Ohms law basic concepts

2. Ohm’s Law
One of the most basic and important laws of
electrical circuits is Ohm’s law.Ohms law establishes these relationship
between voltage (V) , Current (I) and Resistance (R)
Definition:-
Ohms states that, the current in a conductor is directly
proportional to voltage and inversely proportional to its resistance at
constant temperature.
Ohm’s law formula
I = V / R
where I=Current in amperes
V=applied voltage in volts
R=resistance in ohms
Formula variations of ohm’s law
1. I = V/R for finding current
2. R = V/I for finding resistance
3. V= I/R for finding voltage

3. Formula memorisation

4. Example with solution
• eg. If circuit is having 48 Ω resistance and 6V
is applied to it, what is the current in that
circuit.
• here V = 6V & R = 48 Ω
• I = V / R = 6 V / 48 Ω = 1 / 8 = .125 A

5. Resistance in Series & Parallel
• Two or more resistors are said to be
connected in series when the same amount
of current flows through all the resistors.

6. Resistance’s in Series
When resistance’s are connected end to end (as shown in
figure) They are in series.
R
V
Rn
R3
R2
R1
Vn
V3
V2
V1
When resistance’s are in series the characteristics are:-
Total resistance the circuit equals the sum of individual resistance
i.e. total resistance ( R ) = R1 + R2 + R3 + RN …….
Current through all resistance is same.
Sum of voltage drops across all individual resistance
(IR drop i.e. I x R1, I x R2, I x R3) equals the applied voltage
(V = IR1 + IR2 + IR3 + IR n)

7. Resistance in Parrallel
When two or more resistance are side by side to a single
source, they are in parralel as shown in figure.
R1
R2
R3
8Ω
12Ω
24Ω
12V
+ ve
_ ve
Resistors in Parallel
1/RT = 1/R1 + 1/R2 + 1/R3 + . . .

8. In parallel the characteristic:-
The reciprocal of net resistance (I/R) equals
Sum of reciprocals of individual resistance (1/R1 + 1/R2 + 1/R3)
Hence in given example, 1/R = 1/R1 + 1/R2 + 1/R3
= 1/8 + 1/12 + 1/24 = 6/24
 R = 24/6 = 4Ω
Voltage across each branch is same
Total current equals sum of branch currents. In other words the total current
is distributed in all breaches as each breach provides path to the current.
For the given circuit, total current = Total voltage / Total resistance = (V / R)
= 12 V / 4Ω = 3 Amp
This 3 Amp current is divided as :-
(Hence V is common:-)
I in R1 = I (R1) = V / R1 = 12V / 8Ω = 1.50 Amp. = ( 1 ½ A)
I in R2 = I (R2) = V / R2 = 12V / 12Ω = 1 Amp.
I in R3 = I (R3) = V / R3 = 12V / 24Ω = .5 Amp.
Total current = I R1 + IR2 + IR3
= 1.5 + .5 + 1A = 3A

9. Various Parallel Resistor Networks

10. This much quicker product-over-sum
method of calculating two resistor in
parallel, either having equal or unequal
values is given as:

11. Series and Parallel resistance in same circuit
In such conditions the problem should be worked out using both methods
used for parallel and series circuits. First the equivalent resistance of parallel
combinations is calculated and added with the series
Work out in given circuit:-
R1
R2
R3
8Ω
12Ω
24Ω
24 V
+ ve _ ve
R4
R5
4Ω
4Ω
R6
2Ω
R7
8Ω