# Did I use Laplace correctly?

I haven't done Laplace transforms in a while and I wanted to know if I did this right. I start out with the expression

$$\tau\frac{dT}{dt}+T(t)=T_{a}$$

I took the Laplace of this expression and got $$\tau[sT(s)-T(0)]+T(s)=\frac{T_{a}}{s}$$

which simplified to $$T(s)[s\tau+1]-\tau T(0)=\frac{T_{a}}{s}$$

Upon further simplification I wound up with

$$T(s)=\frac{T_{a}+\tau sT(0)}{s\tau+1}$$

would this be the right final answer? Again, I have not done Laplace in a while and I may have messed up algebraically somewhere so I just wanted some help on this. Thanks everyone

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What is $\tau$? Is it a constant? –  Babak S. Nov 12 '12 at 8:17
Is $T_a$ also just a constant? –  Christopher A. Wong Nov 12 '12 at 8:42
Yes I am sorry. This is the equation for an object's response to a step change in temperature. So $T_{a}$ is a constant as the ambient temperature and $\tau$ is the time constant. They are both just constants –  Greg Harrington Nov 12 '12 at 8:52

What you have done so far looks correct. However, I would strongly suggest that when you take the Laplace transform of the function $T(t)$, you use a new function symbol to indicate the Laplace transform, rather than just changing the variable from $t$ to $s$. The reason for this are many-fold; in final answer, for example, you have a $T(0)$; is that $T(t = 0)$ or $T(s = 0)$? It is not clear from the notation.

If you are trying to solve the differential equation for $T(t)$, once you have your final expression for the Laplace transform, then you have to use the inverse Laplace transform to obtain the solution, and taking the inverse Laplace transform is not always easy. If your homework asks you to take the Laplace transform, then I suppose that is what you'll need to do, but if you're just trying to solve the differential equation, then the Laplace transform is not the best way, because your differential equation is just an inhomogeneous linear ODE with constant coefficients, so it can be solved by more classical methods, like using the method of undetermined coefficients, for example.

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Thank you for the clarifying that I need to differentiate between t and s domains. Actually my homework involved finding the transfer function to this differential equation. I didn't think many on the math page knew about transfer functions but they could at least let me know if I was doing the Laplace correctly. –  Greg Harrington Nov 12 '12 at 21:01