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Current Electricity Cheat Sheet by

current electricity cheat sheet

Basic notations

current (I)
terminal voltage
resistance (R)
V/I or ρ l/A
in the formula for resist­ance, the first formula has current and the second one has length of the resistor

Effect of stretching of wire on resistance

L -- original length of wire
A -- original cross-­sec­tional area
L' -- length of wire after stretching
A' -- cross-­sec­tional area of wire after stretching

final resist­ance, R' = n2R

Resistors in Series

V = IR(s) ......[­R(s) is equivalent resistance for resistors in series]

general formula for 'n' resistors in series --
R(s) = R(1) + R(2) + R(3) + ......... + R(n)
∴ R(s) = n R
n --- no. of equal resistors

Resistors in Parallel

I = V/R(p) .....[R(p) is equivalent resistance for resistors in parallel]

general formula for 'n' resistors in parallel --
1/R(p) = 1/R(1) + 1/R(2) + 1/R(3) + .... + 1/R(n)
∴ R(p) = R/n

Kirchoff's Laws

1st Law :-
At any junction of several circuit elements, the sum of currents entering the junction must be equal to sum of currents leaving it.

2nd Law:-
The algebraic sum of changes in potential around any closed resistor loop must be zero.

Relation b/w R(s) & R(p)

R(s)/R(p) = n2

Wheatstone Bridge

Here if,
P/R = Q/S or
P/Q = R/S, then,
we can simply remove G.
Now, P & Q are in series and R & S are in series, and the combin­ation of the two are in parallel


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