Resistance and the ohm
How much a wire resists the flow of charge.
Two wires, one cell
Wire A and wire B are joined to the same cell, one at a time, at the same temperature. The current in A is 4 A and the current in B is 1 A.
Which wire resists the flow of charge more?
What this lesson covers
The idea
Resistance R = V/I is a conductor's property of resisting the flow of charges, constant for a given wire at a given temperature; its SI unit is the ohm (Ω), with 1 Ω = 1 V/1 A.
Two wires, one cell
Wire A and wire B are joined to the same cell, one at a time, at the same temperature. The current in A is 4 A and the current in B is 1 A.
Which wire resists the flow of charge more?
- Wire B, because less current flows through it
- Wire A, because more current flows through it
- Both resist equally, because the cell is the same
Read the meters
Pick a wire and a potential difference. Look at the ratio V ÷ I for each wire.
A fixed ratio
For each wire, V ÷ I did not change when V changed. A wire that let less current through at the same V had a bigger ratio. That ratio is the resistance.
Resistance R = V/I is a conductor's property of resisting the flow of charges. It is constant for a given wire at a given temperature. Its SI unit is the ohm (Ω): 1 ohm = 1 volt per 1 ampere.
Notes
Resistance R = V/I is the property of a conductor to resist the flow of charges; it is constant for a given wire at a given temperature. Its SI unit is the ohm (Ω), 1 Ω = 1 V per 1 A.
Check yourself
The potential difference across a conductor is 12 V and the current through it is 3 A. What is its resistance, in Ω?
Answer: 4 Ω
R = V/I = 12 V divided by 3 A = 4 Ω.
A resistor of 5 Ω has a potential difference of 10 V across it. What current flows through it, in A?
Answer: 2 A
I = V/R = 10 V divided by 5 Ω = 2 A.
What is a resistance of 1 ohm?
For a given wire at a given temperature, the potential difference across it is doubled. What happens to V divided by I?
- When 1 A of current gives 1 J of work. That mixes up units. One ohm is the resistance when 1 V across the conductor sends 1 A through it.
- When 1 V across the conductor gives a current of 1 A — correct. Yes!
- When 1 C of charge flows each second. That is the definition of 1 ampere, a unit of current, not of resistance.
- It doubles. The current also doubles, so the ratio V ÷ I does not change. It is constant for the wire.
- It halves. The current doubles when V doubles, so V ÷ I stays the same.
- It stays the same — correct. Yes!