Electric Fields

Question 1

Define Electric Field

Answer 1

A region of space in which charged particles are subject to an electrostatic force.

Question 2

What shape of field do point charges have?

Answer 2

Radial fields

Question 3

How can you model uniformly charged spheres?

Answer 3

As a point charge at the centre of the sphere.

Question 4

What do field lines show?

Answer 4

The path a positive test charge would take when placed in an Electric Field.

Question 5

Which direction do the field lines point?

Answer 5

Positive to negative - the lines always point away from a positive charge and towards a negative charge

Question 6

What effect does distance have on the

strength of the electrostatic force?

Answer 6

The greater the distance, the weaker the force.

Question 7

How is the strength of an Electric Field

represented in a diagram?

Answer 7

By how close together the field lines are - the closer the lines, the stronger the field.

Question 8

Define Electric Field Strength

Answer 8

Force per unit charge on a positive test charge placed in the field

Question 9

What is the formula for Electric Field Strength?

Answer 9

E = F ÷ Q
E = Electric Field strength (NC-1)
F = Force (N)
Q = Charge (C)

Question 10

What is Coulomb’s Law?

Answer 10

The force between any two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.

Question 11

What is the formula for the force
between two point charges?
(Coulomb’s Law)

Answer 11

F = Q1Q2/4π ε0 r^2

Q = chargers of the particles
ε0 = permittivity of free space, constant
r^2 = distance of between the charges

Question 12

Define permittivity

Answer 12

The ability of a medium to store energy in an Electric Field (how easily the atoms become polarised).

Question 13

What is the formula for the Electric Field Strength of a point charge?

Answer 13

E = Q/4π ε0 r^2

Q = chargers of the particle
ε0 = permittivity of free space, constant
r^2 = distance of between the charges

Can be derived from E=F/Q

Question 14

Name some similarities between

Gravitational and Electric Fields

Answer 14

Name some similarities between Gravitational and Electric Fields
- Both follow the inverse square law for the force.
- Point masses and point charges both produce a radial field.
- Newton’s law and Coulomb’s law formulae for force are very
similar
- Field strength is defined by force per unit charge/mass.

Question 15

Name some differences between

Gravitational and Electric Fields

Answer 15

• Gravitational fields are always attractive, Electric Fields can be attractive or repulsive (depending on the charge)
• The constants of proportionality in Newton’s Law and Coulomb’s Law are different:

Question 16

What is the formula for the work done when moving a charge in an Electric
Field?

Answer 16

Work done = Force x Distance moved

Question 17

Define the potential at a point in an Electric Field

Answer 17

The work done per unit charge in moving a positive test charge from infinity to that point in the Electric Field. V = W/Q

Question 18

What is the formula for the potential at a

point in an Electric Field?

Answer 18

V= W ÷ Q

V = Potential (V)
W= Work done in moving the particle(J)
Q= Charge of the particle (C)

Question 19

What is the formula for the potential

between two parallel plates?

Answer 19

V = E x d

V= Potential (V)
E= Electric Field strength (NC-1)
d = distance between the plates (m)

Question 20

What is the formula for the capacitance of a parallel plate capacitor?

Answer 20

C = A ε0 εr /d

A = area of the plates (m^2)
ε0 = permittivity of free space
εr = relative permittivity of dielectric
d = distance between the plates (m)

Question 21

What can the motion of charged particles in an Electric Field be modelled as?

Answer 21

Projectile motion: the two components of velocity are independent of each other.
Velocity perpendicular to the field is not affected, velocity parallel to the field is

Question 22

How do you calculate the parallel component of velocity for a charged particle in a uniform Electric Field?

Answer 22

1. First, calculate the time the particle is in the field (using time = distance/speed, where distance = length of charged plates and speed = velocity perpendicular to the field)
2. Use a = F/m and F=Eq to calculate the acceleration of the particle while it is in the field (a = Eg/m)
3. Substitute these values into V = u + at where u is the initial parallel velocity and V is the final parallel velocity.

Question 23

What is the formula for the potential near a point charge?

Coulomb’s Law

Answer 23

V = Q/4π ε0 r

Q = charge of point charge
ε0 = permittivity of free space
r = distance from point charge

Question 24

What does the force-distance graph for a point/spherical charge look like?

Answer 24

Force
I`
I.`
I...`
I......`
I.............`
I--------------------`  radius

Force is inversely proportional to the square of the distance.

Question 25

What does the area under a force-distance graph for a point/spherical charge represent?

Answer 25

Force 
I`
I.`
I...`
I......`
I.............`
I--------------------`  radius

Area under graph = work done

The work done in moving the moving

Question 26

What is the formula for electric potential energy near a point charge?

Answer 26

E= Vxq
E = electric potential energy (J)
V = potential (V)
q = charge of the point charge (C)

Question 27

What is the formula for the capacitance of an isolated sphere?

Answer 27

Isolated spheres can store charge, so technically they can
be classed as capacitors
Using C= Q/V and the formula for V (in terms of Coulomb’s law), you can derive the formula for capacitance

C = 4π ε0 r

Question 28

What is the formula for electric potential energy near a point charge?
(Coulomb’s Law)

Answer 28

E = Q1Q2/4π ε0 r

Q = chargers of the particles
ε0 = permittivity of free space, constant
r = distance of between the charges