# Homework Help: Solving for RMS current of a resistor, help?

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1. Mar 6, 2017

### clamatoman

1. The problem statement, all variables and given/known data
A resistor connected across an AC power supply has a current given by I=(1.35A)cos(300t) when connected to a power supply with emf 120 V rms.
Find the RMS current.

2. Relevant equations
IRMS=Imax/√2

3. The attempt at a solution
IRMS=Imax/√2
IRMS=0.477 A
INCORRECT
Not exactly sure what to do? I am not given any other data besides what i have written above. I do not have the Power, or the Resistance, and in fact will have to solve for those in part B. and C. of this problem. I know I am missing something here, and i have re-read the chapter in my textbook but it is not helping.

2. Mar 6, 2017

### Staff: Mentor

How did you establish the value of $I_{max}$ that you used?

3. Mar 6, 2017

### clamatoman

I used the value of "I=(1.35A)cos(300t)" they gave as Imax.

4. Mar 6, 2017

### Staff: Mentor

Okay, but that's not a value. What value (single number) did you use?

5. Mar 6, 2017

### clamatoman

I used (1.35A)cos(300t) = 0.675
I think what I am missing is what the "t" component means?

6. Mar 6, 2017

### Staff: Mentor

That is incorrect for a couple of reasons. First, the expression cos(300t) is a function of time t. Second, the "300" in the expression would have units of radians per second, not degrees per second.

Read the maximum value from the coefficient: 1.35 A. The cosine function makes the current vary between the bounds -1.35 A and +1.35 A over time.

7. Mar 6, 2017

### clamatoman

Ahh Okay.
So.
Imax=1.35 A
IRMS=1.35/√2=.955 A
Excellent, thank you.