Electric Charge (Edexcel GCSE Physics)

Charging by Friction

• When certain insulating materials are rubbed against each other they become electrically charged

  • This is called charging by friction

• The charges remain on the insulators and cannot immediately flow away

  • One becomes positive and the other negative

•  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

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

• This occurs because negatively charged electrons are transferred from one material to the other

• The material, in this case, the rod, loses electrons

• Since electrons are negatively charged, the rod becomes positively charged

  • As a result, the cloth has gained electrons and therefore is left with an equal negative charge

Charge & Electrons

• All objects are initially electrically neutral, meaning the negative (electrons) and positive charges are evenly distributed

• However, when the electrons are transferred through friction, one object becomes negatively charged and the other positively charged

  • The object to which the electrons are transferred becomes negatively charged

  • The object from which the electrons leave becomes positively charged

• This difference in charges leads to a force of attraction between itself and other objects which are also electrically neutral, by attracting the opposite charge to the surface of the objects they are attracted to

• In the example below, when the cloth and rod are rubbed together, the electrons are transferred to the cloth and leave from the rod

Electrons are rubbed onto the cloth leaving the cloth negatively charged and the rod positively charged

Electric Forces Between Charges

• The charge of a particle is either:

  • Positive

  • Negative

  • Neutral (no charge)

• Electrons are negatively charged particles, whilst protons are positive and neutrons are neutral

• This is why in a neutral atom, the number of electrons is equal to the number of protons

  • This is so the equal (but opposite) charges cancel out to make the overall charge of the atom zero

The number of negative electrons in an atom balances the number of positive protons

• Therefore, an object becomes negatively charged when it gains electrons and positively charged when it loses electrons

• When two charged particles or objects are close together, they also exert a force on each other

• This force could be:

  • Attractive (the objects get closer together)

  • Repulsive (the objects move further apart)

• Whether two objects attract or repel depends on their charge

  • If the charges are the opposite, they will attract

  • If the charges are the same, they will repel

Opposite charges attract, like charges repel

Attraction or Repulsion Summary Table

• Attraction and repulsion between two charged objects are examples of a non-contact force

  • This is a force that acts on an object without being physically in contact with it