6.2 Electric Fields

Question 1

Electric Field

Answer 1

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

Question 2

How can we prove that static electricity and induced charges exist?

Answer 2

By generating a static charge on an object. For example, rubbing a glass rod with a silk cloth.

Question 3

A glass rod is rubbed with a silk cloth. What occurs?

Answer 3

The glass rod loses electrons to the silk cloth, becoming slightly positive.

Question 4

Protons and electrons are charged particles that can be modelled as …?

Answer 4

Point charges.

Question 5

What type of field do point charges have?

Answer 5

Radial fields (which look like the spokes of a wheel).

Question 6

In which direction do electric field lines point? What does it represent?

Answer 6

Outwards from a positive charge or inwards towards a negative charge.

This is because the direction of the field represents the direction of the electrostatic force on a positive charge

Question 7

How does the strength of an electric field change when moving away from the charge?

Answer 7

It decreases (field line density decreases).

Question 8

Why can the metal sphere of a Van De Graaff generator be modelled as a point charge?

Answer 8

Outside the sphere, the electric field is radial and decreases in strength with distance away from the surface of the sphere.

Question 9

Electric field strength, E

Answer 9

The force applied per unit charge on an object.

E = F/Q

Question 10

What are the units for electric field strength?

Answer 10

NC-1 or Vm-1 (these are interchangeable)

Question 11

Is electric field strength scalar or vector?

Answer 11

Vector

Question 12

State coulomb’s law

Answer 12

Any two point charges exert an electrostatic force between them that is proportional to the product of their charges, and inversely proportional to the square of the distance between them.

Question 13

Coulomb’s law is similar to Newton’s law of gravitation. What is different between them?

Answer 13

The charges are replaced with masses.
Gravity is a solely attractive force hence the negative sign used in the gravitation law.

Question 14

What is permittivity?

Answer 14

A constant that defines the ability of a material to become polarized and store charge.

Question 15

What is the permittivity of free space?

Answer 15

The permittivity of a vacuum.

Question 16

Why is the permittivity of free space the lowest value possible?

Answer 16

There is no matter in a vacuum to become polarized.

Question 17

How can the permittivity be found?

Answer 17

ε = εr X ε0

ε: permittivity
εr: relative permittivity (or dielectric constant)
ε0: permittivity of free space

Question 18

Use coulomb’s law and the formula for electric field strength to form an expression for the electric field of a point charge.

Answer 18

Imagine bringing a small test charge of charge q towards a large point charge of charge Q, such that the effect of the test charge on the point charge is negligible (ie the test charge doesnt cause the point charge to move).

E = F/q

Using coulomb’s law

E = Q / 4πr²ε₀

Question 19

What is a uniform field?

Answer 19

A field which has no dependence upon position (ie the field is constant in space)

Question 20

What type of field is produced between two parallel oppositely charged plates?

Answer 20

A uniform field.

Question 21

Does the force on a particle between two parallel oppositely charged plates change depending on where it resides between the plates?

Answer 21

No. The force is constant.

Question 22

Potential difference

Answer 22

The work done per unit charge in moving a charge from one point in an electic field to another.

V = W/Q

Question 23

Define electric potential at a single point

Answer 23

The work done per unit charge to bring a positive charge from infinity to that point.

Question 24

What is the electric potential at infinity?

Answer 24

0

Question 25

What is the potential difference between two points, A and B?

Answer 25

The difference between the electric potential at A and the electric potential at B.
V = Vb - Va

Question 26

What type of field can E = V / d be used for?

Answer 26

Uniform electric fields.

Question 27

How do we get the formula for electric energy, using the formula for work done and coulomb’s law?

Answer 27

W = Fd

F = Qq / 4πr²ε₀

Ee = Qq / 4πrε₀

Question 28

Electric potential is defined as electrical potential energy per unit charge. How do we get an equation for V using this?

Answer 28

V = Ee / q
Ee = Qq / 4πrε₀

V = Q / 4πrε₀