# Motion of Charges in Combined Electric and Magnetic Fields; Measurement of the Ratio of the Electron Charge to the Electron Mass

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1 Motion of Charges in Combined Electric and Magnetic Fields; Measurement of the Ratio of the Electron Charge to the Electron Mass Object: Understand the laws of force from electric and magnetic fields. You will measure the radius of the path of an electron in a magnetic field; you will demonstrate how to counteract that motion with an electric field; and you should understand the conditions (electric field E, magnetic field B, and electron velocity v) under which this happens. Equipment: Evacuated glass bulb containing an electron gun (to produce a beam of electrons with controllable kinetic energy) and parallel plates to create an electric field perpendicular to the beam of electrons; pair of magnet coils; variable-voltage power supply for the electron beam; dual power supply to provide (1) current to the magnet coils, and (2) voltage to the parallel plates; multimeter to be used as voltmeter measure the voltage between the plates; multimeter to be used as ammeter (10 A) setting to measure current to magnet coils; wires with banana plug connectors. Sections in Book: 20.5, (see Example 20.12); 21.5, 24.5 (Knight, Jones, and Fields) Introduction It has long been known that charged particles such as electrons and protons move in circular paths in a magnetic field. The Van Allen radiation belts in space around the earth consist of energetic electrons and protons trapped in the magnetic field of the earth. Magnetic fields are also used to guide the motion of protons and electrons in accelerators for research and medical purposes, and the orbiting motion of charges in a magnetic field is the basis for measuring the mass of atoms in a sample in order to identify what s in it (mass spectrometry). A particle with charge q moving in an electric field E and a magnetic field B experiences a force F: F = qe + qv B (1) where you ve learned to determine the directions the electric and magnetic forces in class. In a vacuum in which collisions between particles are not very frequent, a particle with charge q, mass m, and velocity v perpendicular to a uniform magnetic field B (no E) moves in a circular path with radius mv r = (2) qb One can also deflect the trajectory of a charged particle with an electric field, although not into a circular path. If the electric force on the particle is set to be equal and opposite to the magnetic force, the net force on the particle will be zero. From Eq. (1), this will happen if v = E B. (3) 3/30/2011 Rev3 Page 1 of 12

2 For the correct orientation of the fields (use the right hand rule). For this velocity only, the particle travels in a straight line in the combined electric and magnetic fields. So we can measure its velocity if we know the strength and orientation of the electric and magnetic fields. How do we give velocity to a charged particle? We accelerate it through a known voltage difference V A. If it starts from rest, it gains kinetic energy K = ½ mv 2 = qv A or a velocity v = 2qV A (4) m Equating Eq. (3) and (4) and solving for q/m, we find q m = v 2 = 1 2 E 2V A 2V A B (5) Why is this of interest? Even by the end of the 19 th century, people didn t know either the mass or the charge of the electron, but were able to determine the ratio of e/m with an experiment like this one. As you do this more modern experiment, you may appreciate some of the difficulties these scientific pioneers faced in trying to measure things they didn t yet understand with hand-made glass tubes, sealing wax, leaky batteries, etc. all while dressed in coats and ties. More importantly, the same or similar technique can be used to measure the charge-to-mass ratio of any charged particle, and can thus be used to identify the chemical constituents of all kinds of mixtures (pollutants, explosives, food you name it). Equipment The central piece of equipment is a nearly spherical glass tube that has been evacuated of air. The narrow neck of the tube is plugged into a socket through which electrical connections are made to conductors inside the tube. See Fig. 1 and Fig 2 for a drawing of the tube and a wire diagram. Inside the narrow neck of the tube is a filament that is heated by a current passing through it. When it glows white hot, it emits electrons. When emitted, the electrons have very little kinetic energy (typically a few ev or less). Using a power supply with a variable DC voltage of 0 to 5000V, a voltage difference V A is applied between the filament (also referred to as the cathode ) and a metal plate with a slit opening in it ( anode ) located where the tube neck joins the spherical tube. This voltage accelerates the electrons toward the anode. They gain a velocity given by Eq. (4) by the time they hit the anode. Although most of the electrons will hit the anode, a small fraction of them instead go through the open slit into the spherical volume of the tube. Thus this electron gun generates a ribbon-shaped beam of electrons, with each electron having kinetic energy K = ev A. You can t see the electrons directly, but you can see where they hit the card inside the tube. The square card is coated with a chemical that emits light ( fluoresces ) when struck by energetic electrons (like a TV tube in the pre-flat screen days). As viewed from above the bulb, the card is angled slightly to intercept the ribbon of electrons at different points along its trajectory indicating where the beam of electrons is located. See Fig. 3. 3/30/2011 Rev3 Page 2 of 12

3 Figure 1 (1) Figure 2 (2) 3/30/2011 Rev3 Page 3 of 12

4 Also inside the tube is a pair of metal plates separated by a distance y = 8 mm (according to the specifications that came with the tube). The electron beam normally passes between this pair of plates. When a voltage difference V P is set between these plates, it generates an electric field E between the plates, perpendicular them. If the electric field can be assumed to be nearly uniform, you can calculate the strength of the electric field E if you know V P. (See Ch. 16) Lastly, there is a pair of circular magnet coils that bracket the spherical tube. With these coils connected in series, a current through the coils generates a horizontal magnetic field perpendicular to the direction of the beam. Each coil has N turns of wire, a radius a, and is also separated from its partner by the same distance a. This is the so-called Helmholtz configuration, which generated a nearly uniform magnetic field between them (more on this later). We don t measure the magnetic field directly, but we do know that its strength is proportional to the current through the wire. For the Helmholtz configuration, it is fairly easy to calculate (albeit with a little more physics) the magnetic field between the coils to be B = 4 5 for a current I H through the coils. 3/2 μ 0 N a I H (6) SAFETY You will be using high voltage power supplies that are potentially lethal if you misuse them. This apparatus is safe and it would be very difficult to get a shock from this experiment. Nonetheless, I m sure you could do it if you try, or are careless. So here are some rules to follow: 1. If you have to unplug wires from a power supply or what it is connected to, always turn OFF the power supply first. If there is any doubt in your mind, turn it off. You should not have to unplug any wires during this lab, so this will not likely be an issue for you. 2. The only things you can get a shock from are the exposed conductors on and within the banana plugs at the ends of the wires. So avoid touching them when the power supplies are on. It is perfectly safe to touch the tube, coils, and the knobs and switches of power supplies. 3. When working with electric circuits, it s a good idea to keep your hands dry. Wet skin has lower resistance than dry skin. 3/30/2011 Rev3 Page 4 of 12

6 Part I. Now you will measure the radius r of the electron orbit for different values of B and v. It would seem straightforward to measure the radius of a circle. Oh, if it were that simple! First, the electron beam is inside a glass tube where you can t put a ruler wherever you want. Second, you can only see part of a circle made by the beam. You can see where the beam enters at one vertex of the card and crosses the scale (large ticks are 1 cm apart; small ticks 2 mm apart) on the adjacent side of the card. The distance along this side from the bottom vertex is labeled e in Fig. 3. The scale on the card begins at e = 10 mm and stops at e = 70 mm. One can use geometry to show that the radius can be found from e as follows: r = s2 + e 2 2 s e ( ) (7) where s is the length of the side of the card. For further details on the geometry, you can look at the 3B Scientific Physics datasheet that comes with the experiment. slit y = 8 mm filament electron beam V A = V r r e s = 80 mm Figure 3 1. Set the accelerating voltage at the lowest value for which can see the beam (around V A = 1.5 kv). Record this voltage on your datasheet. Be sure that V P is set to zero for all of Part I. One group member should then increase the current I H to set the value of e to the largest value for which the beam crosses the scale (probably around e = 40 mm). Set the value of e to a convenient value that is easy to measure, e.g., 40 mm. Another of you should record the current I H for this value of e, then increase the current until e = 30 mm. After recording the current for e = 30mm, determine the current necessary for e to equal to 20mm, 10mm, and 0mm (beam crosses top vertex of the card). 3/30/2011 Rev3 Page 6 of 12

7 Set the accelerating voltage to 2.0 kv and repeat; then 3.0 kv, and 4.0 kv, recording your data of V A, I H,, and e. 2. Using each pair of e and I H, calculate values for B and r from Eqs. (6) and (7). 3. Using a spreadsheet program or a piece of graph paper, create a plot of r vs qb m, and use Eq. (2) to determine a velocity that corresponds to each V A. All four plots should be on the same graph. 4. Why is the velocity of the electron different for each accelerating voltage? Use Eq. (4) to calculate the expected value of v for each V A, and compare it with what you obtain from your plots. Calculate the percent difference between the two values of v for each setting of V A. Part II 1. Reduce I H to zero, and set V A to 2.0 kv. Increase the plate voltage to create an electric field between the plates. Note how the beam deflects in the opposite direction. Again set V P to zero. 2. Set V A to the lowest value you used in Part I. Increase I H until you see a noticeable deflection of the beam. Then increase V P until the beam goes straight through the gap between the plates and intersects the far vertex of the card. You ll probably note that the beam actually doesn t go straight through, but first bends one way, then the other, then back again. Record the values of V A, V P, and I H. Repeat for about 5 different values of I H. Then set V A to 2.0 kv and repeat; set V A to 3.0 kv, then 4.0 kv, collecting up to 5 sets of data for each setting of V A. For the higher voltage settings, you will find that you cannot set V P high enough to counteract the magnetic force for all settings of e. 3. Using each pair of V P and I H, calculate E and B, and then on one graph, plot E vs. B for each setting of V A. Using your plots and Eq. (3) determine v for each setting of V A. Compare these values to your calculated values of v from Eq. (4) by calculating the percent difference between them. Part III We believe we know the mass and charge of the electron quite well these days, but that wasn t true about 115 years ago when people weren t yet sure electrons were identical particles. So this experiment confirmed that the ratio of the electron charge to the electron mass was the same for all electrons in the beam. How? By using Eq. (5). These days, we use this technique and similar ones to identify the mass of an unknown particle when we are fairly sure of its charge. 1. For each of your data sets in Part II, calculate the value of e/m (electron charge-to-mass ratio) in units of C/kg. Average them, and compare them to what you d calculate using the known values listed in your book. Calculate the percentage difference. 3/30/2011 Rev3 Page 7 of 12

8 Part IV 1. Slowly return all of the knobs on the power supplies to 0V by turning the knobs counterclockwise. 2. Turn off both multimeters. 3. Turn off both power supplies. 4. Unplug the power cords of both power supplies from the AC outlets. (1), (2) Figures 1 and 2 were copied from 3B Scientific Physics Thomson Tube S U18555, (2008). The remainder of this page was left blank to allow the Data Sheet to begin on a new page when printing on both sides of the page. 3/30/2011 Rev3 Page 8 of 12

9 Name: Name: Name: Name: Data Sheet Motion of Charges in Combined Electric and Magnetic Fields; Measurement of the Ratio of the Electron Charge to the Electron Mass Apparatus Constants Coil Radius (a) Number of Turns for Each Coil (N) Separation between the Upper and Lower Deflection Plates ( y) Length of the Side of the Fluorescent Card (s) 68 (mm) 320 turns 8 x 10-3 (m) 80 (mm) Part I V A = e (mm) 40 KV I H (A) V A = 2.0 KV e I H (A) (mm) 40 V A = 3.0 KV e I H (A) (mm) 40 V A = 4.0 KV e I H (A) (mm) Using each pair of e and I H, calculate values for B and r from Eqs. (6) and (7). Using a spreadsheet program or a piece of graph paper, create a plot of r vs qb m, and use Eq. (2) to determine a velocity that corresponds to each V A. All four plots should be on the same graph. To compare the velocities you determined from your plots with theory, calculate the theoretical velocities using eqn. (4) and determine the % Difference. Tabulate your results and attach to this data sheet. Although you may use Excel to make the repetitive calculations and to create your tables and graphs, you must provide at least one sample calculation for each of the physical characteristics (v (4), B (6), r (7), and the % Difference) that you have been requested to calculate. 3/30/2011 Rev3 Page 9 of 12

10 Part II V A = V P (V) KV I H (A) V A = 2.0 KV V P (V) I H (A) V A = 3.0 KV V P (V) I H (A) V A = 4.0 KV V P (V) I H (A) Using each pair of V P and I H, calculate values for E and B. (Hint: the magnitude of the E field should be calculated using the physical parameters of the parallel plates in the bulb and the potential across these plates.) Using a spreadsheet program or a piece of graph paper, create a plot of E vs B, and use Eq. (3) to determine a velocity for each V A. All four plots should be on the same graph. To compare the velocities you determined from your plots with theory, calculate the theoretical velocities using eqn. (4) and determine the % Difference. Tabulate your results and attach to this data sheet. Although you may use Excel to make the repetitive calculations and to create your table and graphs, your must provide at least one sample calculation for each of the physical characteristics (E, B, and the % Difference) that you have been requested to calculate. Part III For each of your data sets in Part II, calculate the value of e/m (electron charge-to-mass ratio) in units of C/kg. Average them, and compare the average to the known value listed in your book. Calculate the percentage difference. Tabulate your results and attach to this data sheet. Again, you may use Excel, but sample calculations must be provided. 3/30/2011 Rev3 Page 10 of 12

11 Part IV Final Check List Return voltages to zero on both power supplies by turning the knobs counterclockwise. Turn off both Multimeters Turn off both Power Supplies Unplug the power cords of both power supplies from the AC outlets. Questions: 1. Given that you know that these particles are negatively-charged electrons, what must be the direction of the magnetic field produced by the Helmholtz coils? Toward you or away from you? What must be the direction of the current in the coils, clockwise or counterclockwise? What must be the direction of the electric field you apply, up or down? 2. In Part I, you were requested to plot r vs qb m and to determine the velocities from the plots. Briefly explain how determining the velocities from the plots would have been different if r vs B 1 had been plotted instead. Briefly explain why plotting r vs B would be less desirable than plotting r vs B 1. 3/30/2011 Rev3 Page 11 of 12

12 3. In Part II, you notice that the beam didn t go straight across the tube, but wobbled up and down. This probably results from the fact that the magnetic field produced by the Helmholtz coils is not quite uniform within the bulb. From the directions of the wobble and using what you learned from the deflection in Parts I and II, determine whether the actual magnetic field is stronger near the edge of the bulb compared to the center, or weaker. Explain your reasoning. 4. Errors: It is likely you did not get perfect agreement between your measured and calculated values. List at least three reasons why you think there might be differences. For example, consider the possible complication of the earth s magnetic field (about 3 x 10-5 T) in Auburn. Based on your own measurements of the magnetic field strengths produced by your coils, could this lead to an error of the magnitude you measure? 3/30/2011 Rev3 Page 12 of 12

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