Variable resistance and rheostat
Changing the current without changing the battery.
A dimmer for a circuit
In many practical cases it is necessary to increase or decrease the electric current in a circuit. The battery in a circuit cannot be changed during the work.
How can the current be changed while the battery stays the same?
What this lesson covers
The idea
A variable resistance regulates the current in a circuit without changing the voltage source; a rheostat is a device that changes the resistance in a circuit.
A dimmer for a circuit
In many practical cases it is necessary to increase or decrease the electric current in a circuit. The battery in a circuit cannot be changed during the work.
How can the current be changed while the battery stays the same?
- By changing the resistance in the circuit
- By changing the colour of the wires
- It cannot be changed, because the battery alone fixes the current
Turn the rheostat
Choose the rheostat setting. The battery stays at 6 V.
A knob for the current
With the battery fixed at 6 V, a bigger resistance gave a smaller current and a smaller resistance gave a bigger current. A component that can be adjusted like this is a variable resistance.
A variable resistance is a component used to regulate the current in a circuit without changing the voltage source. A rheostat is a device that changes the resistance in a circuit.
Notes
A variable resistance regulates the current without changing the voltage source; a rheostat is a device that changes the resistance in a circuit.
Check yourself
What does a rheostat do in a circuit?
A 6 V battery is joined to a rheostat set at 3 Ω and nothing else. What is the current, in A?
Answer: 2 A
I = V/R = 6 V divided by 3 Ω = 2 A.
The rheostat is changed from 6 Ω to 12 Ω while the 6 V battery stays the same. What happens to the current?
Which is the purpose of a variable resistance?
- It changes the resistance in the circuit — correct. Yes!
- It changes the voltage of the battery. A variable resistance regulates the current without changing the voltage source.
- It measures the current. Measuring the current is the job of an ammeter. A rheostat changes the resistance.
- It becomes twice as large. When the resistance doubles, I = V/R halves for the same V.
- It becomes half as large — correct. Yes!
- It stays the same, because the battery did not change. The current depends on the resistance as well. With the same V, a larger R gives a smaller I.
- To change the voltage source without changing the current. It is the other way round. The voltage source stays unchanged while the current is regulated.
- To stop all current from flowing. A variable resistance regulates the current. A break in the circuit is what stops it.
- To regulate the current without changing the voltage source — correct. Yes!