Demo Equipment
-
Magnetic induction
- bar magnet, solenoid, wires, galvanometer
Demo: induction
- Solenoid connected to galvanometer.
- Galvanometer measures small currents (μA)
- Current: + means current flows out of right side of solenoid, into left
- For each demo, observe what needle does, draw in what needle does.
Solenoid towards magnet
- needle goes right (+)
- current is going out of right of solenoid, into + on galv.
- current is going inwards at top of solenoid, up at front, out at bottom
Solenoid away from magnet
- needle goes left (-)
- current is going out of left of solenoid, into - on galv.
- current is going out at top, down at front, in at bottom
Magnet towards solenoid
Magnet placed at rest in solenoid
- needle does not move → no current.
Goal is to explain these reactions
- NOTE: there is no battery
- Energy to create current (emf) came from the moving magnet!
Magnetic Flux
- Due to a magnet near a loop of wire
- Can be thought of as the number of magnetic field lines passing through the loop.
Definition
- ΦB
- = B A cos(θ)
- B = field strength (T)
- A = area of loop (m2)
- Note: T·m2 = N / (A m) * m2 = J / A = V·s
- θ = angle between B and NORMAL to loop.
Determining θ
- You must pick the n. You have 2 choices.
- Example: loop is horizontal. n could be up or down.
- Example: loop is vertical, edge-on. n could be left or right.
- If B is up, and loop is horizontal, then θ = 0 (n is up) or 180°.
- Choose n, then be consistent.
Example
- Loop is vertical, face-on, r = 20cm
- B is horizontal, 1/2 between in and right, 0.06 T
- What is ΦB?
- A = 0.126 m2
- Choose n to be out of board
- ΦB = 0.06 T × 0.126 m2 × cos(135°) = -0.00533 T·m2
Lenz's
- There is an induced emf (ℰ) when the magnetic flux in a loop changes.
- The induced emf causes a current (Iind) to flow.
- The flowing current generates a magnetic field (Bind).
- The induced magnetic field leads to an induced flux (Φind)
- The direction of the induced current is such that the Φind has the opposite sign from ΔΦB.
Application of the law
- 1) Choose n. Draw it next to the loop.
Remember, n must be perpendicular to the loop, giving 2 choices.
- 2) Determine whether ΦB will be positive or negative.
Positive if θ < 90&Deg; (B and n are in same direction).
- 3) Determine what is changing (A, B, or θ) and whether that will cause the flux to go up or down.
Example: if B becomes stronger, ΦB is driven away from 0. A positive ΦB will increase (ΔΦB > 0), and a negative ΦB will decrease.
- 4) Find the sign of Φind. If ΔΦB > 0, Φind < 0. If ΔΦB < 0, Φind > 0.
- 5) Find the direction of Bind.
If Φind > 0, Bind is in the same direction as n.
- 6) Find the direction of the current using RHR 3.
Practice on demo examples
- Choose n to be to the right for each example.
- Ex. 1: ΦB > 0, B increases, ΔΦB > 0, Φind < 0, Bind to left, I is in at top, up in front, out at bottom.
- Ex. 2: ΦB > 0, B decreases, ΔΦB < 0, Φind > 0, Bind to right, I is down in front.
- Ex. 3: ΦB > 0, B increases, rest is the same as first.
- Ex. 4: ΦB > 0, nothing changes → ΔΦB = 0, no induced current.