Demanding task in the field of electromagnetism

Thread Starter


Joined Jan 3, 2021

''A potentiometer with R = 100 Ohm is connected to an operating voltage of Ub = 10 V. At the center tap, 6V are measured against ground. At time t = 15 ms, an inductance to ground with L = 1 H is connected to the center tap. ''

2.1: Calculate the current Iv that the voltage source must deliver at time t = 0ms.

2.2: At time t = 40ms, calculate the current in the coil IL, its voltage UL and the current that the voltage source must supply.

2.3: Calculate for t-> infinite the current Iv that the voltage source has to deliver.

2.5 After a long time (t-> infinite) the voltage source is suddenly separated from the potentiometer. What happens to the inductance, what voltage is on the coil (calculation!) And how does it work?

My questions:

1.) The center tap probably means the center tap of a trimming potentiometer, right?

2.) Is it always about a real voltage source when talking about an operating voltage? Depending on that, I would have to draw another equivalent circuit diagram.

3.) Does it have an impact on my circuit if the technical current has to flow through my coil first before it goes through? a resistance can flow? So what would happen if I swapped coil and resistor in the circuit in a series connection?

I hope you can help me because I'm very confused about this.

Moderator edit: Moved to Homework Help

Thread Starter


Joined Jan 3, 2021
I found a picture on the internet maybe this can help to explain.

My first idea was a voltage divider. So I use this circuit diagramm for my calculations.


My solutions for:

2.1: Iv = 0.25 A
2.1 UL(0,04s)= 2.01 V and IL(0,04)=0.199 A and the current that the source must supply to that time Iv=50.25 mA
2.3: Iv= 0
2.5 The process ist self indcution.


Thread Starter


Joined Jan 3, 2021
I also dont believe that Iv=0.25 A is not correct. It should be Iv=0.1 A. But then I cant understand the sense of the information that 6V is messured against the ground at the time t=0ms.

But how I'm arrive Iv=0.25 A:

First I calculated R1, that must be R1=40 Ohm. And then Iv= Ub/R1=10V/40 Ohm = Iv = 0.25 A. Because the current which is on R1, muss be the same one that the source supplies.

But please correct me If I'm wrong.
Last edited:

Thread Starter


Joined Jan 3, 2021
Okay but why ? I mean you If you read, the text above.... They messured 6 V against the ground. Otherwise, can you draw it ? Maybe then I can understand what I did wrong.


Joined Oct 2, 2009
R1= 10V
R2= 10V
R1+R2 = 10V


But so Iv must be 0.1 A right ?

How can 10V + 10V = 10V?

Voltage is always a measure of potential difference.

where VA and VB are voltage measurements with respect to some common reference point.