Physics 20: 1-4 Magnetic Fields
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 20: 1 to 4
- 20.1 Magnet and Magnetic Fields: As with electricity, magnetism is a property of matter that exerts both attractive and repulsive forces on other magnetized matrials. Its field of influence can be described using magnetic field lines which by convention point from "NORTH" into "SOUTH" areas of the compass. North is defined as the end of the magnet that lines up with the geographic north-south axis of the earth, in alignment with the earth's magnetic field.
- 20.2 Electric Currents and Magnetic Fields: Magnetic fields arise from moving electrical charges, so current in a wire produces a magnetic field. We use one "right hand rule" to determine the direction field lines (see discussion below).
- 20.3 The magnetic field B: We use another right hand rule to determine the direction of the force Fmag exerted by a magnetic field B on an electrical charge in the field. This force is always at right angles to the magnetic field. The strength of the force depends on the strength of the field, the amount of electrical charge present and its rate of motion (current), and the direction (or angle) of motion of the charge relative to the magentic field lines.
- 20.4 Electric charges moving in a magnetic field: If we replace i (current) with charge / unit time, we can express magnetic force as dependent on the velocity of the charged particle, and use our right hand rule (point in the direction v of the charge, rather than the direction i of the current) to determine the resulting force on a charge moving through a magnetic field B.
|Force on electric current in a uniform magnetic field||Note that the Force is a vector and requires a direction, given by the right-hand rule. This relationship is a vector cross-product, written F = lI ⊗ B. The magnitude of a cross-product is calculated usng the sine of the angle θ between I and B, which is maximum when θ is 90°. |
|Force on a moving charge in a magnetic field||
||The direction of the cross-product is at right angles to the plane containing the vectors involved.|
Whether it is up or down depends on the right-hand rule.
Read the following weblecture before chat: Magnetic Fields
Use the physics simulation for a Charged Particle in a Magnetic Field.
- Start the simulation using default values. What happes to the charged particle? Retain the trace and compare paths as you make the following changes.
- Double the mass of the particle. How does the particle's path change?
- Reset and double the velocity of the particle. How does the particle's path change?
- Reset and double the charge on the particle. How does the particle's path change?
- Reset and double the magnetic field strength. How does the particle's path change?
- What happens if you change the sign on the charge from positive to negative?
Interactive Physics Simulations are provided at the GeoGebra site GeoGebra site.
Chat Preparation Activities
- Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- The chapter quiz is not yet due.
If you want lab credit for this course, you must complete at least 12 labs (honors course) or 18 labs (AP students). One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!
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