Electric Potential and Superposition of Electric Potential

[spark2flame]

1. Three charges are placed at the corners of a rectangle (one charge of -3.3e-6 C is placed on the bottom left hand corner, one charge of 2.7e-6 C on the upper right hand corner, and one charge of -6.6e-6 C on the upper left hand corner.) of length x = 0.65 m and height y = 0.43 m. How much work must be done to move the three charges infinitely far from one another?



2. U = (k*q*qo)/r



3. I tried using the superposition of the electric potential energy, but this did not yield the right answer. I don't know how to find the energy to push the charges to infinity!

 

[Doc Al]

Find the electric potential energy of the configuration by considering all the charge pairs.

 

[spark2flame]

Yes, I tried that using U = (k*q*qo)/r while considering the three different charges (which means that there would be three different U values). Would you simply use this formula for the three different charges and add the values? Does this account for the fact that you are trying to push the charges to infinity?

 

[Doc Al]

Would you simply use this formula for the three different charges and add the values?

Use the formula for three different charge pairs.

Does this account for the fact that you are trying to push the charges to infinity?

You'll compare the electric PE before and after you've moved the charges to infinity.

 

[spark2flame]

Okay that makes sense about the charge pairs. So would you simply add the U values found before and after pushing the charges to infinity? Or would you do U(initial) - U(final), or would you do U(final) - U(initial)?

 

[Doc Al]

The amount of work required to move the charges will equal the change in PE.

 

[spark2flame]

Ohhh okay so its U(initial)-U(final)?

But how would you calculate the U(final) for each charge pair, which would be U = (k*q*qo)/r, if the r approaches infinity and makes U = 0? Wouldnt that make the change in PE the same thing as U(initial)?

 

[Doc Al]

Ohhh okay so its U(initial)-U(final)?

Change is always final - initial.

But how would you calculate the U(final) for each charge pair, which would be U = (k*q*qo)/r, if the r approaches infinity and makes U = 0?

Exactly.

 

[spark2flame]

hahaha FINALLY i got the right answer. thanks SO much!