Energy Transferred by Working
Electricity and Magnetism

Why do car headlamps go dim when starting a car?

Teaching Guidance for 14-16 Supporting Physics Teaching

A selection of answers to an on-line question

Wrong Track: Because the starter motor uses a lot of electricity. There's only so much to go around and so your headlights dim.

Wrong Track: The car needs electrical power to start, therefore the car dumps the power going to the headlights. That's why.

Wrong Track: It's the current or power drain from the battery when you turn the ignition.

Wrong Track: When you start the car you are using only battery power and the battery can only deliver so many amps to the starter motor to turn the flywheel.

Right Lines: When the car engine is started, there's a very large current in the starter motor. So there's a very large current in the battery and a significant potential difference is produced across the internal resistance of the battery.

Building up an explanation: avoiding short cuts

Thinking about the learning

This question was posed on a web forum and gave rise to some interesting answers, which were more often than not going down the wrong track. The wrong track statements (which were presumably written by people with some interest or expertise in these matters) are interesting for the way in which they include the same kinds of vague or incorrect ideas as those reported in episode 01 from 11–14-year-old students.

So, we have:

  • The ideas of current and power being used interchangeably.
  • The idea that the battery is the source of current (or power), which can be drained.
  • Reference to electricity and electrical power, and battery power.
  • The battery as a source of amps.

Each of these statements suffers from a lack of clarity (from a scientific point of view) caused by the incorrect usage of technical terms such as current and power.

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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