Energy Transfers in Circuits Practice Questions

State what is meant by the power of an electrical device, and give the unit of power.

The lamp in the torch transfers $360\,\mathrm{J}$ of energy in $2.0$ minutes.

Calculate the power of the lamp.

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

Describe the energy transfers that take place in the torch when it is switched on.

Calculate the energy transferred by the battery.

energy transferred = ............................................... J

The charge flows through the headlamp in $600\,\mathrm{s}$.

Calculate the power of the headlamp.

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

Explain, in terms of the electrons and the metal lattice, why the cable becomes warm.

After the charger is unplugged, the cable cools down again. Explain what has happened to the energy that was transferred to the cable.

Describe one advantage and one disadvantage of the heating effect of an electric current. Give an example of each.

Calculate the power of the microwave oven on its highest setting.

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

The oven is used on its highest setting for $5.0$ minutes.

Calculate the energy transferred in this time.

energy transferred = ............................................... J

Inside the oven, a motor turns the glass turntable while the food is heated. Describe the energy transfers taking place in the oven while it is in use.

The oven is switched to a lower setting. The potential difference stays at $230\,\mathrm{V}$ but the current falls to $2.6\,\mathrm{A}$.

Explain the effect of this on the power transferred by the oven.

Calculate the power dissipated in the extension cable.

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

The user replaces the extension cable with one of the same length that has thicker copper conductors.

Explain why this reduces the unwanted energy transfer from the cable.

Calculate the energy transferred by the kettle.

energy transferred = ............................................... J

Show, by calculation, which of the three appliances transfers the most energy during the evening.

A student says, "The kettle has the highest power rating, so it must transfer the most energy."

Explain why the student is wrong.

Calculate the power of the heating element.

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

Calculate the current in the heating element.

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

Use the equation $P = I^{2}R$ to calculate the resistance of the heating element.

resistance = ............................................... $\Omega$

Calculate the total energy that the cell can transfer.

energy transferred = ............................................... J

Calculate the total time, in seconds, for which the cell can run the motor.

time = ............................................... s

Not all of the energy transferred from the cell ends up in the kinetic store of the toy car. The wires of the motor coil become warm while the car is running.

Explain why this happens.