1. 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 Last week’s lectures

2. 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 B 1 due to current I 1 is perpendicular to current I 2. The force on current I 2 is towards current I 1. There is an equal an opposite force exerted by current I 2 on I 1. The wires thus attract each other. If current I 1 is reversed, B 1 would be in opposite direction. Therefore we find that antiparallel currents repel.

6. 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. where is the magnetic moment At great distances from the loop, this becomes OR

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? 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.

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