Electric Fields

Question 1

What is charging by friction?

Answer 1

The action of rubbing causes a transfer of electrons from the surface atoms of one of the rubbed materials to the other. The material which gains electrons acquires a negative charge and that which loses electrons becomes positively charged

Question 2

How does a conductor work?

Answer 2

contains free electrons, therefore electrons gained by a conducting material are able to move throughout the material, and if electrons are lost those which remain redistribute in the material. If the conductor is connected to Earth, the charge acquired can flow to the Earth

Question 3

How does an insulator work?

Answer 3

Doesn’t contain free electrons, therefore if one region of an insulating object becomes charged it does not cause a movement of electrons within the body. The charge stays in the region in which it has been placed (it is static).

Question 4

What are electric fields?

Answer 4

• an electric field is the region of space in which a charged body exerts an influence
• the direction of an electric field is the direction of the force experienced by a positively charged particle, a ‘test charge’, when placed at that point in the field

Question 5

Define electric field strength

Answer 5

The electric field strength, E, at a certain point is the force per unit charge acting on a positive charge placed at that point in the field

E = F/Q

Question 6

What is the unit for electric field strength?

Answer 6

Newton per coulomb (N/C)
or
Volt per metre (V/m)

Question 7

Explain electric field lines

Answer 7

In an electric field, the shape of the field can be shown using electric field lines. These are the paths along which a small point positive charge would move it if was to be placed in the field and allowed to move. The closer the spacing of the lines, the greater the strength of the field

Question 8

State Coulomb’s law

Answer 8

The force between two charged particles is directly proportional to the product of the two charges and inversely proportional to the square of their separation

F ∝ Qq/r²

Question 9

What is the constant of proportionality added to Coulomb’s law, and what equation does it produce?

Answer 9

Constant of proportionality = 1/4πε

Full equation - F = Qq/4πε₀r²

Question 10

What is the permittivity of free space (ε₀)?

Answer 10

ε₀ = 8.85 x 10^-12 F/m

Question 11

How do you calculate the permittivity of a material?

Answer 11

ε = εᵣε₀ , where εᵣ is the relative permittivity (dielectric constant) and ε₀ is the permittivity of free space

Question 12

What equation is formed by combining E = F/Q and F = Qq/4πε₀r²?

Answer 12

E = Q/4πε₀r²

Question 13

State the relationship between potential and electric field strength

Answer 13

E = -V/d , where V is the potential difference between 2 plates, and d is the separation of the plates

Question 14

What happens to a charged particle entering a field perpendicular to the field lines?

Answer 14

The charged particle will experience a force parallel to the field lines
- positively charged particles experience a force in the same direction as the field lines
- negatively charged particles experience a force in the opposite direction to the field lines

Question 15

What path will a particle moving through a uniform field follow?

Answer 15

As the field lines are uniform, but the direction of the particle changes, the particle will follow a parabolic path through the field.

Question 16

State the two equations for capacitance

Answer 16

C = Q/V

C = εA/d

Question 17

Define electric potential

Answer 17

The electric potential at a point is defined as the work done in bringing unit positive charge from infinity to that point; electric potential is zero at infinity

Question 18

What are the two equations for electric potential, and how can they be combined to form an equation for energy?

Answer 18

V = Q/4πε₀r

V = E/q , where E is energy transferred

Combines to form E/q = Q/4πε₀r

Rearranges to form energy = Qq/4πε₀r

Question 19

What is an equipotential?

Answer 19

An equipotential surface is one over which the potential in an electric field remains constant

Question 20

What are properties of equipotentials?

Answer 20

• no work is done in moving a charge along an equipotential
• equipotentials always cut across lines of force at right angles
  Close to the charge, equipotentials are close together, showing a large potential gradient

Question 21

Define electric potential energy

Answer 21

The electrical potential energy, E, is equal to the work done in bringing two positively charged particles from infinity to a separation, r