Natural radioactivity was discovered by
Answer: C
Henri Becquerel discovered natural radioactivity, in 1896.
He found that uranium salts fogged a wrapped photographic plate without any exposure to sunlight, so the radiation came from the element itself. He shared the 1903 Nobel Prize with Pierre and Marie Curie, who coined the word radioactivity.
Please log in or register to report this question.
If no external force acts on a system of bodies, the total linear momentum of the system of bodies remains constant. Which law states this ?
Answer: D
This is the principle of conservation of linear momentum.
It follows from Newton's second and third laws: internal forces come in equal and opposite pairs and cancel, so with no external force \[\frac{d\vec{p}}{dt} = 0 \Rightarrow \vec{p} = \text{constant}\] It explains recoil, rocket propulsion and collisions alike.
Please log in or register to report this question.
The hydraulic brake works on the principle of
Answer: A
The hydraulic brake works on Pascal's law.
Pressure applied to an enclosed fluid is transmitted undiminished throughout it, so a small force on the master cylinder produces a large force at the wide brake cylinders: \[\frac{F_1}{A_1} = \frac{F_2}{A_2}\] It also ensures all four wheels are braked equally.
Please log in or register to report this question.
Ohmic conductor among the following is _____.
Answer: C
Constantan is the ohmic conductor.
An ohmic conductor obeys \(V = IR\) with a constant resistance, so its V-I graph is a straight line. Constantan, a copper-nickel alloy, is prized because its resistance hardly changes with temperature. A transistor, an electrolyte and a thermistor are all non-ohmic.
Please log in or register to report this question.
Which law is also called the law of inertia ?
Answer: A
Newton's first law is the law of inertia.
It states that a body remains at rest or in uniform motion in a straight line unless an external force acts on it, so it defines inertia and identifies the inertial frames in which the other two laws hold.
Please log in or register to report this question.
An aeroplane is flying horizontally with a velocity of 600 km/h and at a height of 1960 m. When it is vertically at a point A on the ground a bomb is released from it. The bomb strikes the ground at point B. The distance AB is
Answer: C
The bomb strikes the ground 3.33 km from A.
Vertically it starts with no downward velocity: \[t = \sqrt{\frac{2h}{g}} = \sqrt{\frac{2\times1960}{9.8}} = 20\ \mathrm{s}\]
Horizontally it keeps the aircraft's speed of \(166.7\ \mathrm{m\,s^{-1}}\): \[x = 166.7\times20 \approx 3.33\ \mathrm{km}\]
Please log in or register to report this question.
A potentiometer wire is 100 cm long and a constant potential difference is maintained across it. Two cells are connected in series first to support one another and then in opposite direction. The balance points are obtained at 50 cm and 10 cm from the positive end of the wire in the two cases. The ratio of the emf's is :
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 \(E_1 - E_2\).
\[\frac{E_1+E_2}{E_1-E_2} = \frac{50}{10} = 5\]
\[5E_1 - 5E_2 = E_1 + E_2 \Rightarrow 4E_1 = 6E_2 \Rightarrow \frac{E_1}{E_2} = \frac{3}{2}\]
Please log in or register to report this question.
Stars twinkle because
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.
Please log in or register to report this question.
The capacity of a pure capacitor is 1 farad. In D.C circuit, its effective resistance will be
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.
Please log in or register to report this question.
Two electric bulbs marked 25W ? 220V and 100W ? 220V are connected in series to a 440Vsupply. Which of the bulbs will fuse?
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.
Please log in or register to report this question.
