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

• bkl4life
In summary: It has more information on potential energy.In summary, The potential energy of a system consisting of two identical charges is 4.5 mJ when their separation is 38 mm.
bkl4life

## 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.)

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!

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.)

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.

## 1. What is potential energy?

Potential energy is the energy that an object or system possesses due to its position or state. It is often referred to as stored energy, as it has the potential to do work when it is released.

## 2. How is potential energy different from kinetic energy?

Potential energy is the energy that an object or system has due to its position, while kinetic energy is the energy an object or system has due to its motion. Both forms of energy are interrelated and can be converted into one another.

## 3. What factors affect the potential energy of a system?

The potential energy of a system is affected by the mass of the object, the height or distance of the object from a reference point, and the strength of the force acting on the object. The type of potential energy also varies depending on the system, such as gravitational potential energy, elastic potential energy, or chemical potential energy.

## 4. How is the potential energy of a system calculated?

The potential energy of a system can be calculated using the formula PE = mgh, where PE is potential energy, m is the mass of the object, g is the acceleration due to gravity, and h is the height or distance from a reference point. Alternatively, specific formulas can be used for different types of potential energy.

## 5. Can potential energy be negative?

Yes, potential energy can be negative. This typically occurs when the zero point or reference point is chosen in a way that makes the potential energy of the system negative. For example, if the reference point for gravitational potential energy is set at the top of a cliff, objects below the cliff will have negative potential energy.

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