W11D3Magnetic Forces

EXAMINATION #3Wednesday

November 9th

Reminder

Calendar Thing Today (Watch for last WA before exam. Sorry about the

intensity of these!) Quiz Who wants to present their experimental results?

Collected now! Some Problems (No Evan Show! He is cutting class today)) Continue with Forces

Next Week Monday - As much of remainder of chapter as possible.

Nothing DIFFICULT from this session will be on exam. Wednesday Exam

Second Hour & Friday … moving along!

FINAL EXAMINATION Monday morning – December 3rd 7:30AM Physical Science Building First Floor Conference Room Location is SECRET!! TWO index cards allowed Calculator Writing Instrument YOU. More

Info Later

In the circuit shown below, the emf of the battery is 7.6 volts. Resistor R1 has a resistance of 33 ohms, resistor R2 has a resistance of 47 ohms, and resistor R3 has a resistance of 57 ohms. A steady current flows through the circuit.a) What is the equivalent resistance of R1 and R2?

(b) What is the equivalent resistance of all the resistors: R1, R2, and R3

(c) What is the conventional current through R3?

Switch S in the figure below is closed at time t = 0, to begin charging an initially uncharged capacitor of capacitance C = 10.0 µF through a resistor of resistance R = 16.0 W . At what time is the electric potential across the capacitor equal to that across the resistor? t = 0.111 ms

In the circuit of the figure below, = 2.0 kV, C = 5.5 µF, R1 = R2 = R3 = 0.63 MΩ. With C completely uncharged, switch S is suddenly closed (at t = 0).

a) At t = 0, what is current i1 in resistor 1? 0.00212 A

(b) At t = 0, what is current i2 in resistor 2? 0.00106 A

(c) At t = 0, what is current i3 in resistor 3? 0.00106 A

Repeat for t = infinity (that is, after many time constants.)

(d) What is current i1? 0.00159 A

(e) What is current i2? 0.00159 A

(f) What is current i3? 0 A

(g) What is the potential difference V2 across resistor 2 at t = 0? 667 V

(h) What is V2 at t = ? 1000 V

(i) Sketch V2 versus t between these two extreme times. (Do this on paper. Your instructor may ask you to turn in this sketch.)

In the figure below, the battery has a potential difference of 10.0 V and the five capacitors each have a capacitance of 16.0 µF.

(a) What is the charge on capacitor 1? 0.00016 C

(b) What is the charge on capacitor 2? 3.2e-05 C

MORE ON FORCESRemember Bil?

Bil

Opposites don’t always attract!FORCES BETWEEN WIRES

The Wire in More Detail – Conventional

:

in the direction of the motion of POSITIVE charge (I).

d

d dd

Lq I t I

vL

F qv B I v B ILBv

vectorI

Vector

F L B

L

B out of plane of the paper

Assume all electrons are moving with the same velocity vd.

L

Think “BIL”

I

We usually calculate force per unit length.

TWO WIRES

0 24

IB

r

Now we can calculate the magnitude of the magnetic force F21 exerted on the lower wire by the field produced by the upper wire:     

u sing the right-hand rule with , the direction of the force is up, so that the lower wire is attracted to the upper wire.

IΔL×B

0 121 2

24

IF I L

d

TWO WIRES

B

Let’s Get Dizzy!

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

vB

F

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

vB

F

(B field points into plane of paper.)

Magnetic Force is a centripetal force

Trajectory of Charged Particlesin a Magnetic Field

16

Review of Rotational Motion

atar

at = r tangential accelerationar = v2 / r radial acceleration

The radial acceleration changes the direction of motion,while the tangential acceleration changes the speed.

r s = s / r s = r ds/dt = d/dt r v = r

= angle, = angular speed, = angular acceleration

Uniform Circular Motion

= constant v and ar constant but direction changes

ar = v2/r = 2 rF = mar = mv2/r = m2r

KE = ½ mv2 = ½ mw2r2

v

ar

Radius of a Charged ParticleOrbit in a Magnetic Field

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

vB

F

r

2 vv

v

mq B

r

mr

qB

Centripetal Magnetic Force Force =

Note: as , the magneticforce does no work!

Fv

Cyclotron Frequency

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

vB

F

r

The time taken to complete one orbit is:

qBm

rT

vv

2v

2

mqBf

mqB

Tf

c

2

21

More Circular Type Motion in a Magnetic Field

19

Magnetic Sector - Mass Spectrometer20

vmr

qB

VelocitySelector

How Old is That?? Activity

Low Resolution Mass Spectrum

SIMS

THAT’S ALL THERE IS!Magnetic Forces on Charges