Unit 11: Transfer of Thermal Energy – Short Questions & Answers

The three primary types of heat transfer are conduction, convection, and radiation.

When two objects with different temperatures come into contact, thermal energy flows from the hotter object to the cooler one, a process known as heat transfer.

Conduction is the process by which heat is transferred through direct contact between particles in a material, without any actual movement of the material.

Metals have free electrons that can move freely within the material. These fast-moving electrons carry energy quickly from the hot areas to the cooler areas of the metal. This is why heat moves more rapidly through metals than non-metals.

Convection is the transfer of heat through the actual movement of molecules from a hot area to a cold area.

When gases get heated, they expand, and this creates convection currents due to the differences in air density at various points in the atmosphere. As the air warms up, it becomes lighter and less dense, causing it to rise. As it rises, it cools down, becomes denser, and sinks back toward the surface.

During the day, the land heats up faster than the sea due to the land's lower specific heat capacity. The warm air above the land rises, and cooler air from the sea moves in to replace it, creating a sea breeze.

At night, the land cools down more quickly than the sea. As a result, the air above the sea becomes warmer and rises, while cooler air from the land moves towards the sea, creating a land breeze.

A glider (which is like a small plane without an engine) relies on the upward movement of warm air currents, or thermals, created by convection. Gliders take advantage of these rising currents of warm air to stay in the air for extended periods.

Birds use rising warm air currents (thermals) to stay aloft for long periods without flapping their wings.

These currents continuously move water up and down, helping to mix oxygen from the surface into deeper layers and bringing nutrients from the ocean floor to the surface.

When the heater is turned on, it warms the air near it. This warm air becomes lighter (less dense) and rises toward the ceiling. As the hot air moves up, cooler, heavier air moves down to take its place near the heater.

In most homes, hot water is provided using a storage tank water heater. Cold water enters the heater and is heated using an electric heating element or a gas burner. As the water heats up, it becomes less dense and rises to the top, while cooler, denser water sinks to the bottom.

In radiation, heat transfer occurs through electromagnetic waves without requiring a medium.

Heat does not reach us via conduction through the air because air neither conducts heat efficiently.

The radiation emitted is influenced by various factors, including: i. the colour and texture of the surface, ii. the temperature of the surface, iii. the surface area.

If the temperature of an object is higher than that of its surroundings, it radiates more heat than it absorbs. Consequently, its temperature decreases until it matches the temperature of the surrounding environment.

If the temperature of the object is lower than its surroundings, it absorbs more heat than it radiates, causing its temperature to rise until it reaches equilibrium with the environment.

Leslie's cube is a metal box. It has faces made of different materials, such as: i. A shiny silver surface, ii. A dull black surface, iii. A white surface, iv. A coloured surface.

i. Dark and dull surfaces absorb and give out infrared radiation very well. ii. Light and shiny surfaces do not absorb or give out much infrared radiation; they reflect most of it. iii. Rough surfaces absorb and emit radiation better than smooth surfaces. iv. Smooth surfaces reflect more radiation instead of absorbing it.

When the surface temperature of an object increases, it radiates more energy because hotter objects emit more heat than cooler ones. If an object has a larger surface area, it can emit or absorb more heat faster.

An infrared thermometer measures the temperature of objects without touching them. It detects the heat (infrared radiation) emitted by the object. To use it, we point the thermometer at the object, press the trigger, and read the temperature on the screen.

When a pan is placed on a stove, heat from the flame or heating element is transferred to the pan mainly by conduction. The metal of the pan, being a good conductor, quickly absorbs the heat and spreads it through the pan. This makes the whole pan hot, so the food inside cooks evenly.

The vacuum stops heat transfer by conduction and convection because there is no air or material to move heat. The inside walls of the flask are coated with a reflective layer (silvered coating) that bounces back radiation to keep the liquid warm.

By adding things like thick curtains, foam in the walls, double-glazed windows, or false ceilings, we can reduce heat loss through conduction, convection, and radiation.

The greenhouse effect refers to the process by which the Earth's atmosphere traps some of the Sun's heat, maintaining a temperature that is suitable for life. Without this effect, the planet would be too cold to support life as we know it.

The Sun gives short-wavelength light and ultraviolet radiation, as well as long-wavelength thermal radiation (infrared). Transparent materials like glass or plastic let short-wavelength radiation pass through but block long-wavelength infrared radiation. This traps heat inside the greenhouse. The Sun's rays warm objects inside, and these objects emit long-wavelength radiation. The glass or plastic prevents this radiation from escaping, reflecting it back and keeping the greenhouse warm, which helps plants grow well.

The greenhouse effect occurs when greenhouse gases trap heat from the Sun, maintaining a temperature suitable for life. Without this effect, Earth would be too cold to sustain living organisms.

In recent years, the amount of carbon dioxide in the atmosphere has increased, which strengthens the greenhouse effect. This traps more heat, causing the Earth's average temperature to rise, a phenomenon called global warming.