# laplace assignment, lots of problems

Discussion in 'Homework Help' started by ninjaman, Apr 29, 2015.

1. ### ninjaman Thread Starter Member

May 18, 2013
306
1
please ignore this, the file has different formatting. its taken from a web site my tutor sent me.

hello

I have to do an assignment for my hnc electronics. im off college at the moment with depression and anxiety problems. I cant go in and my tutor is being a bit awkward about helping me via distance learning.
I just got this through email

"Multiplying throughout by10−6gives:

Taking Laplace transform:

Now in this example, we are toldVc(0)=1.0.
So

That is:Therefore:
NOTE:

CollectingIterms and subtractingfrom both sides:

Multiply throughout bys:

Solve forI:

Finding the inverse Laplace transform gives us the current at timet:

List of transforms"

and that's it!

nothing else, its on word and I have open office. I think there must be some formatting issues as there is no list of transforms. I have sent off an email to my tutor about this.
im not sure what im required to do. I have never done this before. im not asking for an answer.
I just want to know if this makes sense to anyone else.
he has also asked me to do it slightly differently? I don't know why. he hasn't specified anything.

any help would be great

thanks
simon

Last edited: Apr 29, 2015
2. ### WBahn Moderator

Mar 31, 2012
18,089
4,917
What is 10-6? Is it 4? Is it a reference to an equation? Need some context here.

As for a table of transforms, you should expect him to send to a list with each assignment. There are plenty of tables of Laplace transforms around.

3. ### ninjaman Thread Starter Member

May 18, 2013
306
1
sorry WBahn, it was a copy of a web site that didnt open using open office. i found the site and got the example. its not my assignment just an example. i have no idea how im going to do it. i have no clue with maths
i was looking at multisim though. do you know of a way of displaying or using laplace in multisim.

this is the site with the example and a circuit schematic with values
http://www.intmath.com/laplace-transformation/10-applications.php

i made this circuit on multisim
i then did an analysis on this circuit
i got this with transient analysis

the answer should be 4 mA
this is on the web site
so the graph looks similar but the current is wrong i get 5mA
not sure what is wrong here.
also, does anyone know how to use the arbitrary laplace function in multisim?

thanks
simon

4. ### WBahn Moderator

Mar 31, 2012
18,089
4,917
When you say the answer should be 4 mA, what should be 4 mA and when should it be 4 mA? Do you mean that the initial current should be 4 mA?

If the voltage across the capacitor is initially 1.0V and when you close the switch the voltage at the top of the resistor is 5.0V, what is the voltage across the 1kΩ resistor at that moment? How much current will flow?

You need to set up your simulation so that it is faithful to the initial conditions stated in the problem.

5. ### ninjaman Thread Starter Member

May 18, 2013
306
1
i got this

im not sure how to do it. i have to rearrange it, i got an example and from my example i think it is this

any help would be great!!!

thanks
simon

6. ### ninjaman Thread Starter Member

May 18, 2013
306
1
hello
I want to plot a graph in excel like this
I am not to sure about how to do this.
I have current in A going down one column from 0.001 to 0.004, im thinking about time in the next column but Im not sure what to put for that I cant think at the moment. should it just be the formula above?

7. ### WBahn Moderator

Mar 31, 2012
18,089
4,917
You generally put your x-axis values down one column and the corresponding y-axis values down the column to the right of it. Normally you simply space the x-axis values uniformly over the range of interest using enough values to give an acceptably smooth curve. For a plot like this you can usually figure that 100 rows is a minimum and that 500 to 1000 will give nice results and anything over 1000 is probably overkill. You then use the formula to calculate the y-axis value for a given row using the x-axis value in that row.

8. ### MrAl Distinguished Member

Jun 17, 2014
2,567
521

Hi,

Are you trying to find the Inverse Laplace Transform here?
If so, you would use one of the properties and find the inverse for the reduced form in a table.

The property here is that a constant in the frequency domain is the same as the constant in the time domain, so if you have a function like:
A*F(s)
where A is a true constant and F(s) is your reduced Laplace, the inverse transform is:
A*f(t)
where
f(t)=the Inverse Laplace Transform of F(s)

F(s) for your problem here is 1/(200+s) and A is 5e-3 so we have:
A*F(s)=5e-3*(1/(200+s))

and that is the impulse response for your circuit (response of the circuit to an impulse).

9. ### ninjaman Thread Starter Member

May 18, 2013
306
1
thank you mr AI
im not sure on all the calculus stuff i dont get it in the slightest but thanks for that

10. ### MrAl Distinguished Member

Jun 17, 2014
2,567
521
Hello,

You're welcome, but i wasnt entirely sure what you wanted to do here because the question changed several times.

If you have a 5v power source and a 1k resistor and that 1uf cap, the current at t=0 will be 5/1000=5ma, provided there is no charge on the cap at t=0.

11. ### amilton542 Active Member

Nov 13, 2010
494
64
Stay away from alcohol. I became a full-blown alcholic in a matter of weeks. I've only just recovered and my passion for EE is firing on all four-cylinders again.

You need to talk to someone ASAP, trust me.

12. ### ninjaman Thread Starter Member

May 18, 2013
306
1
thanks for the support amilton542!
im getting the opposite from college at the moment.

I go to a group on a Monday that I find is helping. I think when my course is out of the way I will feel better.