Problem solving equation with negative exponent

In summary, the problem is that the derivative of the equation is undefined, so it can't be solved for time.
  • #1
CentreShifter
24
0
I'm trying to calculate maximum power. Once I computed power as a function of time I took its derivative and set it equal to zero. Now it need to solve for time and I can't seem to get it done. What I have is...

[tex]-1000e^{-500t}-2000e^{-1000t}=0[/tex]

My first inclination is to use ln, but ln(0) is undefined. Any tips?
 
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  • #2
Are you sure that equation is right? Looks to me like you have -(something which is always positive) - (something else that is always positive) = 0, but the LHS will always be negative, and never zero, unless t is infinite?
 
  • #3
That would seem to indicate that the power has no maximum; perhaps it increases without bound.
 
  • #4
CentreShifter said:
I'm trying to calculate maximum power. Once I computed power as a function of time I took its derivative and set it equal to zero. Now it need to solve for time and I can't seem to get it done. What I have is...

[tex]-1000e^{-500t}-2000e^{-1000t}=0[/tex]

My first inclination is to use ln, but ln(0) is undefined. Any tips?

As it is now, there's no solution. You probably made a sign error while computing the derivative.
If that is indeed the case you can try substituting [itex] e^{-500t} = u [/itex]
 
  • #5
Thanks to all who responded.

I believe I was doing my computations incorrectly. I am given voltage:

[tex]100e^{-500t}V[/tex]

And current:

[tex]20-20e^{-500t}mA[/tex]

Power being the product:

[tex]2e^{-1000t}(e^{500t}-1)W[/tex]

And derivative:

[tex]-1000e^{-1000t}(e^{500t}-2)[/tex]

To find out when power is maximized:

[tex]-1000e^{-1000t}(e^{500t}-2)=0[/tex]

[tex]2000e^{-1000t}=1000e^{-500t}[/tex]

[tex]ln2=500t[/tex]

Putting t at about .0013863 s.
 
  • #6
I can't find any error so far.

Just out of curiousity, what is the purpose of the circuit?
 
  • #7
It isn't real. This was one part of a practice problem I was solving as to lead me up to DEs, which I need to reacquaint myself with.
 
  • #8
Oh. Questions like that should get posted in the Homework & Coursework Questions area, even if it's for personal study and not an actual homework assignment.

I have moved this thread.
 
  • #9
Cool. Thanks for the info.
 

What is a negative exponent?

A negative exponent is a way of representing fractions or decimals in exponential form. It is written as a number with a negative sign in front of it, such as -3. It indicates the number of times the base should be divided by itself when written in fractional form. For example, 2-3 can be rewritten as 1/(2*2*2), which equals 1/8.

How do I solve an equation with a negative exponent?

To solve an equation with a negative exponent, you can use the rule that states a-n = 1/(an). This means that you can move the base with the negative exponent to the denominator and change the exponent to positive. Then, you can solve the equation as you normally would.

Can a negative exponent be a whole number?

Yes, a negative exponent can be a whole number. This is because a whole number can be written as a fraction with a denominator of 1. For example, 3-2 can be rewritten as 1/(3*3), which equals 1/9.

Why do negative exponents result in fractions or decimals?

Negative exponents result in fractions or decimals because they represent the inverse of a number. When a number is raised to a negative exponent, it is being divided by itself a certain number of times. This is equivalent to finding the reciprocal of the number, which will result in a fraction or decimal.

What are some real-life applications of solving equations with negative exponents?

Solving equations with negative exponents can be useful in many fields, such as physics, chemistry, and finance. For example, in physics, negative exponents are used to represent exponential decay, which is the decrease in the amount of a substance over time. In finance, negative exponents are used to calculate compound interest, which is the interest earned on an initial investment over time.

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