Magnetic Fields Chapter 24

Magnetism Since ancient times, certain materials, called magnets, have been known to have the property of attracting tiny pieces of metal. This attractive property is called magnetism. N S Bar Magnet N S

Magnetism The strength of a magnet is concentrated at the ends, called north and south “poles” of the magnet.

Any magnet cut in half will become two independent magnets Magnets are always Dipoles (two poles), magnetic Monopoles (either just north or south) have never been found

Magnetic Poles Like poles repel ( N repels N and S repels S) Opposite poles attract (N attracted to S)

Newton’s 3rd Law and Magnetic Field Strength Two interacting magnets will put an equal force on each other Even if one is a stronger magnet than the other

Magnetic Field Diagrams Similar to electric field diagrams What similarities do you see?

Magnetic Field Diagrams North = outwards arrows (just like positive particles) South = inwards arrows (just like negative particles) Magnetic fields created by a bar magnet interacting with itself:

Magnetic Field Diagrams: Attraction Magnetic attraction diagrams look similar to particle attractions

Magnetic Field Diagrams: Repulsion Magnetic repulsion diagrams look similar to particle repulsions

Magnetic Field Diagrams: Strength Denser lines = stronger magnetic field Always strongest at the poles of a magnet

Charged Particles and Magnets Charged particles experience an electric force when in an electric field regardless of whether they are moving or not moving There is another force that charged particles can experience even in the absence of an electric field but only when they are motion Magnetic Interactions are the result of relative motion ***In other words: In order for a charged particle to feel the force of a magnetic field, it must be moving.

How Compasses Work Why does a compass point North? The red/north end is always attracted to the south magnetic pole of the Earth (which is the geographical north pole)

Practice Which of the below shows: Repulsion Attraction Bar Magnet interacting with itself

Practice At which point is the magnetic field the weakest? At which point is the magnetic field the strongest?

Practice Determine what, if anything, is wrong with the situation presented below. If something is wrong, identify it and explain how to correct it. If nothing is wrong, explain why the situation is correct: A stationary positively charged particle placed at the position shown in the figure below will start moving to the left because it will be repelled by the N pole of the magnet. The statement is incorrect. A positively charged particle placed at the position in the figure would not feel any magnetic force unless it was moving and hence would not move at all.