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Lecture 5: Magnetic Forces
February 3, 2026
Reading Assignment
- Read: 26.1-26.4 (stop at “The Hall Effect” on p. 495). 26.6 (start after Eq. 26.12 and skip “Dipoles and Monopoles”)
- Study: Eqs 26.1, 26.5, 26.13, 26.15; Exs 26.1, 26.5; CEx 26.1
Objectives
- (Continuing objective) Describe applications of the concepts of electricity and magnetism to everyday “real-life” situations.
- Calculate the force (magnitude and direction) acting on moving charges and current-carrying conductors in a magnetic field.
- Starting from Newton's 2nd law, relate the velocity, magnetic field strength, and radius of curvature for a particle moving in a uniform magnetic field.
- For a current loop or coil in a uniform magnetic field, calculate the magnetic moment, the torque on the coil, and the magnetic energy.
Homework
- Wednesday's Assigned Problems:
A18, X4 (below); CH 26 1, 2, 13, 17, 21, 31, 55
Problem X4 A
particle of positive charge $q$ enters a region of uniform
electric and magnetic fields with $E=170\,\text{N/C}$ pointing
upward and $B=0.042\,\text{T}$ pointing out of the page, as
shown in the figure.(a) At what speed $v$ will the particle experience no net force? (b) Would your answer change if the particle had charge $-q$? (c) For the positive charge, which way does it deflect if it is traveling slower than your answer to part (a)?
Answers: X4 (a) $4050\,\text{m/s}$, (b) no, (c) upward
Notes: For CH 26 #17 the answer in Wolfson is incorrect. It should be 311 ns.
- Monday's Hand-In Problems:
A17, X5 (below); CH 26 18, 32, 54
Problem X5 A single-turn wire loop 10 cm in diameter carries a $12\,\mbox{A}$ current. It experiences a $0.015\,\mbox{N$\cdot$m}$ torque when the normal to the loop plane makes a $25^\circ$ angle with a uniform magnetic field. Find the magnetic field strength.
Notes: Hint for CH 26 #18: The frequency is the reciprocal of the period of the electron's orbit.
Note: this is only the first half of the hand-in set.
Lecture Materials
- Click here for the Lecture overheads. Answers: CT1a - back; CT1b - 3; CT1c - 4; CT1d - back; CT1e - 5; CT2 - 6; CT3 - 3; CT4 - 6; CT5 - 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: Moving electron in a magnetic field
- Example: Prof. Allers' dog Finn demonstrates $q\vec{v} \times \vec{B}$
- Example of torque on a coil with multiple turns
Pre-Class Entertainment
- Losing My Religion - REM
- Ain't No Mountain High Enough - Tammi Terrell & Marvin Gaye
- Cold Shot - Stevie Ray Vaughan
- The Best of What's Around - Dave Matthews Band
- Respect - Aretha Franklin