Magnetic Interaction

• Like poles repel. Opposites attract.
• Magnetic field exits magnet at poles.
• Magnetic domains – small regions that are magnetized
• Sketch magnetic domains in nonmagnetized/magnetized iron (p.429)
• Magnetized: Nonmagnetized:

Magnetism

• Magnetism is the attraction or repulsion of two objects due to magnetic fields
• The strength of the force is greater when the objects are closer
• Proportional to radius² – same as gravity, electricity

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Types of Magnets

• Permanent Magnets
• Like you have on your refrigerator door
• Domains all aligned within
• Temporary Magnets
• Domains have been temporarily rearranged by a magnetic field
• (like induction in electricity)
• Electromagnets
• Moving electric charges create magnetic fields

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• All Magnets have N and S pole
• Impossible to isolate a “Monopole”

http://www.coolmagnetman.com/motion09.htm

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Concept Question

• Given: N attracts S, and S attracts N

(opposites attract, like poles repel)

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1. Is your refrigerator door a magnet?

a. How could you test?

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• Why are magnets attracted to your fridge?

(the answer is similar to why neutral objects may be attracted to charged ones)

Temporary Magnets

• Sketch: What mag. domains look like before / after a magnet is brought near

Everything’s Not Magnetic???

• Electrons have magnetic properties, but in most materials, these cancel out.
• In some metals such as iron, cobalt and nickel, charges don’t cancel.
• Due to where the e^- spin!

• What is a magnetic field?

• Area where magnetic force acts.
• Force field lines surround a magnet; go from N to S pole.

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Taking Care of Magnets

• Magnets used in other classes will be permanently confiscated by teachers.
• What you do reflects on me; please make this a good reflection.
• Dropping/slamming magnets together allows magnetic domains to rearrange
• This makes them lose their magnetism!!!
• Be careful when working with powerful magnets; you can be pinched!
• Once coating on magnets is chipped, magnets will break down quickly… be careful.

Activity

• Making a magnet
• Hold a strong magnet near an unmagnetized paper clip for several seconds. Use it to pick up one or more other paper clips or staples.
• Challenge:
• Make a magnet that will transfer the most staples from one table to another

Earth’s Magnetic Poles

• North Pole (Magnetic South) is in Canada, ~1800km from Geographic North pole (Earth’s axis), South pole is between Australia and Antarctica
• Poles have moved, disappeared, and reversed historically.
• We know this from geology
• Earth’s poles under research
• Several current theories
• Aurora

Electricity and Magnetism

• Current through a wire causes a magnetic field → electromagnet
• Switch direction of current → switch direction of magnetic field

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Electromagnets

• Current in a straight wire → field around wire
• Current in a loop of wire → field inside and outside loop
• Electromagnets have an iron core to focus this field
• Electric motors and generators are electromagnets – just working in opposite directions

Right Hand Rule

• To determine the direction of the magnetic field around a wire, visualize:
• 1. Grab wire with right hand
• 2. your thumb points toward the negative end of the circuit
• 3. Your fingers point in the direction that a magnetic field wraps around the wire
• Remember, magnetic field points from N to S.

Activity: Generating a magnetic field

• We are making short circuits; do not leave them connected more than a second or two.
• Place compass under wire so that compass needle lines up with wire. (remember, Copper is non-magnetic)
• Connect one end of the wire to the – battery terminal
• Predict the direction of magnetic field using R hand rule
• Predict the direction of the compass
• Try it out; tap the other end of wire to + battery terminal
• What happens when you switch the direction of the current?