48 questions. Try each one, then open the answer to check yourself and read the explanation.
A transformer is rated 240 V. If the primary coil is 4000 turns and the secondary voltage 12 V, determine the number of turns in the secondary coil.
Answer: D (200)
Ns/Np = Vs/Vp, so Ns = 4000 × 12/240 = 200.
Which of the following materials is a conductor?
Answer: B (Sodium)
Sodium is a metal with free electrons, so it conducts; plastic, wax and glass are insulators.
The charge carriers in gases are
Answer: D (electrons and ions.)
When a gas conducts, its atoms are ionised into positive ions and free electrons, and both carry the current.
When impurities are added to semi-conductors, the conductivity of the semi-conductor
Answer: C (increases.)
Doping adds extra free electrons or holes, so the semiconductor conducts better.
The ray which causes gas molecules to glow is known as
Answer: D (cathode ray.)
Cathode rays (fast electrons) strike gas molecules in a discharge tube and make them glow.
A proton moving with a speed of 1.0 × 106ms−1 through a magnetic field of 1.0 T experiences a magnetic force of magnitude 8.0 × 10−14 N. The angle between the proton's velocity and the field is [Proton charge = 1.6 × 10−19C]
Answer: B (30°.)
F = qvB sin θ: sin θ = 8.0 × 10−14 ÷ (1.6 × 10−19 × 106 × 1.0) = 0.5, so θ = 30°.
23592U + 10n → 14456Ba + 9036Kr + 2X
In the reaction above, X is
Answer: A (neutron.)
Mass numbers: 236 = 144 + 90 + 2A gives A = 1. Atomic numbers: 92 = 56 + 36 + 2Z gives Z = 0. So X is a neutron.
A silicon material is dopped with an element of a certain group and an n-type semi-conductor is formed. The most likely group of the element is
Answer: C (V.)
n-type silicon is made by adding a group V (pentavalent) element, whose fifth electron becomes a free carrier.
The time it will take a certain radioactive material with a half-life of 50 days to reduce to 1/32 of its original number is
Answer: D (250 days.)
1/32 = (½)5, so 5 half-lives are needed: 5 × 50 = 250 days.
In an a.c. circuit that contains only a capacitor, the voltage lags behind the current by
Answer: A (90°)
In a purely capacitive circuit the current leads the voltage by a quarter cycle, so the voltage lags by 90°.
In the circuit diagram of a transistor above, the terminals P, Q and R represent.
Answer: B (EMITTER, BASE and COLLECTOR.)
The lead with the arrow (P) is the emitter, the lead entering from the side (Q) is the base, and the other lead (R) is the collector.
The amount of energy released when 0.5 kg of uranium is burnt completely is [c = 3 × 108 ms−1]
Answer: A (4.5 × 1016J.)
If all the mass becomes energy, E = mc2 = 0.5 × (3 × 108)2 = 4.5 × 1016 J.
The current in a reverse-biased junction is due to
Answer: D (minority carriers.)
Reverse bias stops the majority carriers; the small leakage current is carried by minority carriers.
In photoelectric effect, which of the following graphs correctly represents the graph of stopping potential V0 against frequency f of incident light?
Answer: B (Graph B)
eV0 = hf − W, so V0 against f is a straight line of positive slope that cuts the f-axis at the threshold frequency and, extended, has a negative intercept on the V0 axis (graph B).
If two inductors of inductances 3H and 6H are arranged in series, the total inductance is
Answer: B (9.0 H.)
Inductors in series add (like resistors): 3 + 6 = 9 H.
Satellite communication network makes use of
Answer: D (radio wave.)
Satellites relay signals as radio (microwave) waves, which pass through the atmosphere and travel through space.
The electric field intensity in a place where a charge of 10−10 C experiences a force of 0.4N is
Answer: C (4.0 × 109 NC−1.)
E = F/q = 0.4 ÷ 10−10 = 4.0 × 109 N C−1.
The resistance of a piece of wire of length 20m and cross-sectional area 8 × 10−6m2 is [Resistivity of the wire = 4 × 10−7Ωm]
Answer: A (1.0Ω.)
R = ρL/A = 4 × 10−7 × 20 ÷ 8 × 10−6 = 1.0 Ω.
The figure above shows three capacitors, 2μF, 3μF and 6μF connected in series. If the p.d. across the system is 12V, the p.d. across the 6μF capacitor is
Answer: D (2V.)
1/C = 1/2 + 1/3 + 1/6 = 1, so C = 1 μF and Q = 12 μC. V across 6 μF = 12 μC ÷ 6 μF = 2 V.
The north pole of a magnet can never be separated from the south pole because of a property known as
Answer: A (magnetic dipole.)
Every magnet is a dipole: cutting it always gives smaller magnets, each with both a north and a south pole.
An electric device is rated 2000 W, 250 V. The correct fuse rating of the device is
Answer: A (8A.)
I = P/V = 2000 ÷ 250 = 8 A.
A car battery rated 45 AH is charged with a charger whose rating is 2.5 A. How long will it take to charge the battery fully?
Answer: D (18 hrs)
Time = capacity ÷ current = 45 A h ÷ 2.5 A = 18 h.
If the distance between two point charges is increased by a factor of four, the magnitude of the electrostatic force between them will be
Answer: B (1/16 of its former value.)
Coulomb's law: F ∝ 1/r2, so making r four times bigger makes F (1/4)2 = 1/16 of its former value.
Two charged particles are projected into a region where there is a magnetic field perpendicular to their velocities. If the charges are deflected in opposite directions, which of the following statements is true of the charges?
Answer: C (The charges must be of opposite sign)
The magnetic force direction depends on the sign of the charge; opposite deflections in the same field mean opposite charges.
The value of F in the figure above when in equilibrium is
Answer: D (20 N.)
10 N and 17 N are at right angles, so their resultant is √(102 + 172) ≈ 20 N. F must be equal and opposite to this resultant: about 20 N.
In the figure above, the coefficient of static friction is 0.5. If the tension in the string is T, the acceleration of the system in motion is [g = 10 ms−2]
Answer: C (5 ms−2.)
Friction on the 3 kg block = 0.5 × 30 = 15 N. Net force = 60 − 15 = 45 N on a total mass of 9 kg, so a = 5 m s−2.
In an ideal wheel and axle system, R stands for the radius of the wheel and r is the radius of the axle. The mechanical advantage is
Answer: A (R/r.)
For an ideal machine MA = VR, and the velocity ratio of a wheel and axle is R/r.
A motorcyclist travelling at 30 ms−1 starts to apply his brakes when he is 50 m from the traffic light that has just turned red. If he just reached the traffic light, his deceleration is
Answer: B (9 ms−2.)
v2 = u2 − 2as: 0 = 900 − 100a, so a = 9 m s−2.
If the system above is in equilibrium, the tension in the string Q is
Answer: D (9 N.)
String Q holds up everything below it: the 4 N block and the 5 N block, so the tension is 9 N.
In the figure above, the work done by the force of 100 N inclined at an angle of 60° to the object dragged horizontally to a distance of 8 m is [Neglect frictional effect]
Answer: C (400 J.)
Only the horizontal part does work: W = F cos 60° × s = 100 × 0.5 × 8 = 400 J.
A force of 200 N acts between two objects at a certain distance apart. The value of the force when the distance is halved is
Answer: C (800 N.)
The force follows the inverse-square law: halving the distance makes it 22 = 4 times bigger, 800 N.
The pair of physical quantities consisting of vectors only are
Answer: B (displacement and torque.)
Displacement and torque both have direction; speed, distance and power are scalars.
A real image three times the size of an object is formed 24 cm from a converging mirror. What is the focal length of the mirror?
Answer: C (6 cm)
m = v/u = 3 with v = 24 cm gives u = 8 cm. 1/f = 1/8 + 1/24 = 4/24, so f = 6 cm.
Marching soldiers crossing a suspension bridge are usually advised to break their steps to avoid damaging the bridge owing to
Answer: D (resonance.)
If the marching rhythm matched the bridge's natural frequency, resonance could build up large, damaging oscillations.
A thin converging lens has a power of 4.0 diopters. Determine its focal length.
Answer: A (0.25 m.)
f = 1/P = 1/4 = 0.25 m.
The velocity of sound in air will be doubled if its absolute temperature is
Answer: D (quadrupled.)
v ∝ √T, so to double v the absolute temperature must be multiplied by 4.
The figure above shows a plot of angles of deviation through a glass prism when light is incident at θ degrees on the prism. The incident angle that produces the minimum deviation is
Answer: A (25°.)
Minimum deviation is the lowest point of the curve, which is at an incident angle of about 25°.
Two liquids L1 and L2 are contained in a U-tube. The height and the density of L1 are 8 cm and 103 kgm−3 respectively. If the density of L2 is 800 kgm−3, its height measured from the same level is
Answer: D (10 cm.)
Pressures balance: h1ρ1 = h2ρ2, so h2 = 8 × 1000/800 = 10 cm.
When the temperature difference between the wet and dry bulbs of a hygrometer is high, this indicates that
Answer: C (the relative humidity is low.)
Water evaporates fast from the wet bulb in dry air, cooling it a lot; a big difference means low relative humidity.
An elastic material has a length of 36cm when a load of 40 N is hung on it and a length of 45 cm when a load of 60 N is hung on it. The original length of the string is
Answer: A (18cm.)
An extra 20 N gives 9 cm more, so 40 N gives 18 cm extension. Original length = 36 − 18 = 18 cm.
I. Reproducibility II. Sensitivity III. High thermal capacity IV. High accuracy
The qualities of a good thermometer include
Answer: B (I, II and IV only.)
A good thermometer gives repeatable, sensitive and accurate readings; a high thermal capacity is a drawback because it takes heat from what it measures and responds slowly.
The process whereby the molecules of different substances move randomly is called
Answer: B (diffusion.)
Diffusion is the random movement of molecules that makes different substances spread into each other.
106J of heat is required to boil off completely 2 kg of a certain liquid. Neglecting heat lost to the surroundings, the latent heat of vaporization of the liquid is
Answer: A (5.0 × 105J kg−1.)
L = Q/m = 106 ÷ 2 = 5.0 × 105 J kg−1.
A 3 m3 volume of liquid W of density 200 kgm−3 is mixed with another liquid L of volume 7 m3 and density 150 kgm−3. The density of the mixture is
Answer: B (165 kgm−3.)
Total mass = 3 × 200 + 7 × 150 = 1650 kg; total volume = 10 m3; density = 165 kg m−3.
I. Melting II. Boiling III. Refraction IV. Conduction
Which combination of the above is evident of the molecular nature of matter?
Answer: A (I, II and IV only)
Melting, boiling and conduction are explained by the behaviour of molecules; refraction is a property of light, not evidence of molecules.
When very hot water is poured into two identical thin and thick glass tumblers in equal volumes, the thick one cracks because
Answer: C (of the uneven expansion of glass.)
Glass conducts heat poorly, so the inside of the thick glass expands before the outside warms; this uneven expansion cracks it.
A body of mass 36 kg falls through a viscous liquid which offers a drag force of 260 N on the body. The upthrust on the body at terminal velocity is [g = 10ms−2]
Answer: D (100 N.)
At terminal velocity weight = drag + upthrust: 360 = 260 + U, so U = 100 N.
The process whereby a liquid turns spontaneously into vapour is called
Answer: A (evaporation.)
Evaporation happens on its own at the surface at any temperature; boiling needs the liquid to reach its boiling point.