# Homework Help: Electric potential energy and particle charge

1. Jan 24, 2005

### figs

I've tried all three of these problems I dont know how mant times, and my work is just not correct. Any clue? The third prob, i really used some awkward equations.

A particle has a charge of +1.53 mC and moves from point A to point B, a distance of 0.197 m. The particle experiences a constant electric force, and its motion is along the line of action of the force. The difference between the particle's electric potential energy at A and B is EPEA-EPEB=+9.39×10-4 J. Find (a) the magnitude of the electric force that acts on the particle and (b) the magnitude of the electric field that the particle experiences.

The inner and outer surfaces of a cell membrane carry a negative and positive charge, respectively. Because of these charges, a potential difference of about 0.0680 V exists across the membrane. The thickness of the membrane is 7.95×10-9 m. What is the magnitude of the electric field in the membrane?

Two hollow metal spheres are concentric with each other. The inner sphere has a radius of 0.1440 m and a potential of 86.0 V. The radius of the outer sphere is 0.146 m and its potential is 82.0 V. If the region between the spheres is filled with Teflon, find the electric energy contained in this space.

2. Jan 25, 2005

### figs

1) A particle has a charge of +1.53 mC and moves from point A to point B, a distance of 0.197 m. The particle experiences a constant electric force, and its motion is along the line of action of the force. The difference between the particle's electric potential energy at A and B is EPEA-EPEB=+9.39×10-4 J. Find (a) the magnitude of the electric force that acts on the particle and (b) the magnitude of the electric field that the particle experiences.

i derived force by solving for E=kq/rsquared, then plugged into F=qE.
i derived electric field by solving for the change in V=change in potentials/q, then i found teh electric field=-change in V/change in distance.

I'm guessing i used the wrong equations somewhere b/c im wrong in my process.

2) The inner and outer surfaces of a cell membrane carry a negative and positive charge, respectively. Because of these charges, a potential difference of about 0.0680 V exists across the membrane. The thickness of the membrane is 7.95×10-9 m. What is the magnitude of the electric field in the membrane?

I thought i could just use E=-change in V/change in distance, but my solution was incorrect.

3) Two hollow metal spheres are concentric with each other. The inner sphere has a radius of 0.1440 m and a potential of 86.0 V. The radius of the outer sphere is 0.146 m and its potential is 82.0 V. If the region between the spheres is filled with Teflon, find the electric energy contained in this space.

This one i played with, and someone gave me an equation to work with but, what this is equal to and where they derived this i have no clue

so i dint bother to deal with this equation, even though it looks all simplified and easy.
b/c i thought i could find a solution using V=(kq/rsquared)+(k/q'/rsquared).
but that's not the easiest direction.

3. Jan 25, 2005

### ehild

You do not need to determine the force field of the moving charge, but the electric field that acts upon it. Just think what is r you used in your formula.

Read the text: the particle experiences a constant electric force.
Now you have to know how to calculate the the difference of the potential energy between points A and B. It is equal the work done by the electric force. It is constant and parallel to the line of motion, so the work is simple force times distance. You know work and distance, you get the force.

You can determine the magnitude of the electric field now. The electric field intensity is defined as the electric force on a charged particle divided by its charge. You know the force and you know the charge. So you can get the field intensity.

ehild