Homework Help: Circuit analsysis problem. Simple

1. Jun 13, 2010

harrisiqbal

1. The problem statement, all variables and given/known data

A device can be modeled using a current source in
parallel with a resistor. This device was connected to
a 5[V] voltage source, as shown in Figure P2.3a), and
a current i5 of 0.93[A] resulted. The same device was
then connected to a 4[A] current source as shown in
Figure P2.3b), and a voltage v4 of –34[V] resulted.
Find the voltage v3 if a 3[Ω] resistor, as shown
in Figure P2.3c), is connected to the same device.

2. Relevant equations

V = iR, KCL, KVL, Sign Conventions

3. The attempt at a solution

[Freaking session expired and I lost whatever I wrote.. writing it again grrrrrr]

http://www.mediafire.com/imageview.php?quickkey=lzuqgmqmitk&thumb=6

I define the device as the following:

-----------------
[ ]
[ ]
Current Source Resistor
[ ]
[ ]
------------------

I connect this device to a 5 Volts voltage source for part a. And Apply KCL to get i2-i1 = 0.93 Amps. i2 is the current going through resistor, and i1 is the current by current source.

I move on to part b, and get equations [ probably the wrong ones ], but now I can't use anything from part a because the values would be different correct?

So I don't really understand how to solve this problem...

Help would be appreciated!

____________________________________________________

***Update***:
Okay I understand it a bit better. For part a I use Kirchhoffs current law to find an expression for the current going through internal resistor. i2 = i1-0.93

I get an expression for Internal resistence: R = (V1 / i2) V1 is the 5 Volt power source in Part a.

Going into part b.

KCL law to get -i2 - i3 = 0 [ i3 is the given current source 4 Amps in part b]

so i2 = -4 amps

Voltage is given and current is i2. Resistence can be calculated to be 8.5 Ohms.

Part c:

Current Divider Rule:

lets me find i2[ which will be different because Resistor was added]

i2= -.396 Amps

V3

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I have another QUICK question unrelated to the above problem.

If a question states that there is a device that is modeled using an ideal voltage source in series with a resistance. And that device is connected to a voltage source in the schematic. Then there will be TWO Voltage sources CORRECT? One inside the device and one that the device is connected to?

Last edited: Jun 13, 2010
2. Jun 14, 2010

Staff: Mentor

Correct. However, the first one you mention sounds like they are modeling a real voltage source as an ideal voltage source in series with its output resistance. The second voltage source might not have been expanded yet into its ideal voltage source and series output resistance, or they may just be assuming its an ideal voltage source. You'll have to figure that out from the context of the problem.