Attracting can

In this class activity, students see that after it’s rubbed against your clothes a balloon will attract a drinks can and make it roll. You can use it to introduce why charged objects exert forces on uncharged objects.

Equipment

Each student will need:

• Empty aluminium soft drink can
• Rubber balloon
• Cloth or woollen clothing

Procedure

Ask students to:

1. Inflate the balloon and tie its neck.
2. Place the empty can on its side on a flat surface.
3. Hold the balloon close to the can. They should see that nothing happens because the balloon is initially uncharged.
4. Rub the balloon on their clothing or a piece of cloth so that it becomes charged.
5. Bring the balloon close to the can. They should see the can start to move towards the balloon.
6. Move the balloon gradually away from the can so that the can rolls along.

Discussion prompts

• After it’s been rubbed, the balloon attracts the can. Have you seen this sort of thing before?
• How can you tell that the forces on the aluminium can are unbalanced?
• How do you think the balloon creates a force on the can?

Teaching notes

Charged objects attracting other objects may be familiar from, for example, a comb attracting hair. You could rub the balloon and show that it also attracts a student’s hair. To help them visualise charging processes, introduce electrons as negatively charged particles that move between the materials.

The balloon becomes charged when it’s rubbed because it’s made of a material that attracts electrons more strongly than the cloth. Electrons are transferred from the cloth to the balloon and so the balloon gains a negative charge overall. Explaining that the cloth is left with a positive charge will help students appreciate that charge is conserved, but there is no need to discuss atomic structure or the nature of the positive charge in the objects.

The charging process for the aluminium can is different. The two objects do not come into contact. Instead, electrons in the can are repelled by the balloon and so move to the part of the can furthest away. The back of the can becomes negatively charged and the front positive, but overall the can remains electrically neutral. The reason the aluminium can starts rolling is because the back of the can is further away and so the repulsive force on the back of the can is smaller than the attractive force on the front.

If students use the phrase ‘static electricity’, explain that it can be a misleading one. The charging process for the balloon involves the transfer of charge between cloth and balloon, and the process for the aluminium can involves charges moving within the can. The charging processes may be different, but in neither are the charges ‘static’.

Learning outcome

Students describe how an object made of an insulating material becomes charged when we rub it and also why it then attracts other objects.

This experiment was safety-checked in March 2020.