101. A compact passage says:
Four field laws are used together. One tells how charges relate to electric flux. One states that net magnetic flux through a closed surface is zero. One connects changing magnetic field to electric field. One connects conduction current and changing electric flux to magnetic field.
The last sentence of the passage describes
ⓐ. Ampere-Maxwell law
ⓑ. Faraday's law
ⓒ. Gauss's law for electricity
ⓓ. Gauss's law for magnetism
Correct Answer: Ampere-Maxwell law
Explanation: The last sentence mentions both conduction current and changing electric flux. These two appear together as \(I_c+I_d\) in the corrected Ampere-Maxwell law. Faraday's law would instead focus on changing magnetic flux producing an electric field. Gauss's laws are flux laws for closed surfaces and do not contain the sum \(I_c+I_d\). The phrase "changing electric flux to magnetic field" is the clue that Maxwell's correction is being used.
102. A row in a student's notes says: "Faraday's law: changing electric flux produces magnetic field." The row needs correction because Faraday's law actually says that
ⓐ. electric flux through a closed surface is always zero
ⓑ. conduction current is equal to wavelength
ⓒ. changing magnetic field produces electric field
ⓓ. displacement current is caused by constant electric flux
Correct Answer: changing magnetic field produces electric field
Explanation: Faraday's law is about the electric field produced by a changing magnetic field or changing magnetic flux. The statement in the notes has confused it with the complementary Maxwell-correction idea. A changing electric flux contributes to magnetic-field circulation through displacement current. Electric flux through a closed surface is not always zero; it depends on enclosed charge in Gauss's law for electricity. Keeping the two induction directions separate prevents mixing Faraday's law with the Ampere-Maxwell law.
103. A corrected table of the four standard textbook laws should place "enclosed charge" mainly under
ⓐ. Gauss's law for magnetism
ⓑ. Gauss's law for electricity
ⓒ. Faraday's law
ⓓ. Ampere-Maxwell circulation law
Correct Answer: Gauss's law for electricity
Explanation: Enclosed charge is the source term in Gauss's law for electricity. This law relates electric flux through a closed surface to the charge enclosed by that surface. Gauss's law for magnetism has no corresponding isolated magnetic charge term and gives zero net magnetic flux. Faraday's law and Ampere-Maxwell law focus on changing fields and field circulation. The word "enclosed" can appear in more than one law, but "enclosed charge" specifically points to the electric Gauss law.
104. If a region has no conduction current but its electric flux changes with time, the Maxwell-equation idea that can still connect it to magnetic field is
ⓐ. Gauss's law for magnetism
ⓑ. Faraday's law only
ⓒ. Ampere-Maxwell law
ⓓ. the relation \(c=\nu\lambda\)
Correct Answer: Ampere-Maxwell law
Explanation: A changing electric flux gives displacement current \(I_d=\varepsilon_0\frac{d\Phi_E}{dt}\). The Ampere-Maxwell law includes this term in \(\oint\vec{B}\cdot d\vec{l}=\mu_0(I_c+I_d)\). If \(I_c=0\), the displacement-current part may still produce magnetic-field circulation. Faraday's law would connect changing magnetic field with electric field, which is the opposite induction direction. The wave relation \(c=\nu\lambda\) does not explain the magnetic effect of changing electric flux.
105. A set of four statements is written about the Maxwell framework.
Statement I: Charges are related to electric flux.
Statement II: Isolated magnetic poles are represented by non-zero net magnetic flux through a closed surface.
Statement III: A changing magnetic field can produce an electric field.
Statement IV: A changing electric field can produce a magnetic field.
ⓐ. I, III and IV only
ⓑ. I and II only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, III and IV only
Explanation: Statement I matches Gauss's law for electricity. Statement III matches Faraday's law, and Statement IV matches Maxwell's correction to Ampere's law. Statement II is false because Gauss's law for magnetism states that net magnetic flux through a closed surface is zero, reflecting the absence of isolated magnetic monopoles in ordinary treatment. The four-law framework connects sources, fluxes, and changing fields, but it does not introduce isolated magnetic charge. The false statement comes from trying to make magnetism exactly parallel to electric charge in a way the standard textbook law does not support.
106. A stationary electric charge mainly produces
ⓐ. a magnetic field only
ⓑ. an electric field only
ⓒ. no field at all
ⓓ. an electromagnetic wave continuously
Correct Answer: an electric field only
Explanation: A stationary charge produces an electric field \(\vec{E}\) around it. Since it is not moving, it is not associated with a magnetic field due to charge motion. It also does not radiate an electromagnetic wave merely by being at rest. Radiation requires acceleration of charge, not just the existence of charge. This distinction helps separate static electric effects from electromagnetic-wave production.
107. A charge moving with constant velocity is compared with a charge that is accelerating. The accelerating charge is special because it can
ⓐ. lose its electric field completely
ⓑ. produce only a gravitational field
ⓒ. stop producing magnetic effects
ⓓ. radiate electromagnetic waves
Correct Answer: radiate electromagnetic waves
Explanation: A charge moving with uniform velocity can be associated with electric and magnetic fields, but uniform motion alone is not the basic condition for radiation. An accelerating charge has a changing state of motion, and this changing motion produces electromagnetic radiation. Oscillating charges are a common practical example because their acceleration changes continuously. The radiation carries energy away through electromagnetic fields. The key word is "accelerating," not merely "moving."
108. Consider the following charge situations.
Statement I: A stationary charge produces an electric field.
Statement II: A steady moving charge produces electric and magnetic fields.
Statement III: An accelerated charge can produce electromagnetic waves.
ⓐ. I, II and III
ⓑ. I and II only
ⓒ. II and III only
ⓓ. I and III only
Correct Answer: I, II and III
Explanation: A stationary charge produces an electric field because charge is the source of electric field. A charge moving with steady velocity is associated with both electric and magnetic fields. When the charge accelerates, the changing fields can detach and propagate outward as electromagnetic radiation. Oscillating charges are repeatedly accelerated, so they are useful sources of electromagnetic waves. These three statements form a gradual ladder from static field to radiation.
109. Match the charge condition with the most suitable field description.
| Column I | Column II |
|---|
| P. Charge at rest | 1. Can radiate electromagnetic waves |
| Q. Charge moving steadily | 2. Produces electric field without magnetic field due to motion |
| R. Accelerated charge | 3. Associated with electric and magnetic fields |
| S. Oscillating charge | 4. Practical time-varying source of radiation |
ⓐ. P-2, Q-3, R-1, S-4
ⓑ. P-3, Q-2, R-1, S-4
ⓒ. P-2, Q-1, R-3, S-4
ⓓ. P-4, Q-3, R-2, S-1
Correct Answer: P-2, Q-3, R-1, S-4
Explanation: A charge at rest produces an electric field but no magnetic field due to motion. A charge moving steadily is associated with both electric and magnetic fields. An accelerated charge can radiate electromagnetic waves because its fields change in a way that can propagate outward. An oscillating charge is a practical example of repeated acceleration. The matching separates field production from wave radiation, which is often confused when all moving charges are treated as radiating sources.
110. A radio transmitting antenna works because charges in it are made to
ⓐ. remain permanently at rest
ⓑ. oscillate and therefore accelerate
ⓒ. move with constant speed in one direction only
ⓓ. disappear into the surrounding air
Correct Answer: oscillate and therefore accelerate
Explanation: A transmitting antenna produces radio waves by making charges oscillate. Oscillation means the charges repeatedly change velocity, so they are accelerated. Accelerated charges radiate electromagnetic waves. The antenna does not work by keeping charges stationary or by making them vanish. The practical source of radio waves is the time-varying motion of charges in the conducting antenna.
111. A claim says, "A charge moving uniformly in a straight line is enough to radiate electromagnetic waves continuously." The best evaluation is that the claim is
ⓐ. correct because every moving charge must radiate
ⓑ. correct only if the moving charge is positive
ⓒ. not correct because moving charges never have magnetic fields
ⓓ. not correct because acceleration is needed for radiation
Correct Answer: not correct because acceleration is needed for radiation
Explanation: Uniform straight-line motion means the velocity of the charge is constant. Such a moving charge may be associated with electric and magnetic fields, but uniform motion is not the basic radiation condition. Electromagnetic radiation is produced when charges accelerate. Oscillation is an important case because velocity changes continuously, giving acceleration. The statement confuses ordinary field association with radiation of electromagnetic waves.
112. Study the table and identify the row that contains an unsuitable conclusion.
| Row | Situation | Conclusion |
|---|
| P | Stationary charge | Electric field exists |
| Q | Steady moving charge | Electric and magnetic fields can be associated |
| R | Accelerated charge | Electromagnetic radiation can be produced |
| S | Charge at rest | Radio waves are radiated continuously |
ⓐ. Row P
ⓑ. Row Q
ⓒ. Row S
ⓓ. Row R
Correct Answer: Row S
Explanation: A charge at rest produces a static electric field around it. It does not radiate radio waves continuously just because it has charge. Radio waves are produced by accelerated charges, commonly by oscillating charges in an antenna. Rows P, Q, and R correctly describe the increasing level of field behavior from rest to acceleration. Row S fails because it replaces the acceleration condition with mere presence of charge.
113. Assertion: Oscillating charges can act as sources of electromagnetic waves.
Reason: An oscillating charge undergoes acceleration during its motion.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false
ⓒ. Both Assertion and Reason are true, and Reason explains Assertion
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: An oscillating charge repeatedly changes its velocity. A change in velocity means acceleration. Accelerated charges can radiate electromagnetic waves. Therefore, oscillating charges are natural sources of electromagnetic radiation, especially in antenna systems. The Reason directly explains why the Assertion is true.
114. Read the passage below
In a conducting antenna, charges are driven back and forth rapidly. The changing motion of these charges produces electromagnetic radiation that travels away from the antenna.
The radiation leaves the antenna mainly because the charges are
ⓐ. accelerated
ⓑ. stationary
ⓒ. neutralized completely
ⓓ. converted into sound waves
Correct Answer: accelerated
Explanation: Charges driven back and forth in an antenna do not move with constant velocity. Their velocity changes in magnitude or direction during oscillation, so they are accelerated. Accelerated charges are the source of electromagnetic waves. The radiation is not produced by converting charges into sound waves or by making them stationary. The antenna example is a practical application of the principle that accelerated charges radiate.
115. In the chain "oscillating charge \(\rightarrow\) accelerated charge \(\rightarrow\) electromagnetic wave," the missing physical link is that
ⓐ. electric flux must remain constant
ⓑ. magnetic flux must always be zero
ⓒ. wavelength must become infinite before radiation
ⓓ. accelerated charge produces radiation
Correct Answer: accelerated charge produces radiation
Explanation: Oscillation involves repeated change of velocity, so an oscillating charge is an accelerated charge. The radiation condition is acceleration of charge. This accelerated motion produces changing electric and magnetic fields that can propagate away as an electromagnetic wave. The chain is not based on constant electric flux or zero magnetic flux. It connects charge motion with the production of travelling field disturbances.
116. A source produces electromagnetic waves, and the fields travel outward through empty space. The best description of the travelling wave is that it is
ⓐ. a disturbance carried only by material particles
ⓑ. a self-propagating pattern of electric and magnetic fields
ⓒ. a region where electric and magnetic fields both permanently vanish
ⓓ. a sound wave with very high frequency
Correct Answer: a self-propagating pattern of electric and magnetic fields
Explanation: An electromagnetic wave is a travelling disturbance of electric and magnetic fields. The fields vary with time and space as the wave propagates. It does not need a material medium, so it is not carried by particles in the way a sound wave is. The wave is not a region where fields vanish permanently; the fields are the wave itself. This self-propagating field pattern is the central idea behind electromagnetic radiation travelling through vacuum.
117. The self-sustaining nature of an electromagnetic wave is mainly based on the fact that
ⓐ. an electric field can propagate without any magnetic field
ⓑ. a magnetic field alone carries all wave energy in vacuum
ⓒ. air molecules must compress and expand along the path
ⓓ. changing electric and magnetic fields can produce each other
Correct Answer: changing electric and magnetic fields can produce each other
Explanation: Faraday's law links a changing magnetic field with an electric field. Maxwell's correction links a changing electric field with a magnetic field. Together, these ideas explain how electric and magnetic fields can support one another as a wave moves through space. This does not require air molecules or any other material medium. The wave is self-sustaining because the time-varying fields are dynamically connected.
118. A plane region in space has a time-varying electric field. According to the Maxwell-correction idea, the immediate magnetic-field-related effect is that the changing electric field can
ⓐ. remove all electric flux from the region
ⓑ. create isolated magnetic poles
ⓒ. change frequency into wavelength
ⓓ. produce magnetic field circulation
Correct Answer: produce magnetic field circulation
Explanation: Maxwell's correction states that a changing electric flux contributes to magnetic-field circulation. This is expressed in the displacement-current term \(I_d=\varepsilon_0\frac{d\Phi_E}{dt}\). The idea is not that electric flux is removed, but that its time variation matters. It also does not introduce isolated magnetic poles. This magnetic response to a changing electric field is one half of the field coupling needed for electromagnetic waves.
119. Consider the following statements about electromagnetic-wave propagation.
Statement I: The wave can travel through vacuum.
Statement II: The wave carries energy in its electric and magnetic fields.
Statement III: The wave requires a material medium because it is a mechanical vibration.
ⓐ. II and III only
ⓑ. I and III only
ⓒ. I and II only
ⓓ. I, II and III
Correct Answer: I and II only
Explanation: Electromagnetic waves can travel through vacuum, unlike sound waves. They carry energy through their electric and magnetic fields. Statement III is false because an electromagnetic wave is not a mechanical vibration of material particles. Mechanical waves require a medium, but electromagnetic waves are field waves. The ability of sunlight to cross space is a familiar example of Statements I and II.
120. Read the situation below.
A spacecraft sends a radio signal to Earth while travelling through the near-vacuum of space. The signal carries information and energy across the empty region.
The signal can reach Earth mainly because radio waves are
ⓐ. longitudinal sound waves carried by air in space
ⓑ. electromagnetic waves that need no material medium
ⓒ. conduction current streams crossing the vacuum
ⓓ. waves that exist only inside conducting metal wires
Correct Answer: electromagnetic waves that need no material medium
Explanation: Radio waves are electromagnetic waves. They can propagate through vacuum because they are made of time-varying electric and magnetic fields rather than vibrations of air particles. The signal carries energy and information through these fields. It is not a conduction current crossing the space between the spacecraft and Earth. This situation illustrates why electromagnetic waves are essential for communication across space.
