Electromagnetic Induction MCQs | Physics MCQs

The induced EMF is proportional to sin(ωt), where ωt is the angle of the coil, resulting in a sinusoidal alternating voltage.

The split-ring commutator reverses the connection to the external circuit every half rotation, which converts the internal AC into a pulsating DC output.

The primary function of a transformer is to step up (increase) or step down (decrease) AC voltages based on the ratio of turns in its coils.

The induced EMF in the secondary coil is given by |ε_s| = M |ΔI_p/Δt|. So, |ε_s| = (0.5 H)(8 A/s) = 4 V.

To 'step up' the voltage, the secondary coil must have more turns to link with more of the changing magnetic flux.

This equation correctly shows that the ratio of the voltages is equal to the ratio of the number of turns in the respective coils.

The energy stored in an inductor is given by E = (1/2)LI². So, E = (1/2) * (8.0 H) * (5.0 A)² = 4 * 25 = 100 J.

For an ideal transformer, power in equals power out (Pp = Ps), so Vp * Ip = Vs * Is. If Vs > Vp, then Is must be less than Ip.

Eddy currents are a direct result of Faraday's law of induction, which requires a time-varying magnetic flux within the bulk of a conductor.

In an induction furnace, a strong alternating magnetic field induces large eddy currents in the metal, and the resistance to these currents generates immense heat, melting the metal.

The peak voltage is V_peak = V_rms * √2. So, V_peak = 230 V * 1.414 ≈ 325 V.

The core is made of thin iron sheets insulated from each other. This breaks up the paths for large eddy currents, significantly reducing their magnitude and the associated energy loss.

The law explicitly states that the magnitude of the induced EMF is directly proportional to how quickly the magnetic flux changes over time (dΦ/dt).

According to the formula ε = -N(dΦ/dt), the induced EMF is directly proportional to the number of turns, N.

To oppose the approaching north pole, the induced current creates a north pole on that face, causing repulsion.

Using the formula |ε| = L |ΔI/Δt|. First, convert inductance L = 50 mH = 0.05 H. The change in current ΔI = 4 A - 0 A = 4 A. So, |ε| = 0.05 H * (4 A / 0.01 s) = 0.05 * 400 = 20 V.

An inductor stores energy in the magnetic field created by the current flowing through it. The formula is E = (1/2)LI².

Mutual inductance depends on the number of turns, area, and length of the coils, as well as their proximity and relative orientation.

50 Hz is the standard mains frequency for AC power in Pakistan, Europe, and many other parts of the world.

Using the transformer equation: Vs = Vp x (Ns/Np) = 240 x (50/200) = 60 V.

Laminating the core increases its overall resistance to the flow of eddy currents, thereby minimizing the energy dissipated as heat.

Transformers operate on the principle of mutual induction, which requires a *changing* magnetic flux to induce an EMF in the secondary coil. A steady DC current creates a constant magnetic flux.

An ideal transformer is a theoretical concept with no energy losses, meaning its output power equals its input power.

As the coil rotates through the magnetic field, the direction of the induced EMF and current reverses every half turn, which is the definition of alternating current.

When the conductor moves, the free charges inside it also move through the magnetic field. They experience a magnetic force (F=qvB) that pushes them to one end of the conductor, creating the potential difference.

The formula for self-induced EMF is ε = -L(ΔI/Δt). The magnitude is |ε| = (2 H)(5 A/s) = 10 V.

Resistance is a property that affects current flow and energy loss (heat), but it does not determine the geometric property of mutual inductance.

When a conductor moves in a magnetic field, the induced eddy currents create a magnetic force that opposes the motion, causing a damping effect.

The formula for energy stored in an inductor is E = (1/2)LI² = (1/2)(1 H)(2 A)² = 2 J.

This is the fundamental principle: the magnetic force on the wires of the coil creates a torque that causes rotation. This is the 'motor effect'.

For an ideal transformer, (Is/Ip) = (Np/Ns). So, Is = Ip x (Np/Ns) = 2 A x (10/1) = 20 A.

Soft iron effectively concentrates the magnetic flux lines, leading to a much stronger magnetic field and a more efficient device.

At the moment it's switched on, the armature is not yet rotating, so its speed is zero. Since back EMF depends on the speed of rotation, the back EMF is initially zero.

This is a statement of Gauss's law for magnetism. Since there are no magnetic monopoles, any magnetic field line that enters a closed surface must also exit it, making the net flux zero.

A metal detector creates a changing magnetic field. When a metal object is near, eddy currents are induced in it. These currents create their own magnetic field that the detector picks up.

The frequency is the number of cycles per second. Doubling the rotational speed doubles the number of rotations completed each second, thus doubling the frequency.

Using the formula ε = BvLsin(θ). Given B=0.4 T, v=5.0 m/s, L=2.0 m, and θ=30°. So, ε = (0.4)(5.0)(2.0)sin(30°) = (4.0)(0.5) = 2.0 V.

Power loss in transmission lines is due to heat (P = I²R). By stepping up the voltage, the current (I) is reduced for the same amount of power, which significantly minimizes the I²R losses.

The negative sign mathematically signifies that the direction of the induced EMF is such that it creates a current whose magnetic field opposes the change in magnetic flux that produced it.

Every real inductor is made of wire, which has some inherent electrical resistance (R) that opposes the flow of any current, DC or AC.

Electromagnetic induction requires a *change* in magnetic flux. If the field is constant and the loop is stationary, there is no change in flux.

First, find the secondary RMS voltage: Vs = 200 V * (2500/500) = 1000 V. Then, find the peak voltage: V_peak = V_rms x sqrt(2) ≈ 1000 V x 1.414 = 1414 V.

From the formula ε_s = -M(ΔI_p/Δt), if (ΔI_p/Δt) = 1, then the magnitude |ε_s| = M.

According to Lenz's law, induced eddy currents create a magnetic force that opposes the motion (a braking force), so the net downward force is less than the gravitational force, and a < g.

Energy is required to re-align the magnetic domains in the core material with each cycle of the AC current. This energy is dissipated as heat and is known as hysteresis loss.

Soft iron is a 'soft' magnetic material with a narrow hysteresis loop. It is easily magnetized and demagnetized, which minimizes the energy lost per cycle.

Using Faraday's Law, |ε| = N |ΔΦ/Δt|. The change in flux ΔΦ = 8.0 Wb - 2.0 Wb = 6.0 Wb. So, |ε| = 100 * (6.0 Wb / 3.0 s) = 100 * 2 = 200 V.

Back EMF is a motional EMF and is directly proportional to the rotational speed of the armature. Therefore, it is at its maximum when the motor's speed is greatest.

Rate of change of current: (ΔI/Δt) = (4 A - 2 A) / 0.1 s = 20 A/s. Then, |ε| = L(ΔI/Δt) = (5 H)(20 A/s) = 100 V.

Total energy is conserved and oscillates between being fully stored in the inductor's magnetic field and fully stored in the capacitor's electric field.

In an inductor, the back EMF causes the current to reach its peak value a quarter of a cycle (90 degrees) after the voltage reaches its peak. We say the current lags the voltage.

The relationship between peak and RMS voltage is V_rms = V_peak / √2. So, V_rms = 340 V / 1.414 ≈ 240 V.

From Faraday's law, ε = (ΔΦ/Δt), we can rearrange to get ΔΦ = ε * Δt. The units are therefore Volts multiplied by seconds.

Magnetic flux is given by Φ = BAcos(θ), where θ is the angle between the field and the normal. So, Φ = (2.0 T)(0.5 m²)cos(60°) = (1.0)(0.5) = 0.5 Wb.

Electromagnetic brakes work by inducing large eddy currents in a rotating metal disc. These currents create a magnetic field that opposes the rotation, producing a braking force.

Since the magnetic flux is Φ = B * A, even if the area A is constant, a change in the magnetic field strength B over time will cause a change in flux and induce an EMF.

A transformer requires a *changing* magnetic flux. A steady DC source creates a constant magnetic flux, so no induction occurs.

Tesla is the unit of magnetic field strength. Tesla / Ampere is not a unit of inductance.

The driving torque on the armature is proportional to the magnetic field strength (B). A stronger field will produce a greater torque for the same current, leading to a higher rotational speed.

High inductance provides a large inductive reactance (X_L = 2πfL) to limit the current. Low resistance ensures minimal energy is wasted as heat.

In an ideal transformer with 100% efficiency, the power input to the primary coil (Pp = Vp * Ip) is equal to the power output from the secondary coil (Ps = Vs * Is).

This demonstrates that magnetic monopoles (isolated north or south poles) do not exist. Any piece of a magnet must have both a north and a south pole.

The RMS value for a sinusoidal waveform is the peak value divided by the square root of 2. V_rms = V_peak / sqrt(2) = 120 V / 1.414 ≈ 84.9 V.

The magnetic energy density is given by the formula u_B = B²/(2μ₀). It is proportional to the square of the magnetic field strength.

Using the transformer equation, Vs/Vp = Ns/Np. Vs = Vp * (Ns/Np) = 110 V * (1200 / 400) = 110 V * 3 = 330 V.

Slip rings provide a continuous electrical connection between the rotating armature and the stationary external circuit.

At the magnetic north pole, the Earth's magnetic field lines are directed vertically. As the plane flies horizontally, its wings do not 'cut' any vertical field lines, so the induced EMF is zero.

As the metallic bob swings through the magnetic field, eddy currents are induced in it. These currents create a magnetic force that opposes the motion, causing the oscillations to be 'damped'.

The time constant represents the time it takes for the current to reach approximately 63% of its final value. It is defined as the ratio of the inductance to the resistance.

An induction coil uses a rapid interruption of current in the primary to cause a very fast collapse of magnetic flux, which induces a very high voltage pulse in the secondary coil.

The time constant (τ) of an RL circuit is τ = L/R. Rearranging for R gives R = L/τ. First, L = 200 mH = 0.2 H. So, R = 0.2 H / 0.02 s = 10 Ω.

Each wire creates a magnetic field that is experienced by the other. Using the right-hand rules, the resulting forces pull the wires together.

This is Oersted's discovery. The current in the coil creates a magnetic field that interacts with the magnetic needle of the compass.

An induced EMF is produced by a *change* in magnetic flux. If a time-varying magnetic field is introduced, the flux will change from zero, and an EMF will be induced.

Using the transformer current relationship: Is = Ip x (Np/Ns) = 3 A x (200/50) = 12 A.

This is a direct consequence of Lenz's Law. The induced current creates a magnetic force that acts in the opposite direction to the velocity.

A GFCI uses a transformer to detect differences between the outgoing and incoming current. A difference creates a changing flux, inducing an EMF that trips the circuit.

The formula for instantaneous EMF is ε = ε_max * sin(θ). So, ε = 170 V * sin(45°) = 170 V * (1/√2) ≈ 170 * 0.707 ≈ 120 V.

This is an example of motional EMF. The moving charges in the liquid experience a Lorentz force that separates them, creating a voltage.

Using the formula |ε| = L |ΔI/Δt|. First, convert inductance L = 50 mH = 0.05 H. The change in current ΔI = 4 A - 0 A = 4 A. So, |ε| = 0.05 H * (4 A / 0.01 s) = 0.05 * 400 = 20 V.

The formula for instantaneous EMF is ε = ε₀ sin(θ). ε = ε₀ sin(30 degrees) = ε₀ / 2.

Mutual inductance (M) is a physical property of the transformer, determined by its construction. It does not depend on the frequency of the AC supply.

As the brushes on the commutator make and break contact, sparking can occur. A small capacitor placed across the motor terminals can suppress this high-frequency noise.

By the Right-Hand Rule, the wire creates a magnetic field directed **into the page** through the loop. Since the wire's current is increasing, this 'into the page' flux increases. To oppose this change, Lenz's Law dictates that the induced current must create a magnetic field **out of the page**. A counter-clockwise current is required to produce this opposing field.

The total charge that flows (Q = ∫I dt) is measured and is proportional to the initial magnetic flux (Φ), and thus the magnetic field B.

An induced electric field forms closed loops. The work done in moving a charge around a closed loop in this field is not zero. Therefore, it is a non-conservative field.

Magnetic moment is pole strength times length. If you break a magnet of length L in half, each new magnet has length L/2, and the new magnetic moment will be half of the original.

The power factor is cos(φ). For a pure inductor or capacitor, the phase angle φ = 90 degrees. Since cos(90 degrees) = 0, the average power consumed is zero.

This is the very definition of self-inductance. It creates a back EMF that opposes any increase or decrease in the current, acting as a form of electrical inertia.

By definition, a step-up transformer has more turns in the secondary coil than the primary, resulting in a higher output voltage.

Using the formula ε = B * L * v * sin(θ): ε = (0.8 T)(0.5 m)(10 m/s) * sin(30 degrees) = (4)(0.5) = 2 V.

High permeability strengthens the magnetic field. Low retentivity means it loses its magnetism quickly when the current is turned off, which is desired for an electromagnet.

Faraday's discovery of electromagnetic induction is the fundamental principle behind every commercial electric generator.

The constant change in current of AC creates a continuous back EMF (inductive reactance) that opposes the flow. A steady DC current has no change, so it only faces the inductor's small ohmic resistance.

The formula is L = (μ₀N²A)/l. The new inductance L' = (μ₀(2N)²A)/(2l) = 2 * (μ₀N²A)/l = 2L.

When pushed in, the flux increases, inducing a current in one direction. When pulled out, the flux decreases, inducing a current in the opposite direction, according to Lenz's law.