Energy Transfers in Circuits Flashcards

All 18 cards in this deck

What happens when there is an electric current in a resistor?

There is an energy transfer that heats the resistor.

In terms of particles, why does an electric current heat a metal wire?

The flowing electrons collide with the ions in the lattice, making the ions vibrate more.

What happens to the electrical energy when a current does work against electrical resistance?

It is dissipated as thermal energy in the surroundings.

Give two household appliances that make use of the heating effect of an electric current.

An electric heater and a kettle (also a toaster, iron or hair dryer).

Give one disadvantage of the heating effect of an electric current in a circuit.

Energy is wasted as thermal energy dissipated to the surroundings, so the device is less efficient.

Why can the heating effect of an electric current be dangerous?

Wires and components can overheat, melting insulation and causing fires.

What is the equation for energy transferred in a circuit, with units?

$E = I \times V \times t$: energy transferred (J) = current (A) × potential difference (V) × time (s).

True or false? In $E = I \times V \times t$ the time may be left in minutes.

False. The time must be in seconds, so minutes must be multiplied by 60.

What is power, and what unit is it measured in?

The energy transferred per second, measured in watts (W).

What is the equation linking power, energy transferred and time, with units?

$P = E \div t$: power (W) = energy transferred (J) ÷ time taken (s).

What is the equation for electrical power in terms of current and potential difference, with units?

$P = I \times V$: power (W) = current (A) × potential difference (V).

How is the power transferred by a circuit device related to the current in it and the p.d. across it?

Power = current × potential difference, so increasing either the current or the p.d. increases the power transferred.

What is the equation for electrical power in terms of current and resistance, with units?

$P = I^2 \times R$: power (W) = current squared (A²) × resistance (Ω).

Describe the energy transfer in a mains-powered electric motor, such as in a washing machine.

Energy is transferred electrically from the a.c. mains to the kinetic energy store of the motor, with some dissipated as thermal energy to the surroundings.

Describe the energy transfer in a battery-powered torch.

Energy is transferred from the chemical store of the battery, electrically, to the lamp, which transfers it to the surroundings by light and heating.

Describe the energy transfer in a mains-powered electric kettle.

Energy is transferred electrically from the mains to the heating element, heating the thermal energy store of the water.

What does the power rating of a domestic appliance tell you?

The energy it transfers each second in watts, so a higher power rating means more energy transferred per second.

Does a higher power rating mean an appliance is more efficient?

No. It transfers more energy per second, but it may transfer a smaller share of that energy usefully, so it can be less efficient.