Solenoids

PH 203

Professor Lee Carkner

Lecture 18

Magnetic Field: Loop

Can apply the right hand rule all the way around

Loop acts as a bar magnet

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)

Solenoid Quantified We can write an expression for the

solenoid magnetic field:

B = 0ni

Note that: N is the total number of turns n = N/L

Toroid

The toroid has a radius r we can use as an Amperian loop

(B)(2r) = 0iN

B = (0iN)/(2r)

B is not constant and depends on where you are radially in the toroid

Magnetic Dipole

We can parameterize the loop with a dipole moment,

= NiA

If the loop is placed in an external magnetic field, it will experience a torque

= B sin where is the angle between and B

Dipole Torque

Use the right hand rule to find the direction of

Dipoles that are not aligned with the field will turn until they are

Dipole Energy

U = -B cos When aligned with the field

the energy is –B

When opposite the field the energy is +B

Field from a Dipole

A dipole is effected by external magnetic fields and also produces its own magnetic field

B = (0)/(2z3) Field falls off rapidly with z

Next Time

Test 2 Friday For Monday

Read 30.1-30.4 Problems: Ch 30, P: 2, 3, 4

A wire is carrying a current straight towards you. What is the direction of the magnetic field of the wire?

A) straight towards you

B) down

C) up

D) circling the wire in a clockwise direction

E) circling the wire in a counterclockwise direction

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?

A) east, west

B) up, down

C) north, south

D) up, west

E) It depends on the magnitude of the current

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?

A) left

B) right

C) up

D) down

E) right at you