ohm’s law & kirchhoff’s laws

8/4/2019 Ohms law & Kirchhoffs 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.

ohm’s law & kirchhoff’s laws

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