yr2 - magnetic fields

Magnetic field strength-formula

B=F/IL

What do magnetic field lines represent?

Shows the direction of force acting upon a free isolating North Pole

What does a moving charge in an electric field create?

Electromagnetic induction

What path does a moving charge in a magnetic field take and why?

Circular because their force and direction of motion are perpendicular

Flemings right hand rule?

Thumb = current

Fingers = direction of the magnetic field round that wire

motor effect?

An external magnetic field interacts with an internal one that travel in the same direction,as they are vectors they “merge” and create a magnetic field around a wire as the magnetic pressure is greater

What happens when there are no forces acting upon the wire?

The two forces travelling in opposite directions cancel out and leaving no force upon the wire to keep it up, hence it falls

Flemings left hand rule?

Shows the direction of the force acting on a positive charge acting in a magnetic field

B = f/IL equation manipulation

B = number of field lines per unit area

BA = phi = magnetic flux which is the number of field lines passing through the surface area perpendicular to the surface area

phi = flux which is measured in Wb (Webber)

Flux linkage?

N phi or flux linkage = BANCos theta which is the total number of field lines passing through the surfaces that is perpendicular to the surface. measured in Wb turns

Faradays Law?

EMF = - N d phi /d t

• which means the rate in change of flux linkage OR how quickly we cut the field lines OR how quickly do the number of field lines change

Lenz’s Law?

• The negative symbol shows how the force applied must be able to overcome the induced force to conserve energy in one direction.

Moving wire in a magnetic field?

Moving the wire creates a flow of electrons in one direction, created a difference in charge between two points (a potential difference/voltage.if we then move the wire to the right across a distance we are doing work for as long as we move the wire in the field, an EMF will be induced

Work done in a moving wire in a magnetic field?

BIL x S

Faradays law and work done derivation? (1)

F = BLV when W = BILS

Faradays law and work done derivation? (2)

F = BQV when F= BIL

AC?

Current flows in both directions and always generates a constantly changing magnetic flux

What are transformers used for?

To increase or decrease the voltage by changing the number of turns in a coil

Induced EMF in transformers?

• -n dphi/dt ₁ = -n dphi/dt ₂

• If we induce 100 field lines then 100 field lines must also be cut

Voltage and number of turns in a transformer?

V_{_}/ V_{_} = N_{_}/ N_{_}

V_P/V_S = N_P/N_S

Transformer efficiency formula?

% = I_S V_S / I_P V_P

Ways to improve transformer efficiency?

Increase efficiency- place coils closer to increase the rate in change in flux linkage to increase the magnetic flux

(or overlap the coils)

Reduce heat energy loss- insulate the iron core with plastic (lamination) to reduce (eddy) currents flowing to the iron core

Reduce resistivity- use wires with a large cross-sectional area as R = rho L /A