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Berkeley Review Quick Question

iamgroot2

Full Member
Nov 6, 2019
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  1. Pre-Medical
In TBR, it seems to says that electrostatic force is always positive, with the equation of Coloumb’s Law being -
F = k | Q1 * Q2 | / r^2

However, the book then presents a graph where two like charges would have an inverse relationship on the positive side of a y-axis going towards y= 0 with distance between the charges. However, if the charges are opposite, there is an inverse relationship plotted along the negative y-axis as a mirror image of first graph along the x-axis. How could we plot this if we are taking the magnitude of both charges? Would we need to include the signs of each charge?

Here is an image of a data of that graph: https://files.mtstatic.com/site_453...3QCcC6VVNy8_&Key-Pair-Id=APKAJ5Y6AV4GI7A555NA
 
Last edited:

Instance_Variable

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Jun 1, 2020
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That image link is broken, but I think I understand your question.

The only reason why it says that the electrostatic force is always positive is because they're taking the absolute value of the product of the two charges. If the charges had opposite signs, they would attract (according to Coulomb's Law). If you weren't taking the magnitude of the product of the charges, you would get a negative (attractive) force. It makes more sense to me to solve the equation without absolute value, because a positive force will indicate repulsion, and a negative force will indicate attraction.

If you're taking the magnitude of both charges, the graph would look the same regardless of the signs of either charge. You would need to indicate if the charges had the same sign or opposite signs, so that whoever is looking at the graph knows if the force is attractive or repulsive. If you weren't taking the absolute value and the charges had opposite signs, you would get a mirror image across the y-axis like you described.
 
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iamgroot2

Full Member
Nov 6, 2019
27
2
26
  1. Pre-Medical
That image link is broken, but I think I understand your question.

The only reason why it says that the electrostatic force is always positive is because they're taking the absolute value of the product of the two charges. If the charges had opposite signs, they would attract (according to Coulomb's Law). If you weren't taking the magnitude of the product of the charges, you would get a negative (attractive) force. It makes more sense to me to solve the equation without absolute value, because a positive force will indicate repulsion, and a negative force will indicate attraction.

If you're taking the magnitude of both charges, the graph would look the same regardless of the signs of either charge. You would need to indicate if the charges had the same sign or opposite signs, so that whoever is looking at the graph knows if the force is attractive or repulsive. If you weren't taking the absolute value and the charges had opposite signs, you would get a mirror image across the y-axis like you described.

Thank you! Try this Image Link or the image is actually found on this page just below "The graphs of potential energy between two charges for like and unlike charges are shown below." However, in the book, the y-axis is "Force" and x-axis is "separation r". So, but the graph wouldn't look the same if you just took the magnitude without the sign (you would need a negative sign for the graph to be flipped into the negative section of the y-axis). Also, shouldn't a person use negative and positive signs, since force is a vector and negative and positive signs represent direction? I have used a few other books and have not come across the the absolute value sign used for the electrostatic force equation. So, is this just something I should remember, that electrostatic force can't be negative?
 

BerkReviewTeach

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What the book says is:

"In equation (6.1), the electrical force F is the magnitude of a vector quantity. It will always be a positive value. However, the product of the charges q1 and q2 can be either positive (if the charges have the same sign) or negative (if the charges have opposite signs). The absolute value bars in equation (6.1) ensure that F will always be positive."

It is saying that the magnitude of the vector quantity will always be positive. The direction can be positive or negative.

It seems like you understand the concept well, so you shouldn't stress about the absolute value signs. The AAMC materials are generally good about using the word magnitude in their questions, so if you think about sign as a direction, like you mentioned, you are set.
 
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iamgroot2

Full Member
Nov 6, 2019
27
2
26
  1. Pre-Medical
What the book says is:

"In equation (6.1), the electrical force F is the magnitude of a vector quantity. It will always be a positive value. However, the product of the charges q1 and q2 can be either positive (if the charges have the same sign) or negative (if the charges have opposite signs). The absolute value bars in equation (6.1) ensure that F will always be positive."

It is saying that the magnitude of the vector quantity will always be positive. The direction can be positive or negative.

It seems like you understand the concept well, so you shouldn't stress about the absolute value signs. The AAMC materials are generally good about using the word magnitude in their questions, so if you think about sign as a direction, like you mentioned, you are set.

Thank you so much!!!! That clarifies this issue!
 
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