Energy Transferred by Working
Electricity and Magnetism

Power and pathways in electric motors

Teaching Guidance for 14-16 Supporting Physics Teaching

Practical limitations on electric motors are related to energy and to power

What's the big drawback to using electric motors in cars? This is a question worth addressing with students and it provides a further opportunity to practise a simple power in pathways calculation.

The BMW Mini E has an electric motor rated at 150 kW.

Teacher: Can you explain what rated at 150 kW tells us about the motor of the car?

Martha: The power of the Mini is 150 kW.

Teacher: There's more to it…

Martha: The maximum possible power is 150 kW.

Teacher: Yes, that's it! When the Mini is moving at its top speed the electrical store of the battery is being emptied at a rate of 150,000 J each second. So if the battery stores about 100,000 kJ how can we calculate how long the car will run for at this top steady speed?

Az: Divide 100,000 kilojoule by 150,000 joule / second.

Teacher: That's exactly right, and if we do the calculation it comes to 100 000 joule150 000 joule / second. That's 667 seconds, or just over 11 minutes.

The message from the calculation is clear: the car doesn't run for long at top speed before needing a recharge.

Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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