Solving Equation Problem: Power Generated by 240V and 12A

[brenfox]

Homework Statement

Determine the power generated by a voltage V = 240∠15° volts driving a current I = 12 ∠75°

Homework Equations

P=VIcos(ø-ø)

The Attempt at a Solution

This is a worked example from my open university course. The answer is 240x12cos(15°-75°) = 1440W. I can not for the life in me work out how they have arrived at this answer. I get cos-60= 0.58. then times 240x12= 1692.9 W. Must be doing something wrong!

 

[gneill]

How is cos(60°) equal to 0.58?

 

[pasmith]

How is cos(60°) equal to 0.58?

It is possible that the OP has their calculator in GRAD mode, since cos(60 grad) = 0.588.

But really one shouldn't need a calculator to calculate cos(60°) = 0.5.

 

[brenfox]

Not sure. After inputting cos 60 it came out with 0.58. I am of the understanding cosine 60 equals half. so 240x12x0.5 does equal 1440 Watts . But cosine 60 equates to 0.58 in my calculator.

 

[gneill]

Not sure. After inputting cos 60 it came out with 0.58. I am of the understanding cosine 60 equals half. so 240x12x0.5 does equal 1440 Watts . But cosine 60 equates to 0.58 in my calculator.

As pasmith indicated, check to make sure that your calculator is in degree mode for angles, not grad mode.