Energy Transfers in Circuits (Edexcel GCSE Physics)

This question is about using the mains electricity supply.

(i) An electric kettle is used to boil some water.

The mains supply voltage is 230 V.

The power supplied to the kettle is 1.9 kW.

Calculate the current in the kettle.

Use the equation

current supplied to the kettle = ............................................ A[2]

(ii) A coffee machine takes 120 s to heat some water.

Mains supply voltage = 230 V.

Current in this coffee machine = 7.4 A

Calculate the energy transferred to the coffee machine in 120 s.

Use an equation selected from the list of equations at the end of the paper.

energy transferred to coffee machine=............................... J[2]

Figure 10 shows the inside of a mains plug.

The neutral wire is labelled.

Figure 10

(i) State the name of wire X and the name of wire Y.

wire X ..............................................................

wire Y ..............................................................

[2]

(ii) State the name of component Z.

component Z ........................................................[1]

A transformer is used to connect a laptop computer to the mains electricity supply.

The input voltage to the transformer is 230 V.

The output current from the transformer is 2.37 A.

The transformer has an output voltage of 19.0 V.

Calculate the input current to the transformer.

Use the equation

input current × input voltage = output current × output voltage

input current = .............................................................. A

A technician investigates different electrical devices that are used in a car.

The technician connects a device to the 12 V car battery.

The technician measures the current in the circuit and the potential difference (voltage) across the device.

Figure 17 shows the car battery and the device that is being tested.

Figure 17

(i) Draw on Figure 17 to show how the circuit should be completed so that the current in the circuit and voltage across the device can be measured.

[2]

(ii) The technician tests a headlamp.

The current in the headlamp is 4.8 A when connected to the 12 V battery.  

Calculate the power supplied to the headlamp.

power = .............................................................. W[2]

(iii) The technician tests an interior light.

The current in the interior light is 600 mA when connected to the 12 V battery.

Calculate the energy transferred to the interior light in 7 minutes.

Use an equation selected from the list of equations at the end of the paper.

energy transferred = ......................................................... J[2]

The technician connects four devices to the car battery.

Each device is connected to its own switch and its own fuse.

Figure 18 shows how the four devices, fuses and switches are connected.

The current in each device is shown next to the device.

Figure 18

(i) Calculate the current in the wires to the battery when all the devices are switched on.

current = .............................................................. A[1]

(ii) State how the overall resistance of the circuit changes when any one of the devices is switched off.

[1]

(iii) There is a current of 2.3 A in the radio when the radio is working correctly.

Which of these should the technician choose to protect the radio circuit?

(iv) Explain why the wires to the battery in a car are thicker than the wires that connect each device to its switch and its fuse.

[2]

Figure 12 shows a graph of current against potential difference for an electrical component.

Figure 12

Which electrical component will show this variation of current with potential difference?

A lamp is connected to a potential difference of 0.24 V.

The current in the lamp is 0.12 A. 

(i) Calculate the power of the lamp.

Use the equation

      P = I × V

power of the lamp = ............................................... W[2]

 (ii) The potential difference is changed to 0.30 V.

The current in the lamp is now 0.13 A.

The lamp is switched on for 35 s.

Calculate the energy that is transferred in this time.

Select an equation from the list of equations at the end of this paper.

energy transferred = ............................................... J[2]

(iii) The current in the lamp stays at 0.13 A.

Calculate the charge that flows through the lamp in 35 s.

Use the equation    

Q = I × t

charge = ............................................... C[2]

A student measures the current in the lamp for several values of potential difference across the lamp.

Figure 13 shows the student’s results.

Figure 13

The student uses the results in Figure 13 to write this conclusion.

‘As the potential difference across the lamp increases, the current in the lamp increases and the relationship is directly proportional.’

Comment on the student’s conclusion.

Table 1 shows a range of household appliances and their power ratings.

Table 1 

Complete Table 1 by adding the values for the current drawn for each appliance.

Give your answers to 2 significant figures.

Calculate the electrical work done by using the toaster on full power for 3 minutes.

   work done = .................................... J

Calculate the amount of charge that flowed through the toaster circuit during the 3 minutes of use.

Give your answer to 2 significant figures.

charge = .................................... C

The toaster is not 100% efficient. 

Using the idea of energy transfers, explain why the toaster is not 100% efficient and state where this wasted energy is transferred to.

During one year, 2.42 × 10¹⁹ J of energy was transferred from National Grid.

Calculate the power rating of the National Grid.

Give your answer in kW to 1 significant figure.

   power = .................................... kW

A step-up transformer sends a current of 63 A at a potential difference of 400 000 V through the transmission cables of the National Grid.

The transmission cables have a resistance of 200 Ω.

Calculate the power dissipated from the heating of the cables.

   power dissipated = .................................... W

Determine the maximum potential power leaving the step-up transformer.

   power = .................................... W

Determine the percentage efficiency of the cables.

Give your answer to the nearest whole percent.

   efficiency = .................................... %