Certain quantity of water cools from $70^{\circ}C$ to $60^{\circ}C$ in the first $5$ min and to $54^{\circ}C$ in the next $5$ min. The temperature of the surroundings is

On observing light from three different stars P, Q and R, it was found that intensity of violet colour is maximum in the spectrum of P, the intensity of green colour is maximum in the spectrum of R and the intensity of red colour is maximum in the spectrum of Q. If $T_P, T_Q,$ and $T_R$ are the respective absolute temperatures of P, Q and R, then it can be concluded from the above observation that

The two ends of a metal rod are maintained at temperatures $100^{\circ}C$ and $110^{\circ}C.$ The rate of heat flow in the rod is found to be $4.0$ Js$^{-1}.$ If the ends are maintained at temperatures $200^{\circ}C$ and $210^{\circ}C,$ the rate of heat flow will be

A black body is at a temperature of 5760 K. The energy of radiation emitted by the body at wavelength 250 nm is U₁, at wavelength 500 nm is U₂ and that at 1000 nm is U₃. Wien’s constant, b = 2.88 × 10⁶ nmK. Which of the following is correct?

A refrigerator works between $4^{\circ}C$ and $30^{\circ}C.$ It is required to remove $600$ calories of heat every second in order to keep the temperature of the refrigerated space constant. (Take 1 cal = 4.2 J). The power required is

Coefficient of linear expansion of brass and steel rods are α1 and α2. Lengths of brass and steel rods are l1 and l2, respectively. If (l2 – l1) is maintained same at all temperatures, which one of the following relations holds good?

A piece of ice falls from a height h so that it melts completely. Only one-quarter of the heat produced is absorbed by the ice and all energy of ice gets converted into heat during its fall. The value of h is (latent heat of ice is $3.4 \times 10^5 Jkg^{-2}$ and $g = 10 Nkg^{-1}$).

Two rods A and B of different materials are welded together as shown in the figure. Their thermal conductivities are $K_1$ and $K_2.$ The thermal conductivity of the composite rod will be

A spherical blackbody with a radius of 12 cm radiates 450 watt power at 500 K. If the radius were halved and the temperature is doubled, the power radiated (in watt) would be

The power radiated by a black body is $P$ and it radiates maximum energy at wavelength, $\lambda_0.$ If the temperature of the black body is now changed so that it radiates maximum energy at wavelength $\frac{3}{4} \lambda_0,$ the power radiated by it becomes $nP.$ The value of $n$ is

A copper rod of $88$ cm and an aluminium rod of unknown length have their increase in length independent of increase in temperature. The length of the aluminium rod is:

The unit of thermal conductivity is:

The quantities of heat required to raise the temperature of two solid copper spheres of radii $r_1$ and $r_2$ ($r_1 = 1.5 r_2$) through $1$ K are in the ratio:

Light with an average flux of $20$ W/cm$^2$ falls on a non-reflecting surface at normal incidence having surface area $20$ cm$^2.$ The energy received by the surface during a time span of 1 minute is:

A cup of coffee cools from $90^{\circ}C$ to $80^{\circ}C$ in $t$ minutes, when the room temperature is $20^{\circ}C.$ The time taken by a similar cup of coffee to cool from $80^{\circ}C$ to $60^{\circ}C$ at a room temperature same at $20^{\circ}C$ is:

The energy that will be ideally radiated by a $100$ kW transmitter in $1$ hour is:

Two resistors of resistances, $100 \Omega$ and $200 \Omega$ are connected in parallel in an electrical circuit. The ratio of thermal energy developed in $100 \Omega$ to that in $200 \Omega$ in a given time is:

Two heaters A and B have power rating of 1 kW and 2 kW, respectively. Those two are first connected in series and then in parallel to a fixed power source. The ratio of power outputs for these two cases is: