The voltmeter has range 10 V and its internal resistance is $50 \Omega$. To increase the range of voltmeter to 15 V , the resistance which is to be connected is

If 120 J of thermal energy is incident on area $3 \mathrm{~m}^2$, the amount of heat transmitted is 12 J , coefficient of absorption is 0.6 , then the amount of heat reflected is

A circular coil of wire consisting of ' $n$ ' tums each of radius 8 cm carries a current of 0.4 A . The magnitude of the magnetic field at the centre of coil is $3.14 \times 10^{-4} \mathrm{~T}$. The value of ' $n$ ' is

[Take $\mu_0=12.56 \times 10^{-7}$ SI unit]