Definitions, Laws, Formulas, etc.

1. 1

2. Ohm's law defines a linear relationship between the voltage and the current in an electrical circuit.
The resistor's voltage drop and resistance set the DC current flow through the resistor.
With water flow analogy we can imagine the electric current as water current through pipe, the resistor as a thin pipe
that limits the water flow, the voltage as height difference of the water that enables the water flow.
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3. The resistor's current I in amps (A) is equal to the resistor's voltage VR=V in volts (V) divided by the resistance R in
ohms (Ω):
I =
V
R
V is the voltage drop of the resistor, measured in Volts (V). In some cases Ohm's law uses the letter E to represent
voltage.E denotes electromotive force.
I is the electrical current flowing through the resistor, measured in Amperes (A) R is the resistance of the resistor,
measured in Ohms (Ω)
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4. When we know the current and resistance, we can calculate the voltage.
The voltage V in volts (V) is equal to the to the current I in amps (A) times the resistance R in ohms (Ω):
V = I R
When we know the voltage and the current, we can calculate the resistance.
The resistance R in ohms (Ω) is equal to the voltage V in volts (V) divided by the current I in amps (A):
V
I
R =
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5. It is obvious from this relation that :
I. the current is directly proportional to potential difference, and
II. the current is inversely proportional to resistance.
Since the current is directly proportional to the potential difference applied across the ends of a conductor, it means
that if the potential difference across the ends of a conductor is doubled, the current flowing through it also gets
doubled, and if the potential difference is halved, the current also gets halved . On the other hand, the current is
inversely proportional to resistance. So, if the current is doubled the current gets halved, and if the resistance is halved,
the current gets doubled. Thus, the strength of an electric current in a given conductor depends on two factors :
I. potential difference across the ends of the conductor, and
II. resistance of the conductor.
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6. The electric current is a flow of electrons through a conductor. When the electrons move from one of the conductor to
the other part, they collide with other electrons and with the atoms and ions present in the body of the conductor. Due
to these collisions, there is some obstructions or oppositions to the flow of electron current through the conductor. The
property of a conductor due to which it opposes the flow of current through it is resistance. The resistance of a conductor
is numerically equal to the ratio of potential difference across its ends to the current flowing through it.
The resistance of a conductor depends on length, thickness, nature of the material and temperature, of the conductor.
A long wire (or a conductor) has more resistance and a short wire has less resistance. Again, a thick wire has less
resistance whereas a thin wire has more resistance. Rise in temperature of a wire (or a conductor) increases its
temperature.
The SI unit of resistance is Ohm which is denoted by a Greek letter omega, Ω. The unit of resistance is ohm, can be
defined by using Ohm's Law as described on the next page.
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7. 7
According to Ohm’s Law :
Potential difference
Current = Resistance (a constant)
V
I
That is, = R
So, resistance, V
I
R =
Now, if the potential difference V is 1 volt and the current I is 1 ampere, then resistance R in the above equation becomes
1 ohm.
1 ohm =
1 volt
1 ampere
That is,

8. This gives us the following definition for ohm : 1 ohm is the resistance of a conductor such that when a potential
difference of 1 volt is applied to its ends, a current of 1 ampere flows through it. We can find out the resistance of a
conductor by using Ohm’s Law equation :
8
V = R
I

9. Georg Simon Ohm, (born March 16, 1789, Erlangen, Bavaria [Germany]—died July 6, 1854, Munich), German
physicist who discovered the law, named after him, which states that the current flow through a conductor is directly
proportional to the potential difference (voltage) and inversely proportional to the resistance.
Ohm became professor of mathematics at the Jesuits’ College at Cologne in 1817. The most important aspect of Ohm’s
law is summarized in his pamphlet Die galvanische Kette, mathematisch bearbeitet (1827; The Galvanic Circuit
Investigated Mathematically ). While his work greatly influenced the theory and applications of current electricity, it
was so coldly received that Ohm resigned his post at Cologne. He accepted a position at the Polytechnic School of
Nürnberg in 1833. Finally his work began to be recognized; in 1841 he was awarded the Copley Medal of the Royal
Society of London and was made a foreign member a year later. The ohm, the physical unit measuring electrical
resistance, also was named for him.
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11. 11
Made By :
Tapan S. Panchal
Class : X
Roll No. 1042
“Physics Project Work for Summer Vacation”
Guided By :
Mr. Pawan Kumar,
Physics Teacher
Source :
NCERT Science Textbook
S. Chand Physics
References :
http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.html
http://www.britannica.com/EBchecked/topic/426058/Georg-Simon-Ohm