Resistance problem (Simple electrical problem)

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Discussion Overview

The discussion revolves around a homework problem involving the calculation of the resistance of a copper wire at different temperatures. Participants explore the application of the temperature coefficient of resistance and the relevant equations to solve the problem.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses difficulty in starting the problem without knowing the temperature coefficient of resistance at the initial temperature.
  • Another participant clarifies that R1 and R2 refer to the same component's resistance at different temperatures, emphasizing the relationship between them.
  • A question is raised about the applicability of the temperature coefficient at 0°C for calculations at 80°C.
  • Some participants suggest that it is reasonable to assume the temperature coefficient remains constant within a certain temperature range, despite acknowledging that this may not be strictly true.
  • A later reply indicates that the original poster has found a method to solve the problem by first determining the resistance at 0°C before calculating the resistance at 80°C.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between R1 and R2, but there is some uncertainty regarding the use of the temperature coefficient at different temperatures. The discussion reflects differing views on the validity of assuming the temperature coefficient is constant over the specified range.

Contextual Notes

There is an implicit assumption that the temperature coefficient can be treated as constant for the calculations, but this may not hold true outside a specific temperature range. Additionally, the discussion does not resolve whether the linear approximation is valid for the temperature change involved.

Rito3d03
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Homework Statement


This question comes from one of my previous exam
and I still couldn't solve it after days of trying
hope someone can guide me through this thanks

A copper wire has a resistance of 10Ω at 10°C.
Determine the resistance of the wire at 80°C.
Given that the temperature coefficient of the wire is 0.00393C-1 at 0°C.

Homework Equations


R2 = R1(1+\alpha1*ΔT)

The Attempt at a Solution


I don't know where to start when I don't have the temperature coefficient of R1

I know this may look silly because it maybe very easy to all of you
but please help me on this
 
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We're not talking about two different components R2 and R1.

We're talking about the same component, which has two different resistances at two different temperatures.

R1 is the resistance at temperature T1

R2 is the resistance at temperature T2.

You can confirm this by noticing that if T2 = T1, then delta T = 0, and hence R2 = R1.

In this case, delta T = T2 - T1 = 80 C - 10 C = 70 C, and you already know R1, which is the resistance at T1 = 10 C. So all you have to do is use the equation to solve for R2.
 
cepheid said:
We're not talking about two different components R2 and R1.

We're talking about the same component, which has two different resistances at two different temperatures.

R1 is the resistance at temperature T1

R2 is the resistance at temperature T2.

You can confirm this by noticing that if T2 = T1, then delta T = 0, and hence R2 = R1.

In this case, delta T = T2 - T1 = 80 C - 10 C = 70 C, and you already know R1, which is the resistance at T1 = 10 C. So all you have to do is use the equation to solve for R2.

but how about the \alpha1
I can use the coefficient at 0 C to solve different temperature?
 
Rito3d03 said:
but how about the \alpha1
I can use the coefficient at 0 C to solve different temperature?

I think you can assume alpha is constant. It's not strictly true, but the linearity holds within a certain temperature range around the temperature at which the coefficient was measured. 80 degrees may seem like a fairly large range, but unless you've been given additional information about how the alpha coefficient itself varies, I don't see any other choice other than using the given value and assuming it to be constant with temperature.
 
I finally find out the way to solve this question
first find out R0 which is the resistance at 0°C
10 = R0 (1+0.00393*(10-0))
then I can use the equation to solve the rest of the problem
and figure out R80
R80= R0 (1+0.00393*(80-0))

I guess i was stuck on some logic problem
glad i can finally finish this and move on
thanks for the help
 

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