• All charged particles produce an electric field around them
  • This field exerts a force on any other charged particle within range

   

• The electrostatic force between two charges is defined by?Coulomb’s Law
  • Recall that the charge of a uniform spherical conductor can be considered as a?point charge?at its centre

   

• Coulomb’s Law states that:

The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation

• The force?FE?between two charges as expressed by Coulomb's Law is given by the equation:

The electrostatic force between two charges is defined by Coulomb’s Law

• Where:
  • FE?= electrostatic force between two charges (N)
  • Q1?and?Q2?= two point charges (C)
  • ε0?=?permittivity of free space
  • r?= distance between the centre of the charges (m)
• The 1/r2?relation is called the?inverse square law
• This means that when the separation of two charges doubles, the electrostatic force between them reduces to (?)2?= ? of its original size
• ε0?is a physical constant used to show the capability of a vacuum to permit electric fields
• If?Q1?and?Q2?are oppositely charged, then the electrostatic force?FE?is negative
  • This can be interpreted as an?attractive force?between?Q1?and?Q2

   

• If?Q1?and?Q2?are the same charge, then the electrostatic force?FE?is positive
  • This can be interpreted as a?repulsive?force?between?Q1?and?Q2

Step 1: Write down the known quantities

• • Distance,?r?= 2.0 mm = 2.0 × 10-3 m

The charge of one proton = +1.60 × 10-19?C

An alpha particle (helium nucleus) has 2 protons

• • Charge of alpha particle,?Q1?= 2 × 1.60 × 10-19?= +3.2 × 10-19 C

The gold nucleus has 79 protons

• • Charge of gold nucleus,?Q2?= 79 × 1.60 × 10-19?= +1.264 × 10-17 C

Step 2: The electrostatic force between two point charges is given by Coulomb’s Law

Step 3: Substitute values into Coulomb's Law