objectives1. Understand the motion of charges relative to each

other produces a magnetic force.

2. For given situations, predict whether magnets will repel or attract each other.

3. Describe the magnetic field around a permanent magnet.

4. Describe the orientation of Earth’s magnetic field.

5. Understand the relative motion between a conductor and a magnetic field may produce a potential difference in the conductor.

4.3.1 Magnetic Fields and Electromagnetic Induction

The Earth’s Shield

Magnetism• Magnetism refers to physical phenomena arising from the force

between magnets, objects that produce fields that attract or repel other objects.

• A magnet is any piece of material that has the property of attracting iron (or steel).

• Magnetism may be naturally present in a material or the material may be artificially magnetized by various methods.

• Magnets may be permanent or temporary.

• Materials which can be magnetized are called ferromagnetic materials.

• The cause of magnetism is from a property of the atoms.

• Atoms have a positively charged center called the nucleus and is orbited by one or more negatively charged particles called electrons.

• The electrons spin as they travel around the nucleus (which contain protons and neutrons) much like the earth spins as it orbits the sun.

• As the electrons spin and orbit the nucleus, they produce a magnetic field.

What is a magnetic field and how is it created?

Moving electrons creates magnetic fields.

Not all atoms have magnetic fields• All the electrons produce a magnetic field as they spin and orbit

the nucleus; however, in some atoms, two electrons spinning and orbiting in opposite directions pair up and the net magnetic field of the atom is zero.

• Materials with one or more unpaired electrons are magnetic. Materials with a small attraction to a magnet are called paramagnetic materials, and those with a strong attraction are called ferromagnetic materials. Iron, cobalt, and nickel are examples of ferromagnetic materials.

•Not all the fields are aligned, but when canceling spins are accounted for, a net magnetic field remains.

MAGNETIC DOMAIN• A magnetic domain is a region in which the magnetic fields of

atoms are grouped together and aligned. • You can think of magnetic domains as miniature magnets within

a material.

One way to magnetize a metal is to rub a magnet over the surface of the metal.

MAGNETIC PROPERTIES

1. Attracts iron containing objects.

2. It has two ends called poles: north pole and south pole. North pole points to North of Earth and south pole points to South of Earth.

3. No matter how many times a magnet is broken, each piece always has a north pole and a south pole.

4. Like poles repel each other, unlike poles attract each other.

• The needle of a compass is a small MAGNET• The north pole of a compass needle

– … is marked with a small “N” or a prominent color– … points toward the geographic north direction,

but magnetic SOUTH poles.

Compass

The geographic NORTH pole of the Earth acts like the SOUTH pole of a

magnet!

Magnetic fields• The region where magnetic force exists around a magnet is

called its magnetic field

• Magnetic field allows magnets interact without touching. Magnetic force is a non-contact force.

• A magnetic field exerts a force on any moving charge and can be measured and detected by this effect.

Magnetic Field Lines• Rules for field line directions

2. Magnetic lines never intersect

3. Magnetic field is strongest where the lines are closest.

1. The direction is defined as the direction the N-pole of a compass would point in the field. • Outside magnet NORTH TO SOUTH• Inside magnet SOUTH TO NORTH

Conventions for the direction of a magnetic field

In the plane of the page

Into the page x

Out of the page

Magnetic Field Maps

N S N S

Magnetic Field Maps

S N N S

Magnetic Field Maps

S

N

N S

• Determine the direction of the magnetic field lines at each point. Where is the field strongest?

Example #1

Field is strongest at point C.

B

A

C

D

• Determine the direction of the magnetic field lines at each point. Where is the field strongest?

Example #2

Field is strongest at point F.

F

E

G

H

S N N S

• Determine the direction in which the compass will point.

Example #3

S N S N

Example #4• The diagram below represents the magnetic field

near point P. If a compass is placed at point P in the same plane as the magnetic field, which arrow represents the direction the north end of the compass needle will point?

A B C D

N

S

• Moving charges produce magnetc field. An electric current are moving charges. An electric current can cause a magnetic field around it just like a magnet causes a magnetic field.

Magnetic field lines

Current produce magnetic field

ELECTROMAGNETS • A conductive wire with a current flowing through it creates a magnetic

field. However, the magnetic field of one wire is small and does not have much strength

• If we take a wire and coil it several times to form a long coiled piece of electrical wire, we would have a magnetic field much bigger and stronger when we turn on the current.

If an iron bar is placed through the center of the coiled wire, it would become a temporary magnet, called an electromagnet, as long as the electric current is flowing through the wire.

Electromagnetic induction• ..\..\labs\phet labs\generator_en.jar

• Just as an electrical current induces a magnetic field, when a magnet is moved in and out of coils of wire or when an electrical wire cuts across magnetic lines of force, a magnetic force acts on the electrons and producing an induced potential difference. As a result, current is generated. This process is called electromagnetic induction.

• It does not matter if the magnet is moved or if the coils of wire are moved. The important thing is that there is motion within the magnetic field, and that the magnetic lines of force are cut.

Generator• The electromagnetic induction is the principle by

which electric generators can make electricity. Through the use of magnets, a generator can convert mechanical energy to electrical energy.

• Inside a generator is a magnet, some electrical wire, and a source of mechanical energy. The mechanical energy moves the wire into the magnetic field of the magnet so that the wire cuts through the magnetic lines of force. As a result, electric current is produced.

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example

• An accelerating particle that does not generate electromagnetic waves could be

1. a proton

2. a neutron

3. an electron

4. an alpha particle

Example• Which procedure will produce the greatest induced

potential difference in the conductor?

1. holding the conductor stationary between the poles

2. moving the conductor out of the screen

3. moving the conductor toward the right side of the screen

4. moving the conductor toward the N-pole