Electric Fields

Electric field

• A region where charged particles experience a force

Electric field strength (E)

• The force experienced per unit positive charge

• E = F/Q

• Electric field strength = force experienced/charge

What is the difference between gravitational field strength an electric field strength in terms of energy?

• Gravitational is always attraction so is negative

• Electric can be either

What do the electric field lines on a point charge look like?

• Straight lines pointing away from the point charge

• When the field lines are closer together, the field is stronger

What do the electric field lines look like between two opposing point charges?

• The particles are attracted to each other, hence using the flow of electrons the positive goes to the negative

• They swerve because they can’t go to the negative side due to the weaker field

What do the electric field lines look between two of the same point charges?

• Both charges repel from each other

• Field lines never cross

What do the field lines look between a charged particle and an oppositely charged wall?

• Field lines from positive to negative normal to the wall

What do the field lines look between two oppositely charged walls?

Coulombs law

• The force experienced by a charge particle is directly proportional to it’s charge

• The forced experienced between two objects is inversely proportional to the distance squared

• F = Qq/4πε₀r²

• ε₀ = Permittivity of free space = 8.85 × 10^-12

Derivation of E = V/d

W = Fd

VQ = EQd (W = VQ, E = F/Q)

E = V/d

Gravitational fields vs electrical fields

• G is caused by mass, E is caused by charge

• G is always attractive, hence is always negative, E is either

• Gravitational field strength = F/m

• Electrical field strength = F/Q

• F ∝ Mm and F ∝ Qq

• F ∝ 1/r² for both

• Equations for force and energy are different

• Field lines are opposite for point masses/charges

Electric potential energy

• The work done to bring particles from infinity to a separation of “r”

• E = Qq/4πε₀r

Electric potential

• The work done per unit charge to bring a positive charge to a point from infinity

• V = E/q = Q/4πε₀r

Where are the points in the electric field where the potential difference is equal to X?

A vertical line at X

Equipotential lines

• Directionless lines which are perpendicular to the electric field lines

How does electric potential vary with distance in a uniform electric field?

Directly proportional

Describe the changes in charge when a negative balloon is put near two metal spheres A and B

• The negative charge is attracted to A, hence making A negative

• The negative charge from A is attracted to B, so B becomes negative whilst A becomes positive as it gives it’s charge away

A carbon-coated ball is between two oppositely charged plates. Describe the motion of the ball if it begins at one of the walls

• The ball is in a uniform electric field, hence it experiences a uniform acceleration towards the other wall

• The velocity increases until it hits the other wall, instantly turning negative and accelerating in the negative direction