Pick out the correct statement from the following;
The number density of free electrons in the valance band decides the strength of the electric current
Valance band is always completely filled, while conduction band is always partially filled
The maximum energy required to shift an electron from the conduction band to valance band is called energy band gap
In a semiconductor no free electrons are found in the conduction band at 0 K
Two neutral bodies of masses $m_1$ and $m_2$ are kept at a distance of $r \mathrm{~cm}$ from one another in a vacuum medium. A gravitational force Fracts between the two bodies. The entire set-up is then transferred, as it is, to a water medium. The force between the bodies is then $\mathrm{F}_{\mathrm{w}}$. The ratio of $F_v$ to $F_w$ is:
$4:3$
$1: 1$
$ 1:1.33$
$3: 2$
An electric coil is rated $400 \mathrm{~W}, 200 \mathrm{~V}$. It is cut into two equal parts and connected in parallel to the same source of 200 V . Calculate the percentage increase in energy produced per second.
$400 \%$
$ 100 \%$
$200 \%$
$300 \%$
Resonance is produced between a turning fork and a resonance column tube with upper end open and lower end closed by water surface. If the frequency of the tuning fork is 800 Hz , and first two resonances are observed at lengths $9.75 \mathrm{~cm}, 31.25 \mathrm{~cm}$,
Find the length at which the third resonance occurs? Also find the speed of sound.
52.75 cm and $344 \mathrm{~ms}^{-1}$
31.25 and $330 \mathrm{~ms}^{-1}$
62.60 cm and $335 \mathrm{~ms}^{-1}$
43.10 cm and $340 \mathrm{~ms}^{-1}$
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