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Magnetic Fields Chapter 26 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. Example 26.5 Magnetic field B due to a current in a straight wire.

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Magnetic Fields Chapter 26 26.5 Sources of the Magnetic Field

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  1. 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. 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. 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 becomes OR 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? 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?

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