magnetism

define magnetic flux density

force per unit current per unit length on a current carrying wire placed perpendicular to the field lines

define magnetic flux

magnetic flux density x perpendicular area it passes through

describes magnetic field passing through an area

define magnetic flux linkage

magnetic flux x number of turns of coil (N)

path of a charged particle moving in a magnetic field

follows a circular path

force exerted is always perpendicular to direction of motion due to Fleming’s left hand rule

how do cyclotrons work

formed of two semi-circular electrodes (dees)

uniform magnetic field acts perpendicular to the plane of the electrodes

high frequency alternating voltage applied between electrodes

charged particles move from centre of one electrode and are deflected in a circular path by the magnetic field

once they reach the edge of the electrode, they begin to move across the gap between electrodes, where they are accelerated by the electric field

radius of path increases as they move through the second electrode

when they reach the gap again, the alternating electric field changes direction allowing the particles to be accelerated again

repeats until they reach the required speed and exit the cyclotron

uses of cyclotrons

radioactive traces

producing ion beams for radiotherapy

Faraday’s law

magnitude of emf = rate of change of magnetic flux linkage

Lenz’s law

negative sign shows that the induced emf is always as to oppose the change causing it

how do transformers work

alternating voltage passes through the primary coil

alternating magnetic field created

induces an alternating emf in the secondary coil

causes of energy loss in transformers

eddy currents

resistance in coils

core is not easily magnetised

explain how energy is lost due to eddy currents in transformers

induced by alternating magnetic field in primary coil and they form a loop

due to Lenz’s law they oppose the field that produced them

reduces flux density

they also generate heat, which causes energy loss

how to reduce the effect of eddy currents in transformers

use a laminated iron core

layers of iron between layers of an insulator

eddy currents can’t pass through the insulator so their amplitude decreases

or use a core of high resistivity metal

how to reduce energy lost due to resistance in the coils of a transformer

use thick wire of low resistance

how to reduce energy lost due to the core of a transformer not being easily magnetised

use a magnetically soft iron core

exam q: explain the purpose of the core in the transformer

the core directs the magnetic field round to the secondary coil

exam q: explain why transformers only work continuously when supplied with an alternating current

to produce a continually changing magnetic flux in the core

exam q: explain how eddy currents arise

changing magnetic field

emf or changing magnetic field is in the core

emf induced due to changing magnetic field not back emf

current flows as core is made from a conducting material

exam q: define the tesla

The tesla is the (strength) of the magnetic field/flux density that produces a force of 1 newton in a wire of length 1m with 1 ampere (flowing perpendicular to the field).  (owtte but must contain 1N, 1A and 1m)

exam q: Explain why it is not possible for the magnetic field to alter the speed of a proton while it is in one of the D-shaped regions of a cyclotron

It is not possible as the force (due to the magnetic field) is perpendicular to the motion/direction of travel/velocity (it can only change the charged particle’s direction or alternatively no work is done on the proton) Or No component of force in the direction of motion.