# 2.4.1 Electric Charge

### 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

#### Exam Tip

At this level, if asked to explain how things gain or lose charge, you must discuss electrons and explain whether something has gained or lost them

Remember when charging by friction, it is only the electrons that can move, not any ‘positive’ charge, therefore if an object gains a negative charge, something else must have gained a positive charge

### 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

#### Exam Tip

Remember the saying: “Opposites attract

Materials only become positively charged because of the loss of electrons, rather than the ‘gain’ of any positive charge, which is a common misconception. ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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