• An electric field is a region of space in which an electric charge “feels” a force
• Electric field strength?at a point is defined as:

The electrostatic force per unit positive charge acting on a stationary point charge at that point

• Electric field strength can be calculated using the equation:

• Where:
  • E = electric field strength (N C^-1)
  • F = electrostatic force on the charge (N)
  • Q = charge (C)
• It is important to use a positive?test charge?in this definition, as this determines the direction of the electric field
• The electric field strength is a?vector?quantity, it is always directed:
  • Away?from a positive charge
  • Towards?a negative charge
• Recall that?opposite charges?(positive and negative) charges?attract?each other
• Conversely,?like charges?(positive and positive or negative and negative)?repel?each other

Step 1:?Write down the equation for electric field strength

Step 2:?Rearrange for charge Q

Step 3:?Substitute in values and calculate

• The electric field strength equation can be rearranged for the force?F?on a charge?Q?in an electric field?E:

F = QE

• Where:
  • F = electrostatic force on the charge (N)
  • Q = charge (C)
  • E = electric field strength (N C^-1)
• The direction of the force is determined by the charge:
  • If the charge is?positive?(+) the force is in the?same?direction as the E field
  • If the charge is?negative?(-) the force is in the?opposite?direction to the E field
• The force on the charge will cause the charged particle to?accelerate?if its in the same direction as the E field, or?decelerate?if in the opposite

An electric field strength E exerts a force F on a charge +Q in a uniform electric field

• Note:?the force will always be?parallel?to the electric field lines

Step 1:?Write out the equation for the force on a charged particle

F = QE

Step 2:?Substitute in values

F = (1.60 × 10^-19) × 5000 = 8 × 10^-16?N

Step 3:?State the direction of the force

Since the charge is negative, the force is?directed against the electric field lines?and decelerates the electron.

• For a point outside a spherical conductor, the charge of the sphere may be considered to be a?point charge?at its centre
  • A?uniform?spherical conductor is one where its charge is?distributed evenly
• The electric field lines around a spherical conductor are therefore?identical to those around a point charge
• An example of a spherical conductor is a?charged sphere
• The field lines are?radial?and their direction depends on the charge of the sphere
  • If the spherical conductor is?positively?charged, the field lines are directed?away?from the centre of the sphere
  • If the spherical conductor is?negatively?charged, the field lines are directed?towards?the centre of the sphere

Electric field lines around a uniform spherical conductor are identical to those on a point charge