Dr.Baba Basha

General Physics I.Electric field

By electric charge.Magnetic field

By moving e. charge·Light

By accelerating charge

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Objectives of the topic:

• Introduction of Electric charges• Static Electricity• Force between electric charges• Coulomb’s law• Insulators and conductors

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Electricity & Magnetism – Lecture 1Electric Charge

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Electricity in Nature

• Most dramatic natural electrical phenomenon is lightning.

• Static electricity (balloons, comb & paper, shock from a door knob)

• Uses—photocopying, ink-jet printing

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Static electricity

• The electricity developed on objects , when they are rubbed together, is known as frictional electricity.

• The frictional electricity is also called static electricity

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Static Charge

. How can I demonstrate static charge using an inflated balloon?

A. Pop it. The sound it makes is due to static charge.B. Rub it on cloth, rug, or hair, then it will stick to a wall.C. Rub it on a metal surface, then use it to pick up bits of

paper.D. Drop it and time its fall. If it falls slower than a rock, it

is affected by static charge.E. Let the air out slowly. It will be larger than its original

size due to static charge.

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Charges

• There are two kinds of charges.• The electric charge developed on glass when

rubbed with silk is different from the charge developed on ebonite rod when rubbed with fur.

• The algebraic signs, positive and negative , were first given by Benjamin Franklin.

• Like charges repel and unlike charges attract each other

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ELECTRIC CHARGES

• Two types of electric charges:• Protons: positive, +e• Electrons: negative, -e• The unit of an electric charge is Coulomb or

C.• Basic charge: e = 1.60 x10-19C

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Electric charges

• Every neutral object contains an equal number of electrons and protons.

• When the number of electrons and protons are not equal, the object possesses a net negative or positive charge.

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Demonstrations of Electrostatics

• Balloon• Glass rod/silk• Plastic rod/fur• Electroscope• Van de Graaf Generator

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Glass Rod/Plastic Rod• A glass rod rubbed with silk gets a positive charge.• A plastic rod rubbed with fur gets a negative charge.• Suspend a charged glass rod from a thread, and

another charged glass rod repels it.• A charged plastic rod, however, attracts it.• This mysterious force is called the electric force.• Many similar experiments of all kinds led Benjamin

Franklin (around 1750) to the conclusion that there are two types of charge, which he called positive and negative.

• He also discovered that charge was not created by rubbing, but rather the charge is transferred from the rubbing material to the rubbed object, or vice versa.

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Forces Between Charges

• We observe thatLike charges repel each other

Opposite charges attract each other

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The Atom• We now know that all atoms are made of positive charges in the nucleus,

surrounded by a cloud of tiny electrons.

Electron

cProton

Neutron

More accurate picture of the atom—the Helium atom

Proton charge +e, electron charge -ewhere e = 1.60210-19 C

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The Atom• We now know that all atoms are made of positive charges in the nucleus,

surrounded by a cloud of tiny electrons.

Neutron

Proton

ElectronAtoms are normally neutral,

meaning that they have exactly the same number of protons as they do

electrons.The charges balance, and the atom

has no net charge.

2. Which type of charge is easiest to

remove from an atom?A.Proton

B.ElectronProton charge +e, electron charge -ewhere e = 1.60210-19 C

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The Atom In fact, protons are VASTLY more difficult to remove, and for all

practical purposes it NEVER happens except in radioactive materials. In this course, we will ignore this case. Only electrons can be removed

3.If we remove an electron, what is the net charge on the atom?A.PositiveB.Negative

Proton charge +e, electron charge -ewhere e = 1.60210-19 C

If we cannot remove a proton, how do we ever make something charged negatively? By adding an “extra” electron.

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Glass Rod/Plastic Rod Again We can now interpret what is happening

with the glass/plastic rod experiments. Glass happens to lose electrons easily,

and silk grabs them away from the glass atoms, so after rubbing the glass becomes positively charged and the silk becomes negatively charged.

Plastic has the opposite tendency. It easily grabs electrons from the fur, so that it becomes negatively charged while the fur becomes positively charged.

The ability to gain or lose electrons through rubbing is called Triboelectricity.

Tribo means rubbing

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Insulators and Conductors Both insulators and conductors can be

charged. The difference is that

On an insulator charges are not able to move from place to place. If you charge an insulator, you are typically depositing (or removing) charges only from the surface, and they will stay where you put them.

On a conductor, charges can freely move. If you try to place charge on a conductor, it will quickly spread over the entire conductor.

Insulator

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Metals and Conduction Notice that metals are not only good electrical conductors, but

they are also good heat conductors, tend to be shiny (if polished), and are malleable (can be bent or shaped).

These are all properties that come from the ability of electrons to move easily.

This iron atom (26 protons, 26 electrons) has two electrons in its outer shell, which can move from one iron atom to the next in a metal.

Path of electron

in a metal

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Van de Graaf Generator Rubber band steals electrons from

glass

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Electric Force and Coulomb’s Law We can measure the force of attraction or repulsion between charges,

call them q1 and q2 (we will use the symbol q or Q for charge).

When we do that, we find that the force is proportional to the each of the charges, is inversely proportional to the distance between them, and is directed along the line between them (along r).

In symbols, the magnitude of the force is where k is some constant of proportionality.

This force law was first studied by Coulomb in 1785, and is called Coulomb’s Law. The constant k = 8.98755109 N m2/C2 is the Coulomb constant.

221

rqq

kF

r

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Electric Force and Coulomb’s Law Although we can write down a vector form for the force, it is easier to simply

use the equation for the magnitude, and just use the “like charges repel, opposites attract” rule to figure out the direction of the force.

Note that the form for Coulomb’s Law is exactly the same as for gravitational force between two masses

Note BIG difference, There is only one “sign”

of mass, only attraction.

Note also that the mass is an intrinsic property of matter. Likewise, charge is also an intrinsic property. We only know it exists, and can learn its properties, because of the force it exerts. Full form of Coulomb’s Law Because it makes other equations easier to write, Coulomb’s constant is

actually written

where e0 = 8.8510-12 C2/N-m2 is called the permittivity constant.

qmkG

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Fe

041e

k

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Spherical Conductors Because it is conducting, charge on a metal sphere will go

everywhere over the surface. You can easily see why, because each of the charges pushes on

the others so that they all move apart as far as they can go. Because of the symmetry of the situation, they spread themselves out uniformly.

There is a theorem that applies to this case, called the shell theorem, that states that the sphere will act as if all of the charge were concentrated at the center.

Note, forces are equal and opposite

These two situations are the same

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Summary• Charge is an intrinsic property of matter.• Charge comes in two opposite senses, positive and negative.• Mobil charges we will usually deal with are electrons, which can be removed

from an atom to make positive charge, or added to an atom to make negative charge. A positively charged atom or molecule can also be mobil.

• There is a smallest unit of charge, e, which is e = 1.60210-19 C. Charge can only come in units of e, so charge is quantized. The unit of charge is the Coulomb.

• Charge is conserved. Charge can be destroyed only in pairs (+e and –e can annihilate each other). Otherwise, it can only be moved from place to place.

• Like charges repel, opposite charges attract.• The electric force is give by Coulomb’s Law:

• Materials can be either conductors or insulators.• Conductors and insulators can both be charged by adding charge, but charge can

also be induced.• Spherical conductors act as if all of the charge on their surface were

concentrated at their centers.

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