Electrical calculations
Design & Technology by BBC Bitesizes
Electronic calculations
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 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 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
Questions
1.
The symbol I in electronic calculations represents:
2.
What unit is resistance measured in?
3.0.00001 farads
3.
The current passing though a resistor is 20 milliamps. The value of the resistor is 100 ohms. Calculate the potential difference across the resistor.
4.
Two resistors, R1 and R2, are in series. R1 has a value of 330 ohms and R2 has a value of 470 ohms. Calculate the combined resistance.