Please enable JavaScript.

Coggle requires JavaScript to display documents.

HEAT - Tanvi :)) - Coggle Diagram

- - - - Any objects that take in heat in the form of radiation
    - - Any objects that bounce off radiation
    - - Any objects that give off heat in the form of radiation
  - - - If an object has a larger surface area, the rate of heat transfer by radiation is faster.
    - - If the surface of an object has a higher temperature, it will radiate heat at a higher rate.
    - - Black, dull and rough surfaces are the best absorbers and emitters of radiant heat. Hence, they are the poor reflectors of heat.
      - White, shiny and smooth surfaces are the poor absorbers and emitters of heat. Hence, they are the good reflectors of heat.
  - - - White colour is a poor absorber of heat. When a person walks under the hot sun, he will not feel overheated if he puts on white clothes.
    - - The hot water containers have a shiny appearance to reduce heat loss by radiation from inside the containers to the surroundings. In this way, water can stay warmer for a longer period of time.
    - - The cooling coils of a refrigerator are painted black to ensure that heat is lost quickly as black colour can radiate heat faster to the surroundings.
  - - - Trapped air
        
        Function
        
        Air is a good insulator of heat.
        
        Reduces heat loss by
        
        Conduction
      - Vacuum
        
        Function
        
        It does not have any particles.
        
        Reduces heat loss by
        
        Conduction
        
        Convection
      - Stopper
        
        Function
        
        It prevents the setup of convection current.
        
        Reduces heat loss by
        
        Convection
      - Silvery glass walls
        
        Function
        
        Silvery wall reflects radiant heat back into the liquid.
        
        Reduces heat loss by
        
        Radiation
- - - - Description
        
        A box with two chimneys is used.
        
        A candle is placed under one of the chimneys.
        
        A smouldering paper is placed above the other chimney.
        
        The smoke from the smouldering paper flows down the chimney. It then moves towards the lit candle and rises up from that chimney.
      - Explanation
        
        ir above the lit candle is heated and expands. As a result, its density decreases and rises out of the chimney.
        
        The cooler surrounding air is denser at the other chimney (representing by the smoke). so it sinks through the chimney to replace the hot air.
    - - Description
        
        A sample of blue crystals is placed in water at the bottom of a beaker.
        
        The bottom of the beaker is then heated gently.
        
        The circulation of blue stream of water moving upwards and downwards is the convection current.
      - Explanation
        
        As water is heated, it expands and its density decreases. As a result, it rises up.
        
        The cooler part of the water, which is denser, sinks to replace the heated water. It is in turn heated and rises.
        
        This cycle is repeated to set up the convection current.
  - - - When electricity is turned on, the heating element heats up water at the bottom.
      - Hot water rises and cold water sinks. Thus, a convection current is set up in the kettle.
      - The heating element is placed at the bottom of a kettle.
      - Water in the kettle is heated up more uniformly and effectively.
    - - Water is heated up in the boiler.
      - Hot water rises to the upper part of a storage tank.
      - Cold water in the water supply tank sinks into the boiler to replace the hot water.
      - The boiler is installed at the lower part of a hot water system.
    - - Cold air from the air conditioner sinks into the room.
      - Hot air rises and is cooled by the air conditioner and then sinks down.
      - It is placed at the top of a wall.
      - The cycle is repeated to set up convection currents in the room. Hence, the room is cooled efficiently.
  - - - The cooler and denser air from the sea flows in to replace the rising hot air.
      - The movement of air from the sea to the land is called a sea breeze.
      - As a result, the air above the land is warmer than above the sea, so it rises up.
      - During the day, the land is heated up faster than the sea.
    - - As a result, the air above the sea is warmer than above the land, so it rises up.
      - The cooler and denser air from the land flows towards the sea to replace the rising hot air.
      - During the night, the land loses heat much faster than the sea.
      - The movement of air from the land to the sea is called a land breeze.
- - - - A woollen jacket is used to trap air. Both wool and air are insulators that can prevent heat loss from the body to the cold surroundings.
      - Handles of the cooking utensils are made of insulators so that the users can be protected from scorching when holding them during cooking.
    - - Cooking utensils are made of metals such as iron, copper or aluminium because they can conduct heat quickly to cook the food.
      - The heating element in electric kettles is made of metal because it can conduct heat quickly to heat up water.
      - In cars and buses, radiators are made of copper to conduct heat from the hot water to the surroundings quickly.
  - - - In metals, there are free electrons that can move about freely.
      - It is a fast process of heat transfer.
      - During heating, these free electrons help to transfer heat energy from the heated region to a cooler region quickly.
      - However, different metals conduct heat at different speeds.
      - The presence of these free electrons make the metals much better heat conductors than non-metals such as plastics, wood or glass.
    - - It is a slow process of heat transfer.
      - Solids conduct heat better than liquids and gases because the particles in solids are closely packed together than those in liquids and gases. As a result, it is easier for the particles to cause their neighbouring particles to vibrate vigorously.
      - When heated, the particles gain heat energy and vibrate more vigorously. These particles will then cause the neighbouring particles to vibrate vigorously too. Hence, heat energy is transferred from one particle to the next in the medium.
      - In solids, the particles are closely packed together and can only vibrate at their fixed positions.
- - - - flow from region of higher temperatures to region of lower temperatures
        
        there is a temperature difference
        
        temperature of a cooler object rises
        
        hotter object falls
        
        same temperature.
  - - - physical state
      - size
- - - - The particles in an object gain more kinetic energy and push each other further apart through their vigorous motion. Hence the volume occupied by the particles in the object increases.
    - - Expansion
        
        Causes
        
        mass remains the same
        
        volume increases
        
        Reason
        
        the particles in a substance gain heat energy during heating, so their motion increases. [Recall that the particles in substances have motion.]
        
        The vigorous motion push one another funher apart. This increases the space they occupy.
        
        Since the mass remains unchanged, the ratio of (mass/volume) decreases. Hence, the density decreases. [Recall that density = mass/volume
        
        Density
        
        Decreases
      - Contraction
        
        Causes
        
        Mass remains the same
        
        Volume decreases
        
        Reason
        
        The less vigorous motion makes them become closer to one another. This decreases the space they occupy.
        
        Since the mass remains unchanged, the ratio of (mass/volume) increases. Hence, the density increases.
        
        The particles in a substance lose heat energy during cooling, so their motion decreases.
        
        Density
        
        Increases
    - - liquids
        
        The water level drops when water is heated initially. This is because glass expands first before water, so the volume of glass increases.
        
        After heating for a while, the water level increases. This shows that liquids expands upon heat gain.
        
        The expansion in liquids is caused by the particles occupying more spaces when their motion increases and move further apart from each other, so the liquid level rises.
      - gases
        
        This shows that gases expands upon heat gain.
        
        The expansion of a gas causes gaseous particles to occupy more spaces, so some of them are forced out of the flask.
        
        Air bubbles are seen in water when air inside the flask is heated.
      - solids
        
        After the metal ring is heated for a while, the metal ball can pass through it easily.
        
        This shows that solids expand upon heat gain.
        
        The metal ball cannot pass through the metal ring initially.
        
        The expansion in solids is caused by their particles pushing each other further apart to occupy more spaces, so the length of solids increases.
    - - Concrete road surfaces
        
        Problem
        
        When the concrete roads are paved in a continuous way, cracks will occur.
        
        The road surfaces expand and contract in different times of a day. The length of the concrete road increases and decreases, causing the road surfaces to crack easily.
        
        Solution
        
        Gaps are left between sections of concrete road surfaces.
        
        The gaps provide room for the concrete roads to expand or contract in different temperatures.
      - Pipes
        
        Problem
        
        When a pipe carrying hot or cold liquids is straight, it bursts easily.
        
        If a straight pipe carries hot liquids, it expands and increases in its volume. This causes the straight pipe to buckle and burst.
        
        When the hot liquids in the pipe cools, it causes the pipe to contract and decrease in its volume. This will shorten the length of the pipe and causes the pipe to break apart.
        
        Solution
        
        Pipes carrying hot liquids have curved joints or are coiled.
        
        The curved joints or coiled pipes provide some spaces for the pipes to expand or contract to prevent them from breaking easily.
      - Railway tracks
        
        Problem
        
        When a railway track is constructed in a continuous way, it will buckle and cause train to derail.
        
        The railway track expands and increases in its volume when heated during hot days. This will increase the length of the railway track and causes it to bend upward or sideway.
        
        Solution
        
        Small gaps are left between sections of a railway track.
        
        The gaps provide spaces for the railway track to expand during hot days.
      - Bridges
        
        Problem
        
        When both ends of a bridge is fixed, the bridge can be damaged easily.
        
        The problem is caused by changes in the size of the bridges during hot days and cold days.
        
        Solution
        
        One end of a bridge has a fixed end, the other is built with a movable joint.
        
        The movable joint provides spaces for the bridge to expand or contract.
    - - Placing a wheel inside a metal tyre
        
        When the metal tyre cools, it contracts and decreases in its size.
        
        Therefore, the wheel is fitted tightly inside the tyre.
        
        A metal tyre is first heated. It expands and increases in its size. e A wheel is then placed inside the metal tyre.
      - Bimetallic strip as a thermostat
        
        A thermostat is a device that can regulate temperatures automatically.
        
        During heating, brass expands faster than iron. Thus, its length is greater than iron, causing the strip to bend.
        
        Brass and iron have different rates of expansion and contraction.
        
        During cooling, brass contracts faster than iron. Thus, its length is shorter than iron, causing the strip to bend in the opposite direction.
        
        It contains a bimetallic strip which is made of brass and iron generally.
        
        The different rates of expansion and contraction of these two metals can connect or disconnect the supply of electricity in a circuit.
        
        It is used in fire alarms, electric ovens or electric irons.
      - Riveting
        
        Riveting is a process of holding two metal plates firmly together.
        
        Rivets are a type of bolts made of steel.
        
        A hot rivet is inserted to the holes of two metal plates. The other end of the rivet is hammered until it secures the metal plates.
        
        When the rivet cools, it contracts and decreases in size. This tightens the metal plates further.
      - Applications of bimetallic strip
        
        Fire alarm
        
        This causes the bimetallic strip to bend and touches the contact point.
        
        Electricity flows through the circuit and the fire alarm goes off.
        
        When a fire occurs, the temperature in the surrounding increases.
        
        Electric iron
        
        When the temperature of the electric iron becomes very high, the bimetallic strip expands and curves to break away from the contact point. The supply of electricity is cut off.
        
        When the temperature drops, it contracts and touches the contact point again. The supply of electricity is turned on again.
        
        he thermostat in an electric iron keeps the temperature constant when the iron is used.