Demo Equipment
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Disk magnet lines
- Strong magnet
- small compass
- paper + something to stand the compass up with
-
Wire as a source of field
- Batteries
- Long cord
- 4 small compasses
Demo: field lines
- A compass ALWAYS points in the direction of the magnetic field
- ... but does not give the field's intensity
Tracing lines
- Note direction of compass needle - mark it on paper
- Move compass a short distance in the direction the needle points
- Repeat
- Make a smooth curve through markings
Demonstrate for disk magnet
- Note: all lines are loops
- Field is strongest when lines are closest together (close to poles)
- Direction of field on a line is tangent to a line
- Direction of field between two lines is somewhere between the two nearest tangents
Notes:
- 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)
Demo: Field due to a current
- Square shaped circuit made of a long wire and a battery
- Place 4 compasses, one on each edge.
Observations
- How is each compass directed before the circuit is closed?
- Record this in a schematic
- How is each compass needle deflected when the current runs? (Ex: E side deflected counter clockwise, S side not deflected, W side deflected clockwise)
What is happening?
- Compass shows net magnetic field, Earth's + current's
- Current generates a magnetic field.
- E side, current field points W. S side, field points N. W side, field E.
Field lines
- Draw field lines
- RHR 2: Thumb = current, curled fingers give direction of field lines (knuckle to tip)
Notes: Field due to a current
- Straight wire: B = μ0 I / (2 π r)
- CENTER of circular coil: B = N μ0 I / (2 r)
- Solenoid (evenly spaced loops): B = N μ0 I / L
- μ0 = 4 π × 10-7 T m / A
Example: Straight wire
- Suppose current of 2.5A flows down a wire. What is the B field 1.0cm away?
- B = (4 π × 10-7 T · m / A × 2.5 A / (2 π × 10-2 m)
- B = 2 × 10-7 T × (2.5 / 10-2) = 5 × 10-5 T
- This is only a little greater than Earth's magnetic field, which is why the compass does not necessarily point straight in the direction of the wire's magnetic field.
Example: Coil
- RHR 3: direction of field at the center of a loop = thumb, fingers curl in direction of I
- Note: lines are loops, so outside the coild, the field will reverse direction
- Note also: B field is additive, so field of coil = field of loop, times number of loops in the coil (N)
- Note finally: the equation does not give the magnitude of B anywhere else besides the center.
Example: solenoid
- Direction of field inside is found the same way as for the coil
- Field outside is 0!
- Field inside is constant, so the equation gives the value of the field anywhere inside, not just at the center.
solenoid field
- A solenoid is constructed from 1000 loops, and has a length of 12.5cm.
- The solenoid has a resistance of 200Ω, and is connected to a 3V battery.
- What is the magnetic field inside the solenoid?
- I = ℰ / R = 0.015 A.
- B = μ0 N I / L = 1.3×10-6 T m / A × 1000 × 0.015 A / 0.125 m = 1.6 × 10-4 T