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Lecture 1: Electric Charges and Forces
January 20, 2026
Reading Assignment
- Read: 20.1, 20.2
- Study: Eq 20.1; Fig 20.6; Ex 20.2
Objectives
- (Continuing objective) Describe applications of the concepts of electricity and magnetism to everyday “real-life” situations.
- Describe the properties of charge, including that there are two types of charge, that opposites attract and likes repel, that electrons and protons have fundamental charge magnitude \(e\), and that charge is conserved. Be able to relate an amount of charge in units of coulombs to a number of fundamental charges.
- 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.
- 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, A109; CH 20: 1, 8, 15, 49, 57
-
A4
[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:
- 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.
- Using the ruler (or the grid), verify the \(1/r^2\) in Coulomb's Law
- 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.
- Monday's Hand-In Problems from Lecture 1:
A3, CH 20: 46, 48, 52
Note: this is only the first half of the hand-in set.
Lecture Materials
- Click here for the Lecture overheads. Answers: CT1 - 4; CT2 - 3; CT3 - 5; CT4 - 5
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.
Pre-Class Entertainment
None.