How come potential energy is negative when charges attract

In summary, the concept of attraction and repulsion in terms of electric potential energy can be confusing. The closer two like charges are, the less potential energy they have, resulting in a positive value. On the other hand, the closer two opposite charges are, the more potential energy they have, resulting in a negative value. This is due to potential energy being based on a reference point of infinity, where the potential energy is set to 0. It is important to understand this concept rather than just memorizing it.
  • #1
x86
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and positive when they repel?

It doesn't make any sense to me at all. To what are they referring to?

The way I see it, if you're got...

+...-

The closer they are, the less electrical potential energy they have. So it should still be positive.

And if you've got two like charges

C...C

The closer they are, they more electric potential energy they have. So it should also be positive.

How come for my first example its negative? Mathematically its negative, ofc, since (-1) * (1) is always negative.. But I can't understand the concept of attraction being negative potential energy. It just doesn't make sense.

I understand potential energy is all based on reference, but I just can't understand it. The book doesn't say why, it just basically wants you to memorize the facts. However, I know that you don't learn physics by memorizing, but by understanding. Hopefully someone can shed some light on this.

Thanks
 
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  • #2
x86 said:
I understand potential energy is all based on reference, but I just can't understand it.
Potential energy depends on your reference point. It is customary, and mathematically advantageous, to take "infinity" as your reference where PE = 0. Then, using your own reasoning, you should be able to see why two similar charges must have a positive PE for any finite distance, while two opposite charges must have negative PE.

(The same is done for gravitational PE between two masses.)
 

FAQ: How come potential energy is negative when charges attract

1. Why is potential energy negative when charges attract?

According to the law of conservation of energy, the total energy of a closed system must remain constant. In the case of two charged particles, the potential energy decreases as they move closer to each other, but this decrease is offset by an increase in kinetic energy. The negative sign in the potential energy equation accounts for this offset, ensuring that the total energy remains constant.

2. Does the negative potential energy mean that the system has less energy?

No, the negative sign in the potential energy equation does not indicate a decrease in the system's energy. It simply represents the direction of the force between the charged particles. The magnitude of the potential energy can be calculated by taking the absolute value of the negative number.

3. How does the distance between the charges affect the potential energy?

The potential energy between two charged particles is directly proportional to the magnitude of the charges and inversely proportional to the distance between them. As the distance decreases, the potential energy becomes more negative, indicating a stronger attractive force between the charges.

4. Can potential energy ever be positive in a system with attractive charges?

No, potential energy can never be positive in a system with attractive charges. As the charges move closer together, the potential energy becomes more negative, approaching an infinitely negative value when the charges are in contact. This indicates an infinitely strong attractive force between the charges.

5. How does potential energy change when the charges have the same sign and repel each other?

In a system with two charges of the same sign, the potential energy is positive. As the charges move further apart, the potential energy decreases, indicating a weaker repulsive force between the charges. However, unlike in the case of attractive charges, the potential energy cannot become infinitely negative or positive, as the charges will never be in contact or infinitely far apart.

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