Demo Equipment
-
Force on wire
- Strong magnets, taped to board
- wire hanger, batteries, cables
Example: Forces on moving charge pt 2
- F = q v B sin(θ)
- Direction from RHR
RHR
- Pointer = q v
- Other fingers close in direction of B
- Thumb = F
- Work examples on board
- Ex: q +, B out, F right, what direction must v be in for strongest F? (up)
- Ex: q -, B right, F up, what direction must v be in for strongest F? (in)
- Ex: q -, v out, F right, what direction must B be in for strongest F? (up)
Demo: Force on wire
- Wire carries a current upwards.
- B field is out
- q v ⇆ I direction
- What direction is the force in?
RHR
- pointer = I = up
- other fingers = B = out
- thumb = F = right
- demonstrate
Flip magnets
- What will happen?
- B direction? (in)
- I direction? (up)
- F direction? (left)
Reverse current too
- What will happen?
- B direction? (in)
- I direction? (down)
- F direction? (right)
Problem: Force on wire
- F = I L B sin(θ)
- Wire carries ~ 1.5 A current.
- Field is 0.020 T, out
- Wire is bent into 3 sided square, each side is 8 cm long.
- first side carries current in, second side carries current up, third side carries current out
- What is the total force on the wire?
Solution
- F = 1.5A × 0.080 m × 0.020 T × sin(90°)
- F = 0.0024 N
- If wire mass is 1 g → w = 0.01 N, then F is more than enough to swing the wire.