# Energy in a circuit

1. Aug 29, 2009

### mba444

1. The problem statement, all variables and given/known data

2. The voltage and current at the terminals of the circuit element in Fig. 1 are zero for t<0. For t>0 they are

V= e^(-500*t)-e^(-1500*t) V
i= 30-40e^(-500*t)+10e^(-1500*t) mA

a) Find the power at t = 1 ms.
b) How much energy is delivered to the circuit element between 0 and 1 ms?
c) Find the total energy delivered to the element.

Figure 1. An ideal basic circuit element

2. Relevant equations

P=V*i
W=integral (P dt)

3. The attempt at a solution

i solved part a by simple substituting the time in both V and i equations then i multipled them giving me a result of 3.1 W, in the second part i used the integral where i substitued the answer i got in the previous part and set the integral to be from 0 to 1 ms but it turns to be wrong !!

2. Aug 29, 2009

### CEL

Voltage and current are variable and so is their product, the power. You must perform the multiplication and integrate it.

3. Aug 30, 2009

### mba444

i did so as you said but i got a negative wrong answer

can you explain more in depth

4. Aug 30, 2009

### Redbelly98

Staff Emeritus
If you show your work, somebody can help figure out where things went wrong.

5. Aug 30, 2009

### CEL

Both voltage and current start with the value 0 at t=0 and them they become positive. The current rises monotonically to 40 and after some time (well after 1 ms) the voltage starts to decay to zero, but still positive.
It is not possible that their product can be negative.