Energy dissipated in a resistor

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Homework Help Overview

The problem involves calculating the energy dissipated in a resistor with a specified resistance and a voltage function over a given time interval. The subject area pertains to electrical circuits and energy calculations.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the use of definite integrals to find energy, questioning which integral is appropriate. There are inquiries about the dimensions of the integral and the relationship between power and energy. Some participants suggest integrating the square of the voltage divided by resistance to find instantaneous power.

Discussion Status

The discussion is ongoing, with participants providing hints and guidance on the correct approach to integrate the voltage function to find energy. There is recognition of the need to clarify the relationship between power and energy, and multiple interpretations of the integration process are being explored.

Contextual Notes

Participants are navigating the constraints of the problem, including the need for correct units and the specific form of the voltage function. There is an emphasis on ensuring that the calculations align with the physical principles governing energy dissipation in resistors.

Bob jefferson
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Homework Statement


A resistor of resistance 10Kohms has voltage across it described by the function
V(t) = 6sin(10t+pi/4)*exp(-2t) Volts
Calculate the energy dissipated in the resistor between t=0 and t=0.5

Homework Equations

The Attempt at a Solution


I have done the definite integral and have got 0.324 but i am not too sure if i am supposed use this answer in P=V^2/R .
 
Last edited:
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Definite integral sounds good, the question is which integral exactly?
 
Also, the units of the answer should be supplied.
 
DrDu said:
Definite integral sounds good, the question is which integral exactly?
I integrated this V(t) = 6sin(10t+pi/4)*exp(-2t)
 
Does the integral ##\int V(t) dt ## have the dimension of an energy?
 
DrDu said:
Does the integral ##\int V(t) dt ## have the dimension of an energy?
No
 
phyzguy said:
Also, the units of the answer should be supplied.
I think the answer should be in joules as its asking for energy dissipated.
 
Bob jefferson said:
I think the answer should be in joules as its asking for energy dissipated.

That's correct. As DrDu pointed out, the integral you did doesn't have these units. Why did you go away from P = V^2 * R? What are the units of power? How do you go from power to energy?
 
Hint: Energy is the time integral of power.
 
  • #10
gneill said:
Hint: Energy is the time integral of power.
so i just multiply 0.5s with the value i obtained from the integration.
 
Last edited:
  • #11
Bob jefferson said:
so i just multiply 0.5s with the value i obtained from the integration.
No, you need to perform another integration where what you are integrating is the instantaneous power.
 
  • #12
gneill said:
No, you need to perform another integration where what you are integrating is the instantaneous power.
Oh so actually putting V^2/r in the integral.
 
  • #13
Bob jefferson said:
Oh so actually putting V^2/r in the integral.

Yes. Try this and tell us what you get.
 
  • #14
gneill said:
No, you need to perform another integration where what you are integrating is the instantaneous power.
so would it be some thing like ##\frac 1 R \int_0^{0.5} V^2(t) dt ##
 
  • #15
Bob jefferson said:
so would it be some thing like ##\frac 1 R \int_0^{0.5} V^2(t) dt ##

Yes.
 
  • #16
phyzguy said:
Yes.
so i need a bit of guidance, would ##sin^2(10t+\frac \pi 4)## equal to ##\frac 1 2(1-cos(2*10t+\frac \pi 4))##
 
  • #17
\sin^2(x) = \frac{1}{2}(1-\cos(2x)), but that's not exactly what you wrote. The 2 should multiply everything inside the cos argument.
 

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