DEPARTMENT OF TECHNICAL EDUCATION TELANGANA

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DEPARTMENT OF TECHNICAL EDUCATION TELANGANA

Name : K.SRINIVAS Designation : Lecturer Branch : Electrical & Electronics Engineering Institution : Govt Polytechnic Kosgi . Year / Semester : II Year ( IV Semester ) Subject : Electrical Engineering & Basic Electronics Subject Code : M-403 Topic : Basic Concepts & Electro-Magnetic Induction Duration : 50 Minutes Sub Topic : Ohm’s Law Teaching Aids : Animations & Figures

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Recap

In last class we have discussed about , Matter Atomic structure Valance electrons Classification of materials

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Objectives

Electric Potential Electro-Motive Force Voltage & Volt Current & Ampere Ohm’s Law Limitations of Ohm’s Law

On completion of this topic, you would be able to know,

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Types of charge

Negative charge Excess of electrons gives negative charge to a material. Positive charge Deficit of electrons gives positive charge to a material.

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Electric Potential

Generally all atoms are electrically neutral. But due to some external Force, some atoms may gain or loose electrons & become excited atoms. The excited atoms will always try to neutralise their charges. contd...

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Thus the excited atoms will create an electric force to move the electrons from one place to other in a conducting material. It means ions will create an electrical pressure.

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Electric Potential

The ability of a charged body to do the work is called electric potential. Electric potential is expressed in Volts or J/C.

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Potential Difference

The difference in the electric potentials of two charged bodies is called as potential difference. Potential difference = A – B = 10 - 4 = 6 V.

10 V

4 V

A

B

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Voltage

Voltage is the work done per unit charge. Voltage = Work done / Charge J/C Where work done is expressed in Joules Charge is expressed in coulombs. NB : Greater the J/C on a charged body, greater is the electric potential. contd..

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Due to this potential difference between two charged bodies, there is a continuous flow of current in the electric circuit. The unit of potential difference is Volt indicated by the letter ‘v’.

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Electro-Motive Force (EMF)

The electric pressure which sets the electrons in motion is called as EMF. (OR) The Electrical Force which is required to over come the resistance of a conductor is called the EMF Cells, Batteries, Generators develop an EMF.

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EMF is the primary cause of the flow of electrons Potential difference is produced by the EMF. EMF is the Cause whereas Potential difference is the Effect of EMF. The unit of EMF is Volt.

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Voltage

The EMF of an electric source (or ) the potential difference across a resistor is called the voltage. (OR) Energy (work done) / unit charge is called as Voltage. Voltage is represented by letter V. The unit of Voltage is Volt.

V = W / Q

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Volt

One Volt : “ Electric Potential at a point is 1 Volt if 1 Joule of work is done in bringing a unit +ve charge of 1 Coulomb from infinity to that point.”

1 Volt = 1 J/C

+1C

A

x

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Example : If 70 Joules of energy is available for every 30 Coulombs of charge. What is the Voltage? Sol : Work done (w) = 70 Joules Charge (Q) = 30 Coulombs Voltage (V) = ? V = W/Q = 70 /30 = 2.33 Volts.

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Basic Concept for Current

There are free flow of electrons available in all semi-conducting & Conducting materials. These free electrons move randomly in all directions within the substance in the absence of external force (or) Voltage as in figure(1).

Fig(1)

Fig(1)

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If a certain amount of voltage is applied across the material, all the free electrons move in one direction depending on the polarity of the applied voltage as Show in fig(2)

V

+ -

Flow of electrons

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Electric Current

Flow of free electrons in a closed circuit is in one direction is called electric current. It is denoted by letter I. Current is measured in Amperes.

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Direction of Electric Current

Green arrow head indicates the Electrons flow direction. (i.e., from +ve to -ve terminal of the battery). Red arrows indicates the Conventional current direction. (i.e., from -ve to +ve terminal of the battery). Direction of Electrons is in opposite to the direction of Conventional Current. In circuits, we always represent the current direction In the conventional form.

V

+

-

+

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The unit of current is Ampere (Amp.) I = charge / Time = Q / T C /sec. Current is the Rate of flow of charge.

Q = IT

i = dq / dt

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Ampere

If one Coulomb of charge flows per one second, it is called 1 Ampere. 1 Ampere = 1 Coulomb / 1 second. NB : 1 Coulomb = charge of 6.25 x 10 18 electrons

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Temperature remaining Constant, “ The Current flowing in a circuit is directly proportional to the Potential difference driving it .” i.e, I = k x V where k is constant of proportionality and is equal to I/R Therefore V/ I = Constant = R (or) I=V/R

V = IR

Ohms Law

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V = I R Where, V = Voltage across the conductor (Volts). I = Current flowing through the conductor (Amp). R = Resistance of the conductor (Ohm).

I R V

Conductor

Fig(4)

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Resistance

In Ohm’s law, the constant ‘R’ is called Resistance. Resistance : Resistance is the property of a material which opposes the flow of current through it. Resistance is represented by a letter ‘R’. It is measured in Ohms ( Ώ ). It is denoted by a symbol

resistor

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I

R

V

How to Remember Ohm’s Law Formulae

V

R

I = V/R

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I

R

How to Remember Ohm’s Law Formulae

V = IR

I

V

R = V/I

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How to Remember Ohm’s Law Formulae

V

I

R

V

I

R

V

I

R

V = IR

I = V/R

R = V/I

V = IR

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Limitations of Ohm’s Law

Ohm’s Law cannot be applied to the Non-linear devices such as  Vacuum Devices  Gas filled Devices  Thermion Devices  Semi-Conductor Devices. It cannot be applied to Arcing Devices. contd..

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Ohm’s law cannot be applied to those electrolytes in which gases are liberated on the electrodes. It cannot be applied to those conductors whose temperature changes due to the flow of current through them.

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Summary

In this chapter we studied about, Charge Electric potential Electric motive force Current Ohm’s law Limitations of ohm’s law

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Quiz

The capacity of a charged body to do work is called (a) Potential difference (b) Electric Potential (c) EMF (d) None

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QUIZ 2. 1 Volt = (a) 1 J/C (b) 1 C/J (c) None

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Quiz

3. Ohm’s law is applicable for (a) linear devices (circuits) only (b) non-linear devices only (c) both linear & non-linear circuits .

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Quiz

4. Ohm’s is applicable to (a) Electrolytes (b) Arc lamp (c) Semi-conductors (d) Linear bilateral circuits

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Quiz

5. Diode is considered as (a) Linear element (b) Non-linear element (c) Unilateral element (d) Active element

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Frequently Asked Questions

Define Resistance. Define (i) Current (ii) EMF Define Electric Potential & state its units. Define Potential difference & state its units. State Ohm’s Law. Write the limitations of Ohm’s Law.

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THANK YOU