Electric Fields

Question 1

Define electric field strength of an electric field at a point in space

Answer 1

Force experienced per unit positive charge at that point

E = F/Q (NC^-1)

Question 2

Is Electric Field strength a vector

Answer 2

Yes, always points to direction in which a positive charge would move when placed at that point

Point away from positive charges and towards negative charges

Question 3

Ideas for electric field patterns

Answer 3

Arrow on an electric field line shows the direction of the field

Electric field lines are always at right angles to the surface of a conductor

Equally spaced, parallel electric field lines represent a uniform field

Closer electric field lines represent greater electric field strength

Question 4

Describe electric field patterns for a point charge and a sphere

Answer 4

Radial, decreases with distance from the centre

Question 5

What is Coulomb’s Law

Answer 5

Any two point charges exert an electrostatic 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

Question 6

Coulombs Law Equation

Answer 6

F = Qq/4pi epsilon0 r^2

Question 7

Electrical field strength in a radial field

Answer 7

Decreases as you move further away from the centre of the sphere

Question 8

Electrical Field strength of a radial field

Answer 8

E = F/q
= Q / 4pi x epsiolon0 x r^2

Question 9

Graph of E against 1/r^2 for a radial field

Answer 9

Straight line through the origin

Question 10

Key difference between a gravitational and an electric field

Answer 10

Masses always produce an attractive field

Charges can create both attractive and repulsive fields

Question 11

Electric field strength between two parallel plates

Answer 11

E = V / d

V m^-1

Question 12

Equation for capacitance

Answer 12

C = epsilon x A / d

Epsilon = Epsilon1 x Epsilon0

where epsilon 1 is the relative permittivity

Question 13

Ideas used to determine the motion of the electron between plates

Answer 13

E = V/d

F = Eq

W = Vq

Question 14

Electrons in between parallel plates

Answer 14

Will travel towards the positive plate, opposing direction of the electric field

Experiences a constant electrostatic force due to the uniform electric field between plates, so it has a constant acceleration

Electrons travelling from positive to negative will experience a deceleration.

Question 15

Charged particles moving at right angles to an electric field

Answer 15

Horizontal motion:
No acceleration so horizontal velocity remains constant
Vertical motion:
a = F/m = Eq/m
initial vertical velocity = u
final vertical velocity = u + at

Leads to parabolic motion

Question 16

How may a uniformly charged sphere be treated as

Answer 16

A point charge

Question 17

Force distance graph for a point and spherical charges

Answer 17

Area under a force-distance graph is equal to work done

Question 18

What is total work done under a force distance graph

Answer 18

Total area under the graph is equal to the total work done to bring the particles from infinity to a separation r

Question 19

Electric Potential Energy equation

Answer 19

Work done = E = Qq/ 4pi x episolon0 x r

Magnitude of E represents external energy required to completely separate the charged particles to infinity

Question 20

What is total work done equal to

Answer 20

EPE

Question 21

Define Electric Potential

Answer 21

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

Question 22

Electric Potential Equation

Answer 22

V = Q / 4pi x epsilon0 x r

Question 23

Electric Potential difference definition

Answer 23

Work done per unit charge between two points around the particle of charge Q

Question 24

What is the capacitance of charged sphere

Answer 24

Ratio of the charge it stores to its potential at its surface

C=Q/V = 4pi x epsilon0 x R x v / V = 4pi x epsilon0 x R