Forgive the title, I was unsure of how to briefly describe the problem.(adsbygoogle = window.adsbygoogle || []).push({});

Attached are diagrams.

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

The device shown in Figure P2.5 can be modeled as a current source in parallel with a resistance. When this device is connected to a 5[mA] current source, so that 5[mA] enters the device at terminal A and leaves at terminal B, then the device delivers 296[mW] to the 5[mA] current source.

Then, when the same device is disconnected from the current source and is connected to a 10[V] voltage source, so that there is a 10[V] voltage at terminal B with respect to terminal A, then the device absorbs 16.5[mW].

Find the power absorbed by the device when the device is connected to a 10[kΩ] resistor.

2. Relevant equations

v=ir, p=iv, KVL, KCL

3. The attempt at a solution

First with the Current Source [Circuit one]

power.del.by.device = power.abs.by.5ma = .296[W]

p=iv so...

v(5ma current source) = 59.2 [V]

KCL on top node...

-.005 + (59.2/Rx) - ix = 0

Now with the Voltage Source [Circuit two]

power.abs.by.device = power.del.by.voltagesoruce = .0165 [W]

p=iv so...

i(voltage source) = .00165[A]

KCL on top node...

-.00165 + (10/Rx) - ix = 0

Solve the linear equations...

Rx = 14686.57 [Ω]

ix = -.000969

Third condition (resistor)

I forgot to make a diagram for this one. The new resistor has a current flowing upward. Its poles are positive on top and negative below.

power.abs.by.device = power.del.by.10kresistor

p=i^2 (r)

we need i at the resistor...

KCL on top node...

-ix - i(10k resistor) +i(middle resistor) = 0

KVL on left loop

-i(10k resistor) +Rx*i(middle resistor) = ...

-i(10k resistor)*10000+14686.57*i(middle resistor) = ...

-i(10k resistor)*10000+14686.57*(ix + i(10k resistor))= 0 (Solvable)

i(10k resistor) = .003037[A]

p= i^2 (r) = .003037^2 (10000)

p.abs.by.dev = 92.2 [mW]

My main concern is that their is something wrong with my understanding of current flow, when to flip signs etc. I know that in all my circuits I have the current iQ (which i refer to as the current across the introduced component) which goes the opposite direction of what it "should" be going (positive to negative). There are several questions like this on my HW which is another reason I want to be sure I get this one right. If something was left unclear please do ask.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: A device connected to a current source and then resistor

**Physics Forums | Science Articles, Homework Help, Discussion**