Physics Week 26: Magnetism

This week of the Ron Paul physics course focused on Magnetism. Here are the main points covered.

Observations on magnets

  • Attract and repel similar to electric charges.

  • If a magnet is broken in half the pieces will attract or repel each other depending on their orientation.

Magnetic poles

  • Every magnet has 2 poles (north and south)
  • Like poles repel
  • Opposite poles attract.

Magnetic fields

  • Magnetic poles have magnetic fields.
  • The direction of the field is from the north to the south pole.
  • In wire coils, each coil has a magnetic field, and they all combine into one big strong magnetic field through the coil.

What causes magnetic fields?

Magnetic fields are created by the motion of electric charge. Magnets are created by spinning electrons. Each electron is a tiny magnet. In a magnet there are enough electrons spinning in the same direction to create a detectable magnetic field.

Create a magnet

Align magnetic domains within iron by exposing the iron to a magnetic field.

Right hand rule

If you wrap your right hand around a wire with your thumb pointing in the direction of the current, the fingers point in the direction of the magnetic field.

Forces on a current carrying wire

Newton’s third law states that a compass’s needle and its magnetic field would also exert a force on the wire’s magnetic field.

Direction of force

If a current flows through a magnetic field, the force on the wire is perpendicular to both the direction of the current and the direction of the magnetic field. It follows the right hand rule.

Right hand rule for force

Use the right hand with the fingers unbent and point the fingers in the direction of the current with the thumb extended in the direction of the force. Bend the fingers so that they are perpendicular to the hand. Now they are pointing in the direction of the magnetic field.

Force on a current carrying wire

F = IlB sin 0, where B = magnetic field, l = length, I = current

Force on a charged particle

F = qvB sin 0, where q = charge, v = velocity, B = magnetic field

Magnetic field created by a current

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