Physics Pre-Lab 212P-3

Physics Pre-lab 212P-3

Electric Fields and Electric Potential

Name:______Section:_____ Date:______

(Read this & answer the questions before coming to lab)

Summary of relevant concepts: a) The electric potential energy U of a charge at any point in space is defined as the negative of the work done by the electric field E when the charge is moved from infinity to that point. Here, we define the electric potential at infinity to be 0. Note that this definition is equivalent to saying: "electric potential energy is the work YOU have to do in bringing a charge from infinity to that point." b) The electric potential V at any point in space is defined as the negative of the work done by the electric field when a charge of +1 C is brought from infinity to that point i.e.

A   V = -тE · ds Ҙ c) We also talk about the potential difference between two points A & B: B   DV = - E · ds тA d) i.e. the negative of the work done by the electric field in moving a charge of +1C from A to B. e) Electric potential and potential differences are measured in VOLTS; electric potential energy is measured in joules. f) Equipotential surfaces are a convenient way of picturing the electric potential in any region. Most often, equipotential LINES are used to portray a cross-sectional view of equipotential surfaces. By definition, all points on an equipotential line/surface have the same potential. Pre-lab Questions:

Q1. What is a "conservative force?" (Recall: Physics 211.)

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Q2. When a mass that is free to move is released in the presence of a gravitational field, does it move from a region of high gravitational potential to a region of low gravitational potential or vice-versa?

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Q3. When a charge that is free to move is released in the presence of an electric field, does it move from a region of high electric potential to a region of low electric potential or vice-versa? Does your answer depend on whether the charge is positive or negative? Why/why not?

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______The figure below shows a region of space with a uniform electric field E. You can move charges from A to B along the three different paths shown. ACBD is a square of side L.

Q4. Suppose a positive charge +q is moved from A to B. Calculate the work W done by the electric field on the charge, if the charge +q were taken:

(a) first from A to C and then from C to B.

(b) first from A to D and then from D to B. Q5. Calculate the work done by the electric field on the charge if the charge were moved along the straight line AB. How does this compare with your answers to Q4?

Q6. From your answers to Q4 & Q5, what is the potential difference V = VB - VA between points A and B ?

Q7. An equipotential surface is defined as a surface on which there is no potential difference between any of the points. What are the equipotential surfaces for the problem above? Sketch a cross-sectional view of a few equipotential surfaces, showing surfaces with a constant potential difference between them.