Magnetic Fields

Question 1

What is a magnetic field and how do you represent it

Answer 1

A region where a force is exerted on magnetic materials

Represented by field lines (flux lines)
-go from north to South Pole of magnet
-closer lines = stronger field
- at a neutral point, magnetic fields cancel out
-shows the direction of force in a MF along which a stationary North Pole would move in the field

Question 2

How is a magnetic field produced in a wire

Answer 2

When current flows in a wire or any long straight conductor, a MF is produced around the wire
-the field lines are concentric circles centred on the wire
-use right hand rule to find the direction of MF around a current carrying wire

Question 3

What happens when you put a current-carrying wire into an external magnetic field (e.g. two magnets)

Answer 3

The field around the wire and the field around the two magnets will add together
-causes a resultant field- lines closer together shows a stronger MF
-these close lines cause a pushing force on the wire

Question 4

Describe Flemings left hand rule

Answer 4

The direction of the force is always perpendicular to both current and MF
If current is parallel to field lines the size of the force is 0N
-there is no component of MF perpendicular to the current

FBI
Force, B=Magnetic field (N to S), I=current (+ to -)

Question 5

Define magnetic flux density

Answer 5

The force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines

Measure of the strength of a magnetic field

Question 6

What is force proportional to in an external MF and when current is 90 degrees to the MF

Answer 6

The force on a current-carrying wire at right angles to an external magnetic field is proportional to the magnetic flux density

When current is 90 degrees to the MF, the size of the force is proportional to the current

Question 7

What type of quantity is flux density and what is it measured in

Answer 7

Vector quantity and is measured in Tesla

1 Tesla= Wb/m^2

Question 8

Define a Tesla

Answer 8

The magnetic flux density if a wire of length 1m carrying a current of 1A has a force of 1N exerted on it in a direction perpendicular to both the flux and the current

Question 9

Where are magnetic fields found

Answer 9

Around permanent magnets, induced magnets or current-carrying wires

Question 10

What is the notation for current going into the page and out of the page

Answer 10

Into the page = cross inside a circle

Out of the page= a dot inside of a circle

Question 11

What will happen if 2 parallel wires are placed near each other and their currents are in the same direction? What if the currents were anti-parallel

Answer 11

Parallel currents attract (weak field between wires, strong outside)
Anti parallel currents repel (strong field between wires, weak outside)

Question 12

What is the motor effect and what is the equation for the force on a current carrying wire of length L

Answer 12

A forced produced when a current carrying wire is placed in a magnetic field and the two magnetic fields interact

F=BIL

Question 13

In the motor effect what will happen if you:
-reverse the direction of current
-flip the magnetic poles
-place the wire parallel to the MF

Answer 13

-causes the force to act in the opposite direction

-causes the force to act in the opposite direction

-there is no force when wire is parallel to MF

Question 14

Explain the structure of a motor and the processes involved in making it spin

Answer 14

In a motor, a rectangular current carrying coil is inside a MF
The sides of the coil perpendicular to the MF experience a force
These forces act in opposite directions and form a coupe as the current is in opposite directions
The couple formed causes the coil to rotate (the sides parallel to the MF do not experience a force)
When the coil reaches its vertical position the magnitude of the couple is zero as the forces are acting through the axis of rotation
The SPLIT RING COMMUTATOR ensures the coil continues to rotate as it reverses the direction of the current in the coil

Question 15

What is the equation for the force on a charged particle moving in a MF, when the field is perpendicular to velocity

Answer 15

F=BQv

Why: a force acts on charged particles MOVING in a MF
Electric current is the FLOW of negatively charged electrons

Question 16

Explain why the path of a particle with a positive charge moving at a velocity v at right angles to a MF at magnetic flux density B is circular?

Answer 16

Since the force acts perpendicular to velocity NO WORK is done on the particle (i.e. kinetic energy is constant, therefore speed is constant
The particle is accelerating so direction mush change
Therefore the particle moves in a circle

BQv=mv^2/r
r=mv/BQ

-r increases (particle deflected less) if the mass or velocity of particle increases
-r decreases (particle deflected more) if the strength of the MF or the charge on the particle increases
-if particles with a larger specific charge (Q/m) are used, r decreases

Question 17

What is a cyclotron

Answer 17

A type of particle accelerator used to accelerate charged particles to large speeds approaching the speed of light
The protons are kept moving in a circular path by a uniform magnetic field at right angles to the plane of the path
An alternating EF is used to accelerate the particles onto spiral paths of increasing radius

Question 18

What does a cyclotron look like and how does it work

Answer 18

-consists of 2 hollow Dees (d shaped electrodes) in a vacuum chamber
-a MF is applied perpendicular to plane of the dees, and the dees are connected to an AC supply which produces an alternating EF between the dees

-charged particles are fired into one electrode (dee). The MF makes them follow a circular path and then leave the electrode
-an applied p.d. between the electrodes accelerates the particles across the gap to the next electrode
-since the speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again
-the p.d. is reversed so the particle accelerates again before entering the next electrode
-this process repeats as the particle spirals outwards, increasing in speed before eventually exiting the cyclotron

Question 19

Explain why frequency is constant in a cyclotron and give the equation that shows this

Answer 19

The time to complete each orbit is independent of their speed because although they travel faster in each successive orbit, they also have further to travel

v=2πr/T 1 T=time for one full cycle of the alternating voltage
r=mv/BQ 2

2—> 1
T=2mπ/BQ
f=BQ/2πm

Question 20

What is magnetic flux passing through an area perpendicular to a magnetic field equal to
(Not the same as magnetic flux density)

Answer 20

Φ=BA where B is normal to A

Question 21

How can you induce an emf in a rod

Answer 21

If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force which causes them to accumulate at one end of the rod
This induces an emf across the ends of the rod

Question 22

How do you induce an emf in a flat coil or solenoid

Answer 22

-moving the coil towards or away from the poles of a magnet
-moving a magnet towards or away from the coil

-the emf is caused by the MF changing
-if the coil is apart of a complete circuit, an induced current will flow through it

Question 23

What is flux linkage and the rate of change of flux linkage

Answer 23

The product of the emf induced when moving a coil in a MF, the magnetic flux passing through the coil and the number of turns in the coil that cuts the flux

NΦ=BAN

The rate of change of flux linkage tells you how strong the emf will be in volts

Question 24

State Faradays Law

Answer 24

The induced emf is directly proportional to the rate of change of flux linkage

EMF=flux linkage change/time taken