1. 26.5 Sources of the Magnetic Field
Magnetic Fields
Chapter 26
26.5 Sources of the Magnetic Field
The magnetic field of moving charges
The magnetic field of currents
Last week’s lectures
Biot-Savart law

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

3. 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.

4. 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:

5. 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.
If current I1 is reversed, B1 would be in opposite direction. Therefore we find that antiparallel currents repel.

6. Find the magnetic field at a point P on the y-axis at y = 6 cm.
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

7. 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?

8. At great distances from the loop, this becomesOR
where is the magnetic moment

9. electric dipole
magnetic dipole

10. Dipole moment
Reading:
26.6 Magnetic dipoles
26.7 Magnetic matter

11. CHECKPOINT: What is the current direction in this loop
CHECKPOINT: What is the current direction in this loop? And which side of the loop is the north pole?
Current clockwise; north pole on top
Current clockwise; north pole on bottom
Current anticlockwise; north pole on top
Current anticlockwise; north pole on bottom
Answer: B.

12. Gauss’ law for magnetism
Got it? Which could be a magnetic field?