Thomson Cathode Ray Tube Experiment

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Los Angeles Valley College *

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101

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Chemistry

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May 24, 2024

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9/20/23, 1:31 PM Beyond Labz 2.0 Assessment https://worksheets.beyondlabz.com/multiembedq.php?id=6776-6777-6778-6779-6780-6781-6782-6783-6784-6785-6786 1/3 Click here to erase all data entry on this page Thomson Cathode Ray Tube Experiment In this video we'll do an overview of the Thomson Experiment. As scientists began to examine atoms, their first discovery was that they could extract negatively charged particles from atoms. They called these particles electrons. In order to understand the nature of these particles, scientists wanted to know how much charge they carried and how much they weighed. John Joseph (J.J.) Thomson was a physics professor at the famous Cavendish Laboratory at Cambridge University. In 1897, Thomson showed that if you could measure how far a beam of electrons was bent in an electric field and in a magnetic field, you could determine the charge-to-mass ratio ( q / m e ) for the particles (electrons). Knowing the charge-to-mass ratio ( q / m e ) and either the charge on the electron or the mass of the electron would allow you to calculate the other. Thomson could not obtain either in his cathode ray tube experiments and had to be satisfied with just the charge-to-mass ratio. If at any point you need to exit the lab and come back at another time, click the Save Lab button below. That will save your progress. Then when you re-enter the lab, open the Presets menu and click on My Saved Labs. You can click the latest link there to start at the point where you left off. 1. To start this activity, click this link for Thomson Cathode Ray Tube Experiment . The lab will load in a new tab. Click back to this tab to read further instructions and complete the questions below. You can follow along with the instructions below in the Procedures tab in the lab. 2. What source is used in this experiment? Drag your cursor over to the source to identify it. Electron gun 01:01
9/20/23, 1:31 PM Beyond Labz 2.0 Assessment https://worksheets.beyondlabz.com/multiembedq.php?id=6776-6777-6778-6779-6780-6781-6782-6783-6784-6785-6786 2/3 What type of charge do electrons have? Negative What detector is used in this experiment? A phosphor screen 3. Turn on the Phosphor Screen by clicking on the red/green light switch on top of the screen or click the slide switch next to the Phosphor Screen label at the top of the Live Data menu in the tray. What do you observe? There's a spot in the center of the phosphor screen. The phosphor screen detects charged particles (such as electrons) and it glows momentarily at the positions where the particles impact the screen. 4. Click the Grid button in the Phosphor Screen menu in the Live Data tab in the tray, and then set the Magnetic Field to 30 µT with the up arrows above the Magnetic Field digits in the Live Data display. What happens to the spot from the electron gun on the phosphor screen? It moves to the right 5. Set the Magnetic Field back to zero by clicking the down arrows under the Magnetic Field display. Then set the Electric Field to 10 V by clicking the up arrows above the Electric Field display in the Live Data menu. What happens to the spot on the phosphor screen? It moves to the left. Where should the signal on the phosphor screen be if the electric and magnetic forces are balanced? At the center of the phosphor screen.
9/20/23, 1:31 PM Beyond Labz 2.0 Assessment https://worksheets.beyondlabz.com/multiembedq.php?id=6776-6777-6778-6779-6780-6781-6782-6783-6784-6785-6786 3/3 6. Increase the voltage of the Electric Field so the spot is 5 cm left of center. What voltage is required? 13 V 7. Increase the magnetic field strength until the spot reaches the center of the screen. What magnetic field creates a magnetic force that balances the electric force? 44 µT 8. In a simplified and reduced form, the charge-to-mass ratio ( q / m e ) can be calculated as follows: where V = the electric field in Volts, d = the deflected distance from center in cm after applying just the voltage, and B = magnetic field in µT. What is your calculated value for the charge-to-mass ratio for an electron (q/m e )? 1.706*10^1 The modern accepted value is 1.76*10 11 . Calculate your percent error as follows: % Error = 3.07 % 9. You may want to repeat the experiment several times using different size deflections.
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