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Le Fevre High School SACE Stage 2 Physics Motion of Charged Particles in Electric Fields – Teltron Tube AIM: : To investigate the effect of electric and magnetic fields on electrons Apparatus High voltage power supply, teltron electron deflection tube, 6 V DC continuously variable power supply, ammeter, bar magnet. Description and Explanation of Experiment: The electron tube consists of 3 essential parts. 1. An electron gun consisting of a heated filament to produce electrons and anode (positive electrode) maintained at a high potential with respect to the filament to accelerate electrons across to the deflecting plates. Top plate connection parallel plates Filament Filament lead connections Anode connection bottom plate connection 2. A pair of parallel plates which produce a deflecting electric field. Note that the plates are 5.2 cm apart and the electric field between them is given by E V V = volt/metre d 0.052 (Equation 1) This equation assumes a uniform field. 3. A pair of coils outside the tube separated by a distance equal to their radius. Such coils are called Helmholtz coils and produce a magnetic field that is nearly uniform in the region between them. r r Coil Sockets For these particular coils the magnetic field strength is given by -3 B = (4.23 x 10 )(IB ) Tesla (Equation 2) Where IB is the Current flowing in the coils. Le Fevre High School NB The coils are connected to a low voltage power supply. (Tesla is the unit of magnetic field strength.) Theory A: Magnetic field only Consider the motion of a beam of electrons in a magnetic field only. Moving electrons experience a force given by: F = q v B sin and is at right angles to v and B in a direction given by the right hand rule. F = e v B as sin = 1 in this case and q = e for electrons. Note electrons are negative in charge F v (mutually perpendicular vectors) B (at 90o and into the plane of the page) This force causes v to change in direction without changing in magnitude and the force always maintains a direction at right angles to v so the electrons are forced into a circular path with the centripetal force being provided by the magnetic force. ie. F cent = F magnetic. m v2 e v BR R the (Equation 3) where BR is the magnetic field required to force electrons into a path of radius R (This idea is used in the Velocity Selector - no longer required in the 1999 syllabus) B: Electric field only Consider the motion of electrons in an electric field only. The parallel plates produce a field (ie. constant in magnitude and direction) so each electron experience a force which is constant in size and direction given by: F = eE eV d (equation 4) + Force v This force causes the electrons to move in a parabolic path as it is constant in size and direction. Le Fevre High School Procedure: 1- Connect the circuit shown in the circuit diagram. Be careful not to knock the anode connection at the side, as any damage is expensive to repair. Have the circuit checked before switching on. EHT POWER SUPPLY 0 6 12 EHT 0 + - Notice that the parallel plates are both connected to the positive terminal of the E H. T. Under these conditions there is no PD between the plates and hence no field. This is always done when no electric field is required between the plates rather than just disconnecting them and leaving them “floating”, because it prevents the build up of charge by collection of stray electrons and the consequent generation of odd electric fields. Later you will set up an electric field between the plates by removing one of the leads from the positive terminal and moving it to the negative terminal Whenever any such changes are made switch of the EHT and wait until the voltmeter reads zero. 2. When the circuit has been checked, switch on the EHT supply. Adjust the anode voltage to 3 000 Volt (using the coarse and fine controls) and observe the path of the undeflected beam. Using the bar magnet provided try observing the effect it has on the beam. Don’t knock the glass tube with the magnet) 3 Magnetic Deflection only. Now Connect the Helmholtz coils to the 6 V DC Supply as shown. Have your circuit checked Use a continuously variable 6 V DC power supply. A A 0V Z Z Z 6 V DC A 6V I A A Z Step 1: Switch on the current (IB) to obtain a magnetic field between the coils. Adjust the current in the coils to 0.1 A. Q1. The magnetic field between the coils deflects the beam into a curved path. What is the shape of this path? Q2. The glowing filament produces a yellow ray of light through the slit of the anode. What is the shape of the path of this light? Q3. What can be concluded about the effects of magnetic fields on light waves? Le Fevre High School Step 2: Increase IB from 0.1 A to 0.3 A with Voltage = 3000 V. Q4. .Describe and explain what happens to the radius of curvature of the paths when I B is increased from 0.1A to 0.3 A with V = 3000 V. Note that the magnetic field is now stronger but the electrons still have the same speed. Step 3: Increase the anode voltage to 3500 V with IB = 0.1 A. Note that this time the magnetic field is the same but the electrons are going faster. Q5. Describe and explain what happens. Use the speed dependence of magnetic force (F = evB) in your answer. ie use m v2 mv e v B R so R R e BR 4. Electrostatic deflection only. Now switch off the magnetic field and swap the lead connected to the bottom deflecting plate from the positive terminal to the negative terminal of the E H T power supply. Switch off the EHT while you do this. This sets up a potential difference between the plates and hence an electric field. Observe the electric field deflects the beam into a curved path. Q6. What is the direction of the deflection under the electrostatic deflection? Q7. What is the shape of the path ? Q8. What is the motion of electrons in an electric field similar to ? What is common to the two situations ? Increase the E H T voltage from 3000 V to 3500 V. Q9. Describe and explain what happens to the trace on the screen when the EHT is raised from 3000 V to 3500 V Q10. What is the shape of the path of the ray of light from the filament? Hence what can you conclude about the effect of E fields on light waves Q11. Is there any other evidence in this experiment for conclusions in Q3 and Q10? Describe and explain the evidence. Requirements Full write up including answers to all questions. This is a SUMMATIVE piece.