Magnetic Fields

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Properties of Magnets

Polarized - magnets have two opposite ends

North pole and south pole

Charges can be separated but magnetic poles cannot be separated

Earth As A Magnet

The north end of a compass points to the geographic north pole

At the geographic north pole is the magnetic south pole

Similarly, at the geographic south pole is the magnetic north pole

How Do Magnets Affect Other Materials

A magnet can cause another metal to become polarized and have magnet properties

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Permanent Magnets

ALNICO V - a permanent magnet alloy (aluminum, nickel, cobalt)

Magnetic Fields Around Permanent Magnets

Magnetic forces can be described by the existence of a field around the magnet

Can be non-contact forces

Magnetic fields are vector quantities that exist where a magnetic force occurs

Magnetic Field Lines

Magnetic field lines are imaginary lines used to visualize a magnetic field

Direction of fields lines are defined as the direction of a compass needle when placed in teh magnetic field

Outside the magnet, field lines leave the magnet from the north pole and end at the south pole

Inside the magnet, the magnet field lines go from the south pole to the north pole to form a closed loop

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Magnetic Flux

Magnetic flux - the number of field lines passing through a surface

Magnetic flux is proportional to the magnetic field strength

The magnetic flux is concentrated at the poles, where the magnetic field strength is the greatest

Electromagnetism

Danish physicist Oersted found that when a current was in a wire, a compass needle rotated until it was perpendicular to the wire

If the needle rotated, it must have been because of a magnetic field

Magnetic field lines around a current carrying wire loop in the same way that field lines about a permanent magnet do

First Right-Hand Rule

Determines direction of a magnetic field relative to the direction of conventional current

Pretend to hold the wire with your right hand

Point thumb in direction of conventional current

Fist points in direction of magnetic field

If it's based on the current of electrons, the opposite direction of the conventional current-based field direction is the electron's direction of field

Magnetic Field Near a Coil

Solenoid - a long coil of wire consisting of many loops

The field of each loop adds to the fields of the other loops and creates a greater total field strength

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Increase Strength of A Solenoid

More loops (longer wire)

Increase current

Add an iron core (increases magnetic field strength)

Second Right-Hand Rule

Determine direction of the field produced by an electromagnet relative to the flow of conventional current

Curl your right hand fingers around the loops in the direction of the conventional current

Thumb points toward the north pole of the electromagnet

Domain

Electrons act like tiny electromagnets

Domain- when the magnetic fields of electrons in a group of atoms are all aligned in the same direction

When a piece of iron is not in a magnetic field, the domains point in random directions and their magnetic fields cancel each other out

In a temporary magnet, an external magnetic field aligns the domains + when the external field is removed, the domains return to random arrangement

In a permanent magnet, the iron keeps the domains aligned after the external magnet is removed

24.2 Forces Caused by Magnetic Fields

A current produces a magnetic field similar to a permanent magnet

When a current moves through a wire or conductor in a magnetic field, a force is produced

The force on the wire is at right angles to both the direction of the magnetic field and the direction of the current

If velocity is parallel to the magnetic field, then the force is zero

Otherwise, the direction of the force can be found by using the right-hand rule

Force On A Wire From A Magnetic Field

The magnitude of the force, F, on the wire is proportional to the strength of the field, B, the current, I, and the length, L, of the wire in the magnetic field

F=ILB

Force = Current x Length x Magnetic Field Strength

B, magnetic field strength, is measured in teslas, T

1 T = 1N/Am)

1 Tesla = 1 Newton / Ampere x meter

Third Right-Hand Rule

Determine the direction of magnetic force

Point right hand fingers in direction of magnetic field

Point thumb in direction of conventional current

Palm points in direction of force

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3D Magnetic Field Symbols

An x in a circle indicates that the magnetic field points into the page

A dot in a circle indicates that the magnetic field points out of the page

The Force On A Charged Particle

Charged particles can move in a vacuum

These are called cathode ray tubes used to form pictures on screens

Magnetic fields control the motion of the beam

The force produced by a magnetic field on a single electron depends on the velocity of the electron and the strength of the magnetic field

F = qvB

F = ILB

q = charge in Coulombs

v = velocity in m/s

B = magnetic field strength in T

The direction of the force is given by the third right-hand rule

The rule is for positively charged particles so the direction of the force for electrons would be opposite