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Physics II PHY 202/222 Magnetism

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Magnetism Test 5

Beiser Chapters 27 MC odd, SP 5 17

odd Beiser Chapters 28 MC odd, SP 1 19

odd Beiser Chapters 29 SP 3 Browne Chapter

26-29 for PHY 222 Students 26 1,5,9

27 2, 28 7 29 1, 2

Chapter 27 Magnetism

Beiser p.319

Magnets

When we think of magnets we either consider

permanent magnets or magnetic effects of moving

charge. Since permanent magnets come from moving

charge, we consider moving charge first.

Beiser p.319

Magnetic Fields

Unit of magnetic field is the Tesla where 1 T 1

N/Am 1 Weber/m2 10,000 Gauss The field

around a strong permanent magnet is 0.1 T. An

MRI is from 0.2 to 1.5 T. A junkyard

electromagnet for lifting cars is 1 T.

Beiser p.319

Magnetic Field of a Straight Current

Every current in a wire generates a magnetic

field. Point the thumb of your right hand in

the direction of the current, and your curled

fingers will point in the direction of the

field. The magnitude at a distance s from the

wire is given by the formula

Beiser p.320

Magnetic Field of a Loop

A current in a loop of wire generates a magnetic

field. Point the fingers of your right hand in

the direction of the current, and your thumb will

point in the direction of the field inside the

coil. The magnitude of the field inside the loop

is given by the formula

Beiser p.321

Earth Magnetism

The Earth has a magnetic field due to currents of

molten material in the core. The magnitude is

around 3 x 10 -5 T

Beiser p.322

Magnetic Force on a Moving Charge

A charge Q moving in a magnetic field B with

velocity v will experience force F. In the

picture, the charge is moving to the right in a

magnetic field into the screen. The magnitude of

the force is given by The force will be upwards

as follows Put the thumb of your right hand in

the direction of v. Put your fingers in the

direction of B. Curl fingers up. Force will be

in direction of fingers for a positve charge, and

opposite for a negative charge.

Beiser p.323

Magnetic Force on a Current

F I L B

The force will be upwards as follows Put the

thumb of your right hand in the direction of I.

Put your fingers in the direction of B. Curl

fingers up. Force will be in direction of

fingers.

Beiser p.323

Force Between two Currents

If currents are in opposite directions, the force

is repulsive same attractive.

Beiser p.324

Ferromagnetism

Beiser p.327

27.6

27.10

27.12

27.14

27.16

Chapter 28 Electromagnetic Induction

Beiser p.335

Electromagnetic Induction

- A current is produced if
- If a conductor is moved in a magnetic field
- If a magnet is moved near a wire, especially a

coil of wire - A magnetic field changes near a conductor/coil.

For a straight conductor moving perpendicular to

a magnetic field.

Beiser p.335

Faradays Law

For a magnet moving in a coil

Lenzs Law an induced current is always in the

direction so that its own magnetic field opposes

the effect that created it. Hence the negative

sign above.

Beiser p.335,6

Transformers

N1

N2

Beiser p.337

Self Induction

A change in current in a conductor causes a

change in magnetic field. A change in magnetic

field causes an self-induced emf.

Where L is the inductance of the circuit

component.

For a solenoid

Beiser p.339

Inductors in Combination

Energy of an Inductor

Beiser p.341

Time Constants and Current

When a switch in an inductive circuit is closed,

the current builds up to its full value

according to the formula

Where the time constant, T L / R.

Beiser p.341-3

28.4

28.6

28.8

28.10

28.12

28.14

28.16

28.18

28.20

Chapter 29 Alternating Current Circuits

Beiser p.350

Alternating Current

AC Alternating Current V Vmax sin ?t I Imax

sin ?t

DC Direct Current V constant

Beiser p.350

Generators

Split ring commutator

Slip rings

Generator Move the coil, electricity out.

Motor Electricity in, motion out.

Effective Values

Since the average AC voltage V Vmax sin ?t is

zero, we need a way to be able to calculate its

capacity to do work. So we use the effective

value or root-mean-square (rms) value.

Beiser p.350

Phase Angle

ELI ICE In an AC circuit with only an inductor

(L) the voltage (E) leads the (I) current by

900. In an AC circuit with only an capacitor (C)

the current (I) leads the voltage (E) by 900. In

AC circuits with both inductors and capacitors

you would have to find the phase angle as shown

in the book.

Beiser p.353

Maxwells Equations

Gausss Law for electricity Electric fields come

from charges

Gausss Law for Magnetism There are no magnetic

charges/monopoles. Any ball has the same B out

as in sum 0

Faradays Law Change in magnetic field makes

electricity.

Amperes Law Change in electric field makes

magnetism.

?E ? ?B ? ?E ? propagates through space as

light or other EM waves. WOW!

Browne p.343