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# 10_MagneticFields-RH.. - Le Moyne

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10_MagneticFields-RH.. - Le Moyne

Magnetic Fields: Right Hand Rules

Magnetic forces on wires, electron beams, coils; direction of magnetic field in a coil

November 14, 2013

Before the lab, read all sections of the

Introduction and the warning at the ______________________________________ beginning of Activity #1. Then

last page of this handout. Be prepared to discuss the Pre-Lab

questions in lab as Activity #1. ______________________________________

______________________________________

You will return this handout to the instructor at the end of the lab period. Table of Contents

0. Introduction 2

2. Activity #2: Lab instructor presents the e/m apparatus to each group 5

3. Activity #3: Force on a current carrying wire. 5

4. Activity #4: Electron beam moving in a magnetic field 7

5. Activity #5: Predicting the magnetic field around a coil of wire 9

6. Activity #6: Tracing the field of a magnet and of a coil with a compass 10

7. When you are done ... 12

Comprehensive Equipment List

Bar magnet (Alnico preferred; N and S taped over with masking tape; ends randomly re-

labeled A and B)

Suspension Loop as in Figure 2. (Bare 22 gauge wire suspended by two lengths of 30

gauge wire. Short lengths of insulated 22 gauge wire connected to the 30 gauge wire for

making the connection to the battery. Wire-to-wire connections are soldered.)

Banana plug wires; two, one red and one black (Radio Shack 270-375C)

Alligator clips (one pair) that can plug onto banana wire plugs

e/m apparatus with power supplies (one setup as a demonstration)

Large coil of wire

Magnetic compass

D-cell (1.5 V) in D-cell holder

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Magnetic Fields: Right Hand Rules

0. Introduction

Electromagnetism is one of the most interesting and intriguing areas of physics to study. Certain kinds of objects are able to exert forces over distance for some reason. Charged rods attract or repel depending on their composition and the material they were rubbed with; magnets attract or repel depending on which poles are brought near to each other. A magnet will always attract a ferrous metal (steel, iron, etc) but has no effect on metals like aluminum or copper; or do they? You will investigate electromagnetic interactions using simple materials like batteries, wires, magnets, and more complex devices like a galvanometer. You will even build a simple motor. Once you understand how this motor works, you will, in principle, understand how all electric motors work.

The following sections in this Introduction cover the concepts and rules you need to complete the activities of this lab on magnetism. There are no formulae. In these lab activities you will be concerned with the directions of currents, magnetic fields, and magnetic forces but not with the magnitudes of the currents, fields, or forces.

The summary here is complete but very terse. For more expansive explanations with examples and diagrams, refer to the general physics text citations given in each section below. 0.1 The nature of electric current

Text Reference

thSection 20.1 Cutnell & Johnson, 6 edition

thPage 841 and the first half of page 842 Serway ? Beichner, 5 edition

Electric current always flows in closed loops.

The direction of electric current in a circuit connected to a battery is always from the plus

terminal of a battery toward the negative terminal.

You should think of electric current as positive charges moving from the plus terminal of

a battery to the negative terminal (even though in wires current is really negative

electrons moving from the negative terminal to the positive terminal).

0.2 The nature of magnetic fields

The lines of the magnetic field always form closed loops.

0.3 The nature of permanent magnets and their magnetic fields

Text Reference

thSection 21.1 Cutnell & Johnson, 6 edition

thPage 905, Figure 29.1 on page 906, Serway ? Beichner, 5 edition Figure 30.6(c) on page 943

Permanent magnets always have two poles, called N and S for North and South.

Opposite magnetic poles attract, and like magnetic poles repel.

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Magnetic Fields: Right Hand Rules

The magnetic field lines in the space outside a permanent

magnet always have a direction that points away from N poles

and towards S poles. (Inside the permanent magnet, they point

from S to N, thus completing continuous closed loops that

leave the permanent magnet at or near the N pole, re-enter the

permanent magnet at or near the S pole, and continue through

the permanent magnet back to the N pole. See the diagram to

the right.)

F

0.4 Compass conventions

Compass Lines of B

The lines indicate the magnetic field,

and the arrowheads show its direction.

On the compass, the arrowhead is the

red end of the pointer needle.

Figure 1 How to use a compass to determine the direction of a magnetic field Each compass used in this lab has a pointer needle. One end of the pointer needle is

painted red, and the other is white.

Figure 1 shows how to use a compass to determine the direction of a magnetic field. The

red end of the pointer needle points in the direction of the magnetic field.

0.5 Right Hand Rule #1: Direction of magnetic forces on charges moving in magnetic fields

Text Reference

thSection 21.2 Cutnell & Johnson, 6 edition

thPage 908 and Figure 29.4 on page 908 Serway ? Beichner, 5 edition

If a charged particle in a magnetic field has velocity zero m/s, the magnetic field exerts

zero force on the particle.

If a charged particle with positive charge is moving in a magnetic field, the relation

between the direction of the particle's velocity, the direction of the magnetic field, and the

direction of the magnetic force on the particle is given by the Right Hand Rule. See the

references for the statement of the Right Hand Rule.

If a charged particle with negative charge is moving in a magnetic field, the magnetic

force is in a direction opposite to the direction given by the Right Hand Rule.

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Magnetic Fields: Right Hand Rules

0.6 Right Hand Rule #2: Direction of magnetic field around a current-carrying wire

Text Reference

thSection 21.7 Cutnell & Johnson, 6 edition

thPage 941, Figure 30.3 on page 941 Serway ? Beichner, 5 edition Figure 30.6(a) on page 945

A wire that carries an electric current has a magnetic field the lines of which loop around

the wire in closed loops.

If you seize the wire with your right hand in a fist and your thumb pointing in the

direction the current is flowing, your fingers wrap around the wire in the direction of the

magnetic field. This is Right Hand Rule #2.

You can also use Right Hand Rule #2 to determine the direction of the magnetic field

generated by a coil of wire. Grab the coil with your right hand in a fist (so that your

fingers poke through the center of the coil) and with your thumb in the direction of the

current. The direction of the magnetic field loops is the same as the direction your finger

point.

0.7 Uniform magnetic fields make charged particles move in circles

Text Reference

thSection 21.3, page 634 Cutnell & Johnson, 6 edition See especially The Circular Trajectory, page 636

thSection 29.4, page 918 Serway ? Beichner, 5 edition Look for a discussion of circular trajectories

A charged particle moving perpendicular to a uniform magnetic field and subject to no

forces other than the magnetic force will have uniform circular motion.

The magnetic force will always be directed towards the center of the particle’s circular

trajectory and will keep the particle moving in a circle. Therefore the magnetic field is

what supplies the particle’s centripetal acceleration.

If R is the radius of the circle, q the particle’s charge, m its mass, and v its velocity,

2Newton’s Second Law, ;F = ma becomes qvB = mv/R, because qvB is the magnetic xx2force, and mv/R is mass times the centripetal acceleration.

2 Solving qvB = mv/R for R gives R = mv/qB, the

Please, No magnets near radius of the circle in terms of particle mass,

velocity, charge and the magnetic field strength. the computer monitor!

monitors. all of you to agree on the answers. Your instructor will

be pleased to assist, if appropriate. Thanks.

1.2 When you and your lab partners have agreed on all

the answers to the pre-lab questions, have the lab

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Magnetic Fields: Right Hand Rules

2. Activity #2: Lab instructor presents the e/m apparatus to each group

As each group of lab partners completes their review of the pre-lab questions, the instructor shows the group the e/m apparatus. The purpose is to ensure that everybody knows what they are seeing when the look at the e/m apparatus.

Outer coils produce a magnetic field parallel to the floor

High-quality vacuum inside the glass bulb

Electron gun (hot cathode boils electrons out of the wire, accelerating potential)

Electrons move in a circle due to the magnetic field

Electron trajectory is visible because the low pressure gas inside the tube glows due to

electron impacts with gas molecules

3. Activity #3: Force on a current carrying wire.

Equipment: Bar magnet (Alnico preferred; N and S taped over with masking tape; ends

randomly re-labeled A and B)

Suspension Loop as in Figure 2. (Bare 22 gauge wire suspended by two lengths

of 30 gauge wire. Short lengths of insulated 22 gauge wire connected to the 30

gauge wire for making the connection to the battery. Wire-to-wire connections

are soldered.)

D-cell in a D-cell holder

Alligator clips (one pair)

BatteryTable top

+-

Tape here

Copper wire

Figure 2 The Suspension Loop when connected to the battery

Do not connect the Suspension Loop to the battery at this time.

The lab instructors do not take off points for

wrong answers to the questions in Activity #3.

Q 1 If the Suspension Loop were connected to the battery and the N pole of a vertical bar magnet were placed directly below the short horizontal wire at the bottom of the loop, which way would the Suspension Loop deflect? Toward the table or away from the table? Explain Page 5 ? Le Moyne Physics Faculty

Magnetic Fields: Right Hand Rules

your answer using Right Hand Rule #1 (the rule which gives the direction of the magnetic force on a moving positive charge).

Q 2 The same question as the previous question, but this time the S pole of a vertical bar magnet is directly below the Suspension Loop.

3.1 When performing the following steps, try to have the suspension loop (see Figure 2) connected to the battery for as little time as possible. This is to keep the battery from running down too soon.

3.2 Without using excess time, but still being careful, do the following.

3.2.1 Use alligator clips to clamp the Suspension Loop leads to the posts on the battery

holder.

3.2.2 With your bar magnet oriented vertically and having the pole labeled A up, slowly

bring the A pole up under the Suspension Loop and observe which way the Suspension

Loop deflects.

Q 3 Which way did the Suspension Loop deflect? Toward the table or away from the table?

Q 4 Based on the answer to Q 3, is the A end of the bar magnet an N pole or a S pole?

3.2.3 Repeat 3.2.2 but with the B end of the bar magnet up.

3.2.4 Disconnect the Suspension Loop from the battery.

Q 5 Which way did the Suspension Loop deflect? Toward the table or away from the table?

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Magnetic Fields: Right Hand Rules

Q 6 Based only on the answer to Q 5, is the B end of the bar magnet an N pole or a S pole?

3.3 Remove the masking tape from your bar magnet, and see if your identification of the polarity of the ends of the bar magnet were correct.

Q 7 Did you correctly identify the polarity of the ends of your bar magnet? (Yes or No)

3.4 If you did not correctly identify the polarity of the ends of your bar magnet, please consult with your lab instructor to determine what went wrong.

4. Activity #4: Electron beam moving in a magnetic field

Equipment: e/m apparatus with power supplies (one setup as a demonstration shared by

everyone in the lab)

Electron beam inside glass

bulb. Inside the bulb is a

near-vacuum, containing

only a low-pressure gas that

glows when electrons

collide with its atoms.

Glass bulb containing

the electron beam ?

Current-carrying coils

+ 150 VDC

(300 VDC Max)

2 A DC

Max

+

~ 6.3 VAC

Figure 3 Electron beam moving in the magnetic field produced by current-carrying coils of wire 4.1 Note the electron beam in a magnetic field prominently displayed in the lab. The direction in which the electrons are moving is as indicated in Figure 3.

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Magnetic Fields: Right Hand Rules

Q 8 The beam is composed of electrons, and the electrons are moving counterclockwise in uniform circular motion. Which direction in the beam does electric current flow? Clockwise or counterclockwise? Explain.

Q 9 At the center of the electrons' orbit (where the ? is in Figure 3 ), which direction does the magnetic field created by the moving electrons point? With respect to Figure 3, choose from (1) up, (2) left, (3) right, (4) down, (5) into the paper, or (6) out of the paper. Explain your answer using Right Hand Rule #2.

Q 10 From the direction in which the electrons move, determine the direction of the magnetic field created by the coils. With respect to Figure 3, choose from (1) up, (2) left, (3) right, (4) down, (5) into the paper, or (6) out of the paper. Explain your answer using the Right Hand Rule #1. Method: You need to begin by identifying the direction of the force that the magnetic field created by the coils exerts on the electrons in the beam. From the direction of the magnetic force on the electrons and their velocity, you can determine the direction of the coil's magnetic field.

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Magnetic Fields: Right Hand Rules

5. Activity #5: Predicting the magnetic field around a coil of wire

Equipment: Large coil of wire

D-cell battery in D-cell holder

Magnetic compass

Banana plug wires; two, one red and one black

Alligator clips (one pair) that can plug onto banana wire plugs

S

F

Figure 4 A compass inside a coil connected to a battery

Do not connect the coil to the battery at this time.

The lab instructors do not take points off for wrong answers to the questions in Activity #5.

5.1 Carefully inspect your coil to determine which way the wire in the coil is wound. If you are not sure, ask your lab instructor to assist you.

Q 11 For someone looking at your coil face-on, like Figure 4, and if the battery were connected as in Figure 4, would the current flow clockwise or counterclockwise?

Q 12 If your coil were connected to the battery as in Figure 4, which way would the magnetic field inside the coil point (for a person looking at the coil face-on)? Out of the paper and toward you? Or into the paper and away from you? Use the Right Hand Rule to explain your answer.

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Magnetic Fields: Right Hand Rules

Q 13 With the compass inside the coil, which way would the needle of the compass point, if the coil were connected to the battery as in Figure 4? With respect to Figure 4, out of the paper toward you, or into the paper away from you? Explain your answer in terms of the way a compass needle responds to a magnetic field.

Q 14 With the compass outside the coil and on top of it, which way would the compass needle point (still pretending the coil is connected to the battery as in Figure 4)? With respect to Figure 4, out of the paper toward you, or into the paper away from you? Explain your answer in terms of the way a compass needle responds to a magnetic field.

6. Activity #6: Tracing the field of a magnet and of a coil with a compass 6.1 Without using excess time (to avoid discharging the battery), but still being careful, do the following.

6.1.1 Use banana plug wires, with an alligator clip plugged onto one end, to connect the

positive terminal of the battery to the socket marked “S” on the coil.

6.1.2 Similarly, connect the negative terminal of the battery to the socket marked “F”

on the coil.

6.2 Use your compass to trace the magnetic field of the coil as follows:

6.2.1 Put the compass inside the coil, and observe the direction in which the compass

needle points.

6.2.2 Put the compass outside the coil but near to the ends of the coil (the end toward

you and the end away from you), and observe the direction in which the compass needle

points in both cases.

6.2.3 Hold the compass above and to the sides of the coil, and again observe the

direction in which the compass needle points.

Page 10 ? Le Moyne Physics Faculty

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