Magnetism

What are magnets?• Let’s first start off with what causes an

magnetic field…

• A magnetic field is created around any moving charged object.

What is charged that is moving within an atom?

• Electrons (e-)• The atoms within most

materials have paired up electrons spinning in opposite directions so the magnetic field that is created by one is cancelled out by the other.

• Some materials like iron, nickel, and colbalt have a single electron or pair of electrons that spin in the same direction creating a magnetic field or a small atomic magnet.

The atomic sized magnets line up to create domains within the material.

The individual domains line up to form a magnet.

A magnet has two ends called poles (dipoles), where the magnetic force is the

strongest.

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

• There is no such thing as a monopole.

A magnetic field exists around a magnet or any moving charged object.

Magnetic fields are like electric fields or gravitational fields in that they allow

magnets to interact without touching.

Imaginary lines that map out the magnetic field (B) around a magnet are known as

magnetic field lines or magnetic flux lines.

Rules for drawing magnetic field lines (same as an electric field):

1. Magnetic field lines always go from the north pole to the south pole (outside a magnet).

The direction of the magnetic field is determined by using a compass. The

needle of the compass points in the direction of the magnetic field.

Rules for drawing magnetic field lines:

2. Magnetic field lines are closed loops and never cross or intersect.

Rules for drawing magnetic field lines:

3. Where the magnetic field lines are closer the magnetic field is stronger.

Draw magnetic flux lines around the magnets below:

S NS N

S N SN

Magnetic Field Strength (B)

The number of magnetic lines of flux per unit area passing through a plane perpendicular to the direction

of the lines is called the magnetic field strength (B).

The weber (Wb) is the unit used for measuring the number of lines of flux.

1 Wb = 1 Tesla (T) m2

Tesla’s are used as the unit for magnetic field strength (magnetic flux density).

The magnetic field strength is a vector like electric field strength.

B = F qv

Feeling the strongest magnetic force…

B

An object that enters this magnetic field will feel a maximum force

only if it is moving perpendicular to the flux lines.

It will feel no magnetic force if is is moving

in line, or parallel, with the flux lines.

Fmax

F = 0

Magnetism

• Magnetism is the force of attraction or repulsion between magnetic poles.

Magnetic Field around the Earth

What we do know…The north magnetic pole and the geographic North

Pole do not coincide. The magnetic pole is about 1500 km (930 mi) south of the geographic North Pole and it wanders.

A compass actually indicates the direction of magnetic north, not true north. Therefore a navigator must need to know the magnetic declination for a specific area. This is the angular

difference between magnetic and true north.

The details and mechanisms of how and why the Earth has a magnetic field are unclear. They do believe that it may have something to do with

motions in the liquid outer core.

Charges moving through a wire:Remember what creates

magnetic fields- a charged object in motion.

Current is the flow of electrons. A moving charged object.

A compass can be used to detect a magnetic field around a wire that

had current flowing through it.

Current in a wire produces a magnetic field around the wire.

SN

LEFT HAND RULE: Helps us to analyze the path of a charged object in a magnetic field.

B

e-

Which way will the magnetic field below push this electron traveling through the field?

We use the LHR for electron flow.

Fingers are aligned with the direction of the magnetic field (B).

Thumb is aligned with the direction of current (I) or movement.

Your palm indicates the direction the object would move do to the magnetic force.

LHR Symbols:

Sometimes the magnetic field or current is going into or out of the page. For this we us the following symbols:

WIRE:

FIELD:

Coming out of page Going into page

LHR with wires:

Effects of two wires:

1. Two wires next to each other that both have currents in the same direction.

WIRES WILL ATTRACT EACH OTHER

Effects of two wires:

2. Two wires next to each other that have currents in opposite directions.

WIRES WILL REPEL EACH OTHER

Effects of two wires:

3. Two wires with currents flowing perpendicular to each other.

NO INTERACTION, F=0

Practice…

The diagram below shows an end view of a current carrying wire between the poles of a magnet. The wire is perpendicular to the magnetic field.

If the direction of the electron flow is into the page, what is the direction of the magnetic force (F) acting on the wire?

F

Practice…

The wire below is moved toward the right through a magnetic field. In which direction will the magnetic force push the wire?

Looped wire

What’s the magnetic field around a loop of wire?

+ -

e-

Where’s the north pole created?

The north pole is located where your finger tips end up.

Behind the wire.

What’s the magnetic field around a loop of wire?

+

-

Where’s the north pole created?

e-

Above the wire.

Looping a wire around several times into a coil will produce an…

…ElectromagnetAn electromagnet is a type of magnet whose

magnetic field is produced by the flow of electric current. The magnetic field disappears when the

current ceases.

An electromagnet is most commonly made by coiling wire around a piece of iron. This electromagnet is called a

solenoid. The shape of the magnetic field is the same as a bar magnet.

As electrons move through the coil of wire, the magnetic field of one electron adds to the field of any others moving in the same direction.

Factors affecting the magnetic field of a solenoid:

1.The magnetic field around a solenoid is directly related to the current through the coil.

2. The magnetic field around a solenoid is directly related to the number of turns or coils around the solenoid.

Factors affecting the magnetic field of a solenoid:

Factors affecting the magnetic field of a solenoid:

3. The magnetic field around a solenoid is directly related to the material around which the wire is coiled. The soft iron is more permeable to the magnetic field than the air is.

Determining the poles of a solenoid:

LHR Modified:1. Have palm away from you.2. Point fingers of left hand in the direction of the current (many coils, many fingers).3. Stick thumb out and that’s where the north pole is.

Determine the poles of this solenoid:

N

S

Electromagnetic Induction

This is the process of generating a potential difference (voltage) in a conductor (wire) due to the motion

of the conductor in a magnetic field.

Generating a voltage in a wire would mean generating areas of uneven charge within a wire.

How can this be done?

What can be forced to move within a wire that will create a potential difference?

Moving a wire through a magnetic field will cause a magnetic force which will move electrons towards one end of the wire

creating a potential difference.

Let’s take a closer look…

wire

B

If this wire is moved up or down (perpendicular to the field), a voltage will be generated and if the wire is part of a complete

circuit then current is induced.

Motion of wire

If this wire is moved left or right (parallel to the field), no current will be generated because no force is generated to

move electrons and create a voltage.

-- -

-

-

-

Other ways to induce voltage…

This is how Niagara Falls was used to produce electricity…

Water flow is used to turn a turbine through a magnetic field

inducing a potential difference and producing electrical current.

This is how an electric motor works…

An electric motor utilizes the property of electromagnetic induction to convert electricity into mechanical energy to make things move. The conductor itself, a coiled wire, will move to oppose the magnetic field. Just when it gets into position the current is reversed, and the coil spins round and round and round full of mechanical energy.

A Generator…

http://www.stmary.ws/highschool/physics/home/java/fendt/ph11e/generator_e.htm

A generator is simply the same process in reverse, converting mechanical energy into electricity. Almost all of the electrical energy we use in our daily lives is supplied by electric generators.

• http://www.physics.sjsu.edu/becker/physics51/mag_field.htm

• http://www.regentsprep.org/Regents/physics/phys03/cintromag/sld001.htm