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Selecting the correct componentscomponent: ###TODO### 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 resistanceresistance: The opposition in an electrical component to the flow of electricity through it. Resistance is measured in ohms. 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-ohmskilo-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
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