6.2 - Electrical 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

Question 12

Define permittivity

Answer 12

The ability of a material to transmit 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

This can be derived using E = F ÷ Q and the formula for force(Coulomb’s Law).

Question 14

Name some similarities between Gravitational and Electric Fields

Answer 14

• Both follow the inverse square law for the force.
• Point masses and point charges both produce a radial field.
• Field strength is defined by force per unit charge/mass.
• Newton’s law and Coulomb’s law formulae for force are very similar:

Question 15

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

Answer 15

Work done = Force x Distance moved

Question 16

Define potential at a point in an Electric Field

Answer 16

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

Question 17

What is the formula for the potential at a point in an Electric Field?

Answer 17

V = W ÷ Q

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

Question 18

What is the formula for the potential between two parallel plates?

Answer 18

V = E x d

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

Question 19

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

Answer 19

Question 20

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

Answer 20

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 21

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

Answer 21

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 theparticle while it is in the field (a = Eq / m).
3. Substitute these values into V = u + at where u is the initialparallel velocity and V is the final parallel velocity

Question 22

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

Answer 22

Question 23

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

Answer 23

Force is inversely proportional to the square of the distance.

Question 24

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

Answer 24

The work done in moving the charge.

Question 25

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

Answer 25

E = V x q ## Footnote E = electric potential energy (J) V = potential (V) q = charge of the point charge (C)

Question 26

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

Answer 26

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

Question 27

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

Answer 27