Loganity
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oops
Last edited by Loganity; 3 months ago
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Joinedup
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Whether you're putting energy in or getting energy out depend on whether the distance between the charges was increasing or decreasing as well as if the charges are the same sign or opposite sign.
(Original post by Loganity)
Question: "Explain why values of gravitational potential are always negative but those of electric potential can be positive or negative."

Answer:
The gravitational force is always attractive. Work got out as masses come together from infinity. The electric force is either attractive or repulsive. For charges of the same sign, work is done on the charges as they come together. For charges of opposite sign, work got out as charges come together.
(Marking scheme)
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I was told instead that work is done on the charges by the electric field in the case of charges of opposite sign. Can someone help me please?
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Eimmanuel
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(Original post by Loganity)
Question: "Explain why values of gravitational potential are always negative but those of electric potential can be positive or negative."

Answer:
The gravitational force is always attractive. Work got out as masses come together from infinity. The electric force is either attractive or repulsive. For charges of the same sign, work is done on the charges as they come together. For charges of opposite sign, work got out as charges come together.
(Marking scheme)
------
I was told instead that work is done on the charges by the electric field in the case of charges of opposite sign. Can someone help me please?
(Original post by Loganity)
Thanks so much for helping
Yes but potential is the work done (by the external force) in moving unit (positive charge/mass) from infinity to the point. So distance decreasing

For repulsive force, work is done on the charges as they come together from infinity. This is clear to me

For attractive force, can we say work is done by the charges? (since applied external force and displacement are opposite)
Work done by external force = negative, so work done by charges = positive? Similar to work done by gas/on gas during change in volume gas?
Is work/energy got out the only acceptable description?
Is work/energy got out the only acceptable description?
No. In fact, you can use negative or positive work done by internal force or external agent instead.
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Eimmanuel
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(Original post by Loganity)
Thanks a lot. Can you please check the following if there is an error? A charge is being moved from infinity to a point in an electric field

For like charges, there is a repulsive force FE on the charge being moved. The external force applied is equal and opposite to this. The external force and displacement are in the same direction so positive work is done by the external force on the charge. Since the external force is equal in magnitude to the electric force, the work done by the external force is converted to solely electric potential energy (not kinetic energy)

For unlike charges, there is an attractive force FE on the charge being moved. The external force applied is equal and opposite to this. The external force and displacement are in opposite directions so negative work is done by the external force on the charge. Since the external force is equal in magnitude to the electric force, the work done by the external force is converted to solely electric potential energy

Because electric potential is defined to be zero at infinity, we have positive and negative values of electric potential.
IMO, it seems that you are explaining the negative or positive work done by the external force without providing the link(s) between electric potential and electric potential energy or between electric potential and work done by the external force.

What is so called MS in your first post is actually comment from the examiner. You need to know how to interpret them correctly, instead of following the writing "blindly".

I doubt that there are so many blank lines for you write such long sentences.
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