Hi everyone, I was having trouble understanding a problem that I have attached. Work for the problem is in the attachment. The problem asks me to find a voltage across a resistor. While doing the problem, I found two different voltages, one positive and one negative. Why does the formula for power absorbed by a resistor give me two voltage values, when only one voltage value is correct?

Could someone please help me understand this?

 

What are you currently studying in this class? There's a much simpler way to determine Va.

 

What are you currently studying in this class? There's a much simpler way to determine Va.

I am currently studying amplifier models. I just was confused as to why trying to solve for va using the equation for power absorbed by a resistor gives me two voltages.

 

Ohms law is all you need, there is no need to bring power into the picture.

 

I am currently studying amplifier models. I just was confused as to why trying to solve for va using the equation for power absorbed by a resistor gives me two voltages.

Post the full text of the problem. There are many ways to solve a problem like this. I'd use one that doesn't require calculating current.

 

Hi everyone, I was having trouble understanding a problem that I have attached. Work for the problem is in the attachment. The problem asks me to find a voltage across a resistor. While doing the problem, I found two different voltages, one positive and one negative. Why does the formula for power absorbed by a resistor give me two voltage values, when only one voltage value is correct?

Could someone please help me understand this?

The basic answer to your problem is that the power dissipated in a resistor is independent of the polarity of the voltage since if you change the polarity of the voltage across the resistor you also change the polarity of the current through the resistor, hence the sign of the product of the voltage and the current will remain unchanged.

But while the power considerations are consistent with either polarity of the voltage, only one of those choices is consistent with KVL as you go around the circuit. The answer you choose must be consistent with ALL of the constraints that apply to the circuit.

 

I see. I understand now about the constraints not being met with the negative voltage. Thank you all for the help!