Chapter 22 - Electric Fields

Question 1

What is an Electric Field?

Answer 1

An electric field is a region of space in which charged particles are subject to an electrostatic force.

Question 2

Static Electricity

Answer 2

· When two insulators are rubbed together.
· One will gain electrons while the other loses them, causing them to become slightly charged.

Question 3

Point Charges

Answer 3

Spherical charges, that produce a radial electric field. Field strength decreases as radial distance increases.
E = Q / (4π r²εₒ)

Question 4

Drawing Electric Fields

Answer 4

Electric field lines are equally spaced, go from positive to negative, perpendicular to the surface.
Line that join positive to negative so attraction, whereas lines that do not join up show repulsion.

Question 5

Define Electric Field Strength

Answer 5

The force applied per unit of charge.
E = F/Q
Is a Vector Quantity.

Question 6

What is Coulomb’s Law?

Answer 6

Any two point charges exert an electrostatic force between them that is proportional to the product of there charges and inversely proportional to the square of the distance between them.
F ⍺ Qq/r²

Question 7

What is ε₀

Answer 7

The permittivity of free space (8.85x10⁻¹²) defines the ability of a vacuum to become polarised and store charge.

Question 8

How to work out Permittivity

Answer 8

ε = εr x ε₀

Question 9

Equation for the Force Between two Point Charges

Answer 9

F = Qq/4π ε₀r²

Question 10

Uniform Electric Fields

Answer 10

A uniform electric field is produced between two parallel oppositely charged plates and the field is constance no matter the position in the field.
E = V/d
Voltage over distance between plates

Question 11

Work Done in an Electrical Circuit

Answer 11

W = VQ

Question 12

Define Potential Difference

Answer 12

V = W/Q
The work done per unit charge in moving a charge from one point to another point.

Question 13

Electric Potential

Answer 13

The electric potential at a single point is defined as the work done per unit charge to bring a positive charge from infinity to that point.
V = Q/4π ε₀r

Question 14

Capacitance of a Isolated Sphere

Answer 14

V = Q/4π ε₀r and V = Q/C
Q/C = Q/4π ε₀r
C = 4π ε₀r

Question 15

Similarities and Differences between Gravitational and Electrical Fields

Answer 15

· Force for both is inversely proportional to the distance squared.
· Force per unit mass or unit charge.
· Gravitational and Electrical Potential.

·Gravitational is always attractive, whereas electrical can be attractive or repulsive.
· Objects can be shielded in an electric field.
· The electrical force depends on the medium between charges.

Question 16

Millikan’s Oil Drop Experiment

Answer 16

·Oil Droplets are charged.
· Fall under gravity in a vertical electric field, which is generated by two oppositely charged parallel plates.
· The voltage between the plates is adjusted so the oil drop hovers between the plates.
· The weight acting downwards must be equal to the upwards acting electric field.
mg = VQ/d
Q = mgd/V
Q = Ae where A is an integer

Question 17

Deriving Electric Potential

Answer 17

F = Qq/4π ε₀r² and W = Fd
Eₑ = Qq/4π ε₀r
V = Eₑ/q
V = Q/4π ε₀r