Magnetic FieldsChapter 26

26.5 Sources of the Magnetic FieldThe magnetic field of moving charges

The magnetic field of currents

Biot-Savart law

Last week’s lectures

Example 26.5

Magnetic field B due to a current in a straight wire.

If the length of the wire approaches infinity in both directions –

Do on board

Magnetic field B due to a current in a straight wire.

If the length of the wire approaches infinity in both directions, we find

We can determine the direction of the

magnetic field due to current-carrying wire using the right hand.

Magnetic field B due to a current in a straight wire.

If the length of the wire approaches infinity in both directions, we find field lines distributed thus:

Magnetic force between parallel wires

The magnetic field B1 due to current I1 is perpendicular to current I2.

The force on current I2 is towards current I1.

There is an equal an opposite force exerted by current I2 on I1. The wires thus attract each other.

1222 BldIdF

If current I1 is reversed, B1 would be in opposite direction. Therefore we find that antiparallel currents repel.

EXAMPLE: Two wires are carrying current of 1.7 A in the positive z-direction, at x = -3 cm, y = 0 and x = +3 cm, y = 0.Find the magnetic field at a point P on the y-axis at y = 6 cm.

This will be done in tomorrow’s workshop lecture

Magnetic dipoles

This is the geometry for calculating the magnetic field at a point on the axis of a circular current loop.

EXERCISE: What is B a long way from the loop?

where is the magnetic moment

20

3

2

4x

I RB

x

2( )I R

At great distances from the loop, this becomes

OR

electric dipole

magnetic dipole

Dipole moment

Reading:

26.6 Magnetic dipoles

26.7 Magnetic matter

CHECKPOINT: What is the current direction in this loop? And which side of the loop is the north pole?

A. Current clockwise; north pole on top

B. Current clockwise; north pole on bottom

C. Current anticlockwise; north pole on top

D. Current anticlockwise; north pole on bottom

Answer: B.

Gauss’ law for magnetism

Got it? Which could be a magnetic field?