Electric Fields

Question 1

Define a field

Answer 1

A field is a region in which a body can experience a non-contact force.

Question 2

What are the rules for drawing electric field lines?

Answer 2

When drawing electric fields: The lines should never cross.

The spacing of the lines represents the strength of the field.

An arrow should be drawn to show the direction of force that a positive charge would feel

Question 3

What is a field line (electric)?

Answer 3

The path a free positive test charge follows.

Question 4

Describe a field between two oppositely charged plates

Answer 4

Field lines run parallel, at right angles to the plates.

Field lines are evenly spaced.

At the edges of the field the lines are slightly curved.

Inside the plates is a uniform field.

Question 5

What kind of a field is it between two oppositely charged plates?

Answer 5

Uniform field

Question 6

What is a point charge?

Answer 6

An expression for a charged object in a situation where distances under consideration are much greater than the size of the object.

Question 7

Sketch the electric field around a point charge?

Answer 7

Radial field with arrows pointing away. See diagram

Question 8

Sketch the field around a negative point charge

Answer 8

Radial field with arrows pointing towards the centre of the charge.

See diagram

Question 9

Sketch the field pattern around two positive charges

Answer 9

See diagram

Question 10

Sketch the field pattern around a positive and negative charge

Answer 10

See diagram

Question 11

Define electric field strength at a point in the field

Answer 11

The force per unit positive charge.

E = F/Q

Question 12

Sketch a graph of electric field strength against separation?

What can be found from this graph?

Answer 12

See diagram

Area = Electrical potential

Question 13

Define Coulombs law

Answer 13

The force of attraction or repulsion between two charges is proportional to the product of the charges and inversely proportional to the separation squared.

Question 14

Sketch a graph of force against separation?

What can be found from this graph

Answer 14

See diagram

Area = work done = Electrical potential energy

Question 15

If two forces are in the same direction, what is the resultant electric force?

Answer 15

F₁ + F₂

Question 16

If two forces are in opposing direction, what is the resultant electric force?

Answer 16

F₁ - F₂

Question 17

If two forces are at right angles to each other, what is the resultant electric force?

Answer 17

Resolve using pythagorus.

(F₁² + F_2²)^0.5

Question 18

State the factors that affect the electric field strength between two parallel plates

Answer 18

1. Potentail difference between the plates (directly proportional)
2. Plate seperation (inversley proportional)

Question 19

Describe the motion of a positive charged particle moving parallel to a uniform electric field.

Answer 19

Uniform acceleration due to uniform field.

Question 20

Describe the motion of a positive charged particle moving perpendicular to a uniform electric field.

Answer 20

Motion perpendicular to the field - constant velocity.

Motion parallel to the field - constant acceleration

This causes the particle to move in a parabolic curve.

Question 21

Define electric potential difference

Answer 21

The total work done to bring a positvely charged particle from infinity to a separation r

Question 22

State the equation for electrical potential difference.

Answer 22

E_p = k Q / r

Question 23

Sketch a graph for electrical potential difference against seperation for like charges.

What can be found from this graph?

Answer 23

See diagram - like charges repel so work is done to bring them together.

There is 0 electrical potential difference at infinity.

Gradient = force

Question 24

Sketch a graph for electrical potential difference against seperation for opposite charges.

What can be found from this graph?

Answer 24

see diagram - Opposite charges attract so work is done moving them apart.

There is 0 electrical potential difference at infinity.

Gradient = Force

Question 25

Define electric potential at a point in an electric field

Answer 25

Work done per unit positive charge on a positive test charge when it is moved from infinity to that point.

V = W/Q

Question 26

What can be found from a graph of electric potential against separation?

Answer 26

Gradient = Electric field strength

Question 27

Sketch a graph of potential vs seperation when moving a positive charge towards another positve charge.

Answer 27

See top half of the diagram (positive y axis only).

Positive charges repel so work is done moving them closer together. As they move closer potential and potential energy would increase.

Potential at infinity is 0.

Question 28

Sketch a graph of potential vs seperation when moving a negative charge towards a positve charge.

Answer 28

See bottom half of the diagram (negative y axis only).

Opposite charges attract so work is done moving them further apart. As they move further apart potential and potential energy would increase.

Potential at infinity is 0.

Question 29

What do lines of equipotentail show?

Answer 29

Lines along which electric potential is constant.

Question 30

What do closer lines of equipotential show?

Answer 30

Closer equipotentials means greater potential gradient at right angles to them.

More work is needed to move a charged particle a set distance

Question 31

How is potential gradient (spacing of lines of equipotential) effected if the field is uniform?

Answer 31

Equipotentials between the plates are equally spaced parallel to the plates.

Potential gradient is constant.

Question 32

Describe the lines of equipotential for a uniform field between two parallel plates

Answer 32

Lines of equipotential are perpendiculer to the electric field and are evenly spaced.

See diagram

Question 33

Describe the lines of equipotential for a radial field from a point charge

Answer 33

Lines of equipotential are circular rings around the charge.

The spacing increases as distance from the charge increases.

See diagram

Question 34

Describe the lines of equipotential between two positive charges

Answer 34

See diagram

Question 35

Describe the lines of equipotential a positive and negative charge

Answer 35

See diagram

Question 36

State some similarities between E fields and g

Answer 36

1. Both have infinite range
2. Both follow the inverse square law with seperation

Question 37

State some differences between E fields and g

Answer 37

1. E depends on charges but g depends on mass
2. E can cause attraction or repulsion. g can only attract.