Charge and Field

Question 1

What has an electric field?

Answer 1

Any charged object.

Question 2

What is Coulomb’s law?

Answer 2

That opposing point charges will experience an attractive force, and vice versa for like point charges.

Question 3

What is the force exerted on a point charge equivalent to?

Answer 3

kQ₁Q₂ / r² Where: k = 1/4πε (ε = permittivity of the material between the charges) Q₁Q₂ = The product of the charge on each point charge. r = the distance between the point charges

Question 4

Is an attractive force between two objects negative or positive?

Answer 4

An attractive force between two objects is negative. A repulsive force is positive.

Question 5

What is the inverse square law?

Answer 5

A law stating that the magnitude of a physical quantity varies inversely with the square of its distance from its source.

Question 6

What is electric field strength equivalent to?

Answer 6

Force per unit charge.

Question 7

What is the equation for the electric field strength of a linear field?

Answer 7

E = F/q Where: F = force q = charge on object E = field strength

Question 8

Is electric field strength a vector or scalar quantity?

Answer 8

It is a vector - it points in the direction a positive charge would move.

Question 9

What is the electric field strength of a radial field equivalent to?

Answer 9

E = kQ/r² Where: E = field strength Q = Charge on fixed charged r = distance of field point from charge centre

Question 10

How is the inverse square law demonstrated on the field diagram of a radial electrostatic field?

Answer 10

By having the field lines grow further apart as they get further from the fixed charge.

Question 11

What is electrical potential energy?

Answer 11

The work required to move a charge q from an infinite distance to a distance r away from a point charge, Q. E = kQq/r

Question 12

What does a graph of electrical potential energy against distance for a repulsive field look like?

Answer 12

Question 13

What is the form factor of a graph mapping electric potential energy against distance from the point charge?

Answer 13

Except for EPE not GPE.

Question 14

What is electrical potential energy measured in?

Answer 14

Volts

Question 15

What is electrical potential energy equal to?

Question 16

What is electric field strength at a point in a linear field?

Answer 16

The electrical potential over the distance between

Question 17

What is an equipotential surface?

Answer 17

An equipotential surface is a surface across which the electrical potential is constant.

Question 18

What is Stoke’s? Law?

Question 19

What is the force experienced by a sphere moving through a fluid equal to?

Answer 19

6πηrv

Where:

η = The viscosity of the fluid

r = radoius

Question 20

Describe Milikan’s oil drop experiment.

Answer 20

Oil droplets were fed between two plates through an atomiser, which ionized the oil droplets via friction as they left it. The terminal velocity of a doplet could be measured, further allowing the radius of the droplets to calculated. This allowed Milikan to calculate the oil droplet’s mass. A potential difference was then created across the plates such that the oil droplet was attracted to the top plate and suspended in mid-air. The charge on the drop could then be calculated by combining F = QV/d and Stoke’s law. Milikan repeated this experiment many times and found that the charges on the droplets were always multiples of -1.6 x 10^-19. This suggested that charge was quantised.

Question 21

What is electrical current in terms of Q, V, and l?

Answer 21

I = QV/l

Question 22

What is the force on a charged particle as it passes through a magnetic field?

Answer 22

F = Bqv

Where:

F = force on the particle

q = charge on the particle

B = magnetic field strength

v = velocity of the particle

Question 23

What is the radius of the circular path of a charged particle in a magnetic field?

Answer 23

r = mv/Bq

Where:

r = path radius

m = particle mass

v = particle velocity

B = magnetic field strength

q = charge on the particle

Question 24

How is the equation for the radius of the path of a charged particle in a magnetic field ( r = mv/Bq ) derived?

Answer 24

F = mv²/r

F = Bvq

⇒mv²/r = Bvq

⇒ r = mv/Bq