6.2 Rules for Resistance and Power

This is the second week of Module 6 in the Navy Basic Electricity and Electronics series, a module
on parallel circuits. The first section was:

6.1 Rules for Voltage and Current

This second section now addresses resistance and power in parallel circuits.

1. In a series circuit, the total resistance is the sum of the individual resistances. In a parallel circuit, the total resistance will actually be lower than the lowest resistance in any of the branches. The total resistance is sometimes called the "equivalent resistance," as if you were to replace all the individual resistors with one.

When you add a resistor to a series circuit it increases the total resistance. When you add a resistor in parallel, the total resistance goes down. Accordingly, total current increases.

2. To find the resistance in any one of the branches, use the formula R=E/I for that branch. There are a number of other ways:

a. When each branch has the same amount of resistance, divide that number by the number of branches and you will have the total resistance (equal branch method).

b. When the branches have different resistance values, another method is the product over the sum: R₁ * R₂/R₁ + R₂.

c. Another method is the reciprocal method. You take the reciprocal of the sum of the reciprocals of all the resistances.

 3. Power dissipation is the same whether the circuit is series or parallel. The three power equations from the previous module were P = EI, P = I²R, and P = E²/R. Because power is the total heat loss, add up the power used by all the resistors.