1. PH 203 Professor Lee Carkner Lecture 18
Solenoids
PH 203
Professor Lee Carkner
Lecture 18

2. Magnetic Field: Loop Can apply the right hand rule all the way around
Loop acts as a bar magnet

3. Magnetic Field: Solenoid
What happens if you stack several loops up?
You produce a solenoid
Field inside the solenoid is strong and uniform (far from the ends)

4. Solenoid Quantified
We can write an expression for the solenoid magnetic field:
B = m0ni
Note that:
N is the total number of turns
n = N/L

5. Toroid The toroid has a radius r we can use as an Amperian loop
(B)(2pr) = m0iN
B = (m0iN)/(2pr)
B is not constant and depends on where you are radially in the toroid

6. Magnetic Dipole We can parameterize the loop with a dipole moment, m
m = NiA
If the loop is placed in an external magnetic field, it will experience a torque
= mB sin q
where q is the angle between m and B

7. Dipole Torque Use the right hand rule to find the direction of m
Dipoles that are not aligned with the field will turn until they are

8. Dipole Energy U = -mB cos q
When aligned with the field the energy is –mB
When opposite the field the energy is +mB

9. Field from a Dipole
A dipole is effected by external magnetic fields and also produces its own magnetic field
B = (m0m)/(2pz3)
Field falls off rapidly with z

10. Next Time Test 2 Friday For Monday Read 30.1-30.4
Problems: Ch 30, P: 2, 3, 4

11. A wire is carrying a current straight towards you
A wire is carrying a current straight towards you. What is the direction of the magnetic field of the wire?
straight towards you
down up
circling the wire in a clockwise direction
circling the wire in a counterclockwise direction

12. A horizontal wire is carrying a current straight north
A horizontal wire is carrying a current straight north. What is the direction of the magnetic field directly above the wire and directly below the wire?
east, west
up, down
north, south
up, west
It depends on the magnitude of the current

13. A horizontal wire is carrying a current to your right
A horizontal wire is carrying a current to your right. What direction would a proton be deflected in if it traveling exactly below the wire also to the right?
left
right up
down
right at you