Demo Equipment
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Magnetic interactions
- Stand, string, 2 long thin bar magnets
- Copper, aluminum, and steel rods
- Plastic / wool, glass / silk OR clear tape
Demo: Magnet
- Magnet hangs from string.
- Predict the outcome (attraction, repulsion, or neither) of the following interactions: N pole with - charge, S pole with - charge, N with +, S with +
Demonstrate interactions
- After first two, have students update their predictions.
- What were the outcomes? (Always attraction)
- Why? - maybe have students discuss the reason. (magnet is neutral, so it polarizes in response to the charge, causing attraction just like all other neutral objects)
- Review with charge diagram.
- Punchline: what you just saw was an ELECTRIC force, not magnetic. We could substitute any other object for the magnet.
Follow up
- Rotate the base of the stand.
- NOTE: magnet remains pointing same direction
- Rotate the magnet and release
- Note: the magnet oscillates back and forth on either side of the original direction
- → there is a force that tries to return the magnet to the original direction (North/South)
Demo: Magnetic force
- Magnet hangs from string.
- Predict the outcome (attraction, repulsion, or neither) of the following interactions: N pole with N pole, N with S, S with S
Outcomes
- Like poles repel, unlike attract.
- NOTE: This COULD NOT BE an electric interaction (the magnet is neutral, and only attracts electrical charge)
- → We have something new on our hands.
Follow up
- Magnet with copper - no interaction
- Magnet with aluminum - no interaction
- → magnets don't interact with ordinary neutral material
- Magnet with steel - attraction with both poles
- → steel is becoming magnetised (ferromagnetism)
New force
- Have students brainstorm some differences between electrical and magnetic force
- Example 1: No object remains charged long - must be charged - while magnets remain magnets forever.
- Example 2: Charges attract any neutral material, while magnets only attract special non-magnetic materials (by making them magnetic first).
Notes: Force on moving charge
- Magnitude: FB = q v B sin(θ)
- Direction: RHR
B field
- Example: bar magnet - direction is out of N pole, into S pole
- Example: Earth - bar magnet lines up with Earth's field, so that N pole of magnet points north
- B field lines are always loops
- Units = T = N·s / (C·m)
- Earth ~ 10-5T, strong bar magnet ~ 0.1 T near pole.
Force
- θ = angle between B field and velocity
- Force = 0 when particle moves in direction of B field, strongest when particle moves at a right angle to B field.
RHR
- Pointer = q v
- Other fingers close in direction of B
- Thumb = F
- Work examples on board: draw q, v, B. Determine F together.
- Ex: q +, v to right, B up → F out of board (•)
- Ex: q +, v up, B out → F right
- Ex: q +, v down, B down → F 0
- Ex: q -, v down, B right → F in (×)
- Ex: q -, v out, B right → F down