Electricity

Question 1

E1-7
1. Define electric current
2. Define power

Answer 1

1. Current is the rate of flow of charge.
2. Power is the rate of transfer of energy.

Question 2

1. What is a coulomb?
2. What is Kirchhoff’s first law?
3. What is Kirchhoff’s second law?

Answer 2

1. One coulomb is the charge that flows past a point in one second when there is a current of one amp.
2. Kirchhoff’s first law states that the sum of the currents coming into and leaving a junction are the same.
3. Kirchhoff’s second law states that the sum of the EMFs is equal to the sum of the p.d.s around any closed loop in a circuit.

Question 3

1. Define potential difference
2. Define EMF
3. What is 1 volt equivalent to?

Answer 3

1. Potential difference is the energy transferred when the charge moves between two points.
2. EMF (electromotive force) is the amount of chemical energy converted to electrical energy around the whole circuit (1) per unit charge. (1) The unit is the volt.
3. 1 Volt is the potential difference between two points when 1 joule of work is done to move a charge of 1 Coulomb.

Question 4

What does the following formulae mean:
1. I (capital i) = △Q/△t
2. Q = Ne

Answer 4

1. Current in Amps (A) = Charge in Coulombs (C)/ Time in seconds (s)
2. Charge in coulombs (C) = Number of electrons x Charge on the electron

Question 5

What do these equations mean:
1. P = ΔW /Δt
2. P = IV

Answer 5

1. Power (P) = Change in electrical energy (W) / Change in time (t)
2. Power (P) = Current (I) x Voltage (V)

Question 6

1. What is this equation:
   ρ = RA/l.
2. Define resistivity.

Answer 6

1. The equation for resistivity: resistivity = (resistance x cross-sectional area) / length
2. Resistivity is the measure of how much a specific metal resists current flow.

Question 7

Explain the IV graph of a silicon diode

Answer 7

A diode is used in a circuit to allow current to flow only in a specific direction. This component only conducts current through it in one direction (when it is ‘forward biased’). Over the voltage conduction limit (0.6V), the current increases by a much greater proportion than the potential difference across it. When the diode is switched around, it does not conduct and is called reverse bias. This is shown by a zero reading of current or potential difference on the left side of the graph.

Question 8

Explain the IV graph of a thermistor

Answer 8

This component is an non-ohmic conductor. At a constant temperature, resistance is constant. Resistance decreases as temperature increases. It is a semi conductor, so the number of electrons increases meaning more charge carriers are able to carry current. This increase in available charge carriers, more than compensates for the increased lattice vibrations, and so resistance decreases as temperature increases. On the graph, it curves downwards (plateauing horizontally), then crossing the origin and curving upwards again.

Question 9

Explain how resistance relates to the length and cross sectional area of an object.

Answer 9

Resistance is directly proportional to the length of the object, and inversely proportional to the cross-sectional area of an object.

Question 10

How do you read a vernier scale?

Answer 10

1. First clamp the jars from the object. Then find the point where the lines on the two scales match, telling you the number of tenths of a millimetre.
2. Read off the centimetre mark to the left of the vernier scale 0.
3. Then read the millimetre mark to the left of the vernier scale zero.

Question 11

1. Define resistance
2. What does ohm’s law state?
3. What is 1 ohm equivalent to?

Answer 11

1. Resistance is the ratio of voltage across a component to current through a component.
2. Ohm’s law states that provided the physical conditions, such as temperature, remain constant, the current through an ohmic conductor is directly proportional to the potential difference across it.
3. A component has a resistance of one ohm if a potential difference of one volt make a current of one amp flow through the component.

Question 12

1. What is an ohmic resistor?
2. Explain the IV graph of a ohmic conductor

Answer 12

1. An ohmic resistor is a resistor that obeys ohm’s law.
2. The current through an ohmic conductor is directly proportional to the voltage. Therefore, the IV graph is a straight line with a positive gradient that goes through the origin.

Question 13

1. What does the IV graph of a filament lamp look like
2. Why does the resistance of a filament lamp depend on the current flowing through it?

Answer 13

1. For an IV graph (where the current is on the y-axis), the graph starts at an almost plateau, then curves inwards and increases in gradient towards the origin, goes through the origin, and then curves outwards like a hill and plateaus again.
2. When a current flows through a metal conductor (like the filament in a filament lamp), some of the electrical energy is transferred into heat energy and causes the metal to heat up. (1) The temperature rise increases lattice vibrations (1), so there are more frequent electron collisions with the positive ions, (1) and the resistance increases. (1)

Question 14

E8-13
1. What is a superconductor?
2. State some uses of superconductors

Answer 14

1. Superconductors are materials which have zero resistivity (1) at and below a critical temperature. (1)
2. The production of strong magnetic fields. The reduction of energy loss / the reduction of dissipation in the transmission of electric power.
   Particles accelerators, MRI scanners, electromagnets, and electrical cables.

Question 15

E1-7
1. Why does light intensity affect the resistance of a light dependent resistor?
2. Why does temperature affect the resistance of a thermistor?

Answer 15

1. As light intensity increases more electrons are liberated. This increase in charge carriers increases current and, therefore, decreases the resistance.
2. Warming the thermistor gives more electrons enough energy to escape from their atoms. This means that there are more charge carriers available, so the resistance is lower. (1) The increase in charge carriers more than compensates for the increased rate of collisions. (1)

Question 16

E8-13
1. Explain internal resistance
2. Define terminal pd
3. How do we calculate the total EMF and internal resistance of two cells in series?

Answer 16

1. Internal resistance is defined as: The resistance of the materials within the battery. It is internal resistance that causes the charge circulating to dissipate some electrical energy from the power supply itself. This is why the cell becomes warm after a period of time. Therefore, over time the internal resistance causes loss of voltage or energy loss in a power supply.
2. The terminal potential difference (p.d) is the potential difference across the terminals of a cell. If there was no internal resistance, the terminal p.d would be equal to the e.m.f, however, since a cell has internal resistance, the terminal p.d is always lower than the e.m.f.
3. For cells connected in series, the total voltage between the ends of the chain of cells is the sum of the potential difference across each cell.

Question 17

E8-13
1. What a does a graph of V against I for a cell look like?
2. How can we find EMF and internal resistance from this graph?
3. Why does terminal pd depend on current?
4. How do we calculate the total EMF and internal resistance of two cells in parallel?

Answer 17

1. The graph is a straight line with a negative gradient, where the maximum voltage is at 0 amps, and goes to zero volts at the maximum current.
2. EMF is the voltage when current is 0. The gradient of the slope is the internal resistance.
3. The battery terminal pd decreases because the more current the battery delivers to a component, the greater the voltage drop across the internal resistance of the battery and the lower the pd read by the voltmeter.
4. The pd across all of the components in each branch of a parallel circuit is equal to the emf of the power supply. The pd across multiple cells in parallel is the same as the pd across one cell in series. You do the reciprocal to find the internal resistance. 1/R total = 1/ R1 + 1 / R2

Question 18

E8-13
1. How do we calculate the total resistance of resistors in series?
2. How do we calculate the total resistance of resistors in parallel?
3. What is the ideal resistance for an ammeter and voltmeter?

Answer 18

1. R total = R1 + R2 + R3
2. 1/ R total = 1/R1 + 1/ R2 + 1/R3
3. The ideal voltmeter has infinite resistance so that no current is drawn from the circuit, giving accurate readings of voltage. The ideal ammeter has zero resistance to give accurate readings of current.

Question 19

E8-13
1. What is a potential divider?
2. What does a potential divider circuit look like?
3. How can we make a sensor from a potential divider circuit?

Answer 19

1. A potential divider is a combination of resistors in series connected across a voltage source (to produce a required pd).
2. A potential divider circuit has a voltage source (such a battery), and two resistors, with a voltmeter across one of the resistors.
3. The resistor that has a voltmeter across it is replaced with a sensor, such as a light-dependent resistor.

Question 20

E8-13
1. Explain how current splits between components in series and parallel

Answer 20

1. For series circuit: the current is the same for all components. For parallel circuits: The higher the resistance of the component, the lower the resistance . Therefore, you need to calculate the voltage across the component, and use V=IR to find the current— you cannot find a ratio of current split between components.

Question 21

E8-13
What is the formula for potential divider circuits?

Answer 21

Where:
R2 is the numerator and the resistance of the resistor over Vout
R1 is the other resistance in series
Vout is the output potential difference
Vin is the input potential difference

Question 22

How do you check if components will operate normally (when given the power and voltage)?

Answer 22

You use the P = IV equation to find their operating currents. If the circuit is series, then the same current will flow through both.

Question 23

E1
1. Explain how a cell creates a potential difference
2. Why are wires often made of copper?

Answer 23

1. A simple cell creates a potential difference through the separation of charge
   One end (terminal) of the cell has an excess of positive charge and the other an excess of negative charge
   Negatively charged electrons are repelled by the negative terminal and attracted to the positive terminal
   Therefore, when a wire is connected between the two terminals, the potential difference causes the flow of electrons (current).
2. Wires are often made of copper because of its low electrical resistance, making it a good conductor.

Question 24

What are some applications of thermistors?

Answer 24

Applications of thermistors include temperature sensors and resistance-temperature graphs.

Question 25

E8-13 1. Explain how the potential difference across each resistor in a potential divider circuit depends upon its resistance

Answer 25

1. The resistor with the largest resistance will have a greater potential difference than the other one from V = IR. If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share. In potential divider circuits, the p.d across a component is proportional to its resistance from V = IR

Question 26

E8-13 Explain the relationship between p.d. and R in a potential divider circuit for different components

Answer 26

As the temperature of a thermistor decreases, its resistance increases. As the light intensity of an LDR decreases, the resistance increases. If an LDR or thermistor's resistance increases, the potential difference through it also increases

Question 27

1. Define lost volts 2. What is the conservation of energy formula for emf?

Answer 27

1. Lost volts is defined as: The work done per unit charge / coulomb to overcome the internal resistance / resistance inside the battery (when current flows). 2. From conservation of energy: v = e.m.f − terminal p.d v = ε – V = Ir (Ohm’s law) Where: v = lost volts (V) I = current (A) r = internal resistance of the battery (Ω) ε = e.m.f (V) V = terminal p.d (V)

Question 28

1. Explain internal resistance 2. How would you find the lost volts from a battery?

Answer 28

1. Internal resistance is defined as: The resistance of the materials within the battery. It is internal resistance that causes the charge circulating to dissipate some electrical energy from the power supply itself, causing the cell becomes warm after a period of time. 2. The lost volts is the voltage lost due to internal resistance. Lost volts = current x resistance.