2.3 Energy Transfers

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

Table 1

The kettle in Table 1 takes two minutes to boil a mug of cold water from the tap. 

Determine how long it would take to boil a mug of tap water in the microwave.

The cost of electricity is 37 p per kilowatt hour (kW hr)

Calculate the cost of using the TV in Table 1 for 4 hours.

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.

When the energy generated by the solar cells is not needed, the energy is stored in a battery. 

A house has solar cells on the roof. 31 000 C of charge flows through the cable from the solar cells to the battery in a 24 hour period.

The potential difference in the circuit is 3.3 V.

Determine the power lost due to heating in the cable.

Give your answer to 2 significant figures.

The electric kettle in Figure 4 below uses the ac mains electricity supply.

Figure 4

The graph in Figure 5 below shows how the energy transferred by the kettle changes with time.

Figure 5

The gradient of the graph gives the power output of the kettle.

Calculate the power output of the kettle.

Calculate the resistance of the kettle.

Give your answer to 2 significant figures.

A wind turbine converts kinetic energy in the wind to electrical energy, which it then transfers to the National Grid.

The wind farm in Figure 5 below contains 5 wind turbines, each with an average power output of 3 MW.

Figure 6

Calculate the total energy generated by the wind farm in 30 minutes.

Show all your working.

In a single turbine, the current through the generator is 450 A.

Calculate the output voltage of one turbine.

A step up transformer is used to increase the output voltage to 132 kV for transmission.

Explain why the voltage is stepped up in this way.

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

Give your answer to 2 significant figures.

The National Grid is a system of transformers and cables that connects power stations to consumers. 

Figure 1 shows a simplified diagram of the National Grid.

Figure 1

State which transformer, X or Y, is the step-up transformer.

The transmission cables in Figure 1 have a current of 63.2 A at a potential difference of 400 000 V when travelling between the two transformers.

The resistance of the cable is 200 Ω.

Calculate the power transferred in heating up the cable.

Which statements are true about step-down transformers?

Tick (✓) three boxes.

Have more turns on the primary coil
Have more turns on the secondary coil
Used to decrease the potential difference
Used to increase the potential difference
Used to reduce wasted energy in transmission
Used to make energy supplied to houses safer