# Homework Help: Lightning strikes tree. Energy is absorbed by tree. Sap evaporates. How much sap?

1. Sep 22, 2009

### vdeity

1. The problem statement, all variables and given/known data

A certain storm cloud has a potential of 1.50 x 10^8 V relative to a tree. If, during a lightning storm, 50.0 C of charge is transferred through this potential difference and 1.00% of the energy is absorbed by the tree, how much sap in the tree can be boiled away? Model the sap as water initially at 26.0°C. Water has a specific heat of 4186 J/(kg·C°), a boiling point of 100°C, and a latent heat of vaporization of 2.26 x 10^6 J/kg.

2. Relevant equations

I'm actually kinda iffy on all the equations, but I got several answers that WebAssign told me was wrong...

3. The attempt at a solution

Energy gain of 50.0 C of charge accelerated through potential difference of (1.50 x 10^8 V) is (50.0 C)(1.50 x 10^8 V) = 7.5 x 10^9 J.

To vaporize 1 kilogram of sap requires mcΔT = (1 kg)(4186 J/kg K)(100 - 26 degrees C) = 3.098 x 10^(5) J to heat to 100 degrees C

An additional 2.26 x 10^6 J for the state change from liquid to gas, for a total required energy of 3.098 x 10^(5) + 2.26 x 10^6 = 2.57 x 10^6 J.

The total energy absorbed is 1% of 7.5 x 10^9 = 7.5 x 10^7 J, and at 2.57 x 10^6 J per kg needed, we are able to vaporize:

(7.5 x 10^7 J)/(2.57 x 10^6 J/kg) = 29.19 kg of sap.

WebAssign says it's wrong.

Please tell me where I went wrong.
It isn't due for a few days, I just wanted to get this assignment out of the way... Last question.

Thanks!

2. Sep 22, 2009

### kuruman

Re: Lightning strikes tree. Energy is absorbed by tree. Sap evaporates. How much s

WebAssign is evil. I see nothing wrong with your solution - your method is OK and I verified your numbers. It may be a matter of sig figs or (perish the thought) the wrong calculated formula has been put into the problem. I suggest that you take your solution to your instructor and ask him/her to find what is wrong with it. Do this before the deadline.

3. Apr 13, 2010

### tom_paine

Re: Lightning strikes tree. Energy is absorbed by tree. Sap evaporates. How much s

Sorry to bring up an old thread, but I was under the impression that the energy absorbed by the tree will be equal to 1/2QV, and not QV. I'd be curious to hear what others have to say about this.

Thanks,

Tom

4. Apr 14, 2010

### willem2

Re: Lightning strikes tree. Energy is absorbed by tree. Sap evaporates. How much s

That would be true if you had a capacitor with 50C on it that you let completely discharge.
The question doesn't say that the stormcloud completely discharges however.

5. Apr 14, 2010

### tom_paine

Re: Lightning strikes tree. Energy is absorbed by tree. Sap evaporates. How much s

I see what you're saying. Thanks.

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