Charging by friction

When insulating materials rub against each other, they may become electrically charged. Electrons, which are negatively charged, may be ‘rubbed off’ one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with a positive charge.

A plastic rod is covered in negative charges (electrons). It is adjacent to a duster covered in positive charges.

When a polythene rod is rubbed with a duster, the friction causes electrons to gain energy. Electrons gain enough energy to leave the atom and ‘rub off’ onto the polythene rod.

  • the polythene rod has gained electrons, giving it a negative charge
  • the duster has lost electrons, giving it a positive charge

If the rod is swapped for a different material such as acetate, electrons are rubbed off the acetate and onto the duster.

  • the acetate rod has lost electrons, giving it a positive charge
  • the duster has gained electrons, giving it a negative charge

Both the rods and the duster are made of insulating materials. Insulators prevent the electrons from moving and the charge remains static. Conductors, on the other hand, cannot hold the charge, as the electrons can move through them.

Electrical forces

A charged object will experience non-contact force from another charged object. The type of force will depend on the type of charge (positive or negative) on the two objects.

The properties of attraction and repulsion are often used to show that an object is charged:

  • a charged rod can pick up small pieces of paper
  • a charged balloon can stick to the wall by attraction
  • a charged rod can pull a stream of water towards it
curriculum-key-fact
Opposite charges attract. Like charges repel.

Example

If a negatively charged plastic rod is brought near to another negatively charged rod, they will move apart as they repel each other.

If a positively charged rod is brought close to a negatively charged rod, they will pull together as they attract each other.

A metal bar lying horizontally is suspended from string. It is adjacent to vertical metal bar. Both bars carry same charge. Bar on the string is repelled by the other, and swings away from it.

The forces of attraction or repulsion are greater when the charged objects are closer.

Question

If a cloth rubs a plastic rod and the cloth is pulled away from the rod slightly, will the rod and cloth attract, repel or experience no force at all?

The rod and cloth will attract. This is true as long as there is enough friction to transfer electrons.

  • if electrons are rubbed off the cloth and onto the rod - the cloth will be positively charged and the rod will be negatively charged
  • if electrons are rubbed off the rod and onto the cloth - the cloth will be negatively charged and the rod will be positively charged

In both cases, the opposite charges will attract.