lecturer

Mr. Bestoon Mustafa

MSc of Nanoscale Science And Technology

Department of Physics

Email: bestoon.taha@tiu.edu.iq

In this semester, you will study: Chapter one: Electric charge and electric field

Chapter two: Electric potential

Chapter three: Capacitance

Chapter four: DC, AC current and resistance

Chapter five: Magnetic field

Chapter six: Electromagnetic field

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Chapter OneElectric charge and electric field

In this chapter you will learn: The nature of electric charge

How objects become Charged electrically

Types of materials in term of conductivity

Coulomb’s law

Electric field lines

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1.1 The nature of electric charge To this point the only one of these forces that we have examined in any

detail is gravity.

Now we are ready to examine the force of electromagnetism, which encompasses both electricity and magnetism.

Electromagnetic interactions involve particles that have a property called electric charge, an attribute that is as fundamental as mass.

These particles as objects with mass are accelerated by gravitational forces,

So electrically charged objects are accelerated by electric forces.

Thus: if a particle is charged electrically called charged particle.

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1.2 Electrostatics The interactions between electric charges that are at rest is called electrostatics.

There are two types of observed electric charge, which we designate as positive(such as protons) and negative charges (such as electrons).

Two positive charges or two negative charges repel each other. A positive charge anda negative charge attract each other.

Plastic rods and fur are particularly good for demonstrating electrostatics.

➢ When two plastic rods are charged by rubbing each of them with a piece of fur, therods will repel each other (figure 1.1a).

➢ When two glass rods are rubbed with a piece of silk, the glass rods also becomecharged and repel each other (figure 1.1b).

➢ But a charged plastic rod attracts a charged glass rod. Furthermore, the plastic rodand the fur attract each other, and the glass rod and the silk attract each other(figure 1.1c).

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fig 1.1 Experiments in electrostatics. (a) Negatively charged objects repel eachother. (b) Positively charged objects repel each other. (c) Positively chargedobjects and negatively charged objects attract each other.

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We can also charge a metal ball using a copper wire and an electrically charged plastic rod, as in Fig. 1.2a.

In this process, some of the excess electrons on the rod are transferred from it to the ball, leaving the rod with a smaller negative charge.

Fig 1. 2 Copper is a good conductor of electricity; nylon is a good insulator. (a) The copper wire conducts charge between the metal ball and the charged plastic rod to charge the ball negatively. Afterward, (b) the metal ball is repelled by a negatively charged plastic rod and (c) attracted to a positively charged glass rod. 7

Images of Static electricity

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1.3 Charging by Induction Induction is a technique in which a material such as plastic rod can

give another body a charge of opposite sign without losing any of itsown charge.

An uncharged metal ball is supported on an insulating stand (Fig. 1.3a).When you bring a negatively charged rod near it, without actuallytouching it (Fig 1.3b), the free electrons in the metal ball are repelled bythe excess electrons on the rod, and they shift toward the right, awayfrom the rod.

We can get excess negative charge at the right surface of the ball and adeficiency of negative charge at the left surface. These excess chargesare called induced charges.

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Continue The system reaches an equilibrium state in which the force

toward the right on an electron, due to the charged rod, is justbalanced by the force toward the left due to the induced charge.

If the plastic rod is nearby, you touch one end of a conductingwire to the right surface of the ball and the other end to the earth(Fig. 1.3c), Some of the negative charge flows through the wire tothe earth.

Now suppose you disconnect the wire (Fig. 1.3d) and thenremove the rod (Fig. 1.3e); a net positive charge is left on the ball.

Note: The charge on the negatively charged rod has notchanged during this process.

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Fig. 1.3 Charging a metal ball by induction

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1.4 Electric Charge and the Structure of Matter

Any material around the universe consists of Molecules.

Molecules are a combination of two or more atoms.

Atoms consist of one nucleus and shells around

the nucleus (figure 1.4).

At the shells the are only electrons.

Inside nucleus there are protons and neutrons.

Electrons are negatively charged,

protons are positively charged and

neutrons are uncharged particles.

The proton and neutron are combinations of

other entities called quarks

Fig 1.4 The structure of an atom 12

➢Note:

. Charge of neutron is zero.

and,Mass of electron (me) =9.1*10^-31 kg

Mass of portion (mp) = Mass of Neutron (mn)= 1.67*10^-27 kg

➢ Principle of conservation of charge:

The algebraic sum of all the electric charges in any closed system is constant. Or,Charge can neither be created nor destroyed. A charge can, however, be transferred from one body to another.

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1.5 Conductors, semiconductors and insulates

According to conductivity, there are three types of materials: conductors, semiconductors and insulators.

Conductors permit the easy movement of charge through them, such as copper.

Insulators do not have ability to transferee free charge carries (electrons) such as wood.

Semiconductors (such as Silicon) are materials that act as insulators at room temperature. However, they can be turned to conductors by adding a motivate in to them.

A motivator can be adding a dopant, affecting by a magnetic field of increasing temperature. 14

Band gap of metals, semiconductors and insulators

Fig. 1.5 Band gap diagram. (CB: Conduction band and VB: Valence band)

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1.6 Coulomb’s Law (detail in 1784)

➢Coulomb established a law called Coulomb’s law:

‘‘The magnitude of the electric force between two pointcharges is directly proportional to the product of thecharges and inversely proportional to the square ofthe distance between them’’.

➢ The magnitude of this force is always positive.

➢ In mathematical terms, the magnitude of the force (F) that each of twopoint charges (q1) and (q2) a distance (r) apart exerts on the other canbe expressed as:

Where k is a proportionality constant

------------ (eq. 1)

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Continue .

𝜀0 is the permittivity of the free space,

The directions of the forces the two charges exert on each other arealways along the line joining them.

When the charges and have the same sign, either both positive or bothnegative, the forces are repulsive; when the charges have opposite signs,the forces are attractive (figure 1.6)

Fig 1.6 direction of coulomb’s forces of (a) similar charges and, (b) opposite charges.

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Problem-Solving Strategy

1. Sketch the locations of the charged particles and label each particle with its charge.

2. If the charges do not all lie on a single line, set up an x,y coordinate system.

3. The problem will ask you to find the electric force on one or more particles. Identify which these are.

4. Calculate the magnitude of that force.

5. If more than one point charges (two or more) affect one charge, the net force on this point charge is the combination of the forces together and the direction of the overall force if going to be due to the largest value of force.

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Example 1

( Figure below),

Solve a)

b)

The attractive force that acts on q1 is to the right, toward q219

Example 2

Solve

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Example 3

Solvethe forces F1 on Q and F2 on Q due tothe identical charges q1 and q2 which are at equal distances from Q. From Coulomb’s law, both forces have the same magnitude of:

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1.7 Electric Field The electric force on a charged body is exerted by the electric

field created by other charged bodies.

E is the force per unit charge exerted by charge A (fig 1.9)

on a test charge at P.

In SI units, the unit of electric field is Newton /CoulombOr N/C

------------ (eq. 2)

------------ (eq. 2)

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Electric Field Electric Field lines

Fig. 1.7 (a) The field produced by apositive point charge points away fromthe charge. (b) The field produced by anegative point charge points toward thecharge.

Fig. 1.9 A charged body creates anelectric field in the space around it.

Fig. 1.8 electric dipole 24

1.8 Electric Forces If the field at a certain point is known, gives the force (F0) experienced

by a point charge q0 placed at that point. This force is just equal to the electric field E produced at that point by charges other than qo, multiplied by the charge q0:

Fig. 1.10

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Example 4 What is the magnitude of the electric field E at a field

point 2.0 m from a point charge q = 4.0 nC?

Solve

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Example 5 A uniform electric field between two parallel conducting plates connected to a 100-volt battery. (The separation of the plates is exaggerated in this figure relative to the dimensions of the plates.) E = 1.00 * 10^4 N/C, (a) If an electron (charge -e = -1.60 *10^-19 C and mass m = 9.11 * 10-31 kg ) is released from rest at the upper plate, what is its acceleration? (b) What speed and kinetic energy does it acquire while travelling 1.0 cm to the lower plate? (c) How long does it take to travel this distance?

Solve

(a)

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Continue example

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