Power in Circuits

Question 1

True or false? The power of a device depends on the potential difference across it and the current through it.

Answer 1

True

The power of a device is the product of these values

Question 2

What is the equation that links power, current and resistance?

Answer 2

Power = (current)^2 x R or P = I^2 x R

Question 3

A 20W bulb is plugged into a 100 V power supply. Calculate the size of the current through the bulb.

Answer 3

0.2 A

P = VI so I = P / V = 20 / 100 = 0.2 A

Question 4

A 750 W microwave has a resistance of 30 Ω. Calculate the current through the microwave when it is turned on.

Answer 4

5 A

P = I^2 x R, so I = √P / R = √750 / 30 = √25 = 5 A

Question 5

Kingslee is a chef. He uses a 1500 W electric hob and a pan to cook pancakes. It takes 1 minute to cook a pancake. How much energy is transferred by the hob in this time?

Answer 5

90000J
1 minute = 60 seconds
P = E / t so E = P x t = 1500 x 60 = 90000J (=90kJ)

Question 6

Would it be quicker or slower for Kingslee to cook a pancake using a 1750 W hob that has the same efficiency? Explain your answer.

Answer 6

It would be quicker. It has a higher power (and the same efficiency), so it will transfer more energy to the pancake per second. It will therefore transfer the total amount of energy needed to cook it more quickly