How Do You Calculate the Magnitude of Charges Based on Their Potential Energy?

Click For Summary

Homework Help Overview

The problem involves calculating the magnitude of identical charges based on their potential energy when separated by a certain distance. The context is centered around electrostatics and the relationship between potential energy and charge interactions.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants express uncertainty about the initial steps needed to solve the problem, particularly regarding the relevance of the charge of an electron or proton. There are discussions about the definition of electrical potential energy and its relation to the work done in separating charges.

Discussion Status

Some participants are attempting to clarify the definitions and relationships involved in the problem. There is an ongoing exploration of how to apply the given potential energy and the implications of using specific charge values. Multiple interpretations of the potential energy concept are being discussed, with some guidance provided on the need to focus on the potential energy of two charges rather than a single charge.

Contextual Notes

Participants are navigating through the definitions and relationships between potential energy, charge, and distance, with some expressing confusion about the application of the given values. There is a reference to external resources for further clarification.

bkl4life
Messages
25
Reaction score
0

Homework Statement



The potential energy of a system consisting of two identical charges is 4.5 mJ when their separation is 38 mm.

a) What is the magnitude of each charge?
b) How many individual charges are present in the system? (You can assume the charges are either electrons or protons.)

Homework Equations


F=Ke(q1*q2)/r^2


The Attempt at a Solution



I'm not really sure where to start.
Do I need the charge of an electron or proton? Any help would be awesome!
 
Physics news on Phys.org
bkl4life said:

Homework Statement



The potential energy of a system consisting of two identical charges is 4.5 mJ when their separation is 38 mm.

a) What is the magnitude of each charge?
b) How many individual charges are present in the system? (You can assume the charges are either electrons or protons.)

Homework Equations


F=Ke(q1*q2)/r^2


The Attempt at a Solution



I'm not really sure where to start.
Do I need the charge of an electron or proton? Any help would be awesome!
The first thing to do is to calculate the potential energy of the system.

What is the definition of [electrical] potential energy?
 
I think it's the amount of work needed to separate the charges.

(9 x 10^9)*(1.6*10^-19)*(-1.6*10^-19)/.038
= -6.06 *10^-27

When does the 4.5 mJ come into play
 
bkl4life said:
I think it's the amount of work needed to separate the charges.
Close but not quite. Notice that the question states that the two charges are equal, in which case they would repel and therefore no work would be required to separate them, they would do so of their own accord.

The potential energy of two charges is the work required to bring the charges from an infinite distance to r = 0.038m.
bkl4life said:
(9 x 10^9)*(1.6*10^-19)*(-1.6*10^-19)/.038
= -6.06 *10^-27
Are you just pulling numbers out of thin air? Where did you get the values from the charges from?
 
I was using the charge of an electron. I was looking at part b when I used those numbers. Should I use the 4.5 mj instead of the charge of electron? The total potential of a system is the sum of all the contributions for each pair of particles. I'm just lost.
 
bkl4life said:
I was using the charge of an electron.
Why?
bkl4life said:
I was looking at part b when I used those numbers.
I say again. Why? You cannot answer part (b) until you have a solution for part (a).
bkl4life said:
Should I use the 4.5 mj instead of the charge of electron?
No, since 4.5 mJ is an energy and not a charge.
bkl4life said:
I'm just lost.
Don't worry we can start again. What does your textbook have to say about the potential energy of two point charges?

If your textbook is of no help, try here: http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/elepe.html

I'm going offline now, but I'm sure someone else will be more than happy to jump in and help you. If not, I'll check in later this evening.
 
My book says:

U=q'*V
U=electric potential energy
V= electric potential
q'= charge

It also says that:

V=U/q'= K(q/r)

Would I be able to do:

2*(8.99*10^9)(charge)/.038= 4.5 mJ
 
bkl4life said:
My book says:

U=q'*V
U=electric potential energy
V= electric potential
q'= charge

It also says that:

V=U/q'= K(q/r)

Would I be able to do:

2*(8.99*10^9)(charge)/.038= 4.5 mJ
You have the right idea, but here you are calculating the potential of a single charge q', rather than the potential energy of two charges.

Check out the link I gave you above.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
1K
  • · Replies 23 ·
Replies
23
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 12 ·
Replies
12
Views
1K
Replies
13
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 18 ·
Replies
18
Views
16K
  • · Replies 13 ·
Replies
13
Views
2K