Physics entrance exam, 2016 E.C. (2024)

Lenz's law says that an induced current always flows in the direction that fights the change which produced it. The induced current sets up its own magnetic field, and that field opposes the change in magnetic flux through the loop. This is option D.

Option A is the common trap. The rule is not about the direction of the applied field. It is about the change in flux. If the flux is decreasing, the induced field actually points the same way as the applied field, to slow the decrease. So the induced field can be with or against the applied field, but it is always against the change.

Lenz's law follows from conservation of energy: if the induced current helped the change instead, the current would grow on its own for free.

Textbook: Physics Grade 12, Unit 4 Electromagnetism, section 4.5 Faraday's Law of electromagnetic Induction (page 129).

The resistance of a wire is given by \(R = \dfrac{\rho L}{A}\), where \(\rho\) is the resistivity of the material, \(L\) is the length, and \(A\) is the cross-sectional area.

From this formula, resistance is directly proportional to the length \(L\) (not its square), and inversely proportional to the cross-sectional area \(A\). A longer wire gives electrons a longer path, so resistance rises. A thicker wire gives them more room to flow, so resistance falls.

Only option C matches the formula. Options A and B insert squares that do not belong, and option D reverses the length relation.

Textbook: Physics Grade 11, Unit 6 Electrostatics and Electric Circuit, section 6.4 Electric Current, Resistance and Ohm's law (page 240).

In an ideal transformer, the same changing magnetic flux passes through both coils. Faraday's law then gives each coil an emf in proportion to its number of turns: the emf per turn is the same on both sides.

So \[\frac{V_s}{V_p} = \frac{N_s}{N_p}\] This is option D. More turns on the secondary means a larger secondary voltage (step-up); fewer turns means a smaller voltage (step-down).

Option C writes the turns ratio upside down, and options A and B square the ratios, which has no basis in the physics.

Textbook: Physics Grade 12, Unit 4 Electromagnetism, section 4.6 Transformers (page 131).

Electrical conductivity depends on how free the electrons are to move.

Option D describes an insulator correctly. Option A actually describes an insulator but calls it a conductor. Option B describes a conductor but calls it a semiconductor. Option C is wrong because even in conductors the inner electrons are not free.

Textbook: Physics Grade 12, Unit 5 Basics of electronics, section 5.1 Semiconductors (page 143).

These two words describe different qualities of a measurement.

A measurement can be precise but not accurate: if a faulty balance always reads 2 g too high, the readings cluster tightly (precise) but they are all wrong (not accurate).

Option D states both definitions correctly. Options A and B swap the two meanings, and option C wrongly ties accuracy to significant figures.

Textbook: Physics Grade 9, Unit 2 Physical Quantities, section 2.2 Measurement and Safety (page 17).

An electromagnetic wave is made of an electric field and a magnetic field that oscillate perpendicular to each other, and both are perpendicular to the direction the wave travels. That is why the wave is transverse. This is option A.

A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field, so the two fields keep regenerating each other and the wave carries itself through empty space with no medium needed.

Options B and D fail because charges at rest produce only steady fields and no wave; a wave needs accelerating (oscillating) charges. Option C fails because parallel oscillating fields do not form an electromagnetic wave.

Textbook: Physics Grade 10, Unit 6 Electromagnetic wave and geometrical optics, section 6.1 Electromagnetic Waves.