What is the resistance of the heater?

Question

An immersion heater used to boil water for a single cup of tea plugs into a 120V outlet and is rated at 500W.

a) What is the resistance of the heater?

b) Suppose your super-size, super-insulated tea mug contains 300g of water at a temperature of 20 degrees C. How long will this heater take to bring the water to a boil? You can ignore the energy needed to raise the temperature of the mug and the heater itself.

Answer 1

a) R = V² / P = (120 V)² / 500 W = 30 Ω

b) Q = mc∆T = 0.3 kg x 4,200 J/kg°C x (100°C - 20°C) = 1.0 x 10^5 J

t = Q / P = (1.0 x 10^5 J) / 500 W = 200 s = 3.3 min

Answer 2

Amps = Watts/Volts = 4.16666...