Charging by Friction (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

• When certain insulating solids are rubbed against each other, they can become electrically charged
  • This is called charging by friction

• The charges remain on the insulators and cannot immediately flow away
  • One gains a net positive charge and the other gains a net negative charge

• An example of this is a plastic or polythene rod being charged by rubbing it with a cloth
  • Both the rod and cloth are insulating materials

Charging solids by friction

A polyethene rod may be given a charge by rubbing it with a cloth

Movement of electrons

• Charging by friction occurs due to the movement of electrons
• When a polyethene rod is rubbed with a cloth, negatively charged electrons are transferred from the rod to the cloth
  • The cloth has gained electrons, so it becomes negatively charged
  • The rod has lost electrons, so it becomes positively charged

Transfer of electrons by friction

When electrons are rubbed onto a cloth, it gives it a negative charge and leaves the rod with a positive charge

• A charged object can
  • attract uncharged objects
  • attract or repel other charged objects

• Some simple methods to determine whether an object is charged are
  • using a charged object to pick up small pieces of paper
  • sticking a charged balloon to the wall by attraction
  • using a charged rod to pull a stream of water towards it
  • bringing two charged rods close to one another

Picking up paper with a charged comb

• Running a plastic comb through hair transfers electrons onto the comb by friction
• The comb becomes negatively charged whilst the hair becomes positively charged
  • This can be observed because hairs start to repel each other

• The pieces of paper are uncharged (neutral), but as the comb is brought close to them, the positive charges are brought to the surface of the paper because they are attracted to the negative charges on the comb
• Since opposite charges attract, the pieces of paper appear to 'jump up' and stick to the comb due to the electrostatic attraction

A negatively charged comb can attract small pieces of paper

Sticking a balloon to the wall

• Rubbing a balloon on a woollen jumper transfers electrons onto the balloon by friction
• The balloon becomes negatively charged whilst the jumper becomes positively charged
• The wall is uncharged (neutral), however, when the balloon is placed near it, the positive charges in the wall are brought to the surface because they are attracted to the negative charge of the balloon
• Since opposite charges attract, the balloon sticks to the wall due to the electrostatic attraction

Electric force between two charged rods

• When two insulating materials are rubbed together, negative charge (electrons) will transfer from one insulator to the other
• A polythene rod gains a negative charge when rubbed with a cloth
  • This is because electrons are transferred to the polythene from the cloth
• An acetate rod gains a positive charge when rubbed with a cloth
  • This is because electrons are removed from the acetate by the cloth

Transfer of electrons between charged insulators

Electrons are transferred to the polythene rod giving it a negative charge, and they move from the acetate rod giving it a positive charge

• When two charged rods are brought near one another, they will experience either
  • attraction
  • repulsion

• This can be demonstrated by setting up the following apparatus:

Demonstrating electrostatic charge using the cradle method

A charged rod is held by a length of thread from a clamp stand so that it can rotate freely

• Suspend one of the rods using a cradle and a length of string so that it can rotate freely
• Rub one end of the rod using a cloth (in order to give it a charge)
• Take a second rod of insulating material and charge one end by rubbing it with a cloth
• Hold the charged end of the second rod close to the charged end of the first rod

• If the first rod is repelled by the second rod, then the materials have the same charge
  • For example, a polythene rod would be repelled by a second polythene rod, as they have the same charge

• If the first rod is attracted to the second rod, then they have opposite charges
  • For example, a polythene rod would be attracted to an acetate rod, as they have opposite charges

Attraction & repulsion of charged rods

If the charged rods have opposite charges, the freely rotating rod will turn toward the second rod. If the charged rods have like charges, the freely rotating rod will turn away from the second rod