# 2 hard physics question?

1. Sep 13, 2006

### smileandbehappy

Hi, I have been set a sheet of questions for homework and there are two which I cannot do. Any help you could give would be greatly appretiated.

1)
I have tried doing this but keep getting into cycles of inputting equations into it and just going around in circles.

2) This is a two part question and I have done the first part.

I = V/R

I = 6/(5.6*10^-3)

I = 1.071*10^-3A

I dont have a clue how to do this!

Last edited: Sep 13, 2006
2. Sep 13, 2006

### Hootenanny

Staff Emeritus
Welcome to the Forums,
Okay, start with the basic equations for capacitance and resistance;

$$R = \frac{V}{I}$$

$$C = \frac{Q}{V}$$

Now using the fact that current is the rate of flow of charge ($I = \frac{Q}{t}$) can you eliminate the current (I) from the first equation?
Do you know the definition of the time constant?

3. Sep 13, 2006

### smileandbehappy

"Do you know the definition of the time constant?"

No.

4. Sep 13, 2006

### Hootenanny

Staff Emeritus
Okay, do you know any equations involving the time constant (besides t = RC)?

5. Sep 13, 2006

### smileandbehappy

"Okay, do you know any equations involving the time constant (besides t = RC)?"

No sorry - t=RC is the only one given in the question.

6. Sep 13, 2006

### Hootenanny

Staff Emeritus
Okay, well in this case the equation you need to know is;

$$I = I_{0}e^{\frac{-t}{RC}}$$

Where I is the current at time t and I0is the intial current value. Has your tutor not given you this information?

7. Sep 13, 2006

### smileandbehappy

No. I have never been shown this equation beofre either. Thanks for all your help, but I think it would be best if I went and asked tomorrow, because I think he may have made a mistake.

8. Sep 13, 2006

### Hootenanny

Staff Emeritus
No problem. Have you any thoughts on the first problem?

Last edited: Sep 13, 2006