Year 2 Chapter 5: Electric Fields

Question 1

What is Coulomb’s Law?

Answer 1

The size of force between two point charges is proportional to the product of their charges and inversely proportional to the square of their separation.
It is attractive for opposite charges and repulsive for like charges

Question 2

What is the equation for Coulomb’s Law?

Answer 2

F= k Q1Q2 / r^2
Q1, Q2: charges of each charge/ C
r: distance between charges/ m
k: constant

Question 3

What is the constant in Coulomb’s Law?

Answer 3

k = 1/4πε0
ε0: permittivity of free space

Question 4

What is permittivity of free space?

Answer 4

A measure of the ability of a vacuum to allow an electric field to pass through it

Question 5

What are the conditions of use for Coulomb’s Law?

Answer 5

Occurs in a vacuum, air can be treated as a vacuum.
Charge for a charged sphere can be assumed to act at the centre of the sphere

Question 6

How do gravitational and electrostatic forces compare for subatomic particles?

Answer 6

Magnitude of electrostatic forces much greater than magnitude of gravitational forces due to very small masses but much larger charges

Question 7

What is an electric field?

Answer 7

A region surrounding a charged object which causes a force to be exerted on any charged object placed within the field

Question 8

What is electric field strength E?

Answer 8

The force per unit charge exerted on a charged object placed at that point in the field. This is a vector acting in the same direction as the force on a positive charge.

Question 9

How does electric field strength differ in uniform and radial fields?

Answer 9

It is constant in a uniform field, eg between two parallel plates, but varies in a radial field, eg about a spherical charge

Question 10

What are two equations for electric field strength in uniform fields?

Answer 10

E = F / Q

E= ΔV / d (for parallel plates)

Question 11

What is the equation for electric field strength in radial fields?

Answer 11

E= kQ / r^2

Question 12

How is Work done = QΔV derived for moving a charged particle between parallel plates?

Answer 12

Work done = F x d
F= EQ ( E = F/Q)
d= ΔV/E ( E= ΔV/d)
Fd = EQΔV/E = QΔV

Question 13

What is the trajectory of charged particles fired parallel in between two charged plates?

Answer 13

Particle is fired at right angles to electric field and charged particles will experience a constant electric force causing it to accelerate and follow a parabolic path.
Positive charge will follow direction of the field lines, negative charge will go opposite direction to field lines

Question 14

What is absolute electric potential?

Answer 14

Potential energy per unit charge of a positive point charge at that point in the field.

Question 15

How is absolute electric potential defined?

Answer 15

Defined to be zero at infinity

Question 16

When is absolute electric potential positive?

Answer 16

When the charge is positive, potential is positive and the force is repulsive

Question 17

When is absolute electric potential negative?

Answer 17

When the charge is negative, potential is negative and the force is attractive

Question 18

How is E found using a graph of potential against distance?

Answer 18

Gradient of the tangent to the graph gives E at that point

Question 19

What is electric potential difference ΔV?

Answer 19

The energy needed to move a unit charge between two points, so ΔW = QΔV

Question 20

What is an equipotential surface?

Answer 20

A surface of constant potential. No work is done by the field when a charge moves along an equipotential

Question 21

How do you find electrical potential difference ΔV using a graph of E against r?

Answer 21

Area under the curve