Magnetism and Magnetic Force

Unit 10 Lecture 1 AP Physics

Magnets

A bit of history …

The ancient Greeks knew about "lodestones“ (rare natural magnets) with the power to attract iron.

One site where such stones were found was near the city of Magnesia in Asia Minor (now Turkey), and from that (perhaps) came the term "magnetism."

But they were not the first to understand how to use it.

A bit of history

Somewhere around 500 ACE, the Chinese built a primitive compass by placing a lodestone on a boat in a bowl of water.

The boat always rotated to point in the same direction, no matter where or how the experiment was performed.

The boat was never pulled in any direction: it only rotated.

William Gilbert

Physician to Elizabeth I

Published On the Magnet and Magnetic Bodies in 1600.

First understood that the earth was itself a magnet.

Summarized all of the knowledge of magnetism up to his day.

Magnets: Overview

Has two “ends” or poles, called N and S

Opposite poles attract; like poles repel

Magnets: Overview

However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.

Sources of Magnetism

A strongly magnetized bar consists of aligned domains.

Sources of Magnetism

A broken magnet

Ferromagnetic material

The domains persist when the external magnetic field is removed

Also called “hard” magnets or permanent

Paramagnetic material

Domains can align with an external magnetic field

The domains do not persist when the external magnetic field is removed

Also called “soft” magnets

Nonmagnetic material

Atom has no net magnetic field so there can be no domain

Exhibits no magnetic effects

Example: copper, gold and silver

Magnets: Overview

Magnetism can be destroyed by heat and restored again by cooling.

Magnetic Earth

The earth acts as a giant magnet.

Geographic north is a south magnetic pole

Freely suspended magnets will align with this earth magnet.

A compass will point downwards near the north pole.

The magnetic pole is not located at the geographic pole. There is a difference!

The earth as a magnet

Magnetic Declination

20.1 Magnets and Magnetic Fields

Magnetic fields can be visualized using magnetic field lines, which are always closed loops.

20.1 Magnets and Magnetic Fields

A uniform magnetic field is constant in magnitude and direction.

The field between these two wide poles is nearly uniform.

Magnetic Forces

Hans Christian Ørsted is credited with discovering the link between current and magnetic forces. (1820)

He was giving a demonstration of the battery when he noticed that a compass always reacted when he turned the electricity off and on.

Force of a Charged Particle

A charged particle (or current) experiences a magnetic force when it is moving through, but not parallel with, a magnetic field.

Magnetic Force

The vector force is given by a vector equation:

The magnitude of the force is given by:

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

F qv B %%%%%%%%%%%%% %

,sin v BF qvB

Right Hand Rule

Point your fingers in direction of velocity

Rotate wrist so you can sweep your fingers in the direction of the magnetic field without moving your palm

Thumb points in the direction of the Force.

Your turn

A positive charge is moving south in a magnetic field that points west. What is the direction of the force on the charge?1. North2. South3. East 4. West5. Up6. Down7. There is no force

Correct Answer: 6

Your turn

A positive charge is moving upwards in a magnetic field that points south. What is the direction of the force on the charge?1. North2. South3. East 4. West5. Up6. Down7. There is no force

Correct Answer: 3

Your turn

A negative charge is moving west in a magnetic field that points north. What is the direction of the force on the charge?1. North2. South3. East 4. West5. Up6. Down7. There is no force

Correct Answer: 5

Your turn

A positive charge is moving east in a magnetic field and the force is downwards. What is the direction of the field?1. North

2. South

3. East

4. West

5. Up

6. Down

7. There is no forceCorrect Answer: 2

Your turn

A positive charge is moving along the +x axis in a magnetic field that points in the –x direction. What is the direction of the force on the charge?1. +z2. -z3. +x 4. -x5. +y6. -y7. There is no force

Correct Answer: 7

Your turn

An electron is moving along the +x axis in a magnetic field that points in the –z direction. What is the direction of the force on the charge?1. +z2. -z3. +x 4. -x5. +y6. -y7. There is no force

Correct Answer: 6

Magnetic Field Defined

The magnetic field strength is defined in terms of the magnitude of the force that is produced on a charged object.

sin

FB

qv

Magnetic Field Defined

4

N N

C m/s A m

Tesla T

1 T 10 G (gauss)

FB

q v

B

Magnetic Fields

Source Field (Gauss)

Earth 0.5

Appliance 10

Bar magnet 100

Human limit 2000

electro-magnet 50,000

Example 1

A proton moves at 8.0E6 m/s along the x-axis. It enters a magnetic field of B=2.5 T, directed at an angle of 60° from the x-axis in the xy plane.

• What is the magnetic force on the proton?

• What is the acceleration of the proton?

Forces on Conductors

A current-carrying conductor is a stream of moving charges.

There will be a force on the conductor if it is placed in a magnetic field.

Representing B

Arrows are used to represent the magnetic field.

Force on a wire

maxForce per charge

Number of charges

F qvB

N nA

maxF BI

If the current is not perpendicular to the field B, then the force will be

sinF BI

Aurora Borealis

Application

The speaker

Motion of Charged Particle

Since F is always perpendicular to v, the motion is circular

2mvF qvB

rmv

rqB

Application

Mass Spectrometer

Applications

Animals and Magnetism: http://videos.howstuffworks.com/hsw/9511-understanding-magnetism-animals-and-magnetism-video.htm

Other topics:

http://videos.howstuffworks.com/search.php?terms=magnetism&x=0&y=0&media=video