Physics Questions and Answers

Answer: D

The ratio of the emfs is 3 : 2.

With the cells assisting, the balancing length measures \(E_1 + E_2\); with them opposing, it measures \(E_1 - E_2\).

\[\frac{E_1+E_2}{E_1-E_2} = \frac{50}{10} = 5\]

\[5E_1 - 5E_2 = E_1 + E_2 \Rightarrow 4E_1 = 6E_2 \Rightarrow \frac{E_1}{E_2} = \frac{3}{2}\]

Answer: C

Because the refractive index of the different layers of the atmosphere keeps changing.

A star is effectively a point source, so its light passes through a narrow column of air. Turbulent cells of differing density bend the ray slightly and randomly, and the apparent position and brightness flicker. This is called atmospheric scintillation.

Answer: B

Its effective resistance in a DC circuit is infinite.

Capacitive reactance is \[X_C = \frac{1}{2\pi f C}\] and for direct current \(f = 0\), so \(X_C \rightarrow \infty\). Once the capacitor is charged no further current flows - a capacitor blocks DC and passes AC.

Answer: A

The 25 W bulb will fuse.

Resistance from the rating: \[R = \frac{V^2}{P} \Rightarrow R_{25} = \frac{220^2}{25} = 1936\ \Omega,\quad R_{100} = \frac{220^2}{100} = 484\ \Omega\]

In series they carry the same current, and the voltage divides in proportion to resistance, so the 25 W bulb takes four-fifths of 440 V - about 352 V, far above its rating. In series, the lower-wattage bulb always gets the larger share.

Answer: C

Correct answer: The lift and the person fall with the same acceleration, so the floor exerts no reaction

Weight is felt through the normal reaction of the floor, and when both fall together at \(g\) that reaction drops to zero. Gravity continues to act throughout, which is precisely why they are falling.

Answer: A

Heat is associated with the kinetic energy of the random motion of molecules.

Temperature measures the average of that random kinetic energy. Orderly motion of the whole body is not heat but bulk kinetic energy: a moving cricket ball is not hotter for moving.

Answer: C

Because of the higher frequency of a woman's voice.

Pitch is determined by frequency. Women's vocal cords are shorter and thinner, so they vibrate faster - typically 165–255 Hz against 85–180 Hz for men. Amplitude governs loudness, not shrillness.

Answer: A

Persistence of vision is the principle behind cinema.

An image lingers on the retina for about a sixteenth of a second after the light is cut off, so still frames shown faster than that merge into apparently continuous motion. Film runs at 24 frames a second for this reason.

Answer: B

The boat does not move.

The fan pushes air backwards and the air pushes the fan - and hence the boat - forwards; but the moving air then strikes the sail and pushes it backwards with an equal force. The two internal forces cancel, and with no external force the boat stays put. Removing the sail would let it move.

Answer: A

This is adhesive force.

Adhesion acts between molecules of different substances - water and glass, ink and paper. Cohesion acts between molecules of the same substance. Whether a liquid wets a surface or beads up depends on which of the two is stronger.