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Lecture 4: Electric Potential and Current
January 29, 2026
Reading Assignment
- Read: 22.1-22.2 (stop at “Continuous Charge Distributions” on p. 422); 24.1 (stop at “Current: A Microscopic Look” on p. 450), 24.3-24.5
- Study: Eqs 22.1a, 22.1b, 22.3, 22.5; Exs 22.1, 22.5; Eqs 24.1a, 24.5, 24.6, 24.7; Ex 24.4
Objectives
- (Continuing objective) Describe applications of the concepts of electricity and magnetism to everyday “real-life” situations.
- For a uniform electric field, relate \(\vec{E}\) to the potential difference between two points in the field.
- Calculate the electric potential for a system of point charges, using superposition.
- For a given physical arrangement of charges and fields, relate electric potential difference, potential energy change, work, and kinetic energy change.
- Use the definition of current and Ohm's Law to relate current, charge, potential difference, and resistance.
- Calculate the power produced or required by an electric system, given an appropriate combination of voltage, current, and resistance.
Homework
- Friday's Assigned Problems:
A9, A11, A12, X3 (below); CH 22 2, 55; CH 24 21, 23, 65
Problem X3 A
120 V battery is connected to two large, parallel conducting
plates with a separation of $2.5\,\text{mm}$, as shown.
(a) Determine the electric field magnitude and direction in
the region between the plates. (b) Determine the charge
density on the top and bottom plates.Answers: X3 (a) $4.8\times 10^{4}\,\text{N/C}$ downward, (b) $\pm 4.25\times 10^{-7}\,\text{C/m$^2$}$, with $+$ on the top and $-$ on the bottom.
- Monday's Hand-In Problems:
A13; CH 22 40, 44, 54; CH 24 28, 66
Notes: For CH 22 #54 the distance from the center of the triangle to one of the charges is $a/\sqrt{3}$.
Note: this is only the second half of the hand-in set.
Lecture Materials
- Click here for the Lecture overheads. Answers: CT1a - 1; CT1b - 3; CT1c - 2; CT2a - 4; CT2b - 3; CT3 - 4; CT4 - 6
- Car battery short circuit video
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.Pre-Class Entertainment
- Loves Me Like a Rock - Paul Simon
- You and I - Ingrid Michaelson
- I'm On My Way - The Proclaimers
- Don't Get Me Wrong - The Pretenders
- Sir Duke - Stevie Wonder