- Home
- > Design & Technology
- > Electronics
- > Electronic calculations
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.
Potential difference
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
Calculating resistance
Resistors in series
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
Resistors in parallel
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
Links
bbc.co.uk navigation
BBC links
BBC © 2013 The BBC is not responsible for the content of external sites. Read more.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.