Ph272- General Physics II (Electricity and Magnetsism)

Lecture 5: Ch. 24 - Electrostatic Potential

Key Topics

Overview

• So far we have discussed the concept of electric field (unit=Newton/Coulomb), but when we go to the store to buy a battery, it is rated in volts. In this lecture, we will relate the electrostatic field to the electrostatic potential (volt). Before we do that, we have to introduce the idea of electrostatic potential energy.

Electrostatic Potential Energy

• Concept: If you move a charge (test charge) against an E-field, you have to do work. Work=energy. The energy you have spent is stored in the change in electrostatic potential energy

Analog: If you stretch a spring, the amount of energy equal to your work done is stored in the change in potential energy of the stretched spring.

Another analog: If you pull two masses apart (eg. lift an object away from the Earth), the amouint of energy equal to your work done is stored in the change in gravitational potential energy of the separated masses.

Statement is the work done = the change in potential energy

-> Q. If the change in potential energy is negative => work done is negative (example a mass is drop from 10 m to 5 m). What does it mean?

-> Do Problem 20-1(a,b), P.685.

• Calculations:

Uniform E-field: Calculate the change in electrostatic potential energy when a test charge, q0 , is moved from r1 to r2 in an uniform E-field.

Non-uniform E-field: Calculate the change in electrostatic potential energy of two point charges from r1 to r2.

Electrostatic Potential

• Concept: If we move a test charge in an E-field (e.g. the E-field produced by a battery), the change in potential energy is proportional to the test charge q0, we can define the change in electrostatic potential (of the battery) by dividing the change in electrostatic potential energy by q0. The change in electrostatic potential is a property of the battery independent of the test charge.
• Unit: Volt = Joule/Coulomb or Electric field x meter.

-> Do Problem 20-1(c-f), P.685.

• Calculation: Calculate the electrostatic potential due to a point charge
• Standard reference point for electrostatic potential.

-> Do Problem 20-6, P. 686

Equip-potential Surface

• Equi-potential map.
• Calculate approximate electric field from equi-potential map.

Review Questions

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