Electric Fields

Question 1

Electrical conductors vs electrical insulators?

Why are some insulators easy to charge?

Answer 1

Electrical conductors contain lots of free electrons.
Electrical insulators contain no free electrons. Some insulators are easy to change bc their surface atoms gain or lose e-s easily.

Question 2

Why must a metal be isolated from the Earth before it can be charged?

Answer 2

Bc otherwise any charge given to it is neutralised by electrons transferring between the conductor and the Earth.

Question 3

What does antistatic material do?

Answer 3

Allows charge to flow across the surface (so prevents earthing of eg microchips, which would otherwise damage them).

Question 4

What’s the line of force/force field?

Answer 4

The path a free positive test charge follows.

Question 5

What’s the electric field strength, E, at a point in the field?
Units?

Answer 5

The force per unit positive charge on a positive test charge placed at that point.
Units: NC^-1 or Vm^-1

Question 6

Electric field strength is in the same direction as the force on the..

Answer 6

+ve test charge.

Question 7

For a charged metal conductor, the charge on it is spread across its surface. Th more concentrated the charge is on the surface,…

Answer 7

the greater the strength of the electric field above the surface.
Eg V shape conductor has charge concentrated at tip.

Question 8

Uniform plates have charge spread evenly across their surface area. Electric field strength between parallel plates, E is (eq)?

Answer 8

E = Q/A !!

Question 9

A Van de Graaff generator can easily produce sparks in air a few cms long. how does it work?

Answer 9

Work is done to charge dome (transfer charge from belt to dome) ∴ electrical potential energy of dome increases as it charges. Some or all of that energy is transferred from the dome when a spark is created.

Question 10

The electric potential at a certain position in any electric field, V, is defined as?
Units?

Answer 10

The work done per unit positive charge on a positive test charge when its moved from infinity to that position.
Units: V or JC^-1

Question 11

What’s the potential gradient at any point in the electric field?

Answer 11

The change of potential per unit change of distance in a given direction.

Question 12

Direction of potential gradient relative to line of force of the electric field + eq?

Answer 12

Because the potential gradient is in the opposite direction to the line of force of the electric field, E = - ΔV/Δx

Question 13

Torsion balance is used to measure force between pith balls. What did we find?
Derive Coulombs law

Answer 13

F ∝ 1/r^2

Question 14

Derive Coulombs law.

Answer 14

F = k Q1Q2/r^2,
where k = 1/4πε0

Question 15

Electric field strength as a vector:

What is the resultant electric field strength at the position of the test charge given by?

Answer 15

The resultant force per unit charge on the test charge gives the resultant electric field strength at the position of the test charge.

Question 16

Electric field strength as a vector:
Forces in same direction?

Answer 16

Resultant E = F/q = qE1 + qE2/q
= E1 + E2

Question 17

Electric field strength as a vector:
Forces is opposite directions?

Answer 17

Resultant E = E1 - E2

Question 18

Electric field strength as a vector:
Forces at ight angles to each other.

Answer 18

Resultant E^2 = E1^2 + E2^2
(bc F^2 = F1^2 + F2^2)

Question 19

What does a negative electric field strength, E indicate?

Answer 19

Indicates a field that acts towards a -ve charge.

Question 20

What does a -ve V indicate?

Answer 20

Indicated a value less than zero.

Question 21

E and V, scalar or vector?

Answer 21

E is vector,
V is scalar.

Question 22

E and V, how do they vary over distance?
Why?

Answer 22

E varies with distance more sharply than V does
E ∝ 1/r^2
V ∝ 1/r