# Van de Graaff generator? adding a electron?

1. Aug 1, 2009

### graphicer89

Van de Graaff generator? adding a electron??

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

A)If a Van de Graaff generator is charged to 5,000 volts, how much energy does it take to add and additional electron to the charge on the sphere? If the charge of an electron is -1.6 x 10 ^-19........

B) How would the voltage of the generator in the part A compare to the voltage of a larger Van de Graaff generator with the same amount of charge? To which generator could an electron be addded with the least expenditure of energy?

2. Relevant equations

Are there any equations i can apply?

3. The attempt at a solution

A) I guessed but i dont know if im right or wrong...i multiplied....

5,000 v x -1.6 x10^-19 C = -8 x 10^-16 energy but i think thats wrong can you please help me with the calculations? i dont know to multiply or divide....

B) I need to do A first to understand B...but my guess is that the voltage is the same but the force or work to include a electron would not be much in a larger generator. Its easier to add a electron to a larger Van de Graaff generator with the least expenditure of energy........

2. Aug 1, 2009

### diazona

Re: Van de Graaff generator? adding a electron??

-8 x 10^-16 Joules. Joules are the units of energy. If you'd written that, it would be correct. (You used the formula $U = qV$)

If the voltage were the same, the work required would be the same. But the voltage is not the same. Here's a hint: what do you know about capacitance?

3. Aug 1, 2009

### graphicer89

Re: Van de Graaff generator? adding a electron??

Honestly nothing...but this is kinda like earth..earth is so large that it consumes an infinite amount of energy and it never over flows..etc...so im thinking...would it require more energy to add a electron to a small generator because its more compact?...i really dont want to go off topic...

4. Aug 1, 2009

### diazona

Re: Van de Graaff generator? adding a electron??

Not actually an infinite amount of energy, more like so-large-that-for-most-cases-it-might-as-well-be-infinite. But anyway: yes it would require more energy to add an electron to a small generator, now can you figure out why? Go back to your references (textbooks, notes, whatever you have) and look up some information on capacitance if you need to.

5. Aug 1, 2009

### graphicer89

Re: Van de Graaff generator? adding a electron??

This is what i found....

The electrical potential energy of a charged particle is increased when work is done to push it against the electric field of something else that is charged. Just as work is done to compress a spring work is done in pushing the charge against the electric field of the sphere. This work is eqqual to the energy gained by the charge.

6. Aug 1, 2009

### diazona

Re: Van de Graaff generator? adding a electron??

OK, that's true, but surely there must be more than that? (You don't need to post it here, but you should read it yourself!) Especially keep your eyes open for a formula involving voltage that has to do with capacitance.

7. Aug 1, 2009

### graphicer89

Re: Van de Graaff generator? adding a electron??

well actually what i tried looking but i just feel like im in a dead end right now

8. Aug 1, 2009

### graphicer89

Re: Van de Graaff generator? adding a electron??

So there's more voltage in the first sphere than in the second...

9. Aug 1, 2009

### graphicer89

Re: Van de Graaff generator? adding a electron??

Nevermind I figured it out thanks though fr your help