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Dust that clings

Electricity and magnetism · level 3, lesson 5 of 6

A question to start

Wipe a TV screen clean and, a few days later, it is dusty again, even on its upright front. Why does dust stick to a screen that does not face up?

The idea

Dust grains rub against the air, against each other and against surfaces, so they pick up static charge. Screens and plastic panels pick up charge too. A charged surface pulls charged grains, and even uncharged ones, the way a balloon pulls bits of paper.

Small grains stick best. The pull of the charge is strong compared with their tiny weight, so a slight push is not enough to knock them off.

This matters for:

  • solar panels, which make less electricity as dust covers them;
  • spacesuits, where fine dust works its way into joints and seals;
  • camera lenses, which get hazy.

On Mars, the dust is very fine and the air is very dry, so dust settles and clings. Dust slowly covered the solar panels of the InSight lander until they could no longer power it, and its mission ended in 2022.

++++ charged grains cling to the panel

Worked examples

1. The screen. The screen charges up as it is used and wiped. Dust in the air is pulled onto it, whichever way it faces.

2. Wiping makes it worse. Rubbing a screen with a dry cloth can charge it more. A damp cloth lets the charge leak away.

3. A dusty panel. A panel with half its surface covered by dust makes roughly half as much electricity.

Try it

  1. How do dust grains get charged?
  2. Why do small grains stick better than big ones?
  3. Why can a dry cloth make a screen dustier later?
  4. Why is clinging dust a problem for a Mars lander?
Answers
  1. By rubbing against the air, each other and surfaces.
  2. The pull of the charge is strong compared with their small weight.
  3. Rubbing charges the screen, so it pulls more dust.
  4. Dust on its solar panels cuts their power.

Common mistake

"Dust only lands on things that face up." Gravity pulls dust down, but static charge pulls it sideways and even upwards onto a surface.

Where it leads