Electromagnetic Waves
Questions for Short Answer
1. In a microwave oven, the food is kept in a plastic container and the microwave is directed towards the food. The food is cooked without melting or igniting the plastic container. Explain.
ANSWER: The food we keep in a plastic container in a microwave oven is generally rich in water content. Water molecules are bipolar but not the molecules of the plastic container or air. When bipolar molecules come in the way of microwaves (Which is electromagnetic radiation), they try to align themselves along the electric field by rotating. But the electric field is an oscillating one. So the bipolar molecules are continuously aligning themselves by rotating. In this process, they hit nearby molecules, thus increasing the kinetic energy of the molecules. It results in increasing the temperature of the water/bipolar materials in the food. The plastic container's molecules are unaffected as there is no polarity.
Note: It is not true that the frequency of microwaves matches the frequency of water molecules and resonates.
2. A metal rod is placed along the axis of a solenoid carrying a high-frequency alternating current. It is found that the rod gets heated. Explain why the rod gets heated.
ANSWER: Due to the alternating current in the solenoid, the magnetic flux inside it is everchanging and its direction is also reversing. In such a magnetic field when the metal rod is placed, emf is induced in it, and current flows on its surface. Since there is no guiding path for the current, it seeks its own path and flows in many irregular loops. These are called eddy currents. Also due to the reversing direction of the alternating current, the direction of eddy currents changes. These produce thermal energy and the metal rod gets heated.
3. Can an electromagnetic wave be deflected by an electric field? By a magnetic field?
ANSWER: An electromagnetic wave do not carry a charge or charged particles, hence it can not be deflected by an electric or magnetic field.
4. A wire carries an alternating current i =iₒ sin ωt. Is there an electric field in the vicinity of the wire?
ANSWER: Due to the moving charge in a wire, a magnetic field is created around it. In alternating current, this magnetic field is always varying. The varying magnetic field creates an electric field in the vicinity of the wire.
5. A capacitor is connected to an alternating current source. Is there is a magnetic field between the plates?
ANSWER: Due to the alternating current, the electric field between the plates changes with time. The changing electric field between the plates produces a magnetic field. Hence there will be a magnetic field between the plates.
6. Can an electromagnetic wave be polarized?
ANSWER: Since an electromagnetic wave is a transverse wave, it can be polarized. In an unpolarized electromagnetic wave, the planes of the electric field and the magnetic field of different waves are randomly arranged. It can be polarized by restricting only the waves to pass through one plane and all these have the same planes of electric and magnetic fields.
7. A plane electromagnetic wave is passing through a region. Consider the quantities (a) electric field, (b) magnetic field, (c) electrical energy in a small volume, and (d) magnetic energy in a small volume. Construct pairs of the quantities that oscillate with equal frequencies.
ANSWER: In a plane electromagnetic wave, the electric field is given as
E =Eₒ sin ω(t -x/c)
and the magnetic field is given as
B =Bₒ sin ω(t -x/c).
We see that both the electric and magnetic fields oscillate with the same angular frequency and hence with the same frequency.
The electrical energy stored in a small volume dV is
Ue =½ϵₒ E²dV
=½ϵₒ {Eₒsin ω(t -x/c)}²dV
=½ϵₒ Eₒ²{½(1-cos 2ω(t -x/c))}dV
=¼ϵₒ Eₒ²{1-cos 2ω(t -x/c)}dV
The magnetic energy stored in a small volume dV is,
Ub =(1/2µₒ)B²dV
=(1/2µₒ){Bₒ sin ω(t -x/c)}²dV
=(1/4µₒ)Bₒ²{1 -cos 2ω(t -x/c)}²dV
We see that both Ue and Ub have the same angular frequency =2ω and hence the same frequency =2ω/2π =ω/π.
So the electrical energy and magnetic energy in a small volume oscillate with the same frequency.
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Links to the Chapters
Links to the Chapters
CHAPTER- 39- Alternating CurrentCHAPTER- 38- Electromagnetic Induction
CHAPTER- 37- Magnetic Properties of MatterCHAPTER- 36- Permanent Magnets
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 39- Alternating Current
CHAPTER- 38- Electromagnetic Induction
CHAPTER- 37- Magnetic Properties of Matter
CHAPTER- 36- Permanent Magnets
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 32- Electric Current in ConductorsCHAPTER- 31- CapacitorsCHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
OBJECTIVE -I
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
EXERCISES - Q-11 to Q-18
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 32- Electric Current in Conductors
CHAPTER- 31- Capacitors
CHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 8 - Work and Energy
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
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CHAPTER- 7 - Circular Motion
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CHAPTER- 6 - Friction
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CHAPTER- 6 - Friction
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CHAPTER- 5 - Newton's Laws of Motion
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Click here for → QUESTIONS FOR SHORT ANSWER
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Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
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Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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CHAPTER- 4 - The Forces
The Forces-
"Questions for short Answers"
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Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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CHAPTER- 3 - Kinematics - Rest and Motion
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Click here for EXERCISES (Question number 1 to 10)
Click here for EXERCISES (Question number 11 to 20)
Click here for EXERCISES (Question number 21 to 30)
Click here for EXERCISES (Question number 31 to 40)
Click here for EXERCISES (Question number 41 to 52)
CHAPTER- 2 - "Physics and Mathematics"
CHAPTER- 2 - "Physics and Mathematics"
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