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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 : M403 Topic : Basic Concepts & ElectroMagnetic 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 ElectroMotive 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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ElectroMotive 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 semiconducting & 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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Limitations of Ohm’s Law
Ohm’s Law cannot be applied to the Nonlinear devices such as Vacuum Devices Gas filled Devices Thermion Devices SemiConductor 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
3. Ohm’s law is applicable for (a) linear devices (circuits) only (b) nonlinear devices only (c) both linear & nonlinear circuits .
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Quiz
4. Ohm’s is applicable to (a) Electrolytes (b) Arc lamp (c) Semiconductors (d) Linear bilateral circuits
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Quiz
5. Diode is considered as (a) Linear element (b) Nonlinear 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.