MagnetismChapter 7

J. C. RoweCourse Instructor

A Brief History Of Electromagnetism

About 2500 yrs ago, Greeks discovered that when Amber resin was rubbed together it attracts leaf pieces

Around the 18th century it was discovered that there are 2 types of electricity; Negative & positive electric currents e.g. rub amber = -ve charge while rub glass = +ve charge

Sir Ben Franklin suggested that all substances possess certain “electric fluids”…

600 BC – The lodestone (Fe3O4) is discovered

Sushruta, ~ 625 BC - 545 BC, Indian surgeon mentioned its use

1175 - First Reference to a Compass

Alexander Neckem an English monk of St. Albans describes the workings of a compass.

1269 - First Detailed Description of a Compass

Petrus Peregrinus de Marincourt, a French Crusader, describes a floating compass and a compass with a pivot point.

1600 - Static Electricity (De Magnete)

W. Gilbert 3 ways to magnetize a steel needle: by touch with a loadstone; by cold drawing in a North-South direction; and by exposure for a long time to the Earth's magnetic field while in a North-South orientation.

1819 with work by Hans Orsted - An understanding of the relationship between electricity and magnetism began its was discovered more or less by accident that an electric current could influence a compass needle

Magnetism Defined1 - phenomenon associated with the motion of

electric charges.

2 - Magnetism is also associated with elementary particles, such as the electron, that have a property called spin.

This motion can take many forms. It can be an electric current in a conductor or charged

particles moving through space, or it can be the motion of an electron in atomic orbit.

Basic Facts:

• Magnet originates from the Greek word “magnetite”

• A magnetite is a naturally magnetic ore.• Natural elements that are strongly attracted

to magnets are called ferromagnetic e.g.. Iron, nickel, cobalt

Are you Curious About:

What is a magnetic field?

The region around a magnetic material or a moving electric charge within which the force of magnetism acts

What is a magnetic force?

This is the attraction or repulsion that arises between electrically charged particles that are in motion. While only electric forces exist among stationary electric charges, both electric and magnetic forces exist among moving electric charge

What is the relationship between magnetism and electricity?

Magnetic properties are due to charges of an element, all electrons in atoms are charges in motion. Therefore all moving electrons can act like tiny magnets. But this isn't true because the magnetic fields produced by individual electrons are very weak but when tiny magnetic fields overlap in proper orientation in areas called magnetic domains, they become a bigger and stronger magnetic field. In natural magnets enough magnetic fields work together to produce a noticeable magnetic properties that we can actually see

• A magnet has 2 poles• South pole or negative pole• North pole or positive pole• Like poles repel while opposite poles

attract• If you break a bar magnet you will

end up 2 magnets

Magnetic Poles

All magnets make a magnetic field. You can not see this field but you can map it by placing a piece of paper over a magnet and pour iron fillings on top. You would something like the picture below.

Earth and Magnetism• The south pole of earths magnetic field is

actually located near the earths geographic north pole.

• The earths magnetic poles change on a daily basis

So • From a magnetic stand point when we head in the

direction of the geographic north, are we headed to the magnetic North or Magnetic South ?

Magnetic field around current carrying wires

Danish physist Hans Christian Oersted discovered the connection between electricity and magnetism. He completed a circuit which caused a deflection of a near by compass

Determining the Direction of the Current by Right Hand Rule:

1. Point the thumb of your hand in the conventional currents direction (into or out of the page) opposite to flow if it is given.

2. Curl fingers in. that is the direction of the magnetic field around the wire

Deduce the Magnetic Field Direction

If you are told electron flow is to the Right of this slide… where is the current direction ?

If you are told the electron flow is towards the ceiling… where is the current flow? Where is the direction of the magnetic flow ? Use the Right Hand Rule…

What if the electron flow is pointing to the ground ?

Solenoidsa coil of wire, usually surrounding a movable iron core, that acts as a magnet when carrying a current: used as an electromagnetic switch or relay

a long, thin loop of wire, which is usually wrapped around a metal and causes a uniform magnetic field to be generated when an electric current is passed through it.

Magnetic Fields Around a Solenoid

A solenoid is a coil around a piece of metal.Ex. A wire around a nail.• Creates a stronger magnetic field• Solenoid acts as a bar magnet (develops

south/north pole)

Right Hand Rule for Solenoids:

1. Find conventional flow direction from the front of 1 of the loops

2. Point your finger in the direction of the conventional current

3. Your fingers will be pointing south4. Curl fingers to show direction of magnetic

field

Magnetic Field Strength• All magnets are a different shape, size, and

strength

The magnitude of the force experienced by a charged object moving perpendicularly through a magnetic field

F=qvB• F is force• Q is particle charge• V is velocity • B is strength of magnetic field

When it is not perpendicularF = qsinX

B=F/qvsinX

HOW TO FIND THE MAGNITUDE OF THE FORCE OF A WIRE IN THE MAGNETIC FIELD:

F= I L B SINX

F = Force I = CurrentL = LengthB = Magnetic FieldX = angle (when force is parallel magnetic force is equal to zero)

Right Hand Rule for Straight Wires:

1. Point thumb in the direction of velocity for positive opposite direction for negative

2. Point index finger to the direction of magnetic field

3. Point middle finger of your right hand in the direction that is perpendicular to both thumb and the index. This gives the direction of force

Extended Right Hand Rule

This is used for Straight Wires

Electromagnetic Induction:

Experiments showed that moving or turning a wire will induce a current. found by Michael Faraday and Joseph Henry. This was one of the biggest discoveries in history.

Magnetic flux:Is the change in current by increasing the number of loops or turning the wire.

Magnetic flux= magnetic field x area where field passes x Cos of angle

Theta angle = angle between the field lines & the surface of the area they pass through

The instantaneous EMF resulting from magnetic induction equals the rate of change of magnetic flux

Or….

Any change in the magnetic environment of a coil of wire will cause a voltage (EMF) to be "induced" in the coil. .. This induced EMF is = rate of change of flux

Faraday’s Law

Faradays law:

Lenz’s Law:

An induced EMF (voltage) gives rise to a current whose magnetic field opposes the change in magnetic flux that produced it.