Help understanding current source

[mango21]

Homework Statement

This is a really basic problem, just a closed circuit with a current source and a resistor. We are meant to give the voltage and the current across the resistor in terms of the current from the source and the resistance of the resistor. (From the diagram, express Vr and Ir in terms of I1 and R). I have never encountered a current source before and am having trouble learning to deal with it.

Homework Equations

Ohm's law: V = IR
Kirchoff's Laws: sum of voltages = 0, and current into a node = current out

The Attempt at a Solution

My thought was to say that I1 = Vs/Rs, so Vs = I1/Rs = Vr = Ir/R.
But then I don't know how to solve these for Ir and Vr in terms of I1 and R.




Any help understanding the concept would be greatly appreciated! Thanks :)

 

[berkeman]

Homework Statement

This is a really basic problem, just a closed circuit with a current source and a resistor. We are meant to give the voltage and the current across the resistor in terms of the current from the source and the resistance of the resistor. (From the diagram, express Vr and Ir in terms of I1 and R). I have never encountered a current source before and am having trouble learning to deal with it.

Homework Equations

Ohm's law: V = IR
Kirchoff's Laws: sum of voltages = 0, and current into a node = current out

The Attempt at a Solution

My thought was to say that I1 = Vs/Rs, so Vs = I1/Rs = Vr = Ir/R.
But then I don't know how to solve these for Ir and Vr in terms of I1 and R.




Any help understanding the concept would be greatly appreciated! Thanks :)

Welcome to the PF.

There is nothing tricky here. The current source sets the series current that flows around the circuit. The resistor value is what determines what voltage is dropped across it, just based on Ohm's Law V=IR.

 

[AlephZero]

I have never encountered a current source before and am having trouble learning to deal with it.

You are probably familiar with voltage sources, even if you didn't use the name. You can think of a voltage source as being like a battery. It always has the same voltage across its terminals, but the current depends on the circuit that is connected to it.

A current source is the opposite way round. It always produces the same current, and the voltage across it depends on the rest of the circuit.

If you increase the resistance of a circuit connected to a voltage source, voltage stays the same but the current decreases. If you increase the resistance connected to a current source, the current stays he same but the voltage increases.

Unlike a battery, it doesn't make any sense to have a current source "not connected to anything", because its output current has to go somewhere.

In real life you have to make a current source from a circuit containing transistors or op-amps, so the idea is a bit more abstract than a battery. But current and voltage sources are both very useful ideas in circuit analysis.

Note, all the above is a bit over-simplified. For example both current and voltage sources can produce a variable output that is related to something else in the circuit. But the important thing is to understand the basic idea before making it more complicated.

 

[mango21]

Thanks! So in this case Ir = I1?

 

[berkeman]

Thanks! So in this case Ir = I1?

Yep!