# Equation of state of a rubber band

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1. Jan 15, 2015

### Robsta

1. The problem statement, all variables and given/known data
For a stretched rubber band, it is observed experimentally that the tension f is proportional to the temperature T if the length L is held constant. Show that:

a) The internal energy U is a function of temperature only
b) Adiabatic stretching of the band results in an increase in temperature (solved)
c) The band will contract if warmed while kept under constant tension

2. Relevant equations
f = kT
(df/dt)L = k

3. The attempt at a solution
I think I need an equation of state for the band for part (a)

For part (b), adiabatic stretching means stretching with no heat going in or out of the band. To stretch it, the tension must be increased. f = kT:
f1/T1 = f2/T2
T2/T1 = f2/f1 If the band is stretched, f2/f1 is >1 so T2/T1 >1 therefore T2>T1. The temperature has increased.

For part c), I can write the total differential, then let df = 0 but that doesn't give much. Intuitively this makes sense, I just need to formalise it.

2. Jan 15, 2015

### Bystander

Can you write an equation for the internal energy that is analogous to that for an ideal gas?

3. Jan 15, 2015

### Robsta

yes,
du = T ds - f dl.
But that hasn't helped me much.

4. Jan 15, 2015

### Robsta

Alternatively,
du = CldT - f dl.

5. Jan 15, 2015

### Bystander

And if L is held constant?

6. Jan 15, 2015

### Robsta

Do you know how to do this? Because at the moment it looks like internal energy is firmly a function of T and L, not T alone

7. Jan 15, 2015

### Robsta

yes, if L is held constant, U is a function of T. But the question is asking me to show U is a function of just T (presumably with variable L)

8. Jan 15, 2015

### Bystander

"For an ideal gas it is observed experimentally that the _____ __ is proportional to the temperature T if the ______ __ is held constant."

9. Jan 15, 2015

### Robsta

Pressure is proportional to the temperature if the volume is held constant. So you're saying the U is only a function of T when L is constant and not otherwise? This makes sense, I just thought the question was getting at the fact that if you stretch the band it will heat up too.

10. Jan 15, 2015

### Bystander

And if you squeeze a balloon filled with an ideal gas, does its temperature go up?

11. Jan 15, 2015

### Robsta

No (if it's an ideal gas). But why should I treat the band as ideal?

12. Jan 15, 2015

### Bystander

"The thermodynamics of the ideal rubber band" is a popular (not universally used) alternative/generalization of the work term in first law.

13. Jan 15, 2015

### Staff: Mentor

In this equation, what is the partial derivative of u with respect to L at constant s? What is the partial derivative of u with respect to T at constant L? What is the partial derivative of f with respect to T at constant L (in terms of partial derivatives of u)?

Chet

14. Jan 16, 2015

### Staff: Mentor

This equation is incorrect. You need to start with du = T ds - f dl, and then show that:

$du=C_ldT+\left[T\left(\frac{\partial f}{\partial T}\right)_l-f\right]dl$

This is very similar to deriving the general equation for du for a gas:

$du=C_vdT+\left[T\left(\frac{\partial P}{\partial T}\right)_v-P\right]dv$

Chet