Module 4 - Magnetic Fields 3.4.5

Section 3.4.5 - Magnetic effects of currents - click here for an interactive site

- magnetism questions in a 'flip-book' for you to try - electromagnetic induction questions in a 'flip-book' for you to try
Syllabus Extract You should be able to:

Force on a current carrying wire in a magnetic field
F = BI l (field perpendicular to current)

Fleming's Left hand Rule

Magnetic flux density B and definition of the tesla T

Compare and contrast electric and gravitational fields - good table on pg 94.

Motion of charged particles in a magnetic field

Force on charged particles in a magnetic field

F = BQv (field perpendicular to velocity)

Circular path of particles; application, e.g. charged particles in a cyclotron

Use Fleming's left hand rule to establish force at right angles to motion. Relate this to the condition for circular motion.
Equate expression for centripetal force to BQv.
Establish effect on radius of orbit if B, m or accelerating voltage change.
Practical: Could show crossed fields experiment here to find e/m. Value obtained is of the right order of magnitude.

Magnetic flux density B, flux Φ and Flux Linkage NΦ

Magnetic flux defined by Φ = BA, where B is normal to A.

Flux linkage as NΦ, where N is the number of turns cutting the flux Φ.

Flux and flux linkage passing through a rectangular coil rotated in a magnetic field:

flux linkage N = BAN cosθ where θ is the angle between the normal to the plane of the coil and the magnetic field.

Define B and relate the two terms using a diagram, this will also explain the value of when B is not perpendicular to the area concerned ( = BA cos θ )

Electromagnetic induction

Simple experimental phenomena, Faraday's and Lenz's laws

For a flux change at a uniform rate


Magnitude of induced EMF = rate of change of flux linkage.

Applications such as a moving straight conductor, e.g. p.d. between wing-tips of aircraft in flight

Emf induced in a coil rotating uniformly in a magnetic field:

ε = BANω sin ωt

The operation of a transformer;
The transformer equation

Transformer efficiency = Is Vs / Ip Vp
Causes of inefficiency of a transformer.
Transmission of electrical power at high voltage.

Recall Faraday's Law: that the magnitude of the induced EMF in a circuit is directlt proportional to the rate of change of flux linkage (or to the rate of cutting of magnetic flux).

Recall Lenz's Law: that the direction of the induced EMF is such that the current which it causes to flow opposes the change which is producing it.

Know that the induced EMF is therefore:

directly proportional to the number of turns of wire

proportional to the rate of cutting of flux (how fast the flux lines atr cut by the wire)

produced in such a way as to make a current flow that has a magnetic field that opposes the field of the one causing the induction.