Electric Charge (SL IB Physics)

• Charge is measured in coulombs (C). One coulomb is defined as:

The charge carried by an electric current of one ampere in one second 

• Charge is a scalar quantity
  • Electrons have a negative charge 
  • Protons have a positive charge 

The Structure of an Atom

• In neutral (i.e. uncharged) atoms and objects the number of electrons and the number of protons are equal

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

The Direction of Forces

• 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

• 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 to becomes negatively charged
  • The object from which the electrons leave from becomes positively charged

• This difference in charges leads to a force of attraction between itself and other objects which are also electrically neutral
  • This is done by attracting the opposite charge to the surface of the objects they are attracted to
• A stable is atom is always neutral, meaning it has the same amount of positive charge as negative charge
  • This means a stable atom has the same number of protons (positive charge) and electrons (negative charge)

Rubbing the Cloth with the Rod

• In the example below, when the cloth and rod are rubbed together, the electrons are transferred to the cloth and leave from the rod
• The charge is conserved. It is transferred but never created or destroyed

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