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Transcript
Ideas to Implementation 1. Increased understanding of cathode rays led to the development of television 1.4. Identify that charged plates produce an electric field/1.6. Discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates/1.7. Describe quantitatively the electric field due to oppositely charged parallel plates An electric field is a region in which charged particles experiences a force Note: Electric field is a vector; so a magnitude and a direction is required (important in calculations) The direction of an electric field is the direction of force that a positive particle would experience within the field The strength of the field is the force experienced by a unit charge. Mathematically: 𝐸 = 𝐹 𝑞 (where E = Electric field [Vm-1 or NC-1], F = Force [N] and q = charge [C]) However, 𝐹 = 𝑞𝐸 and 𝑊 = 𝑞𝑉 = 𝐹𝑑 = 𝑞𝐸𝑑, ∴ 𝑞𝑉 = 𝑞𝐸𝑑 and dividing both sides by ‘q’ we get 𝑉 = 𝐸𝑑 or 𝑉 E=𝑑 A uniform electric field can be established by supplying two oppositely charged parallel plates with potential difference Electric fields around oppositely charged parallel plates: Electric field between point charges and like/opposite charges (basic year 11 knowledge): 1.3. Identify that moving charged particles in a magnetic field experience a force/ 1.5. Describe quantitatively the force acting on a charge moving through a magnetic field 𝑭 = 𝒒𝒗𝑩 𝐬𝐢𝐧 𝜽 Recall from motors and generators that charged particles moving at a constant velocity emanate a magnetic field When moving charged particles are placed in a magnetic field, the particle’s magnetic field and the external field interact, resulting in the particle experiencing a force Note: The direction of the force on a charged particle can be determined using the right hand grip rule. Remember to reverse that direction that your thumb points in when dealing with an electron Now, the magnitude of the force acting on the charged particle can be determined using the formula: 𝑭 = 𝒒𝒗𝑩 𝐬𝐢𝐧 𝜽 WHERE F = Force [N], v = velocity of the particle [ms-1], B = magnetic field strength [T] and 𝜃 is the angle with which the charged particle enters the magnetic field. Note: DIRECTION IS ALWAYS REQUIRED 2.3 Solve problems and analyse information using: 𝐹 = 𝑞𝑣𝐵 𝑠𝑖𝑛 𝜃 and 𝐹 = 𝑞𝐸 A few tricky examples: 1. Suppose a very strong magnetic field is directed into the page. An electron moving at a constant velocity enters this magnetic field from the right a) Describe the subsequent motion of this electron (1 mark) It’s will undergo circular motion as indicated on the diagram b) Explain why it moves in this manner (3 marks) By applying the right hand palm rule, it can be determined that the electron will experience a force upwards, perpendicular to the direction of its motion, causing it to curve up as indicated on the diagram. By applying the right hand palm rule to its motion as it curves, the direction of the force it experiences is always perpendicular to its linear velocity. Thus, by definition, this is a centripetal force and therefore the electron undergoes circular motion. Note: The magnetic field must be strong for this to occur. If the magnetic field is weak, then the electron will not describe a full circle within the limited magnetic field. Rather, it will curve up as an arc of a circle. 2. A top plate of a horizontal pair of parallel electric plates is positive. The plates produce a uniform electric field of 9500 NC-1 between them. An electron is projected into the electric field horizontally to the right a) Determine the force experienced by the electron due to the electric field as it passes through the field F = qE = (-1.602 x 10-19)(9500) = -1.5219 x 10-15 N downwards = 1.5219 x 10-15 N upwards b) Describe the motion of the electron as it moves through the electric field As the electron moves through the electric field, its horizontal velocity is constant (assuming no friction) and is unaffected by the electric field. However its initial vertical velocity is zero, but it experiences a constant force upwards due to the electric field, and it therefore accelerates. By definition, the electron will follow a parabolic path upwards, like an upside-down projectile. c) Does gravity affect the motion of the electron? Discuss Gravity will affect the motion of the electron and will decrease the acceleration it experiences upwards. Thus, it will take a longer time to hit the plate and will have a greater horizontal displacement. However, the force experienced by the electron = mg = (9.109 x 10-31)(-9.8) = -8.92682 x 10-30 N upwards = 8.92682 x 10-30 N downwards. This is very small compared to the force it experiences due to the electric field. Hence the effect of gravity on the motion of the electron is negligible. 1.2.Explain that cathode rays allowed the manipulation of a stream of charged particles A cathode ray tube (CRT) or discharge tube is a sealed glass tube from which most of the air is removed by a vacuum pump. It contains two electrodes: the positive electrode is called the anode and the negative electrode is called the cathode A high potential difference was applied across the two electrodes, causing the appearance of cathode rays, which are now known to be negatively charged particles called electrons In 1896, Eugene Goldstein proved that the source of radiation in a discharge tube was the cathode (hence named them cathode ‘rays’) Due the obstruction of a few remaining gas molecules, it glows with a pale green light. It appears as on the diagram above Structures built into or around the basic CRT allows the cathode rays (streams of charged particles) to be manipulated. For example, placing another set of electrodes that sets up an electric field that deflects the cathode rays. Likewise, a magnetic field may be set up through the use of electromagnet coils to also deflect the cathode rays. Even solid objects such as paddle wheels and Maltese crosses can be places inside. This is seen in the diagram above 1.1. Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves Today we know that cathode rays are beams of negatively charged electrons. However, a little over 100 years ago (when the atom was the smallest unit of matter known), conflicting experiments caused scientists to vigorously debate whether they were indeed charged particles or electromagnetic waves The Magnetic field experiment: In 1865, H. Sprengel showed that cathode rays were deflected by a magnet. This suggests that there were charged, and must therefore be charged particles (as waves do not carry a charge) The Paddle wheel experiment: In 1875, William Crooke’s designed a cathode ray oscilloscope with a paddle wheel in it. When the cathode rays tube was turned on, the paddle wheel moved and turned from the cathode to anode (like the cathode rays themselves). This suggests that Cathode rays passed their momentum onto the paddle wheel, and therefore must have mass (p = mv) and be charged particles (waves do not have a mass) The Electric field experiment: In 1883, Hertz incorrectly showed that cathode rays could not be deflected by an electric field (this mistake was due to the fact that his CRT had small amounts of gas in it which got ionised upon contact with the cathode rays, neutralising the electric field) suggesting that they must be electromagnetic waves The Maltese Cross experiment: An experiment was conducted where the anode was in the shape of a Maltese cross. A sharp shadow was produced by the cross, suggesting that they travel in straight lines and caused fluorescence, suggesting that they must be waves. The metal foil experiment: In 1892, Hertz directed cathode rays at a thin metal foil and observed that they passed through the foil. The smallest unit of matter known at the time was the atom, and this was too big to pass through the atoms of the metal. Hence, it was believed that they must be electromagnetic waves (as these are small enough to permeate the metal foil) Corrected Electric field and charge/mass experiment: J.J. Thomson repeated Hertz’s experiment with electric fields and cathode rays (but reduced the gas in the CRT) and found that they were deflected by electric fields. He also successfully determined the charge/mass ratio of cathode rays, providing the final proof that cathode rays are chargedparticle Scientific method: A theory is a scientist’s explanation of a principle. Since scientific explanations are provisional, and scientific views at any time depend on the evidence available to support these views, theories may change—we say therefore that science is ‘tentative’. Historically, this debate is an apt example of the ‘tentative’ nature of science and the scientific method, i.e. observations from experiments are interpreted and a hypothesis developed to explain what is thought to be happening. Opposing models are then resolved through improved experimentation, allowing us to gain a greater understanding of the nature of cathode rays. 1.7. Outline Thomson’s experiment to measure the charge: mass ratio of an electron Thomson set up his experiment as shown in the simplified diagram: (refer to book) As can be seen, he arranged the electric field such that the cathode rays would deflect upwards He then set up a magnetic field (using electromagnet coils) such that the magnetic field would produce a downward force on the electron He then altered the strength of the electric field such that the downward force caused by the magnetic field would be completely cancelled by the upward force provided by the magnetic field 𝐸 He then equated the Forces: 𝐹𝐸 = 𝐹𝐵 , ∴ 𝐸𝑞 = 𝑞𝑣𝐵, ∴ 𝑣 = 𝐵 He knew both the value for ‘E’ and ‘B’, allowing him to calculate the velocity of the cathode ray Now, he completely switched off the electric field. Since the force acting on the electron now (due to a known magnetic field) will be perpendicular to its linear velocity, the force due to the magnetic field becomes a centripetal force = 𝑚 𝑣2 . 𝑟 He then measured the displacement of the cathode ray beam (the radius of circular motion) He then equated the centripetal force and the magnetic force: 𝐹𝐵 = 𝐹𝐶 , ∴ 𝑞𝑣𝐵 = 𝑚 𝑣2 , 𝑟 ∴ 𝑞 𝑚 𝑣 = 𝑟𝐵 All the values on the right hand side are known. Despite the cathode material or gases, he always attained a value of 1.76 x 1011 C/kg. This value was 1800 times less that the value for a hydrogen ion. Since the charge of mass could not be determine, there were two possible conclusion he could make: 1. The charge of the cathode ray was a 1800 times greater than the charge of a hydrogen ion 2. The mass of the cathode ray was 1800 less than that of a hydrogen ion He concluded the latter. This undoubtedly contributed to the discovery of the electron and the development of models of atoms: as his results paved the way for his conclusion that cathode rays were a new class of negatively charged particles (later named electrons) 2.1. Perform an investigation and gather first-hand information to observe the occurrence of different striation patterns for different pressures in discharge tubes Aim: To observe the occurrence of different striation patterns for different pressures in discharge tubes Equipment: Induction coil, one set of discharge tubes with varying pressure, two plug-clip leads, two plug-plug leads and a power pack Safety: As the cathode rays hit the glass or metal in the discharge tube, X-rays are generated. Prolonged exposure to these X-rays may cause lethal diseases such as cancer. To prevent this, all students observed the striation patterns from five metres away and the teacher operating the discharge tubes was wearing a lead apron. Touching the induction coil while it is operating may cause electric shock and death. To avoid this, stay a safe distance from the induction coil Take care when handling the discharge tubes, as the low pressure can cause them to easily IMPLODE Method: 1. Connect the power pack to the induction coil and set it at 6V. Adjust the points on the induction coil so that a strong steady spark is produced 2. Attach the negative terminal of the induction coil to the cathode of the discharge tube and marked with the highest pressure (40mmHg) and attach the positive terminal to the anode. Switch on the power pack 3. Sketch a diagram of the pattern observed in this tube and describe it carefully 4. Repeat steps 1-3 for each of the other discharge tubes – tubes to be used should be 10mmHg, 6mmHg, 3mmHg, 0.14mmHg and 0.03mmHg 5. Repeat the experiment ten times Results: Pressure (mmHg) 40 10 6 3 0.14 0.03 Striation pattern observed Flashes of purple at anode and cathode Black in the middle Bright purple at anode and cathode Pink stream of light in the body Purple light at anode and cathode One dark gap near cathode Pink-purple body Pink anode Purple flashes at cathode Pink-orange body with striations Pink-purple glow at anode and cathode Purple body Less striations Glass fluorescing Tube is dark except for the purple fluorescence behind the anode on the glass 1.1.9. Outline the role of the following in the cathode ray tube of conventional TV displays and oscilloscopes Electrodes in the electrode gun: The electron gun produces a narrow beam of electrons. It consists of a heating filament, a cathode, a grid and two open-cylinder anodes (the second one is more positively charged than the first). (Note: There are three electrode guns in a colour TV). The filament heats the cathode, causing thermionic emission. The positively charged anodes help to focus and accelerate the electrons to the required speeds, so that they have enough energy on reaching the screen to cause fluorescence. An electrode called the grid between the cathode and anode helps control the brightness of the spot, by controlling the number of electrons emitted by the gun. The more positive the grid is, the more the electrons get attracted and hit the screen, the greater the brightness of the spot The deflection plates or coils: Are either electrically charged (used in CROs), or electromagnets (used in TV and computer screens) can be used to deflect the cathode ray beam to the required spot on the screen o For CROs: Two sets of parallel deflecting plates are charged to produce an electric field between them. The horizontally placed parallel plates (Y plates) produce a vertical electric field, deflecting the beam up and down. The vertically placed parallel plates (X plates) produce a horizontal electric field, deflecting the beam right and left. The beam is always deflected toward the positive plate, and the greater the voltage between the two plates, the greater the magnitude of the deflection o For TVs: Electric current passing through the coils around the cathode-ray tube produces magnetic field that control the movement of the electron beam. Magnetic coils allow for a wider angle beam than what would be possible with electric fields from plates, which is why they were used in CRT TVs The fluorescent screen: The inner surface of the glass screen is coated with layers of a fluorescent material like Zinc Sulfide. When electrons strike the screen, the coating fluoresces, and a spot of light is seen on the screen. 1.2.2. Perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes: containing a Maltese Cross; containing electric plates; with a fluorescent display screen; containing a glass wheel; analyse the information gathered to determine the sign of the charge on the cathode rays Aim: To determine some of the properties of the rays which come from the cathode of a discharge tube Apparatus: Two power packs, two plug-plug leads, one pair of magnets, induction coil, four plug-clip leads and discharge tubes (ones with a Maltese Cross, containing electric plate, a fluorescent display screen and a glass wheel) Method: 1. Connect the terminals of the induction coils and set it at 6V. Adjust the points on the induction coil so that a strong, steady spark is produced 2. Connect the terminals of the induction coil to the discharge tube containing the Maltese Cross when the cross is down and when it is up 3. Replace the Maltese cross tube with the tube containing electric plates, and connect the terminals of the plate to its high DC voltage supply. Observe the effects of the electric field on the cathode rays 4. Connect the tube with the fluorescent screen display to the induction coil and record the effect of placing a set of bar magnets near the cathode rays 5. Finally, connect the tube containing the glass wheel on tracks to the induction coil and observe the effects that the cathode rays have on the when the tubs is horizontal Results: Maltese cross: a clearly define shadow of the cross appears on the back of the tube opposite the cathode. This showed that the cathode rays travelled in straight lines from the cathode to anode, and are blocked by solid objects Electric plates: pairs of electric plates caused the cathode rays to be deflected toward the positive plate. This showed that they ate negatively charged and consist of particles (since electromagnetic waves are not charged) Fluorescent Screen: a thin straight green beam was formed on the fluorescent screen, and this was deflected when a bar magnet was brought close. The deflection showed that the cathode rays were charged, and the direction of deflection indicated a negative charge. The thin beam demonstrated they travel in straight lines. Additionally, the fact that they cause fluorescence showed that they have energy Paddle Wheel: the lightweight glass paddle wheel, able to rotate freely, is placed in the path of the cathode rays so that the rays strike one edge of the wheel at a tangent. The cathode rays caused the wheel to spin and move away from the cathode. This demonstrates that the cathode rays must have momentum, and therefore mass, and they are emitted from the cathode