Magnetism

AP PhysicsChapter 20

Magnetism

20.1 Mangets and Magnetic Fields

Magnets and Magnetic Fields

Objects that align with the magnetic field of the earth. The north pole is

magnetic southpole

Like poles repel and unlike poles repel

Unlike charges, polescan not be isolate

If a magnet is cut, new poles appear 20.1

Magnets and Magnetic Fields

Magnetic field lines – look similar to electric field lines Point in the direction of a force on a North monopoleInteraction is also similar to

electric fields

20.1

Magnets and Magnetic Fields

The earths magneticfield –

Caused by the iron core

Does not point to geographic North

Provides protectionagainst solar wind

20.1

Magnetism

20.2 Electric Currents Produce Magnetic Fields

Electric Currents Produce Magnetic Fields

Hans Christien Oerseted – 1820 a compass needle deflects when a current is first produced in a wireThe field is in a circle around the

wireWe can determine

the directionof a magnetic field using theright hand rule 20.2

Electric Currents Produce Magnetic Fields

Right hand rule #1 1. the thumb points in the

direction of the current 2. the fingers curl in the

direction of the magneticfield

A magnetic field due to a loop

20.2

Magnetism

20.3 Force on an Electric Current in a Magnetic Field

Force on an Electric Current in a Magnetic Field

Magnets exert force on a current-carrying wire (Newton’s Third Law)The force is 1. perpendicular to the direction of current 2. perpendicular to the direction of the magnetic field

20.3

Force on an Electric Current in a Magnetic Field

Right Hand rule #2 1. fingers, direction of

magnetic field 2. palm, direction of current 3. thumb, direction of forceThe force is defined as

B – magnetic field measure in tesla (T) – vectorl – length of wire in the field 20.3

sinIlBF

Force on an Electric Current in a Magnetic Field

A tesla is also called a weber per meter squared

The units in cgs is called a Gauss

20.3

2/11 mWbT

TG 4101

Magnetism

20.4 Force on Electric Charge Moving in a Magnetic Field

Force on Electric Charge Moving in a Mag Field

The equation can be determine by an analysis of the currentI is the amount of charge that passes a point in a given time

l can be defined asPutting those in the force equation

20.4

tqI

vtl

sinIlBF sinBvttqF

sinqvBF

Force on Electric Charge Moving in a Mag Field

The force is maximum when the charged particle moves perpendicular to the fieldZero when it moves parallel to the fieldThe angle is the angle between the velocity and the field The right hand rule will give you the correct direction for the force on a positive charge

20.4

sinqvBF

Force on Electric Charge Moving in a Mag Field

If we establish a uniform magnetic field and the charge moves perpendicular to the fieldIn our example-Field (palm) is out of the slide-Velocity (pointer) is in the direction of the blue arrow-Force (thumb) – points toward middle (assume positive charge)The force is centripetal 20.4

Force on Electric Charge Moving in a Mag Field

A very common type of problemThe electromagnetic force is the centripetal force

Charged particles from the sun, concentrate at the poles because of this – called the Aurora Borealis

20.4

sinqvBF qvBF rvmqvB2

Magnetism

20.5 Magnetic Field Due to a Long Straight Wire

Magnetic Field Due to a Long Straight Wire

As discussed earlier, a current in a wire produces a magnetic fieldThis can be demonstrated by moving a compass around a current carrying wire.

20.5

Magnetic Field Due to a Long Straight Wire

The field around the wire is directly proportional to the current and inversly to the distance from the wire

0 is called the permeability of free space

20.5

rIB

20

AmTx /104 70

Magnetism

20.6 Force Between Two Parallel Wires

Force Between Two Parallel Wires

Assuming that two wires are parallel to each other

20.6

rIB

2

101

dIB

2

101

sinIlBF 1222 BlIF 2210

2 2l

dIIF

Magnetism

20.9 Torque on a Current Loop; Magnetic Moment

Torque on a Current Loop

Electric Motor

20.9

Magnetism

20.10 Applications

Applications

Galvanometer1. Current flows through

the wire2. Produces a magnetic

field3. The magnetic field is

attracted or repelled by the permanent magnetFound in analog voltmeters, ammeters, and ohmeters 20.10

Applications

Electric Motor1. Current flows through

the wire2. Produces a magnetic

field3. The magnetic field is

attracted or repelled by the permanent magnet

4. Brushes allow the current to switch20.10

Applications

Loudspeaker1. Current from the amp

flows through the wire2. Produces a magnetic

field3. This pushes the coil

away, causing the cone to move outward4. Signal turns off, or switches, the cone

moves back20.10