Inductance and resistance question help

Hi folks I'm new to this I was wondering if someone can help me with these questions I've answered them but I would like one of you expert folks to tell me if they are correct or where I'm going wrong Or if I have the correct formulas its just a question for an exam revision which I have soon.

A coil of inductance 4H and resistance of 80Ω is connected in parallel with a 200Ω of resistor of negligible inductance across a 200v dc supply. The switch connecting these to the supply is then opened; the coil and resistor remain connected together. State in each case one for immediately before and one for immediately after opening the switch;

The current Through the resistor

Switch closed
200v/200Ω =1a
Switch open
200v/80Ω =2.5a

The current through the coil

Switch Closed
200v/80Ω = 2.5a
Switch open
200v/(200ohm +80Ω) = 0.714a


The e.m.f induced in the coil

Switch closed
(2.5a*80Ω)/200v = 0v
Switch Open
2.5a *(200Ω + 80Ω) = 700v

The voltage across the coil

Switch closed
200v/1a = 200v
Switch open
2.5a * 200Ω = 500v

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gmoon4 years ago
Since it's DC, I assume you're trying to gauge inrush/spike information when you switch on/off.

Simulate it with LTspice. I doubt you'll get more than an approximation, especially since you haven't specified the load. There are more variables at work dealing with inductors than other passive components.

Also, Duncan Amps has a nice power supply simulator, which includes chokes, etc. It's called PSUD2.
There isn't a load. The question invites him to find the current in the inductor AFAIK.
Ah. That 4 Henry inductor is approx what I'd expect a choke in a practical application, like a PS, to be...
The transient response for this follows the classic exponential form. I don't see you using exponentials at all...
Andrew187 (author)  steveastrouk4 years ago
So where have i gone wrong? I know I haven't used exponential formula now, but I really don't know how to tackled this question I desperately need help. to understand it.
Your profile doesn't indicate your age or ability. Its hard to scale the answer to your knowledge base under those circumstances. I could talk first order differential equations, or Laplace transforms, what works ?