Heat Transfer Physics - Heat Transfer

A copper sphere is suspended in an evacuated chamber maintained at 300K. The sphere is maintained at a constant temperature of 500K by heating it electrically. A total of 210W of electric power is needed to do it. When the surface of the copper sphere is completely blackened, 700W is needed to maintain the same temperature of the sphere. Calculate the emissivity of copper.

On a hot summer day we want to cool our room by opening the refrigerator door and closing all the windows and doors. Will the process work ?

Why is a white dress more comfortable than a dark dress in summer?

Does a body at 20^°C radiate in a room, where the room temperature is 30^°C? If yes, why does its temperature not fall further?

An aluminium rod and a copper rod of equal length 1.0m and cross-sectional area 1cm² are welded together as shown in figure. One end is kept at a temperature of 20^°C and the other at 60^°C. Calculate the amount of heat taken out per second from the hot end. Thermal conductivity of aluminium = 200Wm^-1°C^-1 and of copper = 390Wm^-1°C^-1.

Standing in the sun is more pleasant on a cold winter day than standing in shade. Is the temperature of air in the sun considerably higher than that of the air in shade?

The temperature of the atmosphere at a high altitude is around 500^°C. Yet an animal there would freeze to death and not boil. Explain.

The three rods shown in figure, have identical geometrical dimensions. Heat flows from the hot end at a rate of 40 Win the arrangement (a) Find the rates of heat flow when the rods are joined as in arrangement (b) and in (c) Thermal conductivities of aluminium and copper are 200Wm^-1°C^-1 and 400Wm^-1°C^-1 respectively.

1.  

2.  

3.  

Cloudy nights are warmer than the nights with clean sky. Explain.

Consider the situation shown in figure. The frame is made of the same material and has a uniform cross-sectional area everywhere. Calculate the amount of heat flowing per second through a cross section of the bent part if the total heat taken out per second from the end at 100^°C is 130J.

Suppose the bent part of the frame of the previous problem has a thermal conductivity of 780Js^-1m^-1°C^-1 whereas it is 390Js^-1m^-1°C^-1 for the straight part. Calculate the ratio of the rate of heat flow through the bent part to the rate of heat flow through the straight part.

One end of a metal rod is dipped in boiling water and the other is dipped in melting ice:

1. All parts of the rod are in thermal equilibrium with each other.
2. We can assign a temperature to the rod.
3. We can assign a temperature to the rod after steady state is reached.
4. The state of the rod does not change after steady state is reached.

In a room containing air, heat can go from one place to another:

1. By conduction only.
2. By convection only.
3. By radiation only.
4. By all the three modes.

A hot liquid is kept in a big room. Its temperature is plotted as a function of time. Which of the following curves may represent the plot?

The thermal conductivity of a rod depends on:

1. Length.
2. Mass.
3. Area of cross section.
4. Material of the rod.

A solid aluminium sphere and a solid copper sphere of twice the radius are heated to the same temperature and are allowed to cool under identical surrounding temperatures. Assume that the emissivity of both the spheres is the same. Find the ratio of:

1. The rate of heat loss from the aluminium sphere to the rate of heat loss from the copper sphere.
2. The rate of fall of temperature of the aluminium sphere to the rate of fall of temperature of the copper sphere. The specific heat capacity of aluminium = 900Jkg^-1°C^-1 and that of copper= 390Jkg^-1°C^-1. The density of copper = 3.4 times the density of aluminium.

Why does blowing over a spoonful of hot tea cools it? Does evaporation play a role? Does radiation play a role?