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Lecture 4: Electric Current and Magnetic Forces
January 30, 2025
Reading Assignment
- Read: 24.1, 24.3-24.5, 26.1-26.4
- Study: Eqs 24.5, 24.7; Ex 24.4; Eqs 26.1, 26.5; Ex 26.1, CEx 26.1
- Ignore: “The Hall Effect” discussion in Sec. 26.4
Objectives
- (Continuing objective) Describe applications of the concepts of electricity and magnetism to everyday “real-life” situations.
- 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.
- Correctly sketch the direction of the magnetic field in the vicinity of variously shaped magnets, especially near the North or South poles.
- Calculate the cross product of two vectors, determining both its magnitude and direction.
- Calculate the force (magnitude and direction) acting on moving charges and current-carrying conductors in a magnetic field.
Homework
- Friday's Assigned Problems: A12, A18; CH 24: 21, 23, 65; CH 26: 1, 13, 17, 21
- Monday's Hand-In Problems from Lecture 4:
A13, A17; CH 24: 28, 66; CH 26: 54
Note: this is only the second half of the hand-in set.
Lecture Materials
- Click here for the Lecture overheads. Answers: CT1a - 0; CT1b - 3; CT1c - 4; CT1d - 0; CT1e - 5; CT2 - 3; CT3 - 3; 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.- Example: Electrical Power.
- Example: Moving electron in a magnetic field
- Example: Prof. Allers' dog Finn demonstrates $q\vec{v} \times \vec{B}$
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
Assigned Problems Guide
- A12: medium-long toy kit problem. Unpack the batteries, and straighten out both pieces of the nichrome wire. Once you have things assembled, it's pretty quick.
- A18: medium. Figure out the magnetic force and then use work-kinetic energy.
- 24-21: quick Ohm's law problem.
- 24-23: medium. Connecting resistivity to resistance. You'll need to use Table 24.1 on p. 452 to find the resistivity.
- 24-65: medium-long. Take a 1 second interval. How much work does the motor do in 1 second? (Note: 15 N is the weight, not the mass, so don't multiply by g.) Then the power is work/time = the work you calculated / (1 sec). Now use the expression for power in a circuit.
- 26-1: quick conceptual problem on magnetic force.
- 26-13: medium but straightforward computation of magnetic force magnitudes.
- 26-17: long. Find the relation between q, v, B, etc for circular motion, and then substitute in v in terms of R and T. Once you've solved for T, then plug in all the values. You can find the charge and mass of an electron in the inside cover of your textbook.
- 26-21: fairly quick computation of magnetic force on a wire.