Time constant

[sunny1982]

Hi Mr Al and WTP pepper, how are you? I've went away and this is what I've come up with I'm a bit more confident than before. As soon as this question is complete so is the assignment and I will surely post it on here the full assignment just so you can see what you guys have helped me achieve and if I get top marks which is a merit you guys get the credit

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;

a) The current Through the resistor
Immediately before Switch opening
200v/ 80ohm =2.5a
Immediately after Switch opening
200v / 80ohms = 2.5a

b) The current through the coil
Immediately before Switch opening
200v / 80ohm = 2.5a
Immediately after Switch opening
200v / 80 ohm = 2.5a

c) The e.m.f induced in the coil
Immediately before Switch opening
2.5a * 80ohms -(200v) = 0v
Immediately after Switch opening
-2.5a *(200Ω + 80Ω) = -700v

d) The voltage across the coil
Immediately before Switch opening
2.5a * 80 ohm = 200v
Immediately after Switch opening
-2.5a * 200ohms = -500v

Anyway like I said I'm pretty confident on this one but again not 100% lol. So if you do spot something please do tell me.

 

[MrAl]

Hello again,

You're getting much closer now. Sorry to say (d) is not correct yet, but the others look good.
Check part (d) again and see if you can figure this out. If not dont worry, we'll go over it.

Hint for error catching:
When you are solving for voltages, the sum of the voltage drops around a closed loop should equal zero. You can use this to check if you've got the right result because if you sum all your component voltage drops you better come out with zero volts

 

[sunny1982]

I don't quiet understand (d) then Mr Al

 

[MrAl]

Hello again,


Oh wait a minute, that 80 ohms is being considered as part of the coil resistance isnt it? In that case part (d) is correct.
When there is zero resistance then the voltage is zero, but if the coil has resistance then there will still be some voltage drop.
My mistake this time, you did well

 

[sunny1982]

Hi although this won't be the last time you see me lol I just first of all want to give a very big thanks to Mr Al and WTP pepper. In a short amount of time I have learnt so much which has helped me complete my assignment from college. A bit about myself, I understand in life you have to help yourself before anyone else helps you but a major credit goes to you guys for helping me achieve this, I have a passion for engineering and I really want to succeed I have the desire to learn. Ok enough of the sentiment heres my fully complete assignment I'm hoping for full marks