Electric and Magnetic Fields

Question 1

What is an inverse square law?

Answer 1

The electrostatic force between two charged spherical objects

Question 2

What happens between two like and opposite charges

Answer 2

Like- repels opposite- attract

Question 3

What is electric field strength and units? and what type of unit

Answer 3

E, at a point is the force per unit charge at that point Nc^-1, vector

Question 4

Talk about field lines

Answer 4

The close they are, the stronger the field, they point towards the negative charge

Question 5

Which direction is the field around a negative point charge directed

Answer 5

Inwards towards the charge, and around a spherical positive charge distribution it is directed radially outwards

Question 6

What is an example of a non radial field and where is it found

Answer 6

Uniform electric field, it is found in the space between two parallel plates when there is a potential difference applied between the plates

Question 7

What is a capacitor

Answer 7

A device that stores energy by separating charge. Placing charge on the capacitor in a potential difference

Question 8

Units of capacitance? And what is definition.

Answer 8

A farad, 1F, 1CV^-1, it is a huge unit and normally ranges from mewf to pf. Amount of charge stored per volt. C=Q/V

Question 9

When is a capacitor fully charged

Answer 9

When the potential difference across it becomes equal to the emf of the source

Question 10

E

Answer 10

Energy is stored in the capacitor because work is done as charge moves through the net potential difference in the circuit. This becomes electrostatic potential energy

Question 11

How do we measure the work done

Answer 11

The shaded area under the graph of potential difference between the plates v against the charge on the plates q. Area of triange half base times height so w=0.5QV and using C=Q/V you can change to different formulas when you make q and v subjects.

Question 12

How do you discharge a capacitor? and what happens

Answer 12

Move the switch, as the charge decreases, so does the potential difference across the capacitor and hence also the current

Question 13

What is the discharging process an example of?

Answer 13

Exponential decay

Question 14

What is RC

Answer 14

Time constant of the circuit

Question 15

What does a wire carrying a current have around it and what happens when wire is wound into a long solenoid.

Answer 15

a circular magnetic field and when wound into a long solenoid there is a uniform field inside . Parallel equally spaced field lines

Question 16

What are lines representing the magnetic field called

Answer 16

lines of magnetic flux

Question 17

3 things about the lines

Answer 17

Arrows indicate the forces that would act on a magnetic north pole

The lines are continuous and must not cross each other

The closer the lines, the stronger the magnetic field

Question 18

What do the number of lines passing through a unit area perpendicular to the field represent

Answer 18

the flux density beta, and is measured in tesla, T. Flux omega is measured in webers, wb.

Question 19

When will there be a maximum force in f=bilsintheta and what is the magnetic field strength B defined as?

Answer 19

when the current is at right angles to the direction of the field, the force on one metre of wire carrying a current of one amp at right angles to the magnetic field.

Question 20

Charged particle beams- A charge q, moving with speed v through a magnetic field also experiences a magnetic force F, as long as there is a component of the field perpendicular to the direction of charge movement.

Again, when max force?

Answer 20

Maximum force will be when the charge movement is ar right angles to the direction of the field. F=BQvSintheta

Question 21

When is an emf generated

Answer 21

When any conductor experiences a changing magnetic flux

Question 22

What causes the changing magnetic flux

Answer 22

May be due to relative movement between the conductor and a magnetic field (dynamo effect).

Or because the conductor is in a region of varying magnetic field strength (the transformer effect)

Question 23

What is flux linkage

Answer 23

For a conducting coil, the size of the effect increases with increasing turns of wire, knows as flux linkage.

N Omega, where n is the number of turns. The flux through the coil depends upon the angle of the normal to the plan of the coil to the field direction.

Question 24

What is Faraday’s law

Answer 24

States that the magnitude of the emf, is directly proportional to the rate of change of flux linkage.

Question 25

What is Lenz’s law

Answer 25

States that the induced emf must cause a a current to flow in such a direction as to oppose the change in flux linkage that produces it, otherwise energy would appear from nowhere.

Question 26

What do the fingers represent in the right hand rule.

Answer 26

Thumb- current direction

Curled fingers- Point in the direction of the field

Question 27

Explain the dynamo effect

Answer 27

As the coil rotates, the magnetic flux linkage goes from zero (when the coil is parallel with the field) to a maximum (when the coil is

Question 28

Explain the transformer effect

Answer 28

If the current in a coil changes, then so does the magnetic field produced . If the changing magnetic flux links with another coil then there will be an emf induced in that coil. The coil in which current is changing is referred to as primary. The other coil is called the secondary

Question 29

How do you ensure maximum flux linkage between the two coils

Answer 29

They are mounted on a soft iron core. This becomes magnetized when current flows in the primary, and flux is channeled to the secondary.

Question 30

Explain eddy currents

Answer 30

Any metal object placed in a region of changing magnetic field will have an emf induced in it. This emf causes current to circulate in the object . We call such currents eddy currents. The eddy currents in a transformer dissipate energy, so can lead to inefficiency. In situations where relative movement is involved, the eddy currents cause braking.

Question 31

Talk about field strength in a radial field and a uniform field and explain why, also equation for electric field strength

Answer 31

Raidal field: Depends on how far you are from the charge, as you go further away from a point charge q, the field lines get further apart and the field strenght decreases
Unifrom field: the field strength is the same everywhere. The field lines are parallel so always same distance apart so the field strength depends on the potential difference V and distance d E=V/d (Vm^-1)

E=F/Q force per unit charge

Question 32

Formula for field strength

Answer 32

E=kQ/r^2 where k=1/4pieE

Question 33

The time taken to charge and discharge a capacitor depend on what two things?

Answer 33

The capacitance

The resistance

Question 34

What is a magnetic field and example

Answer 34

A region where a force is exerted on a magnetic material. There is a magnetic field around a wire carrying electric current.

Question 35

What is magnetic field strength or magnetic flux density a measure of and what are the units and what type of quantity.

Answer 35

Is a measure of the strength of the magnetic field per unit area. Its a vector quantity with both a direction and magnitude and is measured in teslas

Question 36

What is flux linkage

Answer 36

The product of magnetic flux and the number of turns on the coil. Wb- weber

Question 37

Talk about the process of a wire carrying a current in a magnetic field experiencing a force

Answer 37

A force acts if a current carrying wire cuts magnetic flux lines

If the current is parallel to the flux lines, no force acts

The direction of the force is always perpendicular to both the current direction and the magneti

Question 38

Fleming’s left hand rule finger meanings

Answer 38

First finger- direction of the uniform magnetic field
seCond finger- direction of the conventional current
thuMb- direction of the force in which motion takes place