Third exam Thursday, April 23

105, 107 Heldenfels

7 pm – 8 pm

The force on a charge q moving with a velocity F

v

)( BvEqF

The magnitude of the force

sinqvBF

sec)//(][ meterCoulombNewtonsB

gausssmCNewtonmwteslaT 42 10//1/1)(1

TGaussBEarth4101

left-hand ruleright-hand rule

BsidFd

Small peace of wire

Ampere’s Law

irdB 0

The field produced by an infinite wire

a

iB

2

0

Biot-Savart Law

3

)(

r

rsdiBd

Infinitesimally small element of a current carrying wire produces an infinitesimally small magnetic field

Sd

i

(Also called Ampere’s principle)

30 )(

4 r

rsdiBd

r

0 is called permeability of free space

2770 )/(104)/(104 ampNmeterampwebers

Faraday’s Law of Induction

The induced EMF in a closed loop equals the negative of the time rate of change of magnetic flux through the loop

dt

dEMF B

Test 3 results Average 77Median 849 perfect scores

>90 >80 >70 >60 >50 <50

39

1310 10 9 11

In the circuit below, suppose the switch has been in position A for a very long time. If it is then switched to B at t=0, find the current as a function of time.

A

B+

_

LRV

Make your own MOTOR!

All you need is a battery, a nail, a small magnet, and a wire (foil works better)

http://en.wikipedia.org/wiki/Homopolar_motor

General Atomics is developing a full-scale, 36.5-megawatt, 120-rpm superconducting DC homopolar motor for ship propulsion.

• Significantly quieter, smaller, and lighter than AC motors• More efficient than the AC motor systems• Control is more straightforward and simpler than the AC motor systems• Suited to simpler and less costly ship electrical distribution architectures

http://atg.ga.com/EM/defense/dc-motor/index.php

Magnetic MaterialsDiamagnetism: The magnetic field caused by current loops is always opposite in the direction to that of the external field.

Paramagnetism: When such a material is placed in a magnetic field, the field exerts a torque on each magnetic moment, . These torques tend to align the magnetic moments with the field. In this position, the directions of the current loops are such as to add to the externally applied magnetic field.

Ferromagnetism: iron, nickel.. Magnetic moments line up parallel to each other in magnetic domains even without external field. Without field they are randomly oriented. When field is applied, the domains tend to orient themselves parallel to the field.

Water Nitrogen Oxygen

Boils

Freezes

100 °C (212 F) -196 °C (-322 F) -183 °C (-297 F)

0 °C (32 F) -210 °C (-346 F) -223 °C (-369 F)

Our air is ¾ Nitrogen and ¼ Oxygen

• 1908- liquefied helium produced

• First discovered in mercury by Kamerlingh-Onnes in 1911.

• Critical temperature 4.21K.

• Nobel Prize in 1913.

Superconductivity

High-Tc Superconductivity

Liquid nitrogen temperature 77 K

Complex ceramic materials were discovered in 1986. They exhibit superconductivity at much higher temperatures – above LN temperature!

Muller and Bednortz, Nobel Prize 1987

Superconductivity

Kamerlingh-Onnes 1911

Meissner effect and magnetic levitation

Type I superconductor Type II superconductor

Vortices