coulomb's Law What is it? What is the equation? What assumptions does it make?
electric force between charged objects depends on the distance between the objects and the magnitude of the charges. F = k (qq/r^2) It assumes that the charges are at a point, and there is free space between them
What is the same word as field lines
flux lines
fundamental units of voltage
kg·m^2·s^−3·A^−1
How to increase emf during the generator effect (therefore what does emf depend on)
kinetic -> electrical To increase the emf, you need to cross-cut more flux lines / increase flux linkage by: - the strength of the magnetic field - increasing the coiling of the wire - moving the wire at a faster rate
Law of electrostatics
like charges repel, unlike charges attract
What happens if the force is perpendicular to motion
no work is done
how to calculate resistivity
p = RA / l Resistivity: p Resistance: R Area of the material that the current flows: length Length of material that the current flows: l IN DATA BOOKLET
flux density
the flux per unit area / the number of field lines passing through a unit area, the same as magnetic field strength (Wb⋅m^−2)
magnetic flux (Φ)
the magnetic field strength normal (perpendicular) to a surface multiplied by the area of that surface Φ = BA (Wb)
what is the energy difference in an electric field?
the work done in moving the charge from A to B
flux
total number of field lines Φ = magnetic field strength x area = BA
What does a current (x axis) and terminal potential difference (y axis) graph show you
y intercept = the actual emf of the battery when the current is 0 The gradient of a current and terminal pd graph is the internal resistance V = - rI + ε
What does this equation mean: Φ = BAcosθN
Φ = flux B = magnetic field strength A = area Θ = angle from the normal (if horizontal instead of normal do 90 - Θ ) N = the number of areas the field lines the field lines cross
Resistivity unit
Ω⋅m
How to calculate the emf in the generator effect
ε = Bvl ε = emf B = Magnitude of the magnetic field v = speed of wire l = length of wire
equation to calculate the relative permittivity of a material
ε0 εr = ε ε0 = permittivity of free space (constant) εr = relative permittivity of the material ε = permittivity of the material CHECK THIS - NOT IN DATA BOOKLET
