Cathode Rays and E/m Measurement Lab

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Activity 2:

We adjust the “bulb” so that we see a slightly helical path, and then we measure the radius of the electron beam, to do so, we measure the radius of the beam as we see it on both sides of the scale, then average the result and record them. Next, we record the accelerating voltage and the current supplied to the Helmholtz coils and then estimate the uncertainty in our radius measurement and determine the uncertainty in our voltage and current measurement from multimeter data sheets, and record all these value.

After we having these data above, we combine the 4 relationships given in the beginning of the introduction to find an expression for the ratio e/m in term of the radius r, the voltage V, and the current I. Then calculate the ratio e/m with its uncertainty.

Activity 3:

The fact that the “beam” is a stream of negatively charge electrons was not always known. So we try a few very simple experiments to try and prove that the particles are negatively charged. We now stop the current supplied to the Helmholtz coils so that the green beam is horizontal, and bring a magnet near the bulb without scratching the bulb. And record what happens when we bring the north/south pole of the magnet near the beam. And see whether our observation support our idea or not.

Then we take the magnet away, and experiment with connecting a voltage source to the deflection plate terminals C on the e/m Apparatus. The e/m apparatus labels which terminal corresponds to the upper and lower deflection plates, this allow us to determine the direction of the applied electric field. We draw a picture of what happens to the electron beam, and explain how the relates to the direction of the applied electric field and the sign of the electron charge.

Result)

Activity 2:

[pic 15] [pic 16] [pic 17]

V = 288.2V I = 1.82A

[pic 18]

[pic 19] (0.5%*reading + 2digits)

[pic 20] (2.5%*reading + 5digits)

As we know the expression of [pic 21] from introduction:

[pic 22]

[pic 23]

[pic 24]

And we know:

[pic 25]

So, we have:

[pic 26] = [pic 27][pic 28]

Activity 3:

- when we bring the north pole of the magnet near the beam, it turns into the page.

- when we bring the south pole of the magnet near the beam, it turns out of the page.

[pic 29][pic 30]

- After we connecting a voltage source to the deflection plate terminals C on the e/m Apparatus. The beam becomes:

[pic 31]

Conclusion)

In activity 2, we got an experimental [pic 32]which is [pic 33][pic 34] and it was found to be consistent with the theoretical value of [pic 35] for an electron is [pic 36]. In activity 3, we bring a magnet vertically near the beam, that means we create an electric field which produce point upward near the beam, then we use right hand rule to determine where the Lorentz force point to, and as it the north pole turns into the page while the south pole turns out of the page, then we can conclude the charge inside is negative.

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