Circuit Formulae/Derivations

Question 1

What is the formula for current in a series circuit?

Answer 1

I’ = I’’ = I’’’ = It

i.e. Current is the same at any point in a series circuit

Question 2

What is the formula for total resistance in a series circuit?

Answer 2

R1 + R2 + R3 = Rt

i.e. sum of individual resistors = total resistance of circuit

Question 3

What is the formula for the total voltage of a parallel circuit (or branch)?

Answer 3

V1 = V2 = V3 = Vt

Voltage is the same across all paths, but uses more energy when supplying more paths

Question 4

What is the formula for current in a parallel circuit (or branch)?

Answer 4

Current going into junction = Current going out of junction

Question 5

What is the formula for total voltage (emf) in a circuit?

Answer 5

V1 + V2 + V3 = Vt

i.e. the sum of the voltage drops across a series circuit = the total voltage

Question 6

What is the formula for total resistance of a parallel branch?

Answer 6

1/R1 + 1/R2 + 1/R3 = 1/Rt

Remember to invert answer to find total resistance