Electrical Power Physics Example 4

Follow the full solution, then compare it with the other examples linked below.

medium

A kettle is rated at

2000 \text{ W}

and operates on a

250 \text{ V}

supply. (a) What current does it draw? (b) What is its resistance? (c) How much energy does it use in

3 \text{ minutes}

1
(a) $I = \frac{P}{V} = \frac{2000}{250} = 8 \text{ A}$ .
2
(b) $R = \frac{V}{I} = \frac{250}{8} = 31.25 \text{ } \Omega$ . Or: $R = \frac{V^2}{P} = \frac{250^2}{2000} = 31.25 \text{ } \Omega$ .
3
(c) $E = Pt = 2000 \times (3 \times 60) = 2000 \times 180 = 360{,}000 \text{ J} = 360 \text{ kJ}$ .

Answer

(a)\; 8 \text{ A}; \quad (b)\; 31.25 \text{ } \Omega; \quad (c)\; 360 \text{ kJ}

Power ratings on appliances tell you the energy consumption rate. Combining

P = VI

with Ohm's law gives useful formulas:

P = I^2R

and

P = V^2/R

. Energy is power × time.

About Electrical Power

The rate at which electrical energy is converted to other forms of energy (heat, light, motion). Measured in watts (W).

A device draws [formula] from a [formula] outlet. What is the electrical power consumed?

A [formula] light bulb operates at [formula]. What is its resistance and the current it draws?

An electric kettle rated at [formula] operates for [formula]. How much energy does it use in kJ?