ohm’s law & kirchhoff’s laws
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Ohms law & Kirchhoffs Laws
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Resistance
It may be defined as the property of a
substance due to which it opposes (or
restricts) the flow of electricity (i.e., electrons)
through it
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The Unit of Resistance
The practical unit of resistance is ohm.** A
conductor is said to have a resistance of one
ohm if it permits one ampere current to flow
through it when one volt is impressed across
its terminals
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Laws of Resistance
The resistance R offered by a conductor
depends on the following factors :
(i) It varies directly as its length, I.
(ii) It varies inversely as the cross-section A of
the conductor.
(iii) It depends on the nature of the material. (iv) It also depends on the temperature of the
conductor.
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Units of Resistivity
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Ohm's Law
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Kirchhoffs Laws
Kirchhoffs laws are basic analytical tools in
order to obtain the solutions of currents and
voltages for any electric circuit; whether it is
supplied from a direct-current system or an
alternating current system.
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Kirchhoffs Current Law (KCL):
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Kirchhoffs Voltage Law (KVL):
It states that in a closed circuit, the algebraic sum of
all source voltages must be equal to the algebraic sum
of all the voltage drops. Voltage drop is encountered
when current flows in an element (resistance or load)from the higher-potential terminal toward the lower
potential terminal. Voltage rise is encountered when
current flows in an element (voltage source) from
lower potential terminal (or negative terminal ofvoltage source) toward the higher potential terminal
(or positive terminal of voltage source).
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Resistance in Series
When some conductors having resistances
R1,R2 and R3etc. are joined end-on-end , they
are said to be connected in series. It can be
proved that the equivalent resistance or total
resistance between pointsA and D is equal to
the sum of the three individual resistances
B i
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Being a
series circuit, it should be
remembered that (i) current is the same through all the threeconductors
(ii) but voltage drop across each is different
due to its different resistance and is given by
Ohm's Law
(iii) sum of the three voltage drops is equal to
the voltage applied across the three
conductors
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Characteristics of series circuit
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Resistance in parallel
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(i) p.d. across all resistances is the same
(ii) current in each resistor is different and is
given by Ohm's Law
(iii) the total current I is the sum of the three
separate currents
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Characteristics of parallel circuits
I. same voltage acts across all parts of the
circuit
2. different resistors have their individual
current.
3. branch currents are additive
4. conductances are additive
5. powers are additive.