Calculating power dissipated in a circuit HELP!!

[Jake Wesley]

Hi guys. I have been looking on the website 'swindon massive' (very useful for anyone studying at college) and one of the questions in the mock exam for principles is;

"An inductive load with 10 ohms inductive reactance is connected to a 100v 50 Hz AC supply. Calculate the power dissipated in the circuit (ignoring any conductor resistance.)"

The answer came up as 0kW but I am very unsure how this is. I understand that the forums aren't a place to get answers for college work but I would definitely like to know how to work out this question and whether it's very simple.

Thanks guys.

 

[STM]

In an purely inductive circuit the current lags behind the voltage by 90 degrees.

To calculate power dissipated you use the formula P = V x I x cos(angle). Using your calculator, type in Cos(90). This will give you the answer of 0. In a purely inductive or for that matter capacitive circuit the currents lags and leads by 90 degrees which equals (0).

For your question: P = V x I x cos(angle) -which results in- P = 100V x ? x cos(90)
To find the missing current value we know by ohms law I = V/R -so- I - 100V/10ohms = 10A
So, P = 100V x 10A x Cos(90) = 0kW.

As it is purely inductive in your question the angle can be assumed to be 90degrees.

If resistance was present in the question, whether in the form of a resistor or cable resistivity then your answer would change. Due to it being a RL circuit, the resistance would alter the angle of the voltage and current and would no longer lag or lead by 90degrees - but lie somewhere say in the middle.

If the circuit contains resistance, we cannot assume it to be 90degrees and so you can calculate the phasor angle.

 

[Jake Wesley]

In an purely inductive circuit the current lags behind the voltage by 90 degrees.

To calculate power dissipated you use the formula P = V x I x cos(angle). Using your calculator, type in Cos(90). This will give you the answer of 0. In a purely inductive or for that matter capacitive circuit the currents lags and leads by 90 degrees which equals (0).

For your question: P = V x I x cos(angle) -which results in- P = 100V x ? x cos(90)
To find the missing current value we know by ohms law I = V/R -so- I - 100V/10ohms = 10A
So, P = 100V x 10A x Cos(90) = 0kW.

As it is purely inductive in your question the angle can be assumed to be 90degrees.

If resistance was present in the question, whether in the form of a resistor or cable resistivity then your answer would change. Due to it being a RL circuit, the resistance would alter the angle of the voltage and current and would no longer lag or lead by 90degrees - but lie somewhere say in the middle.

If the circuit contains resistance, we cannot assume it to be 90degrees and so you can calculate the phasor angle.

You have totally simplified that for me, so thank you. My mistake was that I hadn't taken into account that it was purely inductive. Thanks again!