A student measures the power of a microwave oven - AQA - A-Level Physics - Question 9 - 2017 - Paper 2

The initial temperature of water is 23 °C and the final temperature is 79 °C. The change in temperature (ΔT) is calculated as follows:

$$riangle T = 79 °C - 23 °C = 56 °C$$

Using the specific heat capacity formula:

$$Q = mc riangle T$$

where:

• Q = thermal energy (in joules)
• m = mass of water (in kg)
• c = specific heat capacity (4200 J kg⁻¹ K⁻¹)

Substituting the values:

$$Q = 0.2 ext{ kg} imes 4200 ext{ J kg⁻¹ K⁻¹} imes 56 K = 47040 ext{ J}$$

Power (P) is the rate of energy transfer over time, calculated as:

P = rac{Q}{t}

where:

• t = time (in seconds)
• This heating process lasted for 1 minute, which is 60 seconds:

P = rac{47040 ext{ J}}{60 ext{ s}} = 784 ext{ W}

Thus, rounding this value gives an approximate average power of 780 W.

Given the calculated average power of approximately 780 W, the correct answer is:

A) 780 W