Magnetism

Chapter 36

What is a Magnet?

Material or object that produces a magnetic field.

Two types: Permanent Electromagnet

What causes Magnetism?

In order to create a magnetic field, a charged particle must be moving.

Moving and spinning electrons cause magnetic fields in every object.

Domains

A small region of space where the magnetic fields produced by moving electrons are aligned together.

Often, the directions of the domains cancel each other out.

Ferromagnetic material Cancellations do not occur, resulting in a

net magnetic field

Domains

Magnetism

Opposites poles attract Like poles Repel

Magnetic Poles can not be separated Every object that has a north pole has a

south pole

Magnetism

Magnetic Field

Region around a moving charged particle through which a force is exerted on another moving charged particle

Similar to Electric Fields

Magnetic Field Lines

Lines are not real North South (outside magnet)

Can not cross Closer lines mean stronger field

Magnetic Field Lines

Magnetic Field Lines

Magnetic Field

Magnetic Field is a region around a moving charged object through which a force is exerted on another moving charged particle Motion of particle must be perpendicular to the

magnetic field

Magnetic Fields

Magnetic Fields are often illustrated using arrows

Magnetic Fields

What about into the page or out of the page?

Out of PageInto Page

Magnetic Hand Rules

To determine the direction of the force, we use hand rules.

Different hands for different charges Right hand for Positive charges Left hand for Negative charges

Conventional Current

Conventional Current follows the old “convention” that positive charges are the charges that are moving in current

Use Right Hand Rule

Electron Current

Electron Current is the reality that negative charges are the charges that are moving in current

Use Left Hand Rule

Magnetic Hand Rules

Point index finger in direction of motion

Point palm or other fingers in direction of magnetic field

Point thumb in direction of Magnetic Force

Example

An electron is moving through a magnetic field as shown below. In what direction will the magnetic force be?

Out of page e

Another Example

An electron is moving through a magnetic field as shown below. In what direction will the magnetic force be?

Down

e

Force on Wire

Still use hand rule to determine the direction of the magnetic force

Index finger is the direction of the current

Magnetic Fields produced by Currents

A current carrying wire also produces a magnetic field

Direction follows second Hand Rule

Magnetic Fields produced by Currents

Second Hand Rule Thumb in direction of current Curl fingers around wire Curled fingers show direction of

magnetic field

Example

What is the direction of the magnetic force exerted on wire 2 by the magnetic field produced by wire 1?

I-

I-

1

2

Down

Example

What is the direction of the magnetic force exerted on wire 1 by the magnetic field produced by wire 2?

I-

I-

1

2

Down

Magnetic Fields produced by current carrying loops

Imagine current flowing through the loop below

In what direction will the magnetic field be produced inside the loop?

Into PageI-

Electromagnets

Temporary magnet caused by an induced magnetic field from current carrying wires.

Electromagnets

Current carrying wire produces a magnetic field

Coiling the wire bunches up the magnetic field inside the coil

Electromagnets

Increasing the strength of the electromagnet: Increase Current in wire Increase number of coils Add an iron core

Electromagnetic Induction

If charged particle moving through a magnetic field feels a force, shouldn’t a moving magnetic field exert a force on a charged particle?

Electromagnetic Induction

A voltage can be “induced” in a wire by moving a magnet near the wire. often a coil of wire is used

Faraday’s Law Induced voltage is directly proportional to the

number of coils, cross-sectional area of the coils, and rate of change of magnetic field

Electromagnetic Induction

Electromagnetic Induction

Electromagnetic Induction

Inducing a current in a coil of wire creates its own magnetic field

Electromagnetic Induction

Changing direction of magnetic field changes direction of induced voltage Creating an alternating current (AC)

Alternating Current Current alternates direction at a regular rate

Electrical Outlets

Direct Current Current flows in one direction only

Batteries Sim

Generators & Motors

Device to convert between Electrical and Mechanical Energy

Generator Converts Mechanical Energy to Electrical

Energy Motor

Converts Electrical energy to Mechanical Energy

Generator

Motor

Electromagnetic Induction

Current flowing through a coil wires produces a magnetic field

A changing magnetic field induces a current in an adjacent coil

Electromagnetic Induction

Using AC produces a consistent changing magnetic field

Adding an iron core strengthens the magnetic field

Transformer

Device used to increase or decrease voltage using electromagnetic induction

Transformer

Complete loop is more efficient

Transformers

V = voltage N = number of coils

s

p

s

p

N

N

V

V

s

s

p

p

N

V

N

V

Transformers

Step Up Transformer Secondary has more coils than primary Resulting Voltage is larger

Step Down Transformer Secondary has less coils than primary Resulting voltage is lower

Conservation of Energy

Energy transferred must be equal Power is equal

sspp IVIV

Electric Field

An electric field is produced by a charged particle

A changing electric field can be produced by a moving charged particle

A changing electric field produces a magnetic field

Electromagnetic Induction

A changing magnetic field also induces an electric field

When magnetic fields and electric fields are produced they are at right angles to each other

Electromagnetic Wave

Oscillating electric and magnetic fields that regenerate each other (light)

No medium is required

Electromagnetic Radiation

Only one speed can preserve this regeneration

Speed of Light is 300,000,000 m/s 3 x 108 m/s

Discovered by James Clerk Maxwell

Maxwell’s Equations