Chapter 12 PDF - Physics Past Paper

Easy Questions 1. The metal wires from which most tank circuits are made have electrical resistances. Why do these resistances prevent charge from oscillating forever in a tank circuit and what happens to the tank circuit's energy as time passes? (Correct Answer) The wire's resistanc...

Easy Questions 1. The metal wires from which most tank circuits are made have electrical resistances. Why do these resistances prevent charge from oscillating forever in a tank circuit and what happens to the tank circuit's energy as time passes? (Correct Answer) The wire's resistance wastes the tank circuit's energy as thermal energy which reduces the amount of charge sloshing in the tank circuit until the sloshing stops altogether. The wire's resistance wastes the tank circuit's energy as thermal energy which increases the amount of charge sloshing in the tank circuit until the sloshing stops altogether. The wire's resistance wastes the tank circuit's energy as thermal energy which reduces the amount of charge sloshing in the tank circuit until the charges are destroyed altogether. The wire's resistance wastes the tank circuit's energy as thermal energy which increases the amount of charge sloshing in the tank circuit until the charges are destroyed altogether. 1. To add energy to the charge oscillation in a tank circuit with an antenna, at which time during the oscillation cycle should you bring a positively charged wand close to the antenna? (Correct Answer) Bring the positively charged wand close to the antenna each time the antenna reaches its peak positive charge. Bring the positively charged wand close to the antenna each time the antenna reaches its peak negative charge. Bring the positively charged wand close to the antenna each time the antenna reaches its net neutral charge. None of the above choices are correct.  2. When an AM radio station announces that it's transmitting at 950 kHz, that statement isn't quite accurate. Explain why it may also be transmitting at 948 kHz and 954 kHz. (Correct Answer) Amplitude modulation introduces additional frequencies that extend as much as 5 kHz above and below its carrier frequency. Amplitude modulation introduces additional frequencies that extend as much as 10 kHz above and below its carrier frequency. Frequency modulation introduces additional frequencies that extend as much as 5 kHz above and below its carrier frequency. Frequency modulation introduces additional frequencies that extend as much as 10 kHz above and below its carrier frequency. 3. Why are most microwave TV dinners packaged in plastic rather than aluminum trays? (Correct Answer) Aluminum trays would reflect the microwaves which would result in improperly cooked food. Aluminum trays would transmit the microwaves which would result in improperly cooked food. Aluminum trays would refract the microwaves which would result in improperly cooked food. All of the above choices are correct. 4. Why is it so important that a microwave oven turn off when you open the door? (Correct Answer) Releasing the microwaves into the room wouldn't be healthy. The microwaves heat up the walls inside the microwave oven and you might get burned if you touch the wall. Cold air molecules from the room may go into the microwave oven and violently interact with the hot air molecules inside which may cause an explosion. You could expose yourself to harmful x-rays that are also being generated by the microwave.  5. When you're listening to FM radio near buildings, reflections of the radio wave can make the reception particularly bad in certain locations. Compare this effect to the problem of uneven cooking in a microwave oven. (Correct Answer) Both involve destructive interference in electromagnetic waves. Both involve constructive interference in electromagnetic waves. The FM transmission involves destructive interference and the microwave oven involves constructive interference in electromagnetic waves. The FM transmission involves constructive interference and the microwave oven involves destructive interference in electromagnetic waves. 6. Why is the thin metal handle of a Chinese food container dangerous when placed in a microwave oven? The thin metal handle will transmit the microwaves which causes charges on the other side of the metal to jump into the air which creates a spark. The thin metal handle will absorb the microwaves which causes charges on the metal to jump into the air which creates a spark. The thin metal handle has a low electrical resistance which causes the metal to heat up fast which creates a spark. (Correct Answer) None of the above choices are correct. 7. What did James Clerk Maxwell discover in 1865? A changing electric field produces a changing magnetic field. (Correct Answer) A changing electric field produces a magnetic field. An electric field produces a changing magnetic field. An electric field produces a magnetic field.  8. Radio waves are electromagnetic waves that have (Correct Answer) low frequencies and long wavelengths. low frequencies and short wavelengths. high frequencies and long wavelengths. high frequencies and short wavelengths. 9. FM stands for functional modification. frequency modification. functional modulation. (Correct Answer) frequency modulation. 10. If the wavelength of an electromagnetic wave decreases, the speed of the wave decreases. the speed of the wave increases. the frequency of the wave decreases. (Correct Answer) the frequency of the wave increases. 11. A microwave oven is designed to use a fluctuating magnetic field to heat water. (Correct Answer) a fluctuating electric field to heat water. a fluctuating magnetic field to heat a flat metal surface, which heats the food. a fluctuating electric field to heat a flat metal surface, which heats the food.  12. What is the Lorentz force? A charge in a magnetic field feels a force. A charge in an electric field feels a force. (Correct Answer) A moving charge in a magnetic field feels a force. A moving charge in an electric field feels a force. Medium/Hard Questions 1. When a radio signal travels through a coaxial cable, charge moves back and forth on both the central wire and the surrounding tube. Show that both electric and magnetic fields are present in the coaxial cable. (Correct Answer) Whenever the charges on the central wire and surrounding tube are different, electric fields are present that point from positive to negative charges. And whenever charge moves on either of the two conductors, that moving charge produces magnetic fields. Whenever the charges on the central wire and surrounding tube are different, magnetic fields are present that point from positive to negative charges. And whenever charge moves on either of the two conductors, that moving charge produces electric fields. Whenever the charges on the central wire and surrounding tube are different, electric fields are present that point from negative to positive charges. And whenever charge moves on either of the two conductors, that moving charge produces magnetic fields. Whenever the charges on the central wire and surrounding tube are different, magnetic fields are present that point from negative to positive charges. And whenever charge moves on either of the two conductors, that moving charge produces electric fields.  2. The Empire State Building has several FM antennas on top, added in part to increase its overall height. These antennas aren't very tall. Why do short antennas, located high in the air, do such a good job of transmitting FM radio? (Correct Answer) FM radio transmission involves high-frequency, short-wavelength radio waves and since a good antenna is one-quarter wavelength long, FM radio transmission requires relatively short antennas. FM radio transmission involves low-frequency, long-wavelength radio waves and since a good antenna is one-quarter wavelength long, FM radio transmission requires relatively short antennas. FM radio transmission involves high-frequency, short-wavelength radio waves and since a good antenna is one wavelength long, FM radio transmission requires relatively short antennas. FM radio transmission involves low-frequency, long-wavelength radio waves and since a good antenna is one wavelength long, FM radio transmission requires relatively short antennas. 3. Compare how a potato cooks in a microwave oven with how it cooks in an ordinary oven. (Correct Answer) In a microwave oven, the potato's water absorbs microwaves and that microwave energy becomes thermal energy and the potato's temperature rises uniformly. In an ordinary oven, heat flows gradually into the potato through its surface and its temperature rises nonuniformly with the middle of the potato last to become cooked. In a microwave oven, the potato's water absorbs microwaves and that microwave energy becomes thermal energy and the potato's temperature rises nonuniformly with the middle of the potato last to become cooked. In an ordinary oven, heat flows gradually into the potato through its surface and its temperature rises uniformly. In a microwave oven, heat flows gradually into the potato through its surface and its temperature rises uniformly. In an ordinary oven, the potato's water absorbs the heat energy and the potato's temperature rises nonuniformly with the middle of the potato last to become cooked. In a microwave oven, heat flows gradually into the potato through its surface and its temperature rises nonuniformly with the middle of the potato last to become cooked. In an ordinary oven, the potato's water absorbs the heat energy and the potato's temperature rises uniformly. 4. Is a thick, smooth-edged stainless steel bowl dangerous in a microwave oven? (Correct Answer) No, because a thick stainless steel bowl has a low electrical resistance and a smooth-edged stainless steel bowl doesn't produce strong electric fields. No, because a thick stainless steel bowl has a high electrical resistance and a smooth-edged stainless steel bowl doesn't produce strong electric fields. No, because a thick stainless steel bowl has a low electrical resistance and a smooth-edged stainless steel bowl produces strong electric fields. No, because a thick stainless steel bowl has a high electrical resistance and a smooth-edged stainless steel bowl produces strong electric fields. 5. A cyclotron is a particle accelerator invented in 1929 by American physicist Ernest O. Lawrence. It uses electric fields to do work on charged particles as they follow circular paths in a strong magnetic field. Lawrence's great insight was that all the particles take the same amount of time to complete one circle, regardless of their speed or energy. That fact allows the cyclotron to do work on all the particles at once as they circle together. How can a faster-moving electron take the same time to circle as a slower-moving electron? (Correct Answer) The path of a faster moving electron will bend more gradually which results in a large circle and the fast moving electron returns to its starting point at the same time the slower moving electron returns to its starting point. The path of a faster moving electron will bend less gradually which results in a large circle and the fast moving electron returns to its starting point at the same time the slower moving electron returns to its starting point. The path of a faster moving electron will bend more gradually which results in a small circle and the fast moving electron returns to its starting point at the same time the slower moving electron returns to its starting point. The path of a faster moving electron will bend less gradually which results in a small circle and the fast moving electron returns to its starting point at the same time the slower moving electron returns to its starting point.  6. Although a particular AM radio station claims to transmit 50,000 W of music power, that's actually its average power. There are times when it transmits more power than that and times when it transmits less. Explain. (Correct Answer) The power being transmitted is changed in order to represent air pressure fluctuations with the radio wave: compressions are represented by strengthening the radio wave and rarefactions are represented by weakening the radio wave. The power being transmitted is changed in order to represent air pressure fluctuations with the radio wave: compressions are represented by weakening the radio wave and rarefactions are represented by strengthening the radio wave. The power being transmitted is changed in order to represent air pressure fluctuations with the radio wave: both compressions and rarefactions are represented by strengthening the radio wave. The power being transmitted is changed in order to represent air pressure fluctuations with the radio wave: both compressions and rarefactions are represented by weakening the radio wave. 7. An extremely fast-moving charged particle traveling in a magnetic field can radiate X-rays, a phenomenon known as synchrotron radiation. Why is the magnetic field essential to this emission? (Correct Answer) Without the magnetic field, the charged particle would travel at a constant velocity and would not radiate electromagnetic waves because of its contant velocity. Without the magnetic field, the charged particle would accelerate and would not radiate electromagnetic waves because of its acceleration. Without the magnetic field, the charged particle would come to a stop and would not radiate electromagnetic waves because of its zero velocity. Without the magnetic field, the charged particle would turn around and therefore not radiate electromagnetic waves. Chapter 12 Study Guide: Understanding Concepts for Success Key Concepts and Explanations 1. Energy Dissipation in Tank Circuits Question: Why do resistances prevent charge from oscillating forever in a tank circuit? ○ Concept: The resistance in the wires converts the tank circuit’s electrical energy into thermal energy, which dissipates over time. ○ Outcome: The oscillations decrease and eventually stop. ○ Key Point: This is due to energy being wasted as heat, not due to charges being destroyed. 2. Adding Energy to Oscillations with an Antenna Question: When should a positively charged wand be brought near the antenna? ○ Concept: Bringing the wand close during the peak positive charge enhances the oscillation by adding energy at the right phase of the cycle. ○ Key Point: Timing is critical; align with the peak positive charge. 3. AM Radio Frequency Range Question: Why might an AM station transmitting at 950 kHz also transmit at 948 kHz and 954 kHz? ○ Concept: Amplitude modulation (AM) adds sidebands to the carrier frequency, extending the signal by 5 kHz above and below. ○ Key Point: This is a result of the modulation process. 4. Microwave Packaging Question: Why are TV dinners packaged in plastic rather than aluminum trays? ○ Concept: Aluminum reflects microwaves, leading to uneven cooking. ○ Key Point: Plastic allows microwaves to penetrate and cook food effectively. 5. Microwave Safety Question: Why must a microwave turn off when the door is opened? ○ Concept: Microwaves released into the room are harmful to health. ○ Key Point: This safety feature prevents exposure to intense electromagnetic radiation. 6. Destructive Interference in FM Radio and Microwaves Question: How does interference affect FM radio and microwave cooking? ○ Concept: Both phenomena involve destructive interference of electromagnetic waves, which leads to poor reception or uneven cooking.  ○ Key Point: Understanding wave interference is key to solving issues in both contexts. 7. Thin Metal Handles in Microwaves Question: Why is a thin metal handle dangerous in a microwave? ○ Concept: Thin metal does not spark due to heat or resistance but can cause intense electric fields, leading to sparking. ○ Key Point: Sparking is caused by the interaction of microwaves with the metal. 8. Maxwell’s Discovery Question: What did James Clerk Maxwell discover? ○ Concept: A changing electric field produces a magnetic field. ○ Key Point: This fundamental principle underpins electromagnetic wave theory. 9. Radio Wave Properties Question: What defines radio waves? ○ Concept: Radio waves have low frequencies and long wavelengths. ○ Key Point: This property is used for effective long-distance communication. 10. FM Modulation Question: What does FM stand for? ○ Concept: Frequency modulation involves varying the frequency of the carrier wave. ○ Key Point: FM is different from AM, which modulates amplitude. 11. Wavelength and Frequency Relationship Question: What happens if the wavelength of an electromagnetic wave decreases? ○ Concept: Frequency increases as wavelength decreases (inverse relationship). ○ Key Point: This relationship is governed by the wave equation:. 12. Microwave Heating Question: How does a microwave oven heat food? ○ Concept: A fluctuating electric field heats water molecules, which then transfer heat to food. ○ Key Point: This differs from traditional ovens, where heat transfers via conduction. 13. Lorentz Force Question: What is the Lorentz force? ○ Concept: A moving charge in a magnetic field experiences a force.  ○ Key Point: This principle is key in electromagnetism and particle physics. Advanced Topics 14. Electric and Magnetic Fields in Coaxial Cables Question: How do fields arise in coaxial cables? ○ Concept: Electric fields exist due to charge differences, while moving charges produce magnetic fields. ○ Key Point: Both fields are essential for signal transmission. 15. FM Antennas on Tall Structures Question: Why do short antennas effectively transmit FM radio? ○ Concept: FM waves are high frequency with short wavelengths, requiring antennas that are one-quarter wavelength long. ○ Key Point: Antenna height complements the wavelength. 16. Microwave vs. Conventional Cooking Question: How does cooking differ between the two methods? ○ Concept: Microwaves heat uniformly by exciting water molecules, while conventional ovens heat non-uniformly via conduction. ○ Key Point: Microwaves save time and ensure even heating. 17. Stainless Steel Bowls in Microwaves Question: Are stainless steel bowls dangerous in microwaves? ○ Concept: Smooth-edged, thick bowls do not produce sparks or excessive heat due to low resistance. ○ Key Point: Not all metals behave dangerously in microwaves. 18. Cyclotron Mechanism Question: How do fast and slow particles return simultaneously? ○ Concept: Faster particles take larger paths due to gradual bending, synchronizing with slower particles. ○ Key Point: The magnetic field ensures consistent cycling times. 19. AM Power Variability Question: Why does AM radio power fluctuate? ○ Concept: Power changes represent air pressure fluctuations via compressions (high power) and rarefactions (low power).  ○ Key Point: These changes encode sound information. 20. Synchrotron Radiation Question: Why does a magnetic field cause X-ray emission in fast-moving particles? ○ Concept: The magnetic field bends the path of the particle, causing acceleration and the emission of electromagnetic waves. ○ Key Point: Acceleration is essential for wave radiation. Preparation Tips Practice Problems: Solve exercises related to these questions to reinforce understanding. Key Figures: Review Animated Figures 12.1.2 and 14.3.2 for visual aids. Equation Mastery: Familiarize yourself with all equations provided in the practice exams. Concept Application: Focus on applying concepts rather than memorizing answers.