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Selecting the correct components [**component**: *Working part of a product or system.* ] to use in a circuit [**circuit**: *A closed loop through which current flows - from a power source, through a series of components, and back into the power source.* ] depends upon being able to calculate the component values.

The potential difference across a component can be calculated using **Ohm's Law**:

Potential difference (V) = current (I) x resistance (R)

V = I x R

For example, for I = 25 milliamps (mA) and R = 330 ohms, V = 0.025 x 330 = 8.25 volts

Ohm's Law can also be rearranged to identify the current or the resistance:

I = V / R

For example, for V = 9 volts and R = 330 ohms, I = 9 / 330 = 27 mA

R = V / I

For example, for V = 9 volts and I = 18 mA, R = 9 / 0.018 = 500 ohms

The combined resistance [**resistance**: *The degree to which a component impedes the passage of current. Shown by the letter R. The unit of resistance is the ohm.* ] of two resistors in series is the sum of the resistance values of the two resistors in ohms. The formula is:

R total = R1 + R2

- Rtotal = combined resistance value (in ohms)
- R1 = value of resistance in first resistor (in ohms)
- R2 = value of resistance in second resistor (in ohms)

**Example**

- Question
Two resistors with resistance values 1.2 kilo-ohms and 2.2 kilo-ohms [

**kilo-ohms**:*Units of resistance equal to 1000 ohms.*] are connected in series. Determine the total resistance of the network.

- Answer
**R total = R1 + R2**= 1.2 kilo-ohms + 2.2 kilo-ohms = 3.4 kilo-ohms

To find the combined resistance of two resistors in parallel we use the formula:

^{1}/_{R total} = ^{1}/_{R1} + ^{1}/_{R2}

- R total = combined resistance value (ohms)
- R1 = value of resistance in first resistor (ohms)
- R2 = value of resistance in second resistor (ohms).

Alternatively you can use:

R total = ^{R1 x R2}/_{(R1 + R2)}

**Example**

- Question
Two resistors are combined in parallel. If they have values of 100 ohms and 1.2 kilo-ohms, determine the value of their combined resistances.

- Answer
Note that the value of resistances have to be changed so they are both either ohms or kilo-ohms. Ohms have to be used in this example:

R total =

^{R1 x R2}/_{(R1 + R2)}so

R total =

^{100 x 1200}/_{(100 + 1200)}=^{120000}/_{1300}= 92 ohms

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