Students investigate the efficiency of an electric motor - Edexcel - GCSE Physics - Question 4 - 2018 - Paper 1

The graph for an alternating current of frequency 0.5 Hz will show a sine wave oscillating between positive and negative values. The x-axis represents time in seconds, while the y-axis represents current in amperes. The wave will complete one full cycle in 2 seconds, due to the frequency being 0.5 Hz.

An efficiency of 40% means that 40% of the electrical energy supplied to the motor is converted into useful mechanical energy, while the remaining 60% is lost as waste energy, often in the form of heat due to friction and other factors.

To calculate the energy supplied to the motor:

1. Convert the time from minutes to hours: 3 minutes = 3/60 = 0.05 hours.

2. Use the formula for calculating energy:

   $ext{Energy} = ext{Power} imes ext{Time}$

   In this case,

   $ext{Energy} = 15 ext{ W} imes 0.05 ext{ hours} = (15/1000) imes 0.05 imes 20 = 0.015 ext{ kWh}$

   The unit is kilowatt-hours (kWh).

To calculate the cost, use the formula:

$ext{Cost} = ext{Energy} imes ext{Cost per kWh}$

Substituting in the values:

$ext{Cost} = 0.015 ext{ kWh} imes 20 ext{ pence} = 0.30 ext{ pence}$