# Confused about electric field strength and PE

1. Jan 17, 2007

### crossfacer

I have just started to learn these things(electric field strength, electric potnential energy and electric potential). I am not very clear about what they are. I would like to ask a few questions here.

Why is electric field strength somtimes +ve but sometimes -ve? what does each of them imply?

What is electric potential energy? Is it a vector or scalar? does it increase or decrease when it comes closer to a charged object?

What is electirc potential energy and electric potential? Are they related to the electric field strength?

Why is electirc field strength equal to zero inside a charged metal sphere?

May anyone answer my questions? Thank you.

2. Jan 17, 2007

### cristo

Staff Emeritus
Welcome to PF, crossfacer. Please note that, since this is a homework question, we need to see some work from you before we can help you. Do you have any thoughts on any of the questions?

3. Jan 17, 2007

### crossfacer

Electric field strength is the force(acting on a small and positive point charge) per unit charge around a charged object. I guess +ve means attract the point charge towards the charge object and -ve means repulsion.

Electric potential energy is the total work done required to bring a point charge from infinity to somewhere around a charged object. I think it is a scalar.( However, the book said U for two like charges is always +ve and that for two unlike charges is always negative, I don't know what is going on ). I think U decrease when it come closer to the charged object.

Electric potential is U per unit charge. My teacher said when you want to bring a point charge towards a charged object, you need to overcome the electric potential, I don't know what it means.

For the last question, my teacher said if there is charge inside the charged metal sphere(static), there will be current in the sphere. I don't know why:yuck: . Also, although U inside it is zero, the electirc potential is not zero! Why it that so?

Thank you for spending so much time on my questions

4. Jan 17, 2007

### turdferguson

In a good conductor, the field is zero. The free electrons want to get as far away from each other as possible, so they move to the outside edges of the sphere in this case. If there were a field inside the conductor, the electrons would move until they reached equilibrium. No net force means no net field.

Voltage is given in units of J/C, that is, energy per charge. Dealing with electric potential is analogous to gravitational potential. Two masses at the same height have the same potential, but they may have different GPE depending on their masses. Voltage does not take into account the teest charge, but EPE does. Two different charges on an equipotential line have the same voltage, but if one has more charge it will have more EPE.

A positive charge wants to move from high potential to low potential, because positive is associated with positive voltage and negative is associated with negative voltage. When you move a positive charge into higher potential, you do work on it. W=q*deltaV. Similarly, if you move a negative charge from high voltage to low voltage, work is done on the charge. q is negative, but so is the change in voltage. As you mentioned, the voltage at infinity is taken to be zero, which is important in conservation of energy problems

An important difference between electric field/force and voltage is that voltage is a scalar. This means the voltage at a point can be summed up easily without vectors. However, like 2 positive forces can cancel out if they act in different directions. The net force is zero, but the voltage doees not cancel out.

5. Jan 18, 2007

### crossfacer

Thank you very much for your answer, I get it clearer now.

According to the third paragraph, positive charge move from high potential to low potential, if it moves from low to high potential, work is required. Then, will energy be released when it move from high to low potential? If yes, then how to explain that the EPE of the postive charge increase when it moves from high to low potential energy?

6. Jan 18, 2007

### turdferguson

It is just like gravity, you do work on the positive charge to bring it to an area of high potential. Now it has EPE. Once you let it go, this potential energy is transformed to kinetic energy.

For example, a positive charge has 20J of electric potential energy when it is close to a positive charge, when the charges are very far apart (voltage at infinity is zero), the charge will have 20J of kinetic energy, assuming the second charge stays in place

7. Jan 20, 2007

### crossfacer

Thank you very much