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Last week’s
lectures
Magnetic Fields
Chapter 26
26.5 Sources of the Magnetic Field
The magnetic field of moving charges
The magnetic field of currents
Biot-Savart law
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.
dF2  I 2d l 2  B1
There is an equal an opposite force exerted by
current I2 on I1. The wires thus attract each
other.
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 zdirection, 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?
At great distances from the loop,
this becomes
2
OR
0 2 I  R
Bx 
4 x 3
where
  I ( R 2 )
is the magnetic moment
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?