Electrical power & energy transfers

Some types of kettle are battery powered.

The kettle contains 0.50 kg of water at 20 C.

Calculate the minimum change in thermal energy needed to raise the temperature of the water to 100 C.

The specific heat capacity of water is 4200 J / kg C.

Use the equation:

change in thermal energy = mass specific heat capacity change in temperature

Explain why the actual amount of thermal energy needed to raise the temperature of the water would be greater than the calculated value in (a)(ii).

What is the time taken for the temperature of the water in cup B to decrease from 80 C to 60

Time taken =........ minutes

One cup is made of metal and the other cup is made of plastic.

The metal cup is a better conductor than the plastic cup.

Which cup, A or B, is made of metal?

Give a reason for your answer using information from the graph.

Suggest two variables the students should control in their experiment.

The students wrap the cups in thermal insulation and repeat the same experiment, as shown in the diagram.

Predict how these new temperature readings after 5 minutes compare to the original results on the graph.

Give a reason for your answer.

Fig. 16.1 shows a heater plugged into an electrical socket.

Fig. 16.1

The heater is operating normally.

What are the names of the two wires in the plug that carry a current?

Tick (✓) one box.

A live wire is accidentally connected to the earth wire in the house. This causes a large current to flow in the earth wire.

This can cause electrocution.

Suggest another reason why this is dangerous.

A 2.4 kW heater is used for 2.5 hours each day for 1 week.

Calculate the energy transferred in kW h by the heater in 1 week.

Use the equation: energy transferred = power time

Energy transferred in 1 week =........ kW h

A residual current device (RCD) is a safety feature found in homes.

The RCD turns off the electricity quickly if it detects a fault due to a difference between the current in the live wire and the current in the neutral wire.

Fig. 16.2 shows the effects that mains current has on a human body.

Fig. 16.2

Current passing through a human body (mA)

Describe one trend shown by Fig. 16.2.

A current of 10 mA is passing through a human body.

How long can the current pass through the body before the person has difficulty breathing?

Calculate the charge flowing when 150 mA is in the human body for 0.3 s.

Use the equation: charge flow = current time

Sensitive RCDs are designed to shut off the electricity supply within 40 ms.

Suggest why this is an important feature for currents below 200 mA. Use Fig. 16.2.

An electrician has a choice of two different RCDs, A and B, to use in a house.

Suggest which RCD the electrician should use in the house.

Give two reasons. Use Fig. 16.2.

Fig. 11.1 shows a heater plugged into an electrical socket.

Fig. 11.1

The heater is operating normally.

What are the names of the two wires in the plug that carry a current?

Tick (✓) one box.

A live wire is accidentally connected to the earth wire in the house. This causes a large current to flow in the earth wire.

This can cause electrocution.

Suggest another reason why this is dangerous.

A 2.4 kW heater is used for 2.5 hours each day for 1 week.

Calculate the energy transferred in kW h by the heater in 1 week.

Use the equation: energy transferred = power time

Energy transferred in 1 week =........ kW h

A residual current device (RCD) is a safety feature found in homes.

The RCD turns off the electricity quickly if it detects a fault due to a difference between the current in the live wire and the current in the neutral wire.

Fig. 11.2 shows the effects that mains current has on a human body.

Fig. 11.2

Current passing through a human body (mA)

Describe one trend shown by Fig. 11.2.

A current of 10 mA is passing through a human body.

How long can the current pass through the body before the person has difficulty breathing?

Calculate the charge flowing when 150 mA is in the human body for 0.3 s.

Use the equation: charge flow = current time

Sensitive RCDs are designed to shut off the electricity supply within 40 ms.

Suggest why this is an important feature for a current of 200 mA. Use Fig. 11.2.

An electrician has a choice of two different RCDs, A and B, to use in a house.

Suggest which RCD the electrician should use in the house.

Give two reasons. Use Fig. 11.2.

The frequency of electromagnetic waves can be written as 1 × 10^N Hz. 17 N is an integer (a whole number).

The graph shows the typical values of N for different electromagnetic waves.

Which electromagnetic wave has the highest frequency?

Which electromagnetic wave is the most dangerous?

Ultraviolet waves have a greater frequency than visible waves.

How many times greater?

10² 10³ 10¹³ 10¹⁴ 10¹⁶

What is the frequency of a typical radio wave on the graph?

Write your answer as an ordinary number without standard form.

Which sentence is true about electromagnetic waves? Tick (✓) one box.

A 0.8 kW microwave oven is used to cook food. The microwave oven transfers 0.56 kWh when it is used to cook food.

Calculate the time the microwave oven is used for.

Use the equation: energy transferred = power × time

The frequency of electromagnetic waves can be written as 1 × 10^N Hz. 12 N is an integer (a whole number).

The graph shows the typical values of N for different electromagnetic waves.

Which electromagnetic wave has the highest frequency?

Which electromagnetic wave is the most dangerous?

Ultraviolet waves have a greater frequency than visible waves.

How many times greater?

10² 10³ 10¹³ 10¹⁴ 10¹⁶

What is the frequency of a typical radio wave on the graph?

Write your answer as an ordinary number without standard form.

Which sentence is true about electromagnetic waves?

Tick (✓) one box.

A 0.8 kW microwave oven is used to cook food. The microwave oven transfers 0.56 kWh when it is used to cook food.

Calculate the time the microwave oven is used for.

Use the equation: energy transferred = power × time

Student A uses the electric hot plate in Fig. 11.1 to increase the temperature of water in a saucepan.

Fig. 11.1

Student A wants to calculate the thermal energy transferred to the saucepan of water.

These are the steps in their method:

1 Measure the volume of water with a balance. 2 Measure the starting temperature of the water with a thermometer. 3 Use the equation:

change in thermal energy = mass specific heat capacity change in temperature

Student A ’s method is incorrect.

Identify the two mistakes the student has made and write down the correction for each mistake.

Student A suggests wrapping insulation around the saucepan in Fig. 11.1.

Suggest another way student A can improve their experiment.

Student B uses the immersion heater in Fig. 11.2 to increase the temperature of water in a beaker.

Fig. 11.2

Describe an experiment to measure the energy transferred to the immersion heater, using the equipment in Fig. 11.2.

In your answer include:

• a method

• a circuit diagram

• an equation from the Data Sheet

• the symbol for a resistor to represent the immersion heater in your circuit.

A satellite receiver can be used to watch television channels. It works using a remote control.

Channel 4 uses a wave with a frequency of 1.0714 × 10¹⁰ Hz. (a)

The speed of the wave is 3.0 × 10⁸ m / s.

Calculate the wavelength of the wave.

Use the equation: wave speed = frequency × wavelength Give your answer to 3 decimal places.

The remote control emits infra-red or radio waves when a button is pressed.

The remote control also contains a light which flashes when a button is pressed.

Explain why the light is needed to show when a button is being pressed.

A student uses a smart meter to check their energy use at home.

The student thinks they can save energy by unplugging their satellite receiver from the mains supply for 8 hours at night.

Calculate the amount of energy saved by unplugging the receiver compared to switching the receiver off for 8 hours at night. Use the Data Sheet.

Give your answer in kWh.

The government wants all homes to have a smart meter by 2024.

Describe how smart meters may change the way energy resources are used. Use your answer to (c)(i).

A 2.5 kW electric radiator is used for 2 hours.

Calculate the energy transferred.

Use the equation: energy transferred = power × time

A 1.25 kWh

B 5 kWh

C 1250 kWh

D 5000 kWh

Electrical circuits can be used to transfer energy.

Which row of the table correctly shows the information needed to calculate the amount of energy

An 800 W microwave oven is used for 30 minutes.

What is the energy transferred in kWh?

Use the equation: energy transferred = power × time

A 0.4 kWh

B 24 kWh

C 240 kWh

D 400 kWh

A student wants to check the electrical power of a mains filament lamp.

He sets up the circuit in Fig. 11.1.

Complete the table to describe how he sets up the equipment in Fig. 11.1.

The student reads the meters in the circuit in Fig. 11.1.

• The value of the potential difference is 230 V.

• The value on the ammeter is 0.5 A.

Calculate the actual power of the mains filament lamp.

Use the equation: power = potential difference × current

Another lamp has a power of 100 W. 1 kW = 1000 W

What is the power of the lamp in kilowatts (kW)?

0.1 kW 1 kW 100 kW 100 000 kW

Suggest how the student can make his readings more precise.

Fig. 11.2 shows a night-light. It contains a 1.0 W lamp and a 3.0 V battery.

The student compares the mains filament lamp with the night-light. They are both used for the same amount of time.

Fig. 11.2

Use these words to complete the sentences.

You can use each word once, more than once, or not at all.

more than less than equal to

The energy transferred by the night-light is …………………………… the energy transferred by the mains filament lamp.

The power of the night-light is …………………………… the power of the mains filament lamp.

A 1500 W heater is used for 1.5 hours.

What is the energy transferred in kWh?

Use the equation: energy transferred = power × time

A 2.25 kWh

B 135 kWh

C 2250 kWh

D 135 000 kWh

A 30 kW boiler is used for 4 hours.

Calculate the energy transferred.

Use the equation: energy transferred = power × time

A 7.5 kWh

B 120 kWh

C 7500 kWh

D 120 000 kWh

This question is about electrical devices.

The diagram shows two different electrical devices.

Write down the source of electrical energy for each device.

torch........

Describe how energy is dissipated in the electric fan.

The electric fan is rated 230 V, 0.3 A.

Calculate the power of the electric fan.

Use the equation: power = potential difference × current

*(b) Here is a diagram of a tumble dryer and an electric drill. Each appliance is wired differently.

Explain whether the tumble dryer and electric drill are both safe to use.

Include the function of the earth, live and neutral wires in your answer.

For how long can you use a 2 kW electrical heater with 1 kWh of energy?

A 0.0005 hours

B 0.5 hours

C 2 hours

D 2000 hours

The table shows some information about electrical appliances in the home.

Appliance	Power (W)	Current (A)	Resistance ()
Hairdryer	1800	7.8	29.6
Heater	800	3.5	66.1
Iron	2000	8.7	26.4
Kettle	2500	10.9	21.0

A teacher says: ‘As power increases, resistance increases’

Use data from the table to explain your answer.

The kettle is switched on for 60 seconds.

Calculate the energy transferred.

Electricity companies do not use joules (J) in electricity bills. Electricity companies use kilowatt-hours (kWh).

A hairdryer has a power of 1800 W.

What is the power of the hairdryer in kilowatts (kW)?

Suggest why electricity companies charge for electricity using kWh.

The average electricity consumption per household changes every year.

The graph shows how this changes from the year 2008 to the year 2015.

Describe and explain the trend shown by the graph.

The table shows some information about electrical appliances in the home.

Appliance	Power (W)	Current (A)	Resistance ()
Electric drill	800	3.48	66.1
Iron	2000	8.69	26.5
Kettle	2500	10.86	21.1
Security light	500	2.17	105.8
Toaster	1650	7.17	32.1

Use the table to describe the relationship between power and resistance.

Explain this relationship. Use ideas about resistance in your answer.

The security light is switched on for 45 minutes every day for 7 days.

Use the equation: Energy transferred = Power × Time

Calculate the energy transferred in kWh.

Give your answer to 2 significant figures.

Explain the difference between direct voltage and alternating voltage.

The electric drill does not need an earth wire.

Explain why.

Mains electricity can be produced in a power station that burns coal.

An electric iron is plugged into the mains and switched on. The temperature of the iron increases.

Describe this process.

Use ideas about energy stores in your answer.