Fields and Their Consequences

Question 1

The force felt in a field is a non-contact force. True or false?

Answer 1

True

Question 2

Is force a vector or scalar quantity?

Answer 2

A vector

Question 3

What are some similarities between electrostatic and gravitational forces?

Answer 3

• Inverse square law applies
• Both have potentials
• Equipotential surfaces
• Use of field lines

Question 4

What are some differences between electrostatic and gravitational forces?

Answer 4

The gravitational fields from masses always attracts, while charges can attract and repel

Question 5

What is gravity?

Answer 5

The universal force which acts between all matter

Question 6

What is G?

Answer 6

The universal gravitational constant

Question 7

What can field lines tell us about a field ?

Answer 7

Direction of field, strength of field (density of lines)

Question 8

What is g?

Answer 8

The force per unit area in a uniform field

Question 9

What is gravitational potential ?

Answer 9

The potential energy per kilogram at any point in the field

Question 10

What is the work done by moving a mass in a field ?

Answer 10

Mass x change in potential

Question 11

What is the gravitational potential difference ?

Answer 11

The difference in the gravitational potentials of two points in a gravitational field

Question 12

What is an equipotential ?

Answer 12

A surface in which every point on the surface has the same potential

Question 13

How much work is done when you move one km in any direction on an equipotential ?

Answer 13

No work is done when moving across equipotentials, as the potential at each point is the same

Question 14

Why is gravitational potential a negative value?

Answer 14

Work needs to be done to more an object from the inside the field to outside the field. Since outside the field’s potential is defined as zero then the potential inside the field must be negative

Question 15

Outline the right hand screw rule and when you would use it.

Answer 15

It’s used when trying to determine the direction based on a rotational direction or vice versa. Fingers curl round in the direction of positive rotation and the thumb indicated the positive direction

e.g. direction of current in a wire based on the direction of the circular magnetic field

Question 16

Outline the right hand grip rule and when you would use it.

Answer 16

Often used to find the way the field points where your thumb equals the direction of conventional current and the fingers are equal to the direction of the magnetic field

Question 17

Outline Fleming’s left hand motor rule and when you would use it.

Answer 17

Can be used to find the direction of the Lorentz force or other components. Thumb - force, first finger - field, middle finger - current

Question 18

Outline the right hand rule.

Answer 18

Thumb - velocity, first finger - magnetic field, middle finger - force due to magnetic figures

Question 19

What is a unique feature about magnetic fields that is not true about electric or gravitational fields?

Answer 19

You cannot have monopoles in a magnetic field, you always have north and south together

Question 20

What is the relationship between field strength and distance from the wire?

Answer 20

They are inversely proportional

Question 21

How do field lines flow inside and outside of a solenoid?

Answer 21

Inside the solenoid they travel from S to N, outside they travel N to S

Question 22

What type of field is there in the centre of a solenoid?

Answer 22

A uniform field

Question 23

When drawing field lines/current, what is meant by a dot?

Answer 23

line/current is coming out of the page

Question 24

When drawing field lines/current, what is meant by a cross?

Answer 24

line/current is going into the page

Question 25

What are the four factors that affect the magnitude of the Lorentz force?

Answer 25

magnetic field strength (B), current (I), length of wire in the field (L), angle between the wire and the field (sin theta)

Question 26

Define magnetic field strength and give its unit.

Answer 26

Force per unit current length, Tesla, T

Question 27

What form of Tesla, T is most common?

Answer 27

millitesla (mT)

Question 28

Why does a positive charged particle in a uniform magnetic field undergo circular motion?

Answer 28

The Lorentz force always acts at right angles to the velocity of the particle, resulting in circular motion

Question 29

How would the paths of one positive and one negative particle in a uniform magnetic field, travelling with the same velocity differ?

Answer 29

The negative particle would curve in the opposite direction

Question 30

Why does a positive charge moving into a uniform electric field not undergo circular motion?

Answer 30

The force only acts in one direction so the motion of the particle is parabolic, not circular

Question 31

What is the relationship between Lorentz force, magnetic field strength, charge and velocity?

Answer 31

F = BQV

Question 32

What affects the radius of a charged particle’s motion through a uniform magnetic field?

Answer 32

The charge to mass ratio, velocity, magnetic field strength

Question 33

At what angle must a charged particle enter a magnetic field for it to move in a circle?

Answer 33

90ºC

Question 34

What happens to the motion of a charged particle if it enters a magnetic field at an angle not equal to 90ºC?

Answer 34

It moves in a spiral pattern

Question 35

In a cyclotron, how often does the electric potential change?

Answer 35

Every half rotation of the ion

Question 36

What is one benefit to a cyclotron over a linear accelerator?

Answer 36

It takes up far less space

Question 37

What coils were used to produce the magnetic field for Thompson’s crossed fields?

Answer 37

Helmholtz coils