MAGNETIC FIELDS

Pre-Class for Wednesday 5-2-

12True or False: Electricity and Magnetism are

related.• Begin next chapter:

Ch24: Magnetic Fields– Brief history of

magnets

Some Definitions • Magnetic Force– The interaction when two

magnets are brought close together

– Can be attraction or repulsion

• Magnetic Field– Represents the effect of a

magnet on the area around it

• Magnetic Poles– Two for every magnet

– Strongest field found here

– North (out) and South (in)

Observing Observing Magnetic Magnetic

InterferenceInterference

Proceduresa. Clamp a bar magnet

vertically to the ring stand.

b. Tie string around a paper clip.

c. Anchor the end of the string under a book. Slowly pull the string until the paper clip floats in the air.

d. Without touching the paper clip, slide various objects between the magnet and paper clip.

e. Record observations

Material tested

Result

Paper

Fabric

Scissor blade

Aluminum foil

Other:

Equipment needed: paper clip, ring stand,

clamp, bar magnet, string

Pre-class for Thursday 5-3-12

The location of the strongest magnetic field on a magnet is at the _____.

A. Magnetic forceB. Magnetic polesC.Magnetic domainsD.Electromagnets

Why a magnetic field?Why a magnetic field?

• The mantle is on a tectonic plate, which floats on a river of liquid rock/metal

• The magma is flowing, just like a river

• Friction between the plates and magma cause charges to move, creating a magnetic field

MAGNETIC FORCE DUE TO A CURRENT-CARRYING WIRE

F = B I L

F force of magnetic field (N)

B magnetic field strength (T or gauss)

I current in wire (Amp)

L length of wire (m)

#1 Right Hand Rule

• Fingers point direction of magnetic field (curved around the wire)

• Thumb point direction of current

I

B fieldB field

MAGNETIC FORCE DUE TO A CHARGED PARTICLE IN A

MAGNETIC FIELD

F = q v B

F magnetic force (N)

q charge (C)

v velocity (m/s)

B magnetic field (T or gauss)

#2 Right Hand Rule

• Fingers point direction of magnetic field

• Thumb point direction of motion for the particle

• Force on charge due to magnetic field comes out of the palm

Works for positive charges only!!!(use left hand for Works for positive charges only!!!(use left hand for negative)negative)

B field

P+

e-

What direction will the force be?

FF

X FF

P+

e-

X X X X

X X X X

X X X X

X X X X

X X X X

B field

What direction will the force be?

FF

FF

Pre-class for Monday 5+7=12

Objects that keep their magnetic properties for a long time are

called _____.a. Permanent magnetsb. Electromagnetsc. Magnetic domainsd. Temporary magnets

CH 24 HW

59. move left or spin63. 0.04 T65. ccw66. into page; out of

page74. 2.4 N78. 3000 Amps84. .742 T; 2.66x1016

m/s2

92. 2.1x10-12 N, upward

60. 0.5 T61. 0.89 Volts73. 0.13 Volts; 0.13

Amps; cw; opposite75. 1,800 Volts; 30

Amps; 3,600 Watts76. 36 turns; 9.4

mAmps77. 2:1; 5 Amps80. 3600 Volts, 90

Amps; 1.1x104 Watts

CH 25 HW

Electromagnetic Induction

The process of producing a current in a wire using magnetism

ElectromagnetElectromagnet

• A magnet produced by current electricity

• No current, no magnet• Wire must be wrapped

to combine the magnetic field produced

• A metal (Iron, etc.) core can help, but is not necessary

• How magnets are made

Making Your Own Electromagnet

• Materials– Wire– Battery– 10 paper clips– Nail or bolt– Christmas light

• Procedures– Make an

electromagnet– Do not complete the

circuit until you are ready! Battery will get hot!

– Fill out the chart with the proper number of turns and paper clips.

• Objective– Construct an

electromagnet and pick-up paper clips.

# of turns of wire

# of paper clips

5

10

20

30

Electromotive ForceElectromotive Force

• The voltage produced by a battery to move energy around a wire

• It is not a force—it was named this because it was originally thought charges were being “forced” to move around a wire

• Units: Volts

EMF = B EMF = B ll v sin v sin

Pre-class for Tuesday 5-8-12

Find the direction of

the magnetic field on the inside and

outside of the solenoid shown.

2 TYPES OF CURRENT

• Alternating (ACAC) Current– Current changes direction

– Current flows both ways

• Direct (DCDC) Current– Current flows one direction only

– Defined by Ohm’s Law

But, not these Transformers!!!

TRANSFORMERS

• A device used to increase or decrease the voltage in an AC circuit

• 2 basic types– Step-up: increase voltage– Step-down: decrease voltage

• Used to change the voltage sent from the power company to your house

CH 24 HW

59. move left or spin63. 0.04 T65. ccw66. into page; out of

page74. 2.4 N78. 3000 Amps84. .742 T; 2.66x1016

m/s2

92. 2.1x10-12 N, upward

60. 0.5 T61. 0.89 Volts73. 0.13 Volts; 0.13

Amps; ccw; opposite75. 1,800 Volts; 30

Amps; 3,600 Watts76. 36 turns; 9.4

mAmps77. 2:1; 5 Amps80. 3600 Volts, 90

Amps; 10800 Watts for both

CH 25 HW

Pre-class for Wednesday 5-9-12

The function of an electric motor is to _____.a) Change electrical energy to chemical energyb) Change chemical energy to electrical energyc) Change mechanical energy to electrical energyd) Change electrical energy to mechanical energy

Electric Motor or Electric Motor or GeneratorGenerator• MaterialsMaterials

– Battery– Hand crank generator– Light bulbs– Switch

• ObjectiveObjective– To make an electric motor and

generator

• QuestionsQuestions– Which part models an electric

motor? Explain.– Which part models an electric

generator? Explain.– What would happen if a battery

with a larger voltage was used?– Where is the magnetic field the

strongest? Explain.

ProceduresProceduresA. Connect the hand generator,

light bulb, and switch in series. Turn the hand crank and record the light bulb brightness.

B. Remove hand generator and connect battery instead. Record light bulb brightness.

C. Connect battery to hand generator and switch. Close switch. Record observations.