Mr. Rogers' Honors Physics Syllabus 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter Waves (14) Sound (15) E-Fields (21) Electricity (22) & Circuits (23) Magnetics (24) & (25)

Magnetics

Relevance: Electrical equipment is ubiquitous. Some knowledge of it is very helpful for troubleshooting equipment both as an employee, hobbyist or do-it-yourselfer.

 SC Standards : Indicators P-4.9 Explain the effects of magnetic forces on the production of electrical currents and on current carrying wires and moving charges. P-4.10 Distinguish between the function of motors and generators on the basis of the use of electricity and magnetism by each.

 Practice Test Study Guide
 Objectives
 Essential Question: Where is Earth's north pole?

Basic Concepts

1. State the general rule for the forces two magnets exert on each other.

like poles repel

opposite poles attract

2. Draw the magnetic field (B-field) lines on a bar magnet. Arrows go from N to S pole.

3. State that B-field like gravity field (g-field) is a vector.

4. State that Earth's south magnetic pole is found near the (geographic) North Pole and the Earth's north magnetic pole is found near the (geographic) South Pole.

5. Explain what the magnetic field lines indicate. Magnetic fields are draw with rays, similar to the way that E-fields and g-fields are drawn.

arrows: show the direction of the force that would exist on an imaginary north pole of a magnet

spacing between lines: indicates the magnitude of the above force. Closely spaced lines = strong magnetic field.

6. State an important difference between magnetic field lines and electric field lines. Magnetic field lines are closed loops. E-field lines are not.

Homefun (formative/summative assessment): Read sections 24.1

 Formative Assessment: Physics Investigation Title Twidler's Delight (TD) Investigation Research Question What causes the opposite shaft of the TD to rotate when the shaft on the other end is manually spun? Background The scientific method has 4 steps, 1) formulate a research question, 2) hypothesize how the research question can be answered, 3) design an experiment to test the hypothesis, 4) collect and analyze data 5) draw a conclusion. However, scientific experiment often start with observations and informal experiments (a fancy way of saying playing around). Eventually, formal experiments are designed and run, but even though the early stages of investigation use the scientific method, often the steps are taken informally or out of order. The process often has a distinct social element as the scientists discuss hypotheses and compare notes. The twidler's delight investigation is intended to give students an experience with what the early stages of an investigation involve. Have fun but do not modify, disassemble, or in any way harm the twidler's delight. Instructions Play around with the twidler's delights and observe their behavior. Compare your observations with those of others. Write a brief summary of your observations. Formulate and record 3 possible explanations (hypotheses). Design an experiment that could verify or eliminate at least one of the explanations. Deliverables A hand-written sheet with a short paragraph summarizing your observations, a description of the 3 possible explanations, and the procedure for the experiment. Resources/Materials Twidler's Delights

 Essential Question: How can a magnet be used to generate electricity?

How Electricity is Generated -- Magnetic Flux Changing With Respect to Time

Relevance: Sources of electricity from the power plant providing electricity to homes to the alternator charging a car's battery use magnetic flux changing with respect to time as the way to produce the power.

1. Explain what is meant by magnetic flux.

• Roughly speaking, it's the number of magnetic field lines that pass through a given area.

• It can also be thought of as the amount of magnetic field in a given area.

2. Define the term EMF. EMF means electromotive force and is another term for voltage difference. Note, it is not really a force but represents a difference in electrical potential energy.

3. Describe how magnetic flux changes when the surface is tilted with respect to the magnet's axis and when the distance of the surface from the magnet is changed. Relevance: Changes in Magnetic flux through surfaces outlined by a loop of wire are used for generating electricity. Any change in the magnetic flux with respect to time creates an EMF (voltage difference) across the loop of wire and cause a current to flow.

tilting the surface reduces flux

increasing the distance from a magnet reduces the flux

EMF = (change in magnetic flux) / (time interval)

The faster the magnetic flux is changed, the higher the voltage. It makes no difference whether the magnetic flux is increasing or decreasing. Either one will create a voltage difference and subsequent current in a loop of wire.

4. Describe how a magnet falling toward a loop of wire can create a voltage and subsequent current in the loop. The magnetic field gets stronger within the loop of wire as the magnet gets closer. A stronger field means more field lines pass through the area defined by the loop, hence the magnetic flux is changing over time creating a voltage and current in the loop as it does so.

5. Describe how a spinning loop of wire in a magnetic field can produce electrical power. If the magnetic field is oriented in the correct direction, the spinning wire will produce a change in magnetic flux per unit of time, creating a voltage across the loop and a current when the loop is connected to an electrical device.

6. Describe the difference between a DC motor and a DC generator.

Motor: a current is passed through loops of wire in the presence of a correctly oriented magnetic field causing the loops to spin.

Generator: a power source is used for spinning loops of wire in a correctly oriented magnetic field causing a current to flow. A generator is essentially a motor run backwards.

Homefun (formative/summative assessment): Practice problems 2-4 page 647

 Essential Question: Why is Earth's magnetic field important?

Magnetic Forces on moving charges

1. Apply the first right hand thumb rule to determine the force on a moving charge in a magnetic field.

2. Calculate the force on a moving charge in a magnetic field.

F = qvB(sin q)

where:

q = charge

v = velocity

B = magnetic field strength

q = the angle between the velocity and B-field vectors

3. State the reason why the magnetic force on a freely moving particle can never do work. The magnetic force on the moving charge is perpendicular to the charge's displacement.

4. Describe how Earth's magnetic field helps protect us from high energy charged particles emitted by the sun.

Relevance: The deflection of high energy particles by Earth's magnetic field helps preserve our atmosphere and prevent celular damage that could cause human cancer or harm crops or other forms of life.

Homefun (formative/summative assessment) . read 22.2, practice problems 17-20 page 654

 Essential Question: How can magnetic fields be produced using electricity?

Electromagnetism-- Fields around current carrying wires

1. Apply the first right hand thumb rule to determine the direction of the magnetic field around a current carrying wire.

2. Apply a second right hand thumb rule to determine the direction of the magnetic field through the center of a current carrying loop of wire.

3. Describe the direction of the forces acting on two parallel current carrying wires.

Currents in the same direction: the forces pull the wires together

Currents in the opposite direction: the forces push the wires apart

Relevance: Electromagnets are ubquitous. They are found in everything from door bells to the junk yard electromagnets used for picking up wrecked cars. Electromagnets are used for writing data to the hard drives in computers.

Homefun (formative/summative assessment): problems 74, 75, 76, page 666

 Essential Question: How can you best prepare for the test?

Review

Formative Assessments:

1. Work review problems at the board

2. Work practice problems.

Metacognition Problem Solving Question: Can I still work the problems done in class, several hours or days later? Some amount of repetition on the exact same problems is necessary to lock in learning. It is often better to thoroughly understand a single example of a problem type than to work example after example understanding none of them completely.

Relevance: Good test preparation is essential to performance in physics class.

Homefun (formative/summative assessment): turn in on the day stapled to the back of the test.

Summative Assessment: Unit exam objectives 1-23

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