Magnets and Magnetic field and force

Facts about magnets Magnets have 2 poles

(north and south) Like poles repel Unlike poles attract Magnets create a

MAGNETIC FIELD around them

Facts about magnets

When a bar magnet is broken into smaller pieces, each small piece will have its own north and south pole.

Facts about magnets

When suspended, a bar magnet will align itself with the Earth’s magnetic field. The north pole of the magnet points toward the north geographic pole of the Earth.

Magnetic field of a bar magnet

Magnetic FieldA bar magnet has a magnetic field

around it. This field is 3D in nature and often represented by lines LEAVING north and ENTERING south

To define a magnetic field you need to understand the MAGNITUDE and DIRECTION

We sometimes call the magnetic field a B-Field as the letter “B” is the SYMBOL for a magnetic field with the TESLA (T) as the unit.

Magnetic field

The magnetic field of a bar magnet is strongest at the poles.

Earth’s geographic and magnetic poles

Permanent magnets vs ElectromagnetsPermanent magnets retains a magnetic force Can not be turned on and off

Electromagnets Use electricity to create a magnetic field They can be controlled (turned on and off) Their force or strength of field can be

controlled

Magnetic Force on a moving charge If a MOVING CHARGE

moves into a magnetic field it will experience a MAGNETIC FORCE. This deflection is 3D in nature.

N

N

S

S

-

vo

B

sinqvBF

BvqF

B

B

The conditions for the force are:• Must have a magnetic field present• Charge must be moving• Charge must be positive or negative• Charge must be moving

PERPENDICULAR to the field or must move such that its velocity has a component perpendicular to the field.

Direction of the magnetic force?

Right Hand RuleTo determine the DIRECTION of the

force on a POSITIVE charge we use a special technique that helps us understand the 3D/perpendicular nature of magnetic fields.

Basically you hold your right hand flat with your thumb perpendicular to the rest of your fingers

• The Fingers = Direction B-Field• The Thumb = Direction of velocity• The Palm = Direction of the Force

For a NEGATIVE charge the force is in the opposite direction (where the back of your hand is facing)

Magnetic force on a moving charge: Another version of right-hand rule

Comparison between gravitational, electric and magnetic forcesField The force is applied on a: Direction of the force

Gravitational field mass same direction as the field

Electrical force electric charge opposite or in the same direction as the field

Magnetic field electric charge perpendicular to the plane created by v and B

Our direction guides up +z

in:west in:-x

south north -y +y

out:east out:+x

up

down in:in -z

left right

out:out

down

Charges moving in a wireUp to this point we have focused our attention on

PARTICLES or CHARGES only. The charges could be moving together in a wire. Thus, if the wire had a CURRENT (moving charges), it too will experience a force when placed in a magnetic field.

You simply used the RIGHT HAND ONLY and the thumb will represent the direction of the CURRENT instead of the velocity.

ExampleA 36-m length wire carries a current of 22A running from right to left. Calculate the magnitude and direction of the magnetic force acting on the wire if it is placed in a magnetic field with a magnitude of 0.50 x10-4 T and directed up the page.

B

B

B

F

xF

ILBF

90sin)1050.0)(36)(22(

sin4

0.0396 N into the page

Direction of magnetic force on current-carrying wire