PHYS 212 - Lecture 1

<Previous

January | February | March | April | May

Next>

Lecture 1: Electric Charges, Forces, and Fields

January 21, 2025

Reading Assignment

Read: Chapter 20, through most of page 378 (up until “Continuous Charge Distributions”)
Study: Eqs 20.1, 20.2a, 20.2b, 20.3; Exs 20.2, 20.4, and 20.5

Objectives

(Continuing objective) Describe applications of the concepts of electricity and magnetism to everyday “real-life” situations.
Use Coulomb's Law to calculate electric forces. Specifically, for a given configuration of a small number of point charges, calculate the total electric force (magnitude and direction) acting on any chosen charge, due to all the others.
For a point charge or a configuration of several point charges, calculate the electric field (magnitude and direction) at any given location.
Relate the electric force on a charge to the electric field at the location of the charge.
Describe the physical difference between conductors and insulators.
Show (using sketches) how the proximity of a charged object causes redistribution of charge in a nearby object. Explain how this can result in attractive forces.

Homework

Wednesday's Assigned Problems: A2, A4, A5, A6, X1 (below); CH 20: 17, 23, 49
Notes: See below for the links for Problems A4 and A5.
Problem X1 A $65\,\text{\(\mu$C}\) point charge is at the origin. Find the electric field at the points (a) $x=50\,\text{cm}$, $y=0\,\text{cm}$, (b) $x=50\,\text{cm}$, $y=50\,\text{cm}$, (c) $x=25\,\text{cm}$, $y=-75\,\text{cm}$.
Answers: X1 (a) $2.3 \times 10^6 \hat\imath\,\text{N/C}$, (b) $(8.2 \times 10^5 \hat\imath + 8.2 \times 10^5\hat\jmath )\,\text{N/C}$, (c) $(3.0 \times 10^5 \hat\imath - 8.9 \times 10^5\hat\jmath ) \,\text{N/C}$
Monday's Hand-In Problems from Lecture 1: A3, A109; CH 20: 26, 48
Note: this is only the first half of the hand-in set.
A4 - Problem 1 [www.physicsclassroom.com] This simulation contains two point charges, one positive ($q_2$) and one negative ($q_1$). The charges and ruler can be moved by clicking and dragging. The magnitude of $q_1$ and $q_2$ can also be changed using the slider bar. Play around with the the simulation and get a feel for how things work, then answer the following questions:
1. The simulation only shows a single value for the magnitude of the force. Shouldn't it be showing two different forces; i.e, the force on $q_1$ and the force on $q_2$? Explain why this not a problem with the simulation.
2. Using the ruler (or the grid), verify the $1/r^2$ in Coulomb's Law
3. What does Coulombs' law predict should happen to the force if you double the magnitude of the positive point charge? What if you also double the magnitude of the negative point charge? Test these predictions using the simulation.
A5 - Problem 2 [phet.colorado.edu] This simulation will allow you to look at the electric field produced by several point charges. Start by dragging three positive charges (red dots) and an E-Field Sensor (orange) on to the page. Select “Electric Field” and “Values” (Note that the unit of V/m is the same as N/C). Arrange the charges roughly into equilateral triangle.
1. Where do you think the electric field will be the largest? Why? Use the E-Field Sensor to verify your prediction.
2. Where will the electric field be zero? Why? Use the E-Field Sensor to verify your prediction.
Optional Problem 3 [www.physicsclassroom.com] (Warning: this one is a bit addictive -- it's a game.) This simulation contains a “goal” and a small blue positive test charge. By adding and relocating point charges, try to make a goal. You can use “Clear Screen” to remove any point charges on the screen and “Reset Charge” to move the test charge back to its starting position. Arrows show the magnitude and direction of the electric field.
1. Click, “Choose Level” and select “1”. Where do you have to position a positive test charge to score a goal? Now clear the screen. Where do you have to position a negative charge to score a goal?
2. Click “Choose Level” and select “2”. Using a combination of positive and negative charges, position enough charges that you can score a goal. Watch the motion of the test charge and compare it to the direction of the electric field. What can you conclude about the relationship between the electric field and the motion of the test charge?

Lecture Materials

Click here for the Lecture overheads. Answers: CT1 - 5; CT2 - 5; CT3 - 3

Videos of example problems

To see the problem statement, click on the link below. To play the video example, click on the underlined words "Video Demonstration" near the top of the page with the problem statement.

Example #1: Calculating the total force on a charge due to two other charges.
Example #2: Fields and forces on charges.
Example #3: Vector addition of fields.

Pre-Class Entertainment

In My Life - The Beatles
Keep it Coming Love - K.C. and the Sunshine Band
Is This Love? - Bob Marley
Me and Bobby McGee - Janis Joplin

Assigned Problems Guide

A2: Toy kit problem. You'll be getting the kits tomorrow in problem session. Medium problem. Pay careful attention to the free body diagrams.
A4: Fairly quick. Online problem gaining hands on experience with Coulomb's Law.
A5: Also online problem. Quick, but take some time to play with it and gain some intuition.
A6: Medium length. Need the electric field from a point charge and use superposition. A good sketch is crucial to get the distances and directions correct.
X1: Similar to A6 that you need the electric field from a point charge. There's no superposition, but the electric fields aren't all vertical. Good vector components practice problem.
20-17: Quick. Just working with the magnitude of the Coulomb force. Pause to digest the result. It might be surprising!
20-23: Fairly quick. “Field strength” means magnitude of the electric field. Use the relations between $\vec E$ and $\vec F$.
20-49: Medium long. Using Coulomb's law and adding force vectors. Use symmetry to answer part (a) (quick once you see it), and then plug through the vector addition to get part (b).