Energy in Circuits

Question 1

True or false? The higher the current in a circuit, the less energy is transferred to the thermal energy stores of the components.

Answer 1

False. A higher current means more energy is transferred to the thermal energy stores of the components

Question 2

Name one device which makes use of the heating effect of an electric current.

Answer 2

Any one from: e.g. a fuse / a toaster / a filament lamp / an electric heater

Question 3

A series circuit contains a lamp and a resistor. The p.d across the lamp is 8 V and the p.d. across the resistor is 4 V. Which components transfers the most energy?

Answer 3

The lamp transfers the most energy.
E = I x V x t and the p.d. across the lamp is larger than the p.d. across the resistor, whilst the current is the same as it’s a series circuit

Question 4

Describe the main energy transfer that happens when a battery-powered fan is switching on.

Answer 4

Energy is transferred electrically from the chemical energy store of the battery to the kinetic energy store of the fan’s motor.

Question 5

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

Answer 5

The heating effect reduces the efficiency of the components and means less energy is transferred to the useful energy stores of the components. The heating effect can also cause components to melt, which stops the circuit working

Question 6

A dentist’s drill is connected to a 200V power supply. The dentist drills a tooth. The drill transfers 2400 J of energy and a current of 4 A flows through the drill. How long does the dentist drill for? (energy transferred = current x potential difference x time)

Answer 6

3 s

E = I x V x t so E / (I x V) = 2400 / (4 x 200) = 2400 / 800 = 3 s