Magnetism and Electric Feilds

Question 1

What causes a magnetic field

Answer 1

Due to either:

Moving charges
Permanent magnets

Question 2

Describe the magnetic field line patterns for a long straigh current-carrying conducter

Answer 2

Use right hand grip rule:

Thumb points in direction of conventional current
Your fingers curl around the wire, showing the direction of the magnetic field lines.

Question 3

Describe the magnetic field pattern for a flat coil

Answer 3

The lines come out from the end where the current comes out and go into the end which the current goes in

Question 4

Describe Fleming’s left hand rule

Answer 4

A way to show the relationship in the directions of current, magnetic field lines and the force applied

Question 5

Define magnetic flux density

Answer 5

The strength of the magnetic field, measured in teslas, hence:

The amount of force applied by unit current and unit length on an object in a magnetic field.

Question 6

State four factors which effect the magnitude of the force experienced by a wire in an external magnetic field

Answer 6

Current
Length of the wire in the magnetic field
Sin x, where x is the angle between the magnetic field and the current direction
Strength of the magnetic field.

Question 7

Explain how a charged particle can go travel in circular motion due to a magnetic field

Answer 7

If the force applied to the charged particle due to a magnetic field is at right angles to the particle’s velocity, the force acts as the centrapetal force on the object.

Question 8

Define a velocity selector

Answer 8

A velocity selector is a device that uses both electric and magnetic fields to select charged particles of specific velocity.

Question 9

Describe how a velocity selector works

Answer 9

Two parallel horizontal plates are connected to a power supply
They produce a uniform electric field
A uniform magnetic field is also applied perpendicular to the electric field
All particles enter the velocity selector through a narrow slit
In order to be undeflected, only particles with electric force which is equal to its magnetic force will leave through a second narror slit
The desired velocity is chosen by using v= E/B

Question 10

Define magnetic flux

Answer 10

The product of the component of the magnetic field density perpenicular to the area and the crossectinal area

Question 11

Define the unit weber

Answer 11

One weber is equal to 1 Tesla unit area (magnetic flux)

Question 12

What is magnetic flux linkage?

Answer 12

The product of the number of turns in the coil (N) and the magnetic flux

Question 13

What causes an e.m.f. to be induced in a circuit, when related to magnetism?

Answer 13

When there is a change in the magnetic flux linking the circuit

Question 14

Define Faraday’s Law

Answer 14

The magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux linkage

Question 15

Define Lenz’s Law

Answer 15

The direction of the induced e.m.f. or current is always such as to oppose the change producing it

Question 16

Describe how a simple A.C. works

Answer 16

A coil is turned in radical magnetic field
The change in magnetic flux linkage causes an e.m.f to be induced
As e.m.f. ∝ -∆ø, plotting a graph of magnetic flux linkage against time’s negative gradient will equal the e.m.f.
As the change in magnetic flux linkage varies with cos(x) where x is the angle made with the vertical, and the coil is turned at a constant speed, the rate of change of magnetic flux linkage changes in relation to a cos graph, so the gradient will give a sin graph, giving an alternating e.m.f., so an alternating current

Question 17

Define a transformer

Answer 17

An appliance which changes alternating voltages to higher or lower values

Question 18

Suggest why a transformer is made from a laminated soft iron core?

Answer 18

Soft Iron: can easily be magnitised and de-magnitised

Laminated: minimise currents in the core itself, so minimising loses due to heating

Question 19

What causes an electric field?

Answer 19

Any object with a charge

Question 20

Define electric field strength

Answer 20

The force experienced per unit positive charge at that point

Question 21

Describe how an object with an electric field can be modelled

Answer 21

A uniformly charged sphere is often modelled as a point charge at its centre

Question 22

What is the use of electric field lines?

Answer 22

They are used to map electric fields and travel from positive to negative

Question 23

Define coulomb’s law

Answer 23

Any two point charges exert an electrostatic force on each other which is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

Question 24

Give the similarities between electric and gravitational fields

Answer 24

Both inversley proportional to the separation2
Both a point mass/charge which produces a radial field
Field strenght is both a force per unit of something

Question 25

1

Question 26

Describe how the electric field strength of two parallel plates can be determined

Answer 26

The voltatge across the plates can be measured and divided by the distance between the plates in metres.

Question 27

Define electric potential

Answer 27

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

Question 28

What is the electric potential at infinity?

Answer 28

Zero

Question 29

Derive an equation for the capacitance of an isolated sphere

Answer 29

C=Q/V

Q=4πε0RV

Combining gives: C=4πε0RV/V

So C=4πε0R

Question 30

Describe how the work done for a point charge can be worked out

Answer 30

Calculate the area under the force-distance graph for that point charge

Question 31

Define electric potential energy

Answer 31

The work done on a particle to bring it from infinity to a certain point in an electric field