# electric potential energy and work

#### inaccensa

10+ Year Member
7+ Year Member
I have a couple of questions :-

The electric field lines point from positive charge to negative charge and the force follows the same direction, if the test charge is positive. However if the test charge is negative, it will be opp to the direction of electric field?

Is electric PE = -Work a fact?

Thus if an electron ( at a high potential) moves towards a proton (at a lower potential), i think in terms of energy. Thus, the PE will be negative, since it's a more stable system and the work done will be negative ( from PE=-W). Now who exactly is doing the work? Is it the electric field? In terms of electrostatic force, there will be an attraction and thus a -ve value for force, the V = -KQ/R and thus PE = -KQQ/R. The only thing that is confusing is work.

Also, if the electron were moving from low potential( where the protons are) to high potential (where electrons are) the PE= positive and work is negative.

How does a change in Electric field cause a change in magnetic field?

#### inaccensa

10+ Year Member
7+ Year Member
I have a couple of questions :-

The electric field lines point from positive charge to negative charge and the force follows the same direction, if the test charge is positive. However if the test charge is negative, it will be opp to the direction of electric field?

Is electric PE = -Work a fact?

Thus if an electron ( at a high potential) moves towards a proton (at a lower potential), i think in terms of energy. Thus, the PE will be negative, since it's a more stable system and the work done will be negative ( from PE=-W). Now who exactly is doing the work? Is it the electric field? In terms of electrostatic force, there will be an attraction and thus a -ve value for force, the V = -KQ/R and thus PE = -KQQ/R. The only thing that is confusing is work.

Also, if the electron were moving from low potential( where the protons are) to high potential (where electrons are) the PE= positive and work is negative.

How does a change in Electric field cause a change in magnetic field?

can some plz verify this

#### ese

Your seem to be a little confused, look over this and rephrase your question.

Some equations: F=k q1 q2 / r^2
V= k q / r

Since Electric potential is a scalar (positive charges are always at HIGH potential and negative charges are always at LOW potentials)

Work (by E-field) = -(change)PE = (change) KE

Example: An electron moving to a lower potential (negative charge) would be gaining PE since it naturally wants to go towards high potential (positive charges) resisting the E-field, hence the work in this case would be negative.

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#### inaccensa

10+ Year Member
7+ Year Member
Your seem to be a little confused, look over this and rephrase your question.

Some equations: F=k q1 q2 / r^2
V= k q / r

Since Electric potential is a scalar (positive charges are always at HIGH potential and negative charges are always at LOW potentials)

Work (by E-field) = -(change)PE = (change) KE

Example: An electron moving to a lower potential (negative charge) would be gaining PE since it naturally wants to go towards high potential (positive charges) resisting the E-field, hence the work in this case would be negative.
I'm confused here, I thought that the energy of the electron will be negative, since it naturally wants to go towards the protons or in other words, the system more stable and has less energy. Thus the work done will be positive.

#### ese

I'm confused here, I thought that the energy of the electron will be negative, since it naturally wants to go towards the protons or in other words, the system more stable and has less energy. Thus the work done will be positive.
That is correct and the opposite case of what I said. My example posed an electron (negative charge) being moved towards a negative charge (low potential, potential is a scalar). Thus opposite of the E-field force => -work.

What you just said: electron (negative) wants to go toward a positive charge (say a proton, high potential) => work done by the E-field is positive.

Also: Be careful of the use of energy and stability, the energy of the system is constant, PE is converted to KE. The system is not "more stable due to less energy".

#### inaccensa

10+ Year Member
7+ Year Member
That is correct and the opposite case of what I said. My example posed an electron (negative charge) being moved towards a negative charge (low potential, potential is a scalar). Thus opposite of the E-field force => -work.

What you just said: electron (negative) wants to go toward a positive charge (say a proton, high potential) => work done by the E-field is positive.

Also: Be careful of the use of energy and stability, the energy of the system is constant, PE is converted to KE. The system is not "more stable due to less energy".
gr8 thanks