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Magnetism - PowerPoint PPT Presentation

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Magnetism. Magnets have been known for centuries.

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Magnets have been known for centuries.

The Chinese and Greeks knew about the “magical” properties of magnets. The ancient Greeks used a stone substance called “magnetite.” They discovered that the stone always pointed in the same direction. Later, stones of magnetite called “lodestones” were used in navigation.

William Gilbert, an English physician, first proposed in 1600 that the earth itself is a magnet, and he predicted that the Earth would be found to have magnetic poles.


What is Magnetism?

Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons.

All magnetic phenomena result from forces between electric charges in motion.


The ends of a magnet are where the magnetic effect is the strongest. These are called “poles.” Each magnet has 2 poles – 1 north, 1 south.

Like repels like…

Opposites attract!


Poles of a magnet always

Come in pairs! “Law of Poles”

If you cut a magnet in half,







you get 2 magnets!

no monopoles allowed
No Monopoles Allowed

It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole). 

Note: Some theorists believe that magnetic monopoles may have been made in the early Universe. So far, none have been detected.




Magnetic Fields

The region where the magnetic forces

act is called the “magnetic field”

Magnetic fields are vector quantities.

The direction at any location is in the direction that the north pole of a compass would point if at that location

field lines of attracting bars
Field Lines of Attracting Bars

Field Lines of Repelling Bars


Atoms themselves have magnetic properties due

to the spin of the atom’s electrons.

  • Groups of atoms join so that their magnetic fields

are all going in the same direction

  • These areas of atoms are called “domains”

When an unmagnetized substance is placed in a magnetic

field, the substance can become magnetized.

This happens when the spinning electrons line up in the

same direction.


An unmagnetized substance looks like this…

While a magnetized substance looks

like this…


How to break a magnet:

1. Drop it

2. Heat it

This causes the domains to become random again!





























Magnetic Field Lines

  • The direction of the magnetic field at any point is…
    • tangent to the magnetic field line at that point.
  • the direction that the north pole of a compass would point if a compass were at that location
  • -defined as the direction of motion of a charged particle on which the magnetic field would not create a force.


Up until 1820 everyone thought that magnetism and electricity were completely separate. But in that year, the Danish physicist Hans Oersted (1777-1851) discovered that a compass needle was deflected by an electric current.

Magnetic Fields are VECTOR quantities! They are referred to as a B Field and B is the symbol used. These fields have effects on charged particles.


What does a charged particle “feel” in a magnetic field

  • If the charge is NOT moving there is NO force acting on the particle.
  • If the charge is MOVING ALONG a field line there is NO force acting on the particle.
  • If the charge is MOVING ACROSS
  • a field line, it feels a FORCE









Magnetic fields produce forces on moving charged particles. The forces are perpendicular to both the velocity of the particle and the direction of the magnetic field

The size of the force is proportional to the intensity of the field and the speed with which the particle is cutting across







Note: The direction of the field and the velocity determine a plane. The force is perpendicular to that plane



Hold your right hand with your index finger straight out, your middle finger 90o from the index finger and your thumb straight up.

Keep this orientation!

Your index finger represents the velocity of the positively charged particle, your middle finger points the direction of the magnetic field (from the north end of a magnet) and your thumbs shows the direction of the force applied to that positively charged particle.


Field Vectors



Out of Page




Into Page

The convention of showing three dimensions on a two dimensional page.


Find the resultant force under the given conditions…





Magnets exert forces on moving particles….and as Oersted showed, moving charges also created magnetic fields and that’s what deflected Oersted’s compass.

To examine the simplest case, pass a current carrying wire straight through a plane covered with compass needles.

The needles line up in circles around the wire

The magnetic field of a current is circular centered on the wire and lying on a plane perpendicular to the current.


You can find the direction of the magnetic field in a current carrying wire by pointing your thumb of your right hand along the direction of the flow of positive charges. Your fingers curl in the direction of the magnetic field.

This is known as the RIGHT HAND RULE for current carrying wire

If you look at the negative charges flowing than use the left hand rule.


Two parallel currents attract each other. The magnetic field circling each wire causes forces on the current in the other wire, pulling it closer.

Andrea-Marie Ampere, discovered the force between parallel wires


If a current carrying wire is bent into a circle, a magnetic field is produced.

Notice that one side looks just like a NORTH POLE – the field lines are coming out.


The other side looks like a SOUTH POLE – with field lines going in.


By winding many turns, the magnetic field is made proportionally larger. By winding turns along a cylinder, a solenoid coil is produced, with a magnetic field just like a bar magnet


Inserting an iron bar into the coil concentrates and strengthens the magnetic field, the result is an electromagnet.


Faraday Induction

For 12 years after Oersted’s discovery “electricians” looked for the complimentary effect.

How to make a magnetic field produce a current?

In 1832 it was Michael Faraday that suggested moving the magnet!

Thrusting a magnet through a loop of wire connected to a sensitive ammeter, a Galvanometer, deflects the galvanometer needle. Thus showing a current being induced.


When the magnet is held still, the meter registers no current.

Faraday described this effect by saying that ELECTROMOTIVE FORCES (EMF) are generated in the wire whenever magnetic field lines cut across the wire. This is actually not a force but a potential difference measured in volts

It does not matter whether the magnetic field moves or the wire moves with respect to the magnet.


When the magnet is thrust into the loop. It’s field lines cut across the wire, generating an EMF that produces a current.

The same is true when the loop is moved over the magnet

Although Faraday’s discovery was at first received with indifference, today nearly all our electrical power is generated by moving giant coils of wire near magnets.


Induction without Magnets

Another way to induce a current in a wire is to place a second loop of wire nearby the first and energize it with a power source.

When a current in the second loop is switched on or off, a current pulse is induced in the first.

But when the current in the second loop is steady, no current is induced in the first loop


In the case of the two wire loops, when the current is first turned on in one loop, magnetic field lines build up, cutting across the other loop and producing an EMF.

When the current is switched off, the field collapses, again cutting across the loop.


Lenz’s Law

The induced emf creates a current that itself creates a secondary magnetic field. This secondary magnetic field also changes with time and thus creates a changing secondary magnetic flux. The secondary flux changes in such a way to opposes the change in flux creating the emf.

Normally this means that the secondary magnetic field increases or decreases in such a way as to oppose the change in the magnetic field creating the induced emf.

“Induced current flows in a direction to oppose the charge that produced it”


Magnetic Field

F = q vB sinq

F => Force (N)

q => Charge (c)

v=> Velocity of Charge particle (m/s)

B => Magnetic Field (N/Am = T =Tesla)

q => Angle between v and B


Magnetic Forces on Current Carrying Wire

F = I L B sinq





F => Force (N)




I => Current (A)

L = vt (m) => Length of wire a charge would move in a given time

B => Magnetic Field (T =Tesla)

q=> Angle between v and B


Magnetic Field of an Ideal Solenoid

B = m0 nI

B => Magnetic Field (T =Tesla)

m => 4p x 10 -7 (Tm/A)

n => Linear Turn Density (N/L)

# of turns per meter

I => Current (A)


Induced Electromotive Forces - EMFs

Magnetic Flux – A relative measure of the number of field lines passing through an area

F = B A cosq

Axis of Rotation

F => Magnetic Flux (Tm2 = wb (weber))

B => Magnetic Field (T =Tesla)

A => Area Vector (m2)

q => Angle between A and B












I & II If B and A are parallel, q = 0o or 180o then the Magnetic Flux is at maximum.

F = B A cos0 = B A=> maximum # of lines through the loop

III If B and A are perpendicular, q = 90o than the Magnetic Flux is at minimum or zero.

F = B A cos 90 = 0=> No lines pass through the loop

IV If B and A are between parallel and perpendicular then there is a partial Magnetic Flux.

F = B A cosq => maximum # of lines > F > 0


Faradays Law of Induction

The EMF induced in a coil of N loops depends on the time rate change of the number of filed lines through the loop.

Time Rate Change of the Magnetic Flux

x = -N DF/Dt

x => Electromotive Force (v)

N => Number of loops in the wire

DF => Change in the flux through one loop (Tm2 = wb)

Dt => Time (s)


Faradays Law of Induction

The induced electromotive force (EMF) in any closed circuit is equal to the time rate change of the magnetic flux through the circuit.

Or alternatively,

The EMF generated is proportional to the rate of change of the magnetic flux.


Faradays Law of Induction

The induced electromotive force (EMF) is not actually a force but a measure of potential difference. It is measured in volts (V)



  • Generator: converts mechanical energy into electrical energy (AC current)
  • A loop of wire is rotated between the poles of a magnet by a power source (in this case by the water) and the loop moves through the field of the magnet
  • Thus there is a change in the magnetic field resulting in an induced current through the wire


A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.


Step-Up Transformers

  • If there are more loops in the secondary coil the voltage in the second coil is greater
  • This increases the voltage, making it a step-up transformer
  • Used by power companies to transmit high-voltage electricity as well as fluorescent light and X-rays

Step-Down Transformers

  • In a step-down transformer, there are more loops in the first coil than the second coil
  • The voltage in the second coil is less than the first coil
  • Used to lower the voltage of electricity before it can be used in homes or offices as well as doorbells, small radios, and calculators

Electric Motor

An electric motor, is a machine which converts electrical energy into mechanical (rotational or kinetic) energy.   

A current is passed through a loop which is immersed in a magnetic field. A force exists on the top leg of the loop which pulls the loop, while a force on the bottom leg of the loop pushes the loop.

The net effect of these forces is to rotate the loop.


DC Motor

DC motors are in many ways the simples electric motors. All DC "brushed" motors operate in the same way. There is a stator (a larger stationary part) and a rotor (a smaller part spinning on an axis within the stator). There are magnets on the stator and a coil on the rotor which is magnetically charged by supplying current to it. Brushes are responsible for transferring current from the stationary DC voltage source to the spinning rotor. Depending on the position of the rotor its magnetic charge will change and produce motion in the motor. The animation below further explains the basic operation of a DC motor. Utilizing a DC power source, very few controls are needed. To control speed an inline variable resistance can be utilized to change the amount of current reaching the coils.

The animation to the shows a DC motor in operation. The motor shown is a simplified "two-pole" motor which uses just two magnets in the stator. In this case the magnets in the stator are permanent magnets for the sake of simplicity. The brushes deliver current from a DC voltage source which supplies a magnetic field to that end of the rotor. The polarity of the field depends on the flow of the current. As the rotor turns the brushes make contact with one side of the DC source, then briefly do not make contact with anything, then continue making contact with the other side of the DC source effectively changing the polarity of the rotor. The timing of this change is determined by the geometrical setup of the brushes and leads to the DC source. The animation helps to illustrate how at the moment of maximum attraction the current will change direction and thus change the polarity of the rotor. At this moment the maximum attraction suddenly shifts to maximum repulsion which puts a torque on the rotor's shaft and causes the motor to spin.


Top Ten List

1. Magnets make modern life possible.

2. There are North Poles and South Poles.

3. Like poles repel, unlike poles attract.

4. Magnetic forces attract or repel only magnetic materials.

5. Magnetic forces act at a distance.

6. While magnetized, temporary magnets act like permanent magnets.

7. A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving across a field it experiences a force perpendicular to both the field and the direction of motion.

8. A current-carrying wire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field.

9. Magnetic Flux is the relative number of magnetic field lines passing through an area.

10. The EMF induced in a coil of N loops depends on the time rate change of the number of filed lines through the loop.