Electric fields

Question 1

Electric field def

Answer 1

Force per unit positive charge

Question 2

Uniform electric field def

Answer 2

Where the electric field strength is the same at all points in the field

Question 3

Electric field strength of uniform electric field formula:

Answer 3

E= ΔV/ Δd

Question 4

What three factors are the motion of a charged particle in a uniform electric field influenced by?

Answer 4

• Mass ∝ Deflection
• Charge ∝ Magnitude of deflection
• Speed ∝ 1/ deflection

Question 5

What is coulumb’s law

Answer 5

electrostatic force between two point charges is
directly proportional to the product of the two charges and
inversely proportional to the square of seperation

Question 6

Electrostatic force formula

Answer 6

F=qE

Question 7

Electric field per point charge formula

Answer 7

E= kQ/r²

Question 8

Electric potential definition

Answer 8

Work done per unit positive charge in bringing a small test charge from infinity to that point

Question 9

Electric field strength definition

Answer 9

The electrostatic force at a point per unit positve charge at that point

Question 10

When is electric potential positive

Answer 10

when near an isolated positive charge

Question 11

when is electric potential negative

Answer 11

when near an isolated negative charge

Question 12

when is electric potential zero

Answer 12

at infinity

Question 13

Equipotential line

Answer 13

A line joining all points with equal potential

Question 14

When equipotential lines are equally spaced and parallel, what does it represent?

Answer 14

a uniform electric field

Question 15

Explain why, for any point outside a spherical conductor, the charge on the sphere may be considered to act as a point charge at its centre. (2)

Answer 15

• electric field lines are radial/ normal to the surface of the sphere
• electric field lines appear to originate from the centre of the sphere

Question 16

Gravitational fields mentionned

Similarities between fields of force produced by an isolated charge and an isolated point mass

2 marks

Answer 16

• both are radial
• both have inverse square (variations)

Question 17

gravitational fields mentionned

Similarity between gravitational potential and electric potential

3 marks

Answer 17

• inversely proportional to distance
• points of equal potential lie on concentric spheres
• zero at infinite distance

Question 18

Gravitational fields mentionned

Difference between fields of force produced by an isolated charge and an isolated point mass

Answer 18

direction is always/only towards the mass whereas the direction can be towards or away from the charge

Question 19

State the relationship between electric potential and electric field strength at a point

2 marks

w18 41

Answer 19

Field strength= potential gradient
Potential is (-) and field strength isn’t

Question 20

A helium nucleus is between two plates. It is free to move.
By considering energy changes, explain why the speed it reaches the other plate is independant of the seperation of the plates?

2 marks

w18 41

Answer 20

gain in kinetic energy (= loss in potential energy) = charge × p.d. or qV = ½mv
so
v is independent of separation (because separation not in expressions)

Question 21

Explain the negative sign in:
E=- V/d

Answer 21

Force acts in the opposite direction to voltage

Question 22

Magnetic fields mentionned

Two differences between magnetic fields and electric fields

Answer 22

• E field, force is independant of speed whereas for M field force ∝ speed
• E field, force along direction of field but M field, force is normal to direction of field