C22 - Electric Fields

Question 1

What’s an electric field?

Answer 1

A region around a charged particle which will experience a force.

(Can be attractive or repulsive)

Question 2

In which direction does an electric field act?

Answer 2

From positive to negative.

If charges aren’t shown, arrows/field lines will be from positive to negative (so will point to negative).

Question 3

What’s electric field strength?

Answer 3

Force experienced per unit positive charge at that point.

It’s a vector quantity

E = F/Q (SI units are N/C)

Question 4

What do electric field lines show?

Answer 4

Lines of force of electric fields.

• arrows show direction of the field
• electric field lines are always at right angles to the conductor surface
• equally spaced, parallels lines represent a uniform field
• closer field lines represent greater electric field strength

Question 5

What’s Coulomb’s law?

Answer 5

Any 2 point charges exert an electrostatic (electrical) force on each other that is directly proportional to the product of their charges, and inversely proportional to the square of the distance between them.

F = Qq / 4π ε0 r²

Question 6

How do the electric field lines appear between 2 parallel plate?

Answer 6

Uniform - arrows/direction of force from positive to negative

Question 7

How do the electric field lines appear between 2 point charges/spheres?

Answer 7

Radial. Not uniform.

Force is from positive to negative/lines bend towards the negative.

Question 8

How can electric field strength (at distance r) from the centre is a sphere be found?

Answer 8

E = F/q

And F = Qq / 4π ε0 r²

Therefore E = Q / 4π ε0 r²

Question 9

How is the property that creates the field different between gravitational and electric fields?

Answer 9

G.F: field created by a mass

E.F: field created by a charged particle

Question 10

How is the type of field produced different between gravitational and electric fields?

Answer 10

G.F: they’re always attractive (direction of field always towards the object)

E.F: attractive and repulsive
Positive point charges produce a repulsive field (field away from object).
Negative point charges produce an attractive field (field towards the object)

Question 11

How does the field strength (definitions) differ between gravitational and electric fields?

Answer 11

G.F: force per unit mass
g = F/m

E.F: force per unit positive charge
E = F/Q

Question 12

How does the force between particles differ between gravitational and electric fields?

Answer 12

G.F:
force ∝ product of masses
force ∝ 1/separation ²

E.F:
force ∝ product of charges
force ∝ 1/separation ²

Question 13

How does the force and field strength equations differ between gravitational and electric fields?

Answer 13

G.F:
F = -GMm/r ²
g = -GM/r ²

E.F:
F = Qq/4π ε0 r²
E = Q / 4π ε0 r²

Question 14

What type of field forms with gravitational and electric fields?

Answer 14

G.F:
Point masses produce a radial field

E.F:
Point charges produce a radial field

Question 15

What are the 2 (simple) formulae for calculating electric field strength, E?

Answer 15

E = F/Q
Force per unit positive charge

E = V/d
Potential difference per metre
This can only be used in a uniform field

Question 16

What does the capacitance of a parallel plate capacitor depend on? (3)

Answer 16

The separation, d, between plates

The area, A, of overlap between the plates

The insulator (dielectric) used between the plates

Question 17

For plates in a vacuum, how is capacitance related to area and separation?

Answer 17

C ∝ A/d

Question 18

What happens if an electron is fired between plates, parallel to the electric field?

Answer 18

Being negative, it experiences attraction towards the positive plate so will either accelerate towards the positive (if fired in the same direction) or, if the positive plate is behind the electron, will slow down and change direction.

Question 19

What happens if an electron is fired between plates, perpendicular to the electric field?

Answer 19

It acts as a projectile motion and is deflected towards the positive plate.
(Horizontal and vertical components considered independently)

Question 20

How can electrons be accelerated?

Answer 20

Having a -ve charge, an electron between plates will travel away from the -ve plate and towards the +ve plate.
The electron experienced a constant electrostatic force because of the uniform electric field so it as a constant acceleration.

An electron travelling in the direction of the electric field, from +ve to -ve plate, will experience deceleration.

Question 21

What happens to the horizontal and vertical motion of a charged particle moving at right angles to an electric field?

Answer 21

Horizontal:
- There’s no acceleration therefore Vh remains constant.

• Time spent in the field = t = L/v

Vertical:
- Vertical acceleration of the particle = a = F/m = EQ/m

• Initial velocity = 0
• The final component of velocity as the particle exits the field is given by v = u + at = 0 + EQ/m * L/v = EQL/mv

Question 22

What is the area under a force - distance/separation graph represent?

Answer 22

Work done

Question 23

What’s total work done equal to (electric field)?

Answer 23

It’s the same as electric potential energy, E.

Question 24

What’s electric potential, V?

Answer 24

At a point, it’s the work done per unit charge in bringing a positive charge from infinity to that point.

V = E/q
(Like potential difference)

Question 25

What’s a capacitor?

Answer 25

A device that stores charge.

Question 26

How is capacitance calculated?

Answer 26

C = Q/V

Question 27

What is capacitance?

Answer 27

Charge stored per unit potential difference across a capacitor.

Question 28

An isolated metal sphere is charged using a high-voltage supply.
Discuss the factors that affect the charge stored on the surface of the sphere:
(2)

Answer 28

V=Q / 4πε0 r

Therefore charge, Q is proportional to the (surface) potential, V and the radius r of the sphere.

Question 29

Explain the effect on the charge stored by a capacitor when a thick sheet of plastic inserted in the space between capacitor plates:
(2)

Answer 29

The permittivity of the material between the plates increases, hence the charge stored is greater.

Question 30

Define electric potential at a point in space around a charged object:

Answer 30

Work done per unit positive charge in bringing a charge from infinity to the point in space.

Question 31

Define electric field strength at a point in space:

Answer 31

Force per unit ‘positive’ charge