Current, resistance and potential difference

A student investigated how the current in an LED varies with the potential difference across the LED.

Figure 18 shows the circuit used.

Figure 19 shows the results.

Give one way the student could have varied the potential difference across the LED.

Use the Physics Equations Sheet to answer questions 10.2 and 10.3.

Which of the following equations links current (I), potential difference (V) and resistance (R)?

Tick () one box.

Determine the resistance of the LED when the potential difference across the LED was 3.0 V.

Use Figure 19.

The student reversed the connections to the power supply and varied the potential difference across the LED.

Explain why the ammeter displayed a value of 0.0 A for all the values of potential difference the student used.

A student investigated how the current in an LED varies with the potential difference across the LED.

Figure 5 shows the circuit used.

Figure 6 shows the results.

Give one way the student could have varied the potential difference across the LED.

Use the Physics Equations Sheet to answer questions 03.2 and 03.3.

Which of the following equations links current (I), potential difference (V) and resistance (R)?

Tick () one box.

Determine the resistance of the LED when the potential difference across the LED was 3.0 V.

Use Figure 6.

The student reversed the connections to the power supply and varied the potential difference across the LED.

Explain why the ammeter displayed a value of 0.0 A for all the values of potential difference the student used.

Figure 4 shows a student putting a coin into a vending machine that sells food.

The vending machine is connected to the mains electricity supply.

What is the frequency of the mains electricity supply in the UK?

Tick () one box.

What is the potential difference of the mains electricity supply in the UK?

Tick () one box.

The vending machine identifies the value of the coin by measuring the resistance of the coin.

The machine applies a potential difference of 0.45 V across the coin.

The current in the coin is 0.75 A.

Calculate the resistance of the coin.

Use the equation:

The temperature inside the vending machine is monitored using an electrical circuit.

Figure 5 shows part of the circuit.

The circuit symbol for the thermistor is wrong.

What is the circuit symbol for a thermistor?

Tick () one box.

How could the potential difference (pd) across the resistor be calculated?

Tick () one box.

At one temperature, the thermistor in Figure 5 has a resistance of 200 Ω.

What is the potential difference across the thermistor at this temperature?

Give a reason for your answer.

Tick () one box.

Figure 6 shows how the resistance of the thermistor varies with temperature.

When the temperature of the thermistor is 10 °C, the resistance of the thermistor is 600 Ω.

Calculate the change in resistance when the temperature increased from 10 °C to 15 °C.

A student had an unknown electrical component inside a sealed box.

Figure 14 shows the circuit the student used to identify the component.

The student varied the potential difference across the component and measured the current in the component.

Table 3 shows the results when the potential difference across the component was 6.0 V.

Calculate value X in Table 3.

Figure 15 shows the results.

Calculate the power of the component when the potential difference across the component is 3.0 V.

Use Figure 15 and the equation:

power = potential difference × current

Complete the sentence.

Choose the answer from the box.

As the potential difference across the component increases, the gradient of

What is the component in the sealed box?

Tick () one box.

A student investigated how the length of a wire affects the resistance of the wire at constant temperature.

Figure 18 shows the circuit used.

The student plotted a graph of resistance against the length of wire.

Describe a method the student could have used to collect the data needed to plot the graph.

Which graph shows the relationship between the resistance of a wire at constant temperature and its length?

Tick () one box.

The student used a cell that had a potential difference of 1.50 V.

Explain why the cell was not an electrical hazard to the student in the investigation.

A student investigated how the length of a wire affects the resistance of the wire at constant temperature.

Figure 3 shows the circuit used.

The student plotted a graph of resistance against the length of wire.

Describe a method the student could have used to collect the data needed to plot the graph.

Which graph shows the relationship between the resistance of a wire at constant temperature and its length?

Tick (✓) one box.

The student used a cell that had a potential difference of 1.50 V.

Explain why the cell was not an electrical hazard to the student in the investigation.

Figure 7 shows a student putting a coin into a vending machine that sells food.

The vending machine is connected to the mains electricity supply.

What is the frequency and the potential difference of the mains electricity supply in the UK?

The vending machine identifies the value of the coin by measuring the resistance of the coin.

The power dissipated by the coin is 340 mW when the current in the coin is 0.75 A.

Calculate the resistance of the coin.

Use the Physics Equations Sheet.

Coins that are dirty are not recognised by the vending machine.

Suggest one reason why.

Figure 8 shows part of a different circuit that is used to monitor the temperature inside

the vending machine.

The circuit symbol for a thermistor has not been included.

Draw the circuit symbol for a thermistor in the box below.

Figure 9 shows how the resistance of the thermistor varies with temperature.

The cooling system inside the vending machine turns on when the temperature of the thermistor is above 20 °C.

Determine the potential difference across the thermistor when the temperature is 20 °C.

Use the Physics Equations Sheet.

A student investigated how the current in a filament lamp varies with the potential difference across the lamp.

Figure 1 shows the circuit used.

What is component P ?

Complete the sentences.

Choose answers from the box.

The ammeter in the circuit measures.

The voltmeter in the circuit measures.

How will increasing the resistance of the variable resistor in Figure 1 affect each of the following quantities?

Tick (✓) one box in each row.

A charge flow of 15 coulombs passed through the filament lamp in a time of 60 seconds.

Calculate the current in the lamp.

Use the equation:

When the current in the filament lamp is 0.12 A, the potential difference across the lamp is 6.0 V.

Calculate the resistance of the filament lamp.

Use the equation:

The student repeated the investigation after replacing the lamp with a resistor at constant temperature and then a diode.

The student plotted a graph for each component.

Draw one line from each component to its graph.

Figure 2 shows an ammeter.

The ammeter is not connected to a circuit.

What type of error does the ammeter display?

Tick (✓) one box.

A student investigated how the current in a filament lamp varies with the potential difference across the filament lamp. Figure 6 shows the results.

Describe a method the student could use to obtain these results. You should include a circuit diagram.

Determine the resistance of the filament lamp when the potential difference across it is +3.0 V. Use the Physics Equations Sheet. Use Figure 6 on page 18.

The current in the lamp is 0.21 A when the potential difference across the lamp is 6.0 V. Calculate the energy transferred by the filament lamp in 30 minutes. Use the Physics Equations Sheet.

The power output of the lamp is 1.0 W when the potential difference across the lamp is 5.0 V. A student predicts that the power output would be 4.0 W if the potential difference was doubled. Explain why the student is not correct.

Figure 13 shows an electric car being recharged.

The charging station applies a direct potential difference across the battery of the car.

What does ‘direct potential difference’ mean?

Which equation links energy transferred (E), power (P) and time (t)?

Tick (✓) one box.

The battery in the electric car can store 162 000 000 J of energy.

The charging station has a power output of 7200 W.

Calculate the time taken to fully recharge the battery from zero.

Which equation links current (I), potential difference (V) and resistance (R)?

Tick (✓) one box.

The potential difference across the battery is 480 V.

There is a current of 15 A in the circuit connecting the battery to the motor of the electric car.

Calculate the resistance of the motor.

Different charging systems use different electrical currents.

• Charging system A has a current of 13 A.

• Charging system B has a current of 26 A.

• The potential difference of both charging systems is 230 V.

How does the time taken to recharge a battery using charging system A compare with the time taken using charging system B ?

Tick (✓) one box.

Figure 1 shows an electric car being recharged.

The charging station applies a direct potential difference across the battery of the car. What does ‘direct potential difference’ mean?

Which equation links energy transferred (E), power (P) and time (t)? Tick (✓) one box.

The battery in the electric car can store 162 000 000 J of energy. The charging station has a power output of 7200 W. Calculate the time taken to fully recharge the battery from zero.

Which equation links current (I), potential difference (V) and resistance (R)?

Tick (✓) one box.

The potential difference across the battery is 480 V. There is a current of 15 A in the circuit connecting the battery to the motor of the electric car. Calculate the resistance of the motor.

Different charging systems use different electrical currents.

• Charging system A has a current of 13 A.

• Charging system B has a current of 26 A.

• The potential difference of both charging systems is 230 V.

How does the time taken to recharge a battery using charging system A compare with the time taken using charging system B ?

Tick (✓) one box.

A student investigated how the current in a series circuit varied with the resistance of a variable resistor. Figure 8 shows the circuit used.

Figure 9 shows the results.

The battery had a power output of 230 mW when the resistance of the variable resistor was 36 Ω. Determine the potential difference across the battery.

The student concluded: ‘the current in the circuit was inversely proportional to the resistance of the variable resistor.’ Explain how Figure 9 shows that the student is correct.

Explain how closing the switch would affect the current in the variable resistor.

Figure 13 shows some overhead power cables in the National Grid.

Explain the advantage of transmitting electricity at a very high potential difference.

It is dangerous for a person to fly a kite near an overhead power cable. Figure 14 shows a person flying a kite.

The person could receive a fatal electric shock if the kite was very close to, but not touching the power cable. Explain why.

A scientist investigated how the potential difference needed for air to conduct charge varies with the distance between a cable and earth. Figure 15 shows the results.

The data in Figure 15 gives the relationship between potential difference and distance when the air is dry. When the humidity of air increases the air becomes a better conductor of electricity. Draw a line on Figure 15 to show how the potential difference changes with distance if the humidity of the air increases.

Figure 16 shows a cross-section through a power cable.

A 1 metre length of a single aluminium wire is a better conductor than a 1 metre length of the steel wire. The individual wires behave as if they are resistors connected in parallel. Explain why the current in the steel wire is different to the current in a single aluminium wire.

A student investigated how the current in a filament lamp varied with the potential difference across the filament lamp. Figure 11 shows part of the circuit used.

Figure 11

Complete Figure 11 by adding an ammeter and a voltmeter. Use the correct circuit symbols.

Figure 12 shows some of the results.

The student reversed the connections to the power supply and obtained negative values for the current and potential difference. Draw a line on Figure 12 to show the relationship between the negative values of current and potential difference.

Write down the equation which links current (I), potential difference (V) and resistance (R).

Determine the resistance of the filament lamp when the potential difference across it is 1.0 V. Use data from Figure 12.

A second student did the same investigation. The ammeter used had a zero error. What is meant by a zero error?

A student investigated how the current in a filament lamp varied with the potential difference across the filament lamp. Figure 1 shows part of the circuit used.

Figure 1

Complete Figure 1 by adding an ammeter and a voltmeter. Use the correct circuit symbols.

Figure 2 shows some of the results.

The student reversed the connections to the power supply and obtained negative values for the current and potential difference. Draw a line on Figure 2 to show the relationship between the negative values of current and potential difference.

Write down the equation which links current (I), potential difference (V) and resistance (R).

Determine the resistance of the filament lamp when the potential difference across it is 1.0 V. Use data from Figure 2.

A second student did the same investigation. The ammeter used had a zero error. What is meant by a zero error?

Figure 9 shows a circuit diagram.

In which position could a switch be placed so that both lamps can be switched on or off at the same time?

Tick (✓) one box.

Draw the circuit symbol for a switch in the box below.

In 30 seconds, 24 coulombs of charge flow through the battery. Calculate the current in the battery. Use the equation:

There is a potential difference of 3.6 V across the battery. Calculate the energy transferred by the battery when 60 coulombs of charge flows through the battery. Use the equation: energy transferred = charge flow × potential difference

A student built Circuit X and Circuit Y shown in Figure 10. The components used in each circuit were identical.

Figure 10

Circuit X Circuit Y

How would the reading on the ammeter in Circuit Y compare to the reading on the ammeter in Circuit X ?

Tick (✓) one box.

How does the total resistance of Circuit Y compare with the total resistance of Circuit X ?

Tick (✓) one box.

The student built another circuit which is shown in Figure 11.

Write down the equation which links current, potential difference and resistance.

There is a potential difference of 3.6 V across the lamp in Figure 11. The current through the lamp is 0.80 A

Calculate the resistance of the lamp.

Figure 12 shows a student after rubbing a balloon on his hair.

The balloon and hair have become charged.

Describe the force that acts on the student’s hair in Figure 12.

An earthed conductor was brought near the charged student. A spark jumped between the conductor and the student.

The potential difference between the conductor and the student was 2.5 kV The energy transferred by the spark was 0.0050 J

Calculate the charge transferred by the spark.

A defibrillator can transfer a charge to regulate a person’s heartbeat.

Figure 13 shows a defibrillator.

When the defibrillator is in use, a potential difference of 4800 V is applied across the person’s chest.

A charge of 0.16 coulombs passes through the person’s chest in 4.0 ms

Calculate the resistance of the person’s chest.

Draw a diagram to show how 1.5 V cells should be connected together to give a potential difference of 4.5 V.

Use the correct circuit symbol for a cell.

A student built the circuit shown in Figure 3.

Calculate the total resistance of the circuit in Figure 3.

Use the equation:

The resistance of P is 3.5 Ω.

Calculate the resistance of Q.

The student connects the two resistors in Figure 3 in parallel.

What happens to the total resistance of the circuit?

Tick one box.

Give a reason for your answer.

A student investigated how the resistance of a piece of nichrome wire varies with length.

Figure 17 shows part of the circuit the student used.

Figure 17

Complete Figure 17 by adding an ammeter and a voltmeter.

Use the correct circuit symbols.

Describe how the student would obtain the data needed for the investigation.

Your answer should include a risk assessment for one hazard in the investigation.

Why would switching off the circuit between readings have improved the accuracy of the student’s investigation?

Tick one box.

A student investigated how the resistance of a piece of nichrome wire varies with length.

Figure 3 shows part of the circuit that the student used.

Complete Figure 3 by adding an ammeter and a voltmeter.

Use the correct circuit symbols.

Describe how the student would obtain the data needed for the investigation.

Your answer should include a risk assessment for one hazard in the investigation.

Why would switching off the circuit between readings have improved the accuracy of the student’s investigation?

Tick one box.

Complete the sentence. Choose answers from the box.

The current through an ohmic conductor is directly proportional to the

that the
remains constant.

Figure 7 shows a current – potential difference graph for a filament lamp.

Explain how the resistance of a filament lamp changes as the potential difference across it increases.

Many householders are replacing their filament lamps with LED lamps which are more energy efficient.

What does more energy efficient mean?

A Light Dependent Resistor (LDR) is used to turn on an outside lamp when it gets dark.

Part of the circuit is shown in Figure 8.

The light intensity decreases.

What happens to the potential difference across the LDR and the current in the LDR?

Potential difference

What is the resistance of the LDR when the potential difference across it is 4 V?

Give a reason for your answer.

Calculate the current through the LDR when the resistance of the LDR is 5000 Ω.

Give your answer to 2 significant figures.