Power and pathways in electric motors(Teaching tip)

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 / second667 second… 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.