# Homework Help: Simple RL question with laplace

1. Oct 21, 2014

### darwinharianto

1. The problem statement, all variables and given/known data
I have a circuit with resistor and inductor (RL) that is placed serial and a power supply.
the voltage is constant 50 V at t=0 , R=10 ohm, and L=0.2 H.
Find the I using laplace transformation
2. Relevant equations

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3. The attempt at a solution
my attempt is transform everything to S-domain and with nodal analysis
so the equation go like this
50/(10+0.2S)=I(s)
so, 250/(S+50)=I(s)
then i transform it to t-domain
I(t)=250 e^(-50t)
at t=0
I(0)=250 e^0
so I(0)= 250
is this right?
i have V at 50 Volt, but the current become 250 A
am i doing it right?

2. Oct 21, 2014

### Staff: Mentor

If you recall how inductors behave, you wouldn't expect the current to instantaneously go from zero to 250 Amps at time t = 0 when the voltage is switched on. So clearly something has gone awry in your workings.

Your voltage source should "turn on" at time t = 0, so it's really 50u(t) volts. In the s-domain that becomes 50/s ;)

3. Oct 21, 2014

### darwinharianto

ooh, so the equation go like this?

I(s)=250/s(s+50)
I(s)=5/s - 5/(s+50)
I(t)=5 - 5 e^ -50 t
then because t=0
then I(0) = 5-5 = 0
so because it haven't been started so the current would be 0
okay, thanks