Magnetic Fields
(the B-field)

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Prerequisite Materials:

Forces

Vectors

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Examples of Magnetic Fields

The field around the Earth

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The field around a bar magnet

Examples of magnetic materials are iron, steel, nickel, cobalt and neodymium

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The B-field around a current carrying wire

The B-field due to a current carrying wire loop

The B-field due to a long solenoid

Finding North and South of a solenoid

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Describing the B-field

The magnetic field strength (magnetic flux density/flux density/B-field) is a vector field giving both a magnitude and direction of the magnetic field

The magnitude of the force on a current carrying wire due to a B-field is,

$$F=BIl$$

• where $F$ is the magnitude of the force on the wire, $N$;
• $B$ is the magnitude of the B-field, $T$;
• $I$ is the current in the wire which is perpendicular to the B-field, $A$;
• and $l$ is the length of wire, $m$

The direction of the force relative to the current and field directions is,

Fleming's Left Hand Rule - the relationship of directions for a current carrying wire

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Charged Particles in a B-field

For a charged particle travelling through a magnetic field, the force is given by,

$$F=Bqv$$

• where $F$ and $B$ are as before;
• $q$ is the charge on the particle, in $C$;
• and $v$ is the speed of the particle, in $m{s}^{-1}$

As the force and velocity are perpendicular to each other the particles motion will enscribe a circle, the radius of which is,

$$\frac{m{v}^2}{r}=Bqv$$ $$r=\frac{mv}{Bq}$$

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Electromagnetic Induction

The e.m.f. induced by a conductor moving in a magnetic field,

$$\epsilon =B_{\perp }lv$$

Magntic Flux is defined as

$$\varphi =B_{\perp }A$$

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Questions:

GCSE Worksheets, Questions and Revision
(Care of mmerevise.co.uk)

GCSE Magnetism and electromagnetism Question
(Care of www.kirkhallamacademy.co.uk)

A Level Magnetic Fields Questions
(Care of www.savemyexams.com)

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