Physics Quiz: Explain Energy Transfer Via Fields

What this quiz covers

This quiz focuses on Explain Energy Transfer Via Fields, giving you a quick way to practice the rules, question types, and explanations that matter most for Physics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The magnetic field does mechanical work $W=BIL$ on the wire even though the wire does not move.
2. The magnetic field can exert a force $F=BIL$, but with zero displacement the mechanical work done on the wire is $W=0$. (correct answer)
3. The magnetic field must transfer energy to the wire because a force always implies work.
4. Energy is transferred by direct contact between the magnet and the wire.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Magnetic fields can do work on current-carrying wires—when a wire with current I in magnetic field B moves through distance d, the field exerts force F = BIL and does work W = Fd, converting electrical energy to mechanical kinetic energy, and this occurs without direct contact between the magnet and the wire. In this scenario, the magnetic field B = 0.20 T exerts maximum force F = BIL = (0.20)(2.0)(0.50) = 0.20 N on the wire, but with zero displacement (d=0), the work done is W = Fd = 0 J. The field serves as the intermediary, but no energy is transferred mechanically without motion, even though a force is present. Choice B is correct because it recognizes that work requires displacement, so W=0 despite the force, correctly applying W=Fd. Choice C is incorrect because it assumes a force always implies work, ignoring the need for displacement in the work definition. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 2

1. Correct: energy transfer requires electrons to travel directly from pad to phone through the air.
2. Incorrect: energy transfers through a changing magnetic field that induces current in the phone’s coil, without electrons crossing the gap. (correct answer)
3. Correct: the phone receives energy because the magnetic field directly pushes charges along the field lines, doing work $W=qEd$.
4. Incorrect: wireless charging is actually powered by static electric fields only, not changing magnetic fields.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electromagnetic induction allows energy transfer through changing magnetic fields—when current in a primary coil creates a changing magnetic field (by varying the current), this changing field passes through a nearby secondary coil inducing an EMF and current (Faraday's law), transferring electrical energy from primary to secondary without any direct electrical connection, with the field acting as the energy carrier. In this scenario, the student's claim of electricity flowing through air is incorrect, as energy is transferred by the changing magnetic field inducing current in the phone's coil without electrons crossing the gap, distinguishing field-mediated induction from direct conduction. Choice B is correct because it accurately explains that the field acts as energy carrier allowing non-contact transfer and properly identifies the energy conversion pathway through the changing magnetic field. Choice A incorrectly claims direct electron transfer is required, missing that fields transfer energy across gaps and confusing induction with conduction. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 3

1. The solenoid’s changing magnetic field induces a current in the ring; the induced current interacts with the magnetic field, transferring energy to the ring’s kinetic energy. (correct answer)
2. The ring moves because static magnetic fields do work directly on stationary charges in the metal without any current.
3. Energy is transferred only by physical contact, so the ring must be pushed by expanding air.
4. The ring moves because the solenoid creates an electric field that is constant in time, which always induces a current.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electromagnetic induction allows energy transfer through changing magnetic fields—when current in a primary coil creates a changing magnetic field (by varying the current), this changing field passes through a nearby secondary coil inducing an EMF and current (Faraday's law), transferring electrical energy from primary to secondary without any direct electrical connection, with the field acting as the energy carrier. In this scenario, the rapidly increasing current in the solenoid creates a changing magnetic flux through the ring, inducing a current that interacts with the magnetic field to produce a force, transferring energy to the ring's kinetic energy across the gap. Choice A is correct because it accurately explains the induction mechanism and the role of the induced current in energy transfer via the magnetic field. Choice B incorrectly claims static fields do work on stationary charges, confusing magnetic force requirements. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 4

1. The magnet supplies electrons to the loop, increasing the loop’s kinetic energy by conduction.
2. The magnetic field exerts forces on the current in the loop; as the loop moves through a distance, the field does work $W=Fd$, transferring electrical energy to mechanical energy. (correct answer)
3. The rotor gains energy because magnetic fields always create energy from nothing when current flows.
4. Only an electric field can do work; a magnetic field cannot transfer energy in a motor.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Magnetic fields can do work on current-carrying wires—when a wire with current I in magnetic field B moves through distance d, the field exerts force F = BIL and does work W = Fd, converting electrical energy to mechanical kinetic energy, and this occurs without direct contact between the magnet and the wire. In this scenario, the magnetic field exerts forces on the current in the loop, doing work W = Fd as the rotor turns and lifts the mass, transferring electrical energy to mechanical energy. The field serves as the intermediary, storing energy when created by the source and releasing it to the receiver without requiring physical contact. Choice B is correct because it properly identifies the energy conversion pathway through the magnetic field doing work on the moving loop. Choice D is incorrect because it fails to recognize that magnetic fields can transfer energy in motors via forces on currents, confusing it with the fact that magnetic forces do no work on isolated moving charges. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 5

1. $1.2\times10^{-1}\ \text{J}$
2. $1.2\times10^{-2}\ \text{J}$ (correct answer)
3. $3.0\times10^{4}\ \text{J}$
4. $0\ \text{J}$ because there is no contact force

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electric fields can do work on charges and transfer energy—when a charge q moves through distance d in electric field E, the field does work W = qEd, converting field energy to kinetic energy of the charge, and this occurs even though there's no direct contact between the source charges creating the field and the charge being accelerated. In this scenario, the electric field E = 3.0×10^4 N/C does work on charge q = +2.0×10^{-6} C as it moves distance d = 0.20 m, transferring energy W = qEd = (2.0×10^{-6})(3.0×10^4)(0.20) = 1.2×10^{-2} J from the field to the sphere's kinetic energy. The field serves as the intermediary, storing energy when created by the source and releasing it to the receiver without requiring physical contact. Choice B is correct because it accurately calculates the work using W = qEd and recognizes the positive energy transfer for a positive charge moving along the field. Choice D is incorrect because it claims no work is done without contact force, missing that fields transfer energy across gaps. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 6

1. The electric field does work $W=qEd=2.0\times10^{-6}\ \text{J}$ on the droplet, increasing its kinetic energy. (correct answer)
2. The electric field does work $W=E/(qd)$, so the droplet gains $2.0\times10^{13}\ \text{J}$.
3. No energy is transferred because fields cannot store or transfer energy.
4. The droplet loses kinetic energy because it moves in the direction of the electric field.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electric fields can do work on charges and transfer energy—when a charge q moves through distance d in electric field E, the field does work W = qEd, converting field energy to kinetic energy of the charge, and this occurs even though there's no direct contact between the source charges creating the field and the charge being accelerated. In this scenario, the electric field E = 2.0×10^4 N/C does work on the droplet q = +1.0×10^{-9} C as it moves distance d = 0.10 m upward along the field, transferring energy W = qEd = (1.0×10^{-9})(2.0×10^4)(0.10) = 2.0×10^{-6} J from the field to the droplet's kinetic energy. The field serves as the intermediary, storing energy when created by the source and releasing it to the receiver without requiring physical contact. Choice A is correct because it accurately calculates and describes the work using W = qEd for the energy transfer. Choice B is incorrect because it uses an inverted formula W = E/(qd), leading to a wrong value and misunderstanding the work calculation. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 7

1. Electrical energy flows through the iron core as a current from the primary to the secondary.
2. Energy is transferred from the primary to the secondary by the changing magnetic field, which induces an emf and current in the secondary. (correct answer)
3. Energy is transferred only by direct contact forces between the wires of the two coils.
4. The resistor provides the energy, which then travels backward to the primary coil.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electromagnetic induction allows energy transfer through changing magnetic fields—when current in a primary coil creates a changing magnetic field (by varying the current), this changing field passes through a nearby secondary coil inducing an EMF and current (Faraday's law), transferring electrical energy from primary to secondary without any direct electrical connection, with the field acting as the energy carrier. In this scenario, the changing current in the primary coil creates a changing magnetic flux through the secondary, inducing EMF which drives current through the resistor, delivering power to it. The field serves as the intermediary, storing energy when created by the source and releasing it to the receiver across the gap between coils. Choice B is correct because it accurately explains that the changing magnetic field acts as energy carrier allowing non-contact transfer to the resistor. Choice C is incorrect because it claims energy transfer requires direct contact between coils, ignoring the induction mechanism. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 8

1. $0.12\ \text{J}$ (correct answer)
2. $1.2\ \text{J}$
3. $0.60\ \text{J}$
4. $3.0\ \text{J}$

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Magnetic fields can do work on current-carrying wires—when a wire with current I in magnetic field B moves through distance d, the field exerts force F = BIL and does work W = Fd, converting electrical energy to mechanical kinetic energy, and this occurs without direct contact between the magnet and the wire. In this scenario, the magnetic field B = 0.50 T exerts maximum force F = BIL = (0.50)(4.0)(0.30) = 0.60 N on the wire, doing work W = Fd = (0.60)(0.20) = 0.12 J as it moves d = 0.20 m. The field serves as the intermediary, storing energy when created by the source and releasing it to the receiver without requiring physical contact. Choice A is correct because it correctly calculates the work using W = BILd and recognizes the energy conversion pathway through the field. Choice C is incorrect because it makes a calculation error, likely misapplying the formula or units. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 9

1. The bead gains kinetic energy because the electric field does work on it; the energy ultimately comes from the external source that separated charges to create the field. (correct answer)
2. The bead gains kinetic energy because the negative plate pulls it by direct contact forces through the air.
3. The bead gains kinetic energy because magnetic fields always accelerate charges in the direction of motion.
4. The bead gains kinetic energy because energy is created from nothing when charges move in a field.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electric fields can do work on charges and transfer energy—when a charge q moves through distance d in electric field E, the field does work W = qEd, converting field energy to kinetic energy of the charge, and this occurs even though there's no direct contact between the source charges creating the field and the charge being accelerated. In this scenario, the uniform electric field between the plates accelerates the positive bead toward the negative plate, doing work to increase its kinetic energy, with the energy sourced from the external work done to separate the charges on the plates. Choice A is correct because it accurately explains that the electric field acts as the intermediary for non-contact energy transfer from the source to the bead's kinetic energy. Choice B incorrectly claims direct contact forces through air, failing to identify the field's role. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 10

1. $W=5.0\times10^{-3}\ \text{J}$ (correct answer)
2. $W=2.0\times10^{-3}\ \text{J}$
3. $W=5.0\times10^{-9}\ \text{J}$
4. $W=2.0\times10^{-6}\ \text{J}$

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electric fields can do work on charges and transfer energy—when a charge q moves through distance d in electric field E, the field does work W = qEd, converting field energy to kinetic energy of the charge, and this occurs even though there's no direct contact between the source charges creating the field and the charge being accelerated. In this scenario, the electric field E = 5.0×10^3 N/C does work on charge q = 2.0×10^{-6} C as it moves distance d = 0.50 m, transferring energy W = qEd = (2.0×10^{-6})(5.0×10^3)(0.50) = 5.0×10^{-3} J from the field to the ion. Choice A is correct because it accurately calculates the work W = 5.0×10^{-3} J using W = qEd and recognizes the energy transfer to the ion. Choice B incorrectly halves the value, perhaps by misapplying the distance or overlooking the factor of 0.50. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 11

1. $W=0.12\ \text{J}$ (correct answer)
2. $W=0.30\ \text{J}$
3. $W=1.2\ \text{J}$
4. $W=0.60\ \text{J}$

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Magnetic fields can do work on current-carrying wires—when a wire with current I is in magnetic field B, the force F = BIL sinθ does work W = Fd as the wire moves distance d, converting electrical energy to mechanical energy, as in motors, without direct contact between the magnet and the wire. In this scenario, the magnetic field B = 0.50 T exerts force F = BIL = (0.50)(2.0)(0.30) = 0.30 N on the wire, and as it moves d = 0.40 m, the work done is W = Fd = 0.30 × 0.40 = 0.12 J, transferring energy to the wire's motion. Choice A is correct because it accurately calculates the work W = 0.12 J using W = BILd and recognizes the energy transfer by the magnetic force. Choice B incorrectly doubles the value, perhaps by miscalculating F as 0.75 N. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 12

1. $W=-9.6\times10^{-16}\ \text{J}$ (the electron loses kinetic energy)
2. $W=+9.6\times10^{-16}\ \text{J}$ (correct answer)
3. $W=+2.4\times10^{-15}\ \text{J}$
4. $W=+9.6\times10^{-15}\ \text{J}$

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electric fields can do work on charges and transfer energy—when a charge q moves through distance d in electric field E, the field does work W = qEd, converting field energy to kinetic energy of the charge, and this occurs even though there's no direct contact between the source charges creating the field and the charge being accelerated. In this scenario, the electric field E = 3.0×10^4 N/C does work on charge q = -1.60×10^{-19} C as it moves distance d = 0.20 m, transferring energy W = qEd = (-1.60×10^{-19})(3.0×10^4)(0.20) but considering directions, the magnitude is 9.6×10^{-16} J and positive since the field accelerates the electron, increasing its kinetic energy. Choice B is correct because it accurately calculates the positive work W = +9.6×10^{-16} J done by the field, reflecting the kinetic energy gain. Choice A incorrectly claims negative work and energy loss, missing that the field does positive work on the accelerating electron despite its negative charge. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 13

1. Mechanical energy is converted into electrical energy because the spinning coil generates current.
2. Chemical energy in the magnets is converted directly into kinetic energy of the coil.
3. Electrical energy supplied to the coil is transferred via magnetic forces into mechanical (rotational) energy of the motor. (correct answer)
4. Thermal energy in the wire is converted into magnetic field energy, which then becomes electrical energy again.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Magnetic fields can do work on current-carrying coils—electrical energy supplied to the coil creates currents that interact with the magnetic field, producing forces that cause rotation, transferring energy to mechanical (rotational) energy without direct contact. In this scenario, the current in the coil interacts with the magnetic field to produce torque, converting input electrical power to output mechanical power during steady operation. Choice C is correct because it correctly identifies the energy conversion pathway from electrical to mechanical via magnetic forces in the motor. Choice A confuses the process with a generator, reversing the energy flow from mechanical to electrical. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 14

1. $W=5.0\times10^{-3}\ \text{J}$ (correct answer)
2. $W=2.0\times10^{-3}\ \text{J}$
3. $W=5.0\times10^{-9}\ \text{J}$
4. $W=2.0\times10^{-6}\ \text{J}$

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electric fields can do work on charges and transfer energy—when a charge q moves through distance d in electric field E, the field does work W = qEd, converting field energy to kinetic energy of the charge, and this occurs even though there's no direct contact between the source charges creating the field and the charge being accelerated. In this scenario, the electric field E = 5.0×10^3 N/C does work on charge q = 2.0×10^{-6} C as it moves distance d = 0.50 m, transferring energy W = qEd = (2.0×10^{-6})(5.0×10^3)(0.50) = 5.0×10^{-3} J from the field to the ion. Choice A is correct because it accurately calculates the work W = 5.0×10^{-3} J using W = qEd and recognizes the energy transfer to the ion. Choice B incorrectly halves the value, perhaps by misapplying the distance or overlooking the factor of 0.50. Energy transfer via fields: electric fields accelerate charges doing work W = qEd (field energy → kinetic energy), magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), changing magnetic fields induce currents transferring energy between circuits (Faraday's law: electromagnetic induction), and electromagnetic waves carry energy through space at light speed (radiation energy). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion), and explains wireless charging (induction), motors (magnetic force work), particle accelerators (electric field acceleration), and solar panels (EM wave absorption).

Question 15

1. $0.80\ \text{J}$
2. $0.32\ \text{J}$ (correct answer)
3. $3.2\ \text{J}$
4. $0.20\ \text{N}$

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Magnetic forces on currents do work W = Fd with F = BIL, converting electrical energy to mechanical energy through the magnetic field's mediation. In this scenario, the magnetic force on the wire is F = BIL = (0.80 T)(4.0 A)(0.25 m) = 0.80 N, and as the wire moves distance d = 0.40 m in the direction of this force, the magnetic field does work W = Fd = (0.80 N)(0.40 m) = 0.32 J, transferring energy from the field to the wire's kinetic energy. The field serves as the intermediary, converting electrical energy (from the current source) into mechanical energy of the wire's motion without requiring direct contact. Choice B is correct because it accurately calculates work using W = Fd = 0.32 J. Choice A incorrectly gives 0.80 J (possibly confusing force magnitude with work), Choice C shows 3.2 J (order of magnitude error), and Choice D gives 0.20 N which is a force unit, not energy. Energy transfer via magnetic fields is the foundation of electric motors, speakers, and electromagnetic actuators—demonstrating how fields enable conversion between electrical and mechanical energy across air gaps.

Question 16

1. The claim is correct: fields exert forces, but the work done is always zero unless there is contact.
2. The claim is correct for electric fields, but magnetic fields can transfer energy because magnets contain stored mechanical energy.
3. The claim is incorrect: for example, a changing magnetic field from a primary coil can induce current in a nearby secondary coil, transferring energy across an air gap. (correct answer)
4. The claim is incorrect because energy is transferred by air molecules between objects, not by fields.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electromagnetic induction allows energy transfer through changing magnetic fields—when current in a primary coil creates a changing magnetic field (by varying the current), this changing field passes through a nearby secondary coil inducing an EMF and current (Faraday's law), transferring electrical energy from primary to secondary without any direct electrical connection, with the field acting as the energy carrier. The student's claim that fields cannot transfer energy is fundamentally incorrect, as demonstrated by countless technologies from transformers to wireless chargers. Fields serve as the intermediary, storing energy when created and releasing it to receivers across gaps without any physical contact. Choice C is correct because it provides a clear counterexample: electromagnetic induction in transformers proves fields transfer energy across air gaps. Choice A incorrectly supports the false claim; Choice B wrongly suggests only magnetic fields transfer energy and invents "stored mechanical energy" in magnets; Choice D incorrectly attributes energy transfer to air molecules rather than fields. Energy transfer via fields is fundamental to modern technology: wireless charging, radio transmission, solar panels capturing electromagnetic radiation, and particle accelerators all demonstrate non-contact energy transfer through fields.

Question 17

1. $9.0\times10^{-3}\ \text{J}$ (correct answer)
2. $2.25\times10^{-2}\ \text{J}$
3. $9.0\times10^{-2}\ \text{J}$
4. $1.0\times10^{-10}\ \text{J}$

Explanation: This question tests understanding of how electric fields transfer energy without requiring direct physical contact between objects. Electric fields can do work on charges and transfer energy—when a charge q moves through distance d in electric field E, the field does work W = qEd, converting field energy to kinetic energy of the charge, and this occurs even though there's no direct contact between the source charges creating the field and the charge being accelerated. In this scenario, the electric field E = 1.5×10⁴ N/C does work on charge q = 3.0×10⁻⁶ C as it moves distance d = 0.20 m, transferring energy W = qEd = (3.0×10⁻⁶ C)(1.5×10⁴ N/C)(0.20 m) = 9.0×10⁻³ J from the field to the sphere's kinetic energy. The field serves as the intermediary, storing energy when created by the source and releasing it to the charged sphere across the gap. Choice A is correct because it accurately calculates work using W = qEd = 9.0×10⁻³ J. Choice B incorrectly calculates 2.25×10⁻² J (possibly multiplying by extra factor), Choice C gives 9.0×10⁻² J (order of magnitude error), and Choice D shows 1.0×10⁻¹⁰ J (completely wrong calculation). The key insight is fields serve as energy carriers, storing energy when created and releasing it to objects within the field, enabling energy transfer without material contact—this is fundamentally different from conduction (needs contact) or convection (needs fluid motion).

Question 18

1. Energy transfers because the coils must touch so electrons can flow directly from the pad into the phone.
2. Energy transfers via the changing magnetic field, which induces an emf and current in the phone’s coil; the field acts as the intermediary across the air gap. (correct answer)
3. Energy transfers because the pad’s static electric field pushes charges through the phone’s battery even if the magnetic field is constant.
4. Energy is created in the phone’s coil from nothing; induction violates energy conservation at small gaps.

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electromagnetic induction allows energy transfer through changing magnetic fields—when current in a primary coil creates a changing magnetic field (by varying the current), this changing field passes through a nearby secondary coil inducing an EMF and current (Faraday's law), transferring electrical energy from primary to secondary without any direct electrical connection, with the field acting as the energy carrier. In this scenario, the changing current in the primary coil creates a changing magnetic flux through the secondary, inducing EMF ε = N(ΔΦ/Δt) which drives current through the secondary's circuit, delivering power to the phone's battery. The field serves as the intermediary, storing energy when created by the source and releasing it to the receiver across the air gap between coils. Choice B is correct because it accurately explains that the field acts as energy carrier allowing non-contact transfer through electromagnetic induction. Choice A incorrectly claims direct contact is required, missing that fields transfer energy across gaps; Choice C confuses electric and magnetic fields and incorrectly suggests static fields can charge batteries; Choice D absurdly claims energy is created from nothing, violating conservation of energy. Energy transfer via fields enables wireless charging (induction), transformers, and many other technologies where energy must cross gaps without physical connections.

Question 19

1. It is destroyed because induced currents in the secondary coil are not bound by energy conservation.
2. It remains permanently stored in the magnetic field energy density $\propto B^2$ and never leaves the core.
3. It is transferred to the secondary coil but appears as extra electrical power beyond the load’s rating.
4. It is dissipated as thermal energy (and possibly sound) due to resistive losses and magnetic losses, even though energy transfer to the secondary occurs via the field. (correct answer)

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Electromagnetic induction allows energy transfer through changing magnetic fields—when current in a primary coil creates a changing magnetic field, this changing field passes through a nearby secondary coil inducing an EMF and current (Faraday's law), transferring electrical energy from primary to secondary without any direct electrical connection. In this scenario, the transformer receives Pin = 500 W at the primary, transfers 90% (450 W) to the secondary load via the magnetic field, and the missing 10% (50 W) must go somewhere due to energy conservation. The field serves as the intermediary for the 450 W transfer, but real transformers have losses. Choice D is correct because it accurately explains that the missing 50 W is dissipated as thermal energy due to resistive losses in windings (I²R heating) and magnetic losses in the core (hysteresis and eddy currents), even though the main energy transfer occurs via the field. Choice A violates energy conservation; Choice B incorrectly suggests energy remains trapped in the field; Choice C wrongly implies the secondary receives more than 100% power. Real transformers demonstrate both the power of field-based energy transfer and the practical limitations—while fields enable non-contact transfer, material properties cause unavoidable losses manifesting as heat.

Question 20

1. $0.24\ \text{J}$ (correct answer)
2. $2.4\ \text{J}$
3. $0.12\ \text{J}$
4. $1.2\ \text{J}$

Explanation: This question tests understanding of how electric and magnetic fields transfer energy without requiring direct physical contact between objects. Magnetic forces on currents do work W = Fd with F = BIL (electrical → mechanical in motors), converting electrical energy to mechanical energy through the magnetic field's mediation. In this scenario, the magnetic force on the wire is F = BIL = (0.60 T)(5.0 A)(0.40 m) = 1.2 N, and as the wire moves distance d = 0.20 m in the direction of this force, the magnetic field does work W = Fd = (1.2 N)(0.20 m) = 0.24 J, transferring energy from the field to the wire's kinetic energy. The field serves as the intermediary, converting electrical energy (from the current source) into mechanical energy of the wire's motion. Choice A is correct because it accurately calculates work using W = Fd = BILd = 0.24 J. Choice B incorrectly calculates 2.4 J (order of magnitude error), Choice C gives 0.12 J (possibly using wrong length or distance), and Choice D shows 1.2 J (confusing force with work). Energy transfer via magnetic fields enables motors, generators, and electromagnetic actuators—the magnetic field acts as the energy carrier between electrical and mechanical systems without requiring direct mechanical linkages.