Electromagnetism

Lecture 1

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Electricity If one defines electricity, it is a

phenomenon that involves electric charges.

Common examples are static electricity and lightening.

For lightening see next slide. For static electricity there are two ways

to observe, one is voltmeter and other is if you want to personally experience, feel free to do that at your own risk.

Lightening

Explanation The existence of electricity, the

phenomenon associated with stationary or moving electric charges, has been known since the Greeks Discovered that amber rubbed with fur, attracted light objects such as feathers.

Explanation (Continued) Ben Franklin proved the electric nature

of lightning (The Famous Key Experiment) and also established the conventional use of negative and positive types of charges.

Lightning Rod Experiment In 1752, Franklin proposed an

experiment with conductive rods to attract lightning to a Leyden Jar. (An Early form of capacitor)

In Physics and Electrical Engineering, a conductor is an object or type of material which permits the flow of electric charges in one or more direction. For example a wire is a semi conductor that can carry electricity along its length.

A capacitor is a passive two terminal electrical component used to store energy electro-statically in an electric field.

Kite Experiment Franklin Realized the dangers of using

conductive rods. Instead he used a kite. The increased height allowed him to stay

on the ground and the kite was less likely to electrocute him.

According to the Legend, Franklin kept the string on the kite dry at his end while the string was allowed to get wet in the rain to provide conductivity.

A key was attached to the string and connected to a Leyden jar.

Leyden Jar A Leyden Jar is a device that stores

electricity between two electrodes on the inside and outside of a glass jar.

It was originally a form of a capacitor. It consisted of a Glass Vial, partially filled

with sheets of metal foil. The top of which was closed by a cork pierced with wire/ nail.

Kite Experiment (Continued) Franklyn assumed that the jar will would

accumulate electricity from the lighting. The kite wasn’t struck from the visible light

otherwise his nephew would have passed away.

What Franklin noticed was that the strings of the kite were repelling each other and deducted that the Leyden jar was charged.

Franklin reportedly received a mild shock by moving his hands near the key afterwards, because he had estimated, lightning has negatively charged the key and the Leyden jar. Proving the electric nature of the charge.

Controversy In one of the Television shows ‘Myth-

Busters’ the host has tried to emulate a variation on the famous experiment.

"You can get electricity down a [wet] kite string."

Electricity will NOT travel down a dry kite string.

The key will conduct a static charge, even when NOT struck by lightning.

Discussion (Continued) It was also known that certain materials

blocked electric charge. They are called insulators, such as glass

or wood. Other materials that transferred electric

charge with ease, called conductors. Such as metal.

By the 18th century, physicist Charles Coulomb defined the quantity of electricity later known as coulomb and determined the force law between electric charges.

It was known as coulomb law.

Discussion (Continued) Coulomb law is that the electrical forces

are inversely proportional to the distance of the charges squared, and proportional to the product of charges.

By the end of 18th Century, we had determined that electric charge could be stored in a conducting body if it is insulated from its surroundings.

The first device was a Leyden Jar.

Discussion (Continued) Coulomb found that the force F between

one charge q1 and a second charge q2 is equal to the product of the charges divided by the square of the distance r between them, or F = q1 q2 /r

The force can be either attractive or repulsive, because the source of the force, electric charge, exists in two varieties, positive and negative.

The force between opposite charges is attractive, whereas bodies with the same kind of charge experience a repulsive force

Modern Atomic Theory Modern Atomic Theory explains this as

the ability of atoms to either lose or gain an outer electron.

Thus exhibit a net positive charge or gain a net negative charge.

Today we know that the basic quantity of electric charge is the electron and one coulomb is about 6.24 x 10 P 18 Electrons.

The battery was invented in 19th Century.

Electric Fields The mathematical problem of describing

the motion of electric charges leads to the development of the field concept.

The use of fields (as in electric field, gravitational field, force field) is mathematical tool to describe physical events.

The field itself is not observable but its effect is visible.

To Map a Field

Electric Fields (Continued) The shape and resulting strength, of an

electric field is dependent on the number and the position of the electric charges.

Each point is space can be assigned a vector to determine the magnitude and direction of the motion.

See the next slide for diagrammatic representation.

Magnetism Related to electricity, is the phenomenon

of magnetism Magnetism results from the motion of

electric charges, although historically we know of the behavior of magnetic fields from bar magnets

magnetic fields deflect moving charges or other magnets

Magnets have north and south poles, similar to positive and negative charge, but poles can never be separated

Magnetism (Continued) The simplest magnetic field results from

an electric current circulating in a loop bar, or permanent, magnets result from small atomic dipole loops which sum to produce a macroscopic magnetic field

Difference between current and voltage Current is the rate at which electric

charge flows past a point in a circuit. In other words current is the rate of flow

of electric charge. Voltage is the electrical force that would

drive an electric current between two points.

In other words voltage is energy per unit charge.

The Symbol of current is I and Symbol for voltage is V

The unit of current is amps or amperage and for voltage is volts or voltage.

Difference between current and voltage The measuring instrument for current is

Ammeter and for voltage is voltmeter. Current create magnetic field and

voltage create an electrostatic field. Current is the same through all the

components connected in series and distributed if the components are connected in parallel and for voltage is the other way around as compared to the current.