Direct and alternating potential difference

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.

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.

Figure 14 shows an electrical circuit used to heat the windscreen of a car.

Each resistor in the circuit represents a heating element.

The 12 V battery supplies direct potential difference.

What is meant by ‘direct potential difference’?

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

Which equation links charge flow (Q), energy (E) and potential difference (V)?

Tick (✓) one box.

Calculate the charge flow through the 12 V battery when the battery transfers 5010 J of energy.

Ice forms on the windscreen at a temperature of 0 °C.

The electrical circuit transfers 5010 J of energy to the ice.

A mass of 0.015 kg of ice melts.

Calculate the specific latent heat of fusion of water.

Use the Physics Equations Sheet.

The electrical circuit was left switched on while the ice changed from a solid to a liquid and increased in temperature to 5 °C.

Explain the changes in the arrangement and movement of the particles as the ice melted and the temperature increased to 5 °C.

Figure 4 shows an electrical circuit used to heat the windscreen of a car. Each resistor in the circuit represents a heating element.

The 12 V battery supplies direct potential difference. What is meant by ‘direct potential difference’?

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

Which equation links charge flow (Q), energy (E) and potential difference (V)? Tick (✓) one box.

Calculate the charge flow through the 12 V battery when the battery transfers 5010 J of energy.

Ice forms on the windscreen at a temperature of 0 °C. The electrical circuit transfers 5010 J of energy to the ice. A mass of 0.015 kg of ice melts. Calculate the specific latent heat of fusion of water. Use the Physics Equations Sheet.

The electrical circuit was left switched on while the ice changed from a solid to a liquid and increased in temperature to 5 °C. Explain the changes in the arrangement and movement of the particles as the ice melted and the temperature increased to 5 °C.

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.

Figure 5 shows part of a lighting circuit in a house.

What is the frequency of the ac mains electricity supply in the UK? Tick (✓) one box.

The mains electricity supply has an alternating potential difference. Which diagram shows an alternating potential difference? Tick (✓) one box.

The potential difference across the lamp is 230 V. The current in the lamp is 0.020 A. Calculate the power output of the lamp. Use the equation: power = potential difference current

The potential difference across the lamp is 230 V. Calculate the energy transferred by the lamp when 180 C of charge flows through the lamp. Use the equation: energy transferred = charge flow potential difference

An electrician needs to replace the light switch in Figure 5. Describe the possible hazard and the risk to the electrician of changing the light switch.

Figure 13 shows part of a mains electricity lighting circuit in a house.

A fault in the switch caused a householder to receive a mild electric shock before a safety device switched the circuit off. The mean power transfer to the person was 5.75 W. The potential difference across the person was 230 V. Calculate the resistance of the person.

An electrician replaced the switch. The electrician would have received an electric shock unless the circuit was disconnected from the mains supply. Explain why.

The current from an electric shock causes a person’s muscles to contract. The person cannot let go of the electrical circuit if the current is too high. Figure 14 shows how the maximum current at which a person can let go depends on the frequency of the electricity supply.

The UK mains frequency is 50 Hz. Explain why it would be safer if the UK mains frequency was not 50 Hz.

Figure 7 shows a person using an electric lawn mower.

The lawn mower is connected to the mains electricity supply.

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

The lawn mower has a switch on each side of the handle.

Figure 8 shows the circuit diagram for the lawn mower.

The motor in the lawn mower can only be turned on when the person using it holds the handle of the lawn mower with both hands.

Explain why.

The power input to the motor is 1.8 kW

The resistance of the motor is 32 Ω

Calculate the current in the motor.

The useful power output from the motor is 1.5 kW

Calculate the time it takes for the motor to transfer 450 000 J of useful energy.

The plug of an electrical appliance contains a fuse.

What is the correct circuit symbol for a fuse?

Tick one box.

The appliance is connected to the mains electrical supply. The mains potential difference is 230 V.

Calculate the energy transferred when 13 C of charge flows through the appliance.

Use the equation:

energy transferred = charge flow × potential difference

Figure 9 shows the structure of a fuse.

Write down the equation that links charge flow, current and time.

The fuse wire melts when 1.52 coulombs of charge flows through the fuse in 0.40 seconds.

Calculate the current at which the fuse wire melts.

The mass of the fuse wire is 0.00175 kg. The specific latent heat of fusion of the fuse wire is 205 000 J/kg.

Calculate the energy needed to melt the fuse wire.

Use the Physics Equations Sheet.