Video: Calculating the Resistance of a Resistor

A resistor is connected in series with a power supply of 35 V, producing a current in the resistor of 0.25 A. What is the resistance of the resistor?

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Video Transcript

A resistor is connected in series with a power supply of 35 volts, producing a current in the resistor of 0.25 amps. What is the resistance of the resistor?

If we draw a sketch of this circuit, we know it has a resistor, whose value we’ll call 𝑅, in series with the power supply of 35 volts. Along with that, we’re told that a current 𝐼 of 0.25 amps runs through this series circuit.

The question is what is the resistance of this resistor? To figure it out, we can rely on Ohm’s law, which tells us that the potential difference in this circuit is equal to the product of the resistance times the current. We’ll treat this circuit as that the only source of resistance in it is the resistor 𝑅. That is, we’ll assume there’s no resistance in the wires of the circuit and that there’s no internal resistance in the power supply.

Under that assumption and using Ohm’s law in a rearranged form, we can say that the resistance of the resistor is equal to the potential difference 𝑉 divided by the current in the circuit 𝐼. 35 volts divided by 0.25 amps is equal to 140 ohms. That’s the resistance value of the resistor in this circuit.

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