Physics · Thermal Properties Of Matter · NEET
In conduction, heat passes between neighbouring parts of a body by molecular collisions and the material itself does NOT flow (like a hot metal rod). In convection, the hot fluid physically moves and carries heat with it, so bulk matter flows (only possible in liquids and gases). In radiation, heat travels as electromagnetic waves and needs no medium at all. So the quick test is: does the material move? No = conduction, Yes (fluid) = convection, No medium at all = radiation.
Only radiation needs no medium. Conduction and convection both require matter to carry the heat, so they cannot work across empty space (vacuum). Radiation travels as electromagnetic waves at the speed of light, 3 x 10^8 m/s, which is why it works even through vacuum.
Between the Sun and Earth there is mostly vacuum, so conduction and convection cannot carry heat (they need matter). The Sun's heat reaches us by radiation, which is energy carried by electromagnetic waves. These waves travel through vacuum at the speed of light, so we feel the Sun's warmth even across ~150 million km of empty space.
Both happen, but the main heating of the water is convection. The bottom water gets hot, expands, becomes less dense and rises; cooler denser water sinks to take its place, setting up a convection current. Heat first enters the pot base by conduction, but the water itself is heated mainly by this bulk fluid motion, which is convection.
No. In conduction the material stays in place. Heat is passed from one molecule to the next by collisions and vibrations, so energy moves through the body but the atoms only vibrate about fixed positions, they do not travel. This is the opposite of convection, where the matter itself flows and carries the heat along with it.
Convection needs the material to flow freely so hot parts can rise and cold parts can sink. In a solid the atoms are locked in fixed positions and cannot move as bulk. So solids can only pass heat by conduction (and emit radiation), never by convection. Convection happens only in fluids, meaning liquids and gases.
A black body is at a temperature of 5760 K. The energy of the radiation emitted by the body at wavelength 250 nm is U1, at 500 nm is U2 and at 1000 nm is U3. Wien's constant b = 2.88 x 10^6 nm.K. Which of the following is correct?
A spherical black body of radius 12 cm radiates 450 W power at 500 K. If the radius were halved and the temperature doubled, the power radiated (in watt) would be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The three modes are conduction (heat through a solid by molecular collisions, no matter flow), convection (heat carried by the bulk motion of a hot fluid), and radiation (heat carried by electromagnetic waves, needs no medium).
Radiation. It travels as electromagnetic waves at the speed of light (3 x 10^8 m/s) and needs no medium, which is why it is very fast and can cross a vacuum, like the Sun's heat reaching Earth.
No. Convection needs a fluid that can flow (liquid or gas), so it cannot happen in a solid or in a vacuum. In solids heat moves only by conduction, and across a vacuum only by radiation.
A thermos has a vacuum between double walls to stop conduction and convection (both need matter), and shiny reflective walls to stop radiation. It also rests on an insulating cork, so all three modes are reduced together.
It is convection. During the day the land heats faster than the sea, the warm air over the land rises, and cooler air from the sea flows in to replace it, setting up a convection current called the sea breeze.
