7.3 Electric fields

Question 1

What is an electric field?

Answer 1

A region where charged objects will experience a non-contact force.

Question 2

What is the unit for electric charge?

Answer 2

Coulombs (C)

Question 3

What happens when a charged object is placed in an electric field?

Answer 3

It experiences a force.

Question 4

In electric field questions, what can he assumed about a charged object that is a sphere?

Answer 4

All of its charge is at its centre.

Question 5

How can electric fields be represented?

Answer 5

Using field lines.

Question 6

State Coulomb’s law.

Answer 6

The magnitude of the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

Question 7

What type of law is Coulomb’s law?

Answer 7

• Inverse square law
• Since F ∝ 1/r²

Question 8

What is the significance of the ε in Coulomb’s law?

Answer 8

• This is the permittivity of the material the charges are in
• This affects the size of the force between the charges
• If the system is in air, it can be considered the same as in a vacuum

Question 9

What is electric field strength?

Answer 9

The force per unit positive charge exerted at a certain point in an electric field.

Question 10

What is the unit for electric field strength?

Answer 10

N/C

Question 11

What does a field line diagram look like?

Answer 11

Question 12

What happens to the field lines as you get further away from a point charge?

Answer 12

They get further apart.

Question 13

Describe the graph for E against r for an electric field around a point charge.

A

Answer 13

Question 14

How can a uniform electric field be produced?

Answer 14

Connecting two parallel plates to opposite poles of a battery.

Question 15

What can a uniform electric field be used for? How?

Answer 15

• Determining whether a particle is charged.
• If a particle curves in the same direction as the field lines, it is positively charged
• If a particle curves in the opposite direction as the field lines, it is negatively charged

Question 16

How to find out whether a
particle is charged, and whether its charge is negative or positive ?

Answer 16

• This is done by firing the particle
  at right angles to the field and observing its path
• A charged particle will experience a constant
  electric force either in or opposite to the direction of the field (depending on its charge), this
  causes the particle to accelerate and so it follows a parabolic shape

Question 17

What is absolute electric potential?

Answer 17

The electric potential energy that a unit positive charge would have at a point in an electric field.

Question 18

What effects electric potential?

Answer 18

Size of charge creating the electric field and distance from the charge.

Question 19

When is the magnitude of the electric potential around a point charge the greatest?

Answer 19

On the surface of the charge.

Question 20

Describe the graph of V against r around a point charge for a repulsive force.

Answer 20

Question 21

Describe the graph of V against r around a point charge for an attractive force.

Answer 21

Question 22

How can electric field strength be found from a V-r graph around a point charge?

Answer 22

• Gradient of tangent

* Because E = ΔV / Δr

Question 23

How can potential difference between two points be found from an E-r graph around a point charge?

Answer 23

• Area under graph between two points

* Because E = ΔV / Δr so ΔV = E x Δr

Question 24

What is electric potential difference?

Answer 24

The energy needed to move a unit positive charge between two points.

Question 25

What shape are equipotentials around a point charge?

Answer 25

Spherical

Question 26

What is the one important difference between electric and gravitational fields?

Answer 26

Gravitational fields are always attractive, whereas electric forces can be attractive or repulsive.

Question 27

At sub-atomic level, does electrostatic or gravitational force have a greater effect and why?

Answer 27

• Electrostatic
• Because the masses are tiny, so the gravitational force is also tiny
• NOTE: There are other forces that keep the nucleus stable

Question 28

When can charge transfer between two objects?

Answer 28

When they slide past each other

Question 29

When two objects slide past each other, how is charge transferred?

Answer 29

Electrons leave one surface and join the other

Question 30

What are the rules about field lines? (3)

Answer 30

∙ never start or stop in empty space

∙ never cross

∙ density of field lines shows strength of field

Question 31

How can the strength of the uniform field between parallel plates be made stronger?

Answer 31

∙ increase p.d. across plates

∙ moving plates closer together

Question 32

Difference between radial field and uniform field

Answer 32

• A uniform field exerts
  that same electric force everywhere in the field, as shown by the parallel and equally spaced field
  lines
• whereas in a radial field the magnitude of electric force depends on the distance between
  the two charges

Question 33

Define electric potential at a point in an electric field

Answer 33

The electric potential at a point in an electric field is the work done per unit charge when a point positive charge is moved from infinity to the point.