Test 2 results

>80 >70 >60 >50 >40>90

Average 65.2 Median 68 Highest 99 Lowest 8

Score

Electron motion in a microwave oven

A magnetron in a microwave oven emits electromagnetic waves with frequency f=2450 MHz. What magnetic field strength is required for electrons to move in circular paths with this frequency?

Using Crossed and Fields E

B

Velocity selector

0 qEqvB

vBE

B

Ev independent of the mass of the particle!

Mass spectrometer

qB

vmR 1

1

B

Ev

21

1 qB

EmR

22

2 qB

EmR

Thomson’s e/m experiment

1897: Cavendish Laboratoryin Cambridge, England

m

eVveVmv

2

2

1 2

B

Ev

2

2

2

2

VB

E

m

e

m

eV

B

E

Exercise 1

An electron, q=1.6 10-19C moves with velocity

ondpermetersiniiv yx sec104106 55

It enters a magnetic field with What is the force on the electron?

2/1.0 mwebersiB x

Problem 5

Hall effect: The magnetic force on the charge carries in a wire can be used to determine their sign. Show that there will be an electric field, set up inside a wire in a magnetic field, that is perpendicular to the direction of the current. You should be able to show that the sign of the field depends on whether the mobile charges are positive or negative.

nAq

iv

qvBqEHall

nq

jB

nAq

iBEHall

You place a slab of copper, 2.0 mm thick and 1.5 cm wide, in a uniform magnetic field with magnetic field with magnitude 0.40 T. When you run a 75-A current in the +x direction, you find by careful measurement that the potential at the left side of the slab is 0.81V higher than at the right side of the slab. From this measurement, determine the concentration of mobile electrons in copper.

Current carrying wires

1820 Hans Christian Oersted

Hans Christian Ørsted

Exercise 3

A wire of length l and mass m is suspended as shown. A uniform magnetic field of magnitude B points into the page. What magnitude and direction would a current, passing through a wire, have to have so that the magnetic and gravitational forces would cancel?