Resistance is everywhere. It is in all electrical circuits and components. Once you can calculate resistance you can understand circuits and electricity better. It can also help you to calculate current, voltage, and power. The best part is that it is not hard to calculate.
Step 1
Check if you know any two of total current, total voltage, or total power.
- NOTE: It=total current, Vt or E=total voltage, and Pt=total power.
Step 2
If you know total voltage and total current then use the equation, Rt = Vt/It to calculate total resistance.
Step 3
If you know the total power and the total voltage then use the equation Rt = Vt2/Pt to calculate total resistance.
Step 4
If you know total power and total current then use the equation Rt = Pt/It2 to calculate total resistance.
Step 5
If you know the total current or total voltage and the power on all components in the circuit then add all the power values to get Pt.
- Then use either Rt = Vt2/Pt or Rt = Pt/It2 to calculate total resistance.
Step 6
Decide if it is series, parallel, or series-parallel circuit.
Step 7
If it is a series circuit and you know the resistance on all the resistors then you can just add the resistors together to get total resistance.
Step 8
If it is a series circuit then current is the same all through it. Therefore, if you know total voltage or total power then you can use the equations Rt = Vt/It and Rt = Pt/It2 to calculate total resistance.
Step 9
If it is a series circuit then total voltage is all the voltages on the components added together. Therefore if you know total power or total current then you can add all the voltages together to get total voltage then you can use the equations Rt = Vt2/Pt and Rt = Vt/It.
Step 10
If it is a parallel circuit and you know the resistance on all the resistors then you can use the equation Rt = 1/(1/R1+1/R2+...+Rn) to calculate total resistance.
Step 11
In parallel circuits voltage is the same through each component. In the example above voltage through the source is the same as the voltage through R1 and R2. Therefore, if you know total power or total current then you can find total current and use the equations Rt = Vt2/Pt and Rt = Vt/It.
Step 12
If it is a parallel circuit then the current is split throughout the circuit. Every time current confronts two or more attached wires it splits.
Step 13
To add current in a parallel circuit you have to add the current together at the split every time it splits. Then the current added together at the split closest to the source is the total current. Therefore, if you know total power or total voltage then you can find total current and use the equations Rt = Vt/It and Rt = Pt/It2 to calculate total resistance.
Step 14
If it is a series-parallel circuit then you have to use the series equations where the components are in series and the parallel equations where the components are in parallel.
Step 15
If it is a series-parallel circuit and you know the resistance on all the resistors then start with the two resistors the farthest away from the source and add them using either the series method or the parallel method depending on whether they are in parallel or series with each other. Then redraw the circuit as if those two resistors where one. Continue with that method until you have one resistor left. The amount on that resistor will be your total resistance.
Tips
- Current is measured in amps (A).
- Voltage is measured in volts (V).
- Power is measured in watts (W).
Warnings
- Only use the series resistance equation on series circuits and the parallel resistance equation on parallel on circuits.
