Schedule Today: –Check Exam I –Circuit Problems –Magnetism –Turn in Lab 6 Thursday –Do Lab 8 (in lab book)

Schedule

• Today:– Check Exam I– Circuit Problems– Magnetism– Turn in Lab 6

• Thursday– Do Lab 8 (in lab book)

Exam Trouble Spots

• Difference between field, potential, force, potential energy

• Vectors (GPII)

• Conservation of Energy

Electric Field

Starts at +Ends at –

Can also go to “infinity”

Field makes force

Indicates direction of force on + charge

Magnetic Fields

Start at North Go to SouthUnlike charges Every North has a SouthMake a force – but more complicated

Magnetic Fields

• Magnetism discovered ages ago

• Fields like electrostatics

• Terminology “pole”arization

• More complicated than electric forces

• Forces Related (will see, eventually…)

Magnetic Interaction

S

Magnets have poles N & SLike Electrostatic Forces

Opposites Attract, Like Repel, & non-magnetic unaffected“Magnetic” can be polarized…

S S SN N N N

Compass

• Floating Magnetic Needle

• North end attracted to “North Pole”– Points North

• Handy outside

• Work inside?

Field of a bar magnet

• Field lines start at N and end at S– Notice no infinity!

SN

Field of a Horseshoe Magnet

• Stronger at Poles

Fridge Magnet

• Material like many horseshoe magnets• “Domains”

Fridge Magnet:is Fridge Magnetic?

• What are magnetic materials?• Why do magnets stick to “magnetic

materials”?

Magnetism: Macroscopic ViewFerromagnetic Domains

Arrows Point from North to SouthIndividual “Little Magnets”

Unpolarized Ferro-magnet Fridge, Chunk of Iron, etc…

Magnetism: Macroscopic ViewFerromagnetic Domains

Polarized MaterialBar Magnet, Compass Needle, Earth

How to Polarize : Use magnetic Field!Magnet polarizes nearby magnetic material (Think Charge) Always attractive

Non-Magnetic Materials

• Paramagnetism– Some things just don’t care about fields– Wood, Paper, Aluminum

• Diamagnetism– Actually small repulsion does occur

• (non-ferro)– In a large enough field “anti-polarization”– 16 T floats a frog…

Ferromagnetic Properties

• Iron, Nickel, Cobalt are ferromagnets

• Aluminum, silicon, argon are not

• Similar properties between 3 elements?

• Are they neutral?

• Hint: magnetism & electric forces related

• What gives rise to magnetic properties?

Electrons

• 90% of anything’s properties due to Electrons

• Determine insulator / conductor– Heat & Electricity

• Determine Magnetism

• Determine Color, etc…

Magnetism

• Created by & Acts on MOVING charges

• Magnetic Fields B, units T (Tesla)– Tesla Coil

Magnetic Materials must be Metals?

• Magnetic force acts on MOVING charges

• What moves in a metal?

• Total charge of a metal?

• Do protons move?

• Magnetic Force on electrons?

• Magnetic Force on protons?

Moving Charges Make Magnetic Fields (Straight Wire)

0

2

IB

r

0 is magnetic permittivity of free spacer is distance to wireI current in wire

Like Electrostatic Permittivity

Field Obeys R.H.R.

0

2

IB

r

Fielf From a Loop?

Current

B?

B?

CurrentCurrent

Magnetic Field from a Loop

0

2

IB

R

Like a Bar Magnet

Field at Center

Magnetic Field from Many Loops

0

2

IB

R

Field at Center for one loop

What if a small coil, say 5 loops?Hint: Superposition

Solenoids: Capacitor of Magnetism

00

NIB nI

LN

nL

L

Field Constant InsideField ~ zero outsideDirection R.H.R.

What are electrons doing in a bar magnet?

N

S

What are they doingIn a non-magnetic Metal?

Magnetic Force on Moving Charge

sinMAGF Q vB

Q = 0 F = 0

V = 0 F = 0

Force is PerpendicularEverything Perpendicular

• Force perpendicular to BOTH v & B

Magnetic Field from Moving Charge

sin

FB

Q vB

Definition of magnetic Field

Right Hand RuleWorks for wrenches too!

sinMAGF QvB

How to determine directions Quickly

Negative Charges Opposite“Left Hand Rule”

Sign Important

What is trajectory?

Straight Line?Bendy?How?

Force Perp. To v & B

Motion of Charged Particle in a Magnetic Field

• Force perpendicular to velocity & field

Velocity Perp. To B

B is pointing “out”

Circular Motion

R.H.R.

Circular Motion Review

• Centripetal Acceleration / Force

ˆ ˆˆy x x z z yv B z v B y v B x

2

C

va

r

2

C

vF m

r

F mar

If a velocity not quite perpendicular:

• Helical Motion

• Separate Components

• vP2 + vL

2 = v2

2

C

va

r

Mass Spectrometer

Simple Design2 Isotopes, IonizedSame Charge (+e)Different MassSame velocity

2

2

C

c

vF m

r

mvr

F

r1 r2

Force on a Current Carrying WireCharges moving in Wires

sin

sin sin

F qvB

x qqv q x I x

t t

F I x B ILB

Remember Current is positive charge flowElectron flow opposite (negative charges)

Work with current: everything positive & R.H.R.

Force Obeys R.H.R.

Point in current flow direction, rest same

Try Problems

• Circuits

• Magnetism problems Thursday before lab– Read over chapter & lab

Additional Examples

1. In the circuit shown below:A. Rank in order, from most to least bright, the brightness of

bulbs A–D. Explain.B. Describe what, if anything, happens to the brightness of

bulbs A, B, and D if bulb C is removed from its socket. Explain.

Slide 23-41

Additional Examples

2. In the circuit shown below, rank in order, from most to least bright, the brightness of bulbs A–E. Explain.

Slide 23-42

3. In the circuit shown below:A. How much power is dissipated by the 12 Ω resistor?B. What is the value of the potential at points a, b, c, and d?

Slide 23-43

Additional Examples

The diagram below shows a segment of a circuit. What is the current in the 200 resistor?A. 0.5 AB. 1.0 AC. 1.5 AD. 2.0 AE. There is not enough information to decide.

Slide 23-13

Clicker Question

There is a current of 1.0 A in the circuit below. What is the resistance of the unknown circuit element?

What is the current out of the battery?

Slide 23-19