Coulomb’s Law
Electric Field Lines
An electric field is a region of space where a charged particle will feel a force
Electric field lines show the direction of the force that would be exerted on a positive test charge at that point in the electric field
The density (i.e no. per unit area) of electric field lines is proportional to the magnitude of the electric field strength
Electric Field Strength
The electric field strength is the force per unit charge that would act on a positive test charge placed in the field.
It is measured in N C^(−1) or V m^(−1) and is a vector quantity
What direction is the force?
Adding Electric Field Strengths
Electric field strength is a vector quantity, so to find the resultant field strength we must take the vector sum of field strengths due to each individual charge
We must account for the sign of the individual charges to make sure that each individual field has the correct direction
Electric Potential
The electric potential at a point is the work done (against the field) per unit charge to move a positive test charge from infinity to that point
The work done on a charge when moving between two points in the field is
𝑊=𝑞∆𝑉
Potential has units of J C^(−1) i.e V
Electric Potential equation
Uniform Electric Fields
Between two parallel conducting plates (i.e inside a capacitor) or far away from point charges the electric field is approximately uniform
This means that field lines will all be parallel and uniformly distributed, while equipotential surfaces are spaced by equal distances
The field strength is given by
Cathode ray tube
Where would the charges go on the sphere
All charge is located on the surface of the conductor.
This ensures that the electric field strength is zero inside the conductor
This ensures that the entire conductor is at the same potential
Image charges - how to model a force of attraction between charge q at distance r from metal plate
model plate as charge -q at distance 2r from q. F= Q^2/16πε0r
