The Effect of Electric and Magnetic Fields on Cathode Rays (Electron Beams) - The Lorentz Force

Questions:

Examine the connections between the power supply and the cathode ray tube (CRT).

Electrons should flow from the negative terminal to the positive terminal when the power

supply is turned on.

Turn the Van de Graaff generator on briefly. The dome electrode of the Van de Graaff

generator becomes negatively charged. Touch the discharge electrode to the charged dome

electrode to obtain a negative charge on the discharge electrode. Try to deflect the negative

electron beam in the CRT using the electric field of the Van de Graaff discharge electrode.

Turn down the lights so you can see the beam better.

E-Fields 1. What is the direction of the electric field of the discharge electrode? (1)

2. In what direction does the E-field of the discharge electrode deflect the electron beam?

(1)

B-Fields Place the N pole of the bar magnet beside and behind the CRT pointing the N pole at the

beam.

3. Describe the direction of the magnetic field? (1)

4. In what direction does the B-field near the north pole of the bar magnet deflect the

beam? (1)

5. What’s wrong with the drawing above? (1)

1

NAME: __________________________________

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Oersted’s Experiment

RHR – 1 Predicting the Direction of the Magnetic Field Around a Current Carrying Wire

The Oersted Effect Often during his lectures at the University of Copenhagen H. C. Oersted had demonstrated the non-existence of a connection between electricity and magnetism.

He would place a compass needle near to and at right angles to a current carrying wire to show that there was no effect of one on the other.

After one of the lectures a student asked, "but, Professor, what would happen if the compass needle was placed parallel to the current carrying wire?" Oersted said, "Well, let's see," and the rest is written in the history of physics; the student's name is forgotten.

Questions:

Be sure the compass needle and the frame under it both point in the same direction. You may

have to adjust the orientation of the frame slightly. The compass should be pointing north

provided there are no magnets or ferromagnetic objects near by. Examine the connections to

the battery and the frame carefully.

6. What will be the direction of conventional current flow in the aluminum frame just

beneath the compass needle when the switch is closed? (1)

7. What will be the orientation of the magnetic field produced by this current above the

frame where the compass resides? (1)

8. What direction should the compass point when you close the switch? (1)

9. Close the switch on the apparatus. Were you right? (1)

2

The Magnetic Swing

Using RHR-2, Predict the direction of the Magnetic Force, Fm from the B-field and Electric

Current, I

B

B

B

B

Reverse the electrical connections to reverse the direction of the electric current. What

happens? Right Hand Rule #2 is used to find the force on the current carrying wire. The

greater the current, I the greater the force, Fm. The magnetic force the current carrying wire is

also referred to as the Motor Effect. Its this magnetic force which makes the wire coils in

electric motors rotate.

Fm = BIL(sin)

Questions:

10. What happens if instead of three horseshoe magnets you only use only one magnet?

(1)

11. What if you use two magnets but the two magnets are set-up so the top pole of one is

N and the top pole of the other is S? (1)

I/or v

Fm

3

The Magnetic Force on Parallel Current Carrying Wires

The magnetic field, B produced by the current in the first straight wire, I1 is given by

r

IB o

2

1 while the magnetic force, Fm this B-field exerts on the second current, I2 is

given by Fm = BI2L solving for B here gives LI

FB m

2

.

Combining the two B equations gives r

I

LI

F om

2

1

2

and solving for the force gives

r

LIIF o

m

2

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Question:

12. Measure L, r and I to calculate the magnetic force acting on the aluminium foil wires.

(1)

4

Electric Currents and The Magnetic Fields Produced by Moving Electric Charges

When the switch on this apparatus is closed the light bulb should light up indicating an

electric current. At the same time the multimeter should give a current reading in amperes or

amps (A). The longer the bulb is lit the hotter the tungsten filament inside the bulb gets and

the hotter it gets the greater its electrical resistance gets. As its resistance slowly increases the

less electric current will flow.

Questions:

13. What is the initial current in the circuit as shown on the multimeter? (1)

Electric current is defined as the charge that passes through a particular point in the wire per

unit time of t

qI

. 1 amp = 1 coulomb of charge per second

14. How many coulombs of charge pass though the light bulb each second? (1)

A coulomb is a very large amount of charge. In fact 1 coulomb is equivalent to

6.25 x 1018

electrons!

16. How many electrons pass through the light bulb each second? (1)

The magnetic field strength B produced by an electric current I depends on the distance from

the current r according to r

IB o

2 .

17. Using the ruler and the multimeter, determine the magnitude and direction of the

magnetic field at the point labelled B1 . (2)

18. Using the ruler and the multimeter, determine the magnitude and direction of the

magnetic field at the point labelled B2. (2)

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Magnetic Field Lines

Michael Faraday, credited with fundamental discoveries on electricity and magnetism (an electric

unit is named "Farad" in his honour), also proposed a

widely used method for visualizing magnetic fields.

Imagine a compass needle freely suspended in three

dimensions, near a magnet or an electrical current. We can

trace in space (in our imagination, at least!) the lines one

obtains when one "follows the direction of the compass

needle." Faraday called them lines of force, but the term

field lines is now in common use.

Field lines converge where the magnetic force is strong,

and spread out where it is weak. For instance, in a compact

bar magnet or "dipole," field lines spread out from one

pole and converge towards the other, and of course, the

magnetic force is strongest near the poles where

they come together. The behaviour of field lines in

the Earth's magnetic field is very similar.

In space, where magnetic field lines are

fundamental to the way free electrons and ions

move. These electrically charged particles tend to

become attached to the field lines on which they

reside, spiralling around them while sliding along

them, like beads on a wire.

Sprinkle iron filings, (compass needles) on

top of the white paper, above the magnets to visualize the magnetic fields of N, S dipole (2

opposite poles close together), a N, N dipole (two north poles in close proximity), and the

poles of the horseshoe magnet.

Questions:

19. Sketch the fields. How do the three B-fields compare in appearance? (3)

20. Sketch the magnetic field of the solenoid from the position on the compass needles on

a separate sheet of paper. Which of the above fields does the solenoid field most look

like? (1)

N, S dipole N, N dipole horseshoe magnet

6

The Weight is Down – But What’s Up!

“The Jumping Wire”

The electrical resistance is increased or decreased using the slidewire rheostat. Increasing the

electrical resistance, R decreases the electric current, I in the circuit.

According to Ohm’s Law: R

VI . In this case, a constant potential difference is maintained by

the 6 volt dry cell.

Questions:

21. What happens to the magnetic force on the tin foil wire when the current increases?

Remember Fm = BIL! Does the effect follow RHR-2? Why? (2)

22. What happens to the magnetic force on the tin foil wire when the current decreases?

(1)

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A Slidewire Rheostat to

Change the Current, I

Measuring Gravitational Field Strength with a Spring Scale

Questions:

24. Look for the engravings on the slotted masses and the slotted mass holder. There are

two of them marked A and B. What is the total masses of A and B in kilograms? (1)

Gravitational Field Strength, g is defined as the gravitational force or weight per unit mass

according to m

Fg .

25. Weight the four supplied masses using the Newton spring scale. Prepare a Weight

(N) versus Mass in kilograms graph using the supplied grid. What is the slope of your

force of gravity vs. mass graph? Be sure you include the correct unit in your answer. (1)

26. What is the force of gravity acting on the largest mass? (1)

8

g = Force per Unit Mass

Faraday’s Law – Changing Magnetic Flux Induces a Voltage!

The electrical resistance is increased or decreased using the slidewire rheostat. Increasing the

electrical resistance, R decreases the electric current, I in the circuit. Increasing or decreasing

Electric currents results in a magnetic flux change in the primary solenoid.

A primary solenoid – connected to a battery through a switch and a slide wire rheostat is

magneticly connected to a secondary solenoid by a long iron rod.. The secondary solenoid is

hooked up to a galvanometer which is a very sensitive detector of electric currents.

Questions:

27. With the resistance in the variable resistor at a low level, can you detect any

induction in the secondary coil when the switch is closed and opened? Which way does

the needle deflect when the switch is:

a) closed? (1)

b) opened? (1)

28. While one person holds the switch closed have another change the resistance in the

primary coil using the variable resistor. Which way does the needle deflect when the

resistance is:

a) increased? (1)

b) decreased? (1)

29. What effect does removing the magnetically permeable iron rod from the secondary

coil have on the induction? Don’t forget the switch must be closed and the resistance

must be low in the rheostat. Why would removing the rod induce a current? (2)

9

A Slidewire Rheostat to

Change the Current, I

Figure 2: The Leyden Jar

Figure 1: An Electrophorus

Invention of the Leyden jar After the development of static electric generating machines, early electrical experimenters were able to generate high-voltage electrical currents, but they had no way to store this electricity. The Leyden jar is a device that early experimenters used to help build and store electric energy. It was also referred to as a "condenser" because many people thought of electricity as a fluid or matter that could be condensed. Nowadays someone familiar with electrical terminology would call it a capacitor. With a Leyden jar, an experimenter could store electrical charge and move it to another place to use. Soon, Leyden jars were incorporated into the construction of frictional static-generating machines to make larger, longer sparks.

The Leyden jar revolutionized the study of electrostatics. Soon “electricians” were earning their living all over Europe demonstrating electricity with Leyden jars. Typically, they killed birds and animals with electric shock or sent charges through wires over rivers and lakes. In 1746 the abbé Jean-Antoine Nollet, a physicist who popularized science in France, discharged a Leyden jar in front of King Louis XV by sending current through a chain of 180 Royal Guards.

Constructing a Leyden Jar and an Electrophorus

Rub the styrofoam plate with fur for about a minute to give it a large electric charge. This is the

charged cake. Then use the charged Styrofoam cake to charge a disposable aluminum foil pie pan. The

entire apparatus is called an electrophorus, which is Greek for charge carrier. An even larger charge

can be stored in a Leyden jar, made from a plastic photographic film container or plastic peanut butter

jar and lid.

Supply List for Making an Electrophorus:

A Styrofoam dinner plate “cake” (An old LP record will also

work.)

A piece of wool cloth (mitt or sock)

A disposable aluminum pie pan (“charging electrode”)

A Styrofoam cup (electrode handle)

Masking tape

Supply List for Making the Leyden Jar:

A plastic 35 mm film can.

A T-pin or nail slightly longer than the film can (T-pins are used in biology dissections.)

Some aluminum foil.

Tap water

Optional: A neon glow tube (available from Radio Shack)

Building The Electrophorus: Tape the Styrofoam cup to the middle of the inside of the aluminum pie plate. Place the

pie plate electrode on top of the upside-down Styrofoam plate.

Building the Leyden Jar: Push the nail through the centre of the lid of the plastic film can. Wrap aluminum foil

around the bottom two-thirds of the outside of the film can. Tape the aluminum foil in

place. Fill the film can almost full with water. Snap the lid onto the can. The nail should

touch the water.

Storing and Releasing Electric Charge Rub the Styrofoam plate with the wool cloth. If this is the first time you are using the

Styrofoam in an electrostatic experiment, rub it for a full minute.

To charge the electrode follow these steps exactly:

10

Figure 3: Benjamin Franklin

1. Hold the pie plate electrode by the insulating styrofoam handle close to the charged cake.

Try not to touch it to the cake.

2. Briefly touch the electrode with your finger. You will hear the air crack and feel a shock.

3. Remove the electrode holding only the insulating Styrofoam cup handle. You may have to

hold the charged cake down with your other hand.

The electrode is now fully charged!

Discharge the electrode by touching it with your finger. You will feel a shock. You can also discharge

the electrode through a neon glow tube. Hold one of the two metal leads of the neon tube in your

fingers and touch the other lead to the electrode. The neon tube should flash. You can charge the

electrode several times with the cake.

Charge the Leyden jar by touching the electrode of the electrophorus to the T-pin, nail or central post

while holding the Leyden jar by its aluminum foil covering. Make several charge deliveries by

recharging the electrode and touching it to the nail. Discharge the Leyden jar by touching the

aluminum foil with one finger and the T-pin or central post electrode with another. Watch for the

spark.

The Explanation When you rub the cake insulator with a wool cloth, you charge it negatively. That's because the

Styrofoam attracts electrons from the wool. Styrofoam is an insulator; it will hold its charge until it is

discharged by electrons leaking into the air.

When you place the metal electrode on the cake, the electrons on the Styrofoam repel the electrons on

metal. The electrons can't leave the electrode because it is completely surrounded by insulating air and

Styrofoam. The electrode is neutral until you ground it. When you touch the electrode sitting on the

charged cake, mobile electrons in the metal will be pushed off the electrode and pass through you. The

electrons first make a spark as they jump a few millimetres through the air to reach your finger. You

will feel the current of electrons though your finger.

After the electrons leap to your finger, the electrode is left with a positive charge. Physicists say the

electrode has been charged by induction. The electrode’s charge is opposite in sign to the charge on

the cake. You can carry the positively charged electrode around safely by its insulating handle.

When you touch the positively charged electrode to the small nail

electrode on the Leyden jar, electrons from the nail flow onto the big

electrode. The resulting positive charge on the nail attracts electrons

from your hand onto the aluminum foil of the Leyden jar. The

Leyden jar will then have a positive centre separated from the

negative foil wrapped around the outside of the insulating plastic

film can. When you touch one finger to the foil and bring another

finger near the nail electrode of the Leyden jar, a spark jumps the air

gap. The Leyden jar can hold the charge from several charged

electrodes and store it for a long time. It can also produce a powerful

jolt!

Other Things to Try

Give the Styrofoam cake a positive charge by rubbing it with a

plastic bag. Charge the Leyden jar in reverse. While holding the nail,

touch the aluminum foil with the electrode. Will the Leyden jar stay

charged overnight?

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