P2.4-11 Consider the circuit shown in Figure P2.4-11.  (a) Suppose the current source supplies 3.125 W of power. Determine the value of the resistance R.  (b) Suppose instead the resistance is R = 12 W. Determine the value of the power supplied by the current source.

 

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Figure P 2.4-11

 

Remember Current sources provide the current specified and adjust their voltage accordingly.  Voltage sources provide the voltage specified and adjust their current accordingly.  So knowing the power supplied, and the current we can determine the voltage across the current source and then we can determine the voltage across the resistor.

 

Solving for

Note: this must be +2.50 V since the polarity must have – terminal at top and + terminal at the bottom.

So voltages across the three items must add to zero, so

 

 

Again the minus sign is due to proper polarity.  We can now find the resistance from Ohm’s law.

 

If R is 12 Ω, we know the voltage drop across the resistor is found from

 

 

So going around the loop again gives us a voltage across the current source as

 

 

The minus sign means the voltage across the current source is reversed and now has + on top and – on bottom, which means the source is absorbing power and we will get a negative power.

 

 

Now, since the resistance was reduced, less power from voltage source is absorbed by resistor so the remaining power is now absorbed by current source.

 

 

 

 

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This page last updated on December 28, 2018