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Sparks and lightning

Electricity and magnetism · level 3, lesson 3 of 6

A question to start

You walk across a carpet and touch a metal door handle. Snap! A tiny spark, and a little sting. Where did it come from?

The idea

Air is usually an insulator. But when enough charge builds up, the pull on the electrons gets so strong that the air itself starts to carry them. The charge rushes across the gap all at once, and the air glows: a spark.

  • Walking on carpet charges your body. Touching metal lets the charge jump across.
  • In a thundercloud, ice and water drops bump and rub together. The top of the cloud becomes positive, the bottom negative. When enough builds up, a giant spark jumps: lightning.
  • Lightning heats the air so fast that it bursts outwards with a bang: thunder.
+++ −−−− a giant spark: lightning

Worked examples

1. The door handle. Your body has built up charge. The metal handle is a conductor, so the charge jumps to it the moment your hand is close.

2. Counting to the storm. You see lightning at once, but thunder travels at about 340 m/s. A gap of 3 seconds means the storm is about 1 km away.

3. Dry days. Shocks from door handles are worst on cold, dry days. Damp air lets charge leak away before it builds up.

Try it

  1. What is a spark?
  2. Why do you sometimes get a shock from a door handle?
  3. What makes thunder?
  4. A storm's thunder arrives 6 seconds after the flash. About how far away is it?
Answers
  1. Built-up charge jumping through the air all at once, making the air glow.
  2. Your body has built up charge, and it jumps to the metal handle.
  3. Lightning heats the air so fast that it bursts outwards with a bang.
  4. About 2 km.

Common mistake

"Lightning never strikes the same place twice." It often does. Tall towers and lightning rods are struck many times a year.

Where it leads