Magnetism AP Physics Chapter 20. Magnetism 20.1 Mangets and Magnetic Fields.

Magnetism

AP Physics

Chapter 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 south

pole

Like poles repel and

unlike poles attract

Unlike charges, poles

can 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 monopole

Interaction is also similar to

electric fields

20.1

Magnets and Magnetic Fields

The earths magnetic

field –

Caused by the

iron core

Does not point to

geographic

North

Provides protection

against solar wind20.1

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 wire

The field is in a circle around the

wire

We can determine

the direction

of a magnetic

field using the

right 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 magnetic

field

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. thumb, direction of current

3. palm, direction of force

The force is defined as

B – magnetic field measure in tesla (T) – vector

l – 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 current

I is the amount of charge that passes a point in a given time

l can be defined as

Putting those in the force equation

20.4

t

qI

vtl

sinIlBF sinBvtt

qF

sinqvBF

Force on Electric Charge Moving in a Mag Field

The force is maximum when the charged particle moves perpendicular to the field

Zero when it moves parallel to the field

The 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 field

In our example

-Field (fingers) is out of the

slide

-Velocity (thumb) is in the

direction of the blue arrow

-Force (palm) – 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 problem

The 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 r

vmqvB

2

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 field

This 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 inversely to the distance from the wire

0 is called the permeability of free space

20.5

r

IB

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

r

IB

2

101

d

IB

2

101

sinIlBF 1222 BlIF 2210

2 2l

d

IIF

Magnetism

20.9 Torque on a Current Loop; Magnetic Moment

Torque on a Current Loop

Electric Motor

20.9

Magnetism

20.10 Applications

Applications

Galvanometer

1. Current flows through

the wire

2. Produces a magnetic

field

3. The magnetic field is

attracted or repelled by the permanent magnet

Found in analog voltmeters, ammeters, and ohmeters 20.10

Applications

Electric Motor

1. Current flows through

the wire

2. Produces a magnetic

field

3. The magnetic field is

attracted or repelled by the permanent magnet

4. Brushes allow the current to switch

20.10

Applications

Loudspeaker

1. Current from the amp

flows through the wire

2. Produces a magnetic

field

3. This pushes the coil

away, causing the cone to move outward

4. Signal turns off, or switches, the cone moves back

20.10