Electric Fields

• Coulomb's Law: The force between two point charges in a vacuum is directly proportional to the product of the charges and inversely proportional to the square of their separation
• ε0 is the permittivity of free space
• permittivity is how well a material holds an electric field
• Assumptions:

  • Air treated as a vacuum
  • Charge acts at the centre of the sphere
• Coulomb's Law

  
• If charges have the same sign the force will be repulsive. If different it will be attractive
• A positive force means the charges repel
• A negative force means the charges attract
• The magnitude of electrostatic forces between subatomic particles is magnitudes greater than the magnitude of gravitational forces
• The electrostatic force between two protons is 1.24 x 10 ^36 times bigger than the gravitational force
• An electric field is a volume of space within which an electric charge will experience a force
• Electric field lines always go from the positive charge to the negative charge
• Electric field lines show the direction of the force on a positive test charge
• uniform field

  A) +

  B) -

  
• radial fields

  +/-Q

  A) -Q

  B) +Q

  
• Electric field strength is the force per unit positive charge

  
• For a radial field

  A) Q

  B) r

  
• For a uniform field

  A) V

  B) d

  
• Electric field strength units: NC^-1 or Vm^-1
• Electric field strength is a vector
• They are both positive/negative, therefore there is a point between them that the electric field strength is 0. This point will be closer to the charge with a smaller magnitude charge

  
• Electric potential is a scalar, so you can add them up
• Work done by moving a charged particle between the parallel plates of a uniform field

  A) Q

  B) V

  
• Fire a particle at right angles to a uniform electric field between two parallel plates

  • A charged particle will experience a constant electric force, causing the particle to accelerate and so it follows a parabolic shape
  • +ve : follow direction of field
  • -ve: moves opposite direction to the field
• The particle has a positive charge

  
• Electric potential is the work done per unit charge against the field to move a positive point charge from infinity to that point
• Absolute electric potential at a point is the potential energy per unit charge of a positive point charge at that point
• The absolute magnitude of electric potential is greatest at the surface of a charge and decreases as you move away from the charge.
• Electric potential is zero at infinity
• electric potential

  A) Q

  B) r

  
• units of electric potential: JC^-1 or V
• Potential will be positive when work is done against the field (like charges are repelling)
• Potential will be negative when work is done by the field (when opposite charges attract), as work must be done to take it back to infinity
• potential graphs

  A) repulsive

  B) attractive

  
• The gradient of a tangent to a potential against distance graph gives the value of electric field strength at that point

  
• Electric potential difference is the work done per unit charge
• Between two parallel plates, equipotentials are planes which are equally spaced and parallel to the plates

  
• Equipotential surfaces around a point charge form concentric circles

  
• Electric potential difference is the area underneath a field strength v distance graph

  
• Electric potential energy = q x V