PHYS 1407 – Conceptual Physics II

Discovery of the Electron

 

Leader: _________________________          Recorder: __________________________

Skeptic: _________________________         Encourager: ________________________

 

Materials

Lead, Banana/Banana (6)

Pasco e/m Apparatus with hood

Power Supply, Pasco Model SF-9585A

Cow magnet with N pole labeled

 

Safety

This experiment will use a low current high voltage power supply designed specifically for this type of application.  The risk of shock or electrocution is minimal.  Make or change electrical connections with the voltage turned to 0 V and the power supply turned off.

 

Introduction

      The discovery of the electron by J.J. Thomson in 1897 was of fundamental importance because it was the first particle found smaller than an atom – a subatomic particle.  The discovery of subatomic particles suggested then that atoms might have structure.  In other words, atoms might be made up of smaller particles instead of being indivisible pieces of matter.

 

Procedure

1.  Set up

      Make sure that everything is turned off.  Use a banana cable to connect the plug labeled 500 V on the power supply to the red plug labeled electrode on the e/m apparatus and a second cable to connect the black plug labeled electrode to the 0 plug on the power supply.

      Connect one of each plug labeled AC to one of the plugs labeled Heater on the e/m apparatus.  (Order doesn’t matter for this plug.)  The AC setting should set at 6 V and should not be touched during the experiment.  Verify that 500 V adjust and the 50 V adjust knobs on the power supply are both fully clockwise and that power supply switch is in the off position.

      Make sure that the switch on the e/m apparatus is in the e/m Measure position.

 

Have your instructor verify that you have connected the apparatus correctly.

 

2.  Observe the Effect of a Magnetic Field on the Cathode Ray

      Plug in and turn on the power supply.  Make sure that the red LED above the display on the power supply indicates 500 V.  If it doesn’t press the button next to it, so that it does indicate 500 V.  Turn up the power supply slowly so that the meter reads 150 V.  It will take a few minutes for the tube to warm up.  Once the tube has warmed up, a faint beam will appear at the bottom of the circular portion of the tube. 

Q1.  Does the beam seem to be stronger near the electrodes in the tube or near the glass?

 

 

Since the beam is stronger at the cathode, it appeared to come from the cathode (the – electrode in the tube).  Consequently, it came to be known as a cathode ray.

 

Q2.  If the cathode ray originates at the cathode, as you face the apparatus is it moving right to left or left to right?

 

 

 

Move the N pole of the cow magnet towards cathode ray.

 

Q3)  Describe what happens to the beam as you bring the N pole near it?

 

 

Move the S pole of the cow magnet towards the cathode ray.

 

Q4)  Describe what happens to the beam as you bring the S pole near it?  In particular what is different than the observation with the N pole?

 

 

Once you have completed these observations, turn down the voltage and then turn off the power supply to preserve the life of the tube.

 

Here is another right hand rule for your enjoyment.

If you point the fingers of your right hand in the direction that a charge moves so that your palm points in the same direction as the N pole of the magnet, then your thumb will point in the direction of the force on a positively charged particle.

 

Q5)  Apply the right hand rule to the observation you made in Q3.  Does it predict the correct direction of force for a positive particle?

 

 

Q6)  Based on your answer to Q5) what is the sign of the charge of the particles in the cathode ray?

 

 

 

William Crookes could deflect the cathode rays with a magnet, but because his vacuum wasn’t very good, he couldn’t deflect them with an electric field.  Crookes decided that he had discovered a new state of matter he called radiant matter.  J.J. Thomson worked about 20 years after Crookes and vacuum technology had improved considerably by then.

 


3.  Observe the Effect of an Electric Field on the Cathode Ray

Flip the switch on the e/m apparatus to the Electrical Deflect position.  Use a banana cable to connect the socket labeled 0 V to the plug labeled Deflect Plates + and another to connect the plug labeled –50V to the deflect plates – plug.  The banana plugs connect to each other, so more than one can be plugged into the 0 plug on the power supply. 

 

Have your instructor check your wiring again.

 

Turn on the power supply again and once the tube has warmed up, turn up the 500 V adjust knob to about 300 V so that you obtain a beam again.  If you don’t obtain a beam, contact your instructor.  Press the button so that the red LED indicates 50 V.

 

P7.  The deflector plates are connected above and below the beam.  As you turn up the 50 V adjust, the upper plate will become positive and the negative plate.  Predict what effect there will be on the cathode ray as you turn up the 50 V adjust knob if the cathode rays are positive.

 

 

 

P8.  Predict what effect there will be on the cathode ray as you turn up the 50 V adjust knob if the cathode rays are negative.

 

 

Q8.  Slowly increase the 50 V adjust knob and record your observations.

 

 

Q9.  Which of the predictions was correct?

 

 

 

Q10.  What does this part of the experiment say that the sign of the cathode rays are?

 

 

Q11.  Does agree with the result from the first part of the experiment?

 

 

It was Thomson’s ability to deflect the cathode rays with an electric field that convinced the scientific community that cathode rays were very small negatively charged particles.

 

S12.  Summarize your evidence that cathode rays consist of negatively charged particles/