Junior Certificate Science Heat and Temperature

What is Heat? A type of ENERGY!!! Energy is the ability to work!! Work is done when something is moved!! In what ways can Heat do work? Heat makes a Hot Air Balloon rise. Heat in a steam engine makes the train move. Any more?

Heat Transfer Heat will move from a hot object to a cold object. How it moves depends on what kind of substance it is. Three methods of heat tranfer: – Conduction – Convection – Radiation

Conduction Conduction is the movement of heat through a substance without any overall movement of the substance. Conduction of heat occurs in solids, where their particles are very closely packed together.

When heat is added, the molecules get energy and start to vibrate. The vibrations continue on down the solid until all the heat is distributed evenly. This is why the handle of a poker will get hot when you put the other end in the fire. Different substance conduct heat better than others. E.G. Copper is a better CONDUCTOR than wood. Wood is therefore called an INSULATOR

Convection Convection is the movement of heat through liquids and gases by the mass movement of particles.

When the liquid or gas is heated, it rises. The hot liquid or gas moves up and is replaced by cold liquid or gas. This in turn gets heated up. The hot liquid or gas eventually cools and goes down and a current is formed. This is called a CONVECTION CURRENT.

Radiation Conduction and Convection use atoms or molecules to carry heat. Radiation does not need a substance (MEDIUM) to carry the heat. Radiation is the transfer of heat by means of waves that can travel through a vacuum. Any object close to the hot object will warm up when the waves or rays fall on it.

Examples include the heat from the sun or from a bonfire.

Dark surfaces are better radiators than shiny ones.

Conductors and Insulators Materials that allow heat to travel through them are called conductors. Materials that do not allow heat to travel through them are called insulators.

Good conductors include metals like copper and iron.

Good insulators include air and wood.

Is water a conductor or insulator?

Insulation in the Home. Insulators are used to keep heat in you home. Common insulators are : Double Glazed Window – have a layer of air which is a good insulator. Aero-board (Cavity Walls) – trapped air is a good insulator. Fibre Glass in the Attic Lagging Jackets over hot water tanks.

The Bi-metallic Strip The bi-metallic strip is a strip of two metals (copper and iron) riveted together. The copper expands at a greater degree than iron and when heated the bi-metallic strip bends. These are used in thermostats to control heat levels in e.g. kettles, toasters, central heating.

Temperature Temperature is a measure of how HOT or COLD something is. Thermometers are used to measure temperature. There are three types of thermometer: Alcohol, Mercury, Clinical Clinical Thermometer have a constriction o stop the mercury returning to the bulb when removed. Temperature is measured in o C.

Heat Capacity The higher the heat capacity of a medium...the more it can absorb a large amount of heat energy before its temperature starts to rise

Any examples?

Latent Heat Latent Heat is the heat required to change a substance’s state. In other words the heat needed to change a solid to a liquid or a liquid to a gas. Latent Heat is often called “hidden heat”. This is because even though heat is being added to the substance the temperature doesn’t rise until all the substance has changed state.

You can feel the effects of latent heat on a hot day when sweat evaporates from your skin and you feel cooler. This is because the molecules of liquid which evaporate will need more energy when they become water vapor. This heat energy is taken from your skin, reducing its temperaturewater vapor

When water molecules in the air rise up high enough, they become colder and condense into liquid which has less energy. The "spare" energy becomes latent heat and makes the surrounding air warmer. This leads to wind and, when the process happens quickly, can even cause a thunderstorm.

Effect of Pressure on Boiling Point of Water The boiling point of water is 100 o C in normal conditions. If the pressure changes this affects the boiling point. For example in a pressure cooker the steam in the container cannot escape, so the pressure builds up. This extra pressure causes the boiling point to rise to 120 o C. This means food in a pressure cooker cooks faster.

Exp: To investigate the expansion of solids when heated, and contraction when cooled Method: 1.Take a ball and ring apparatus and check that the ball passes through the ring. 2.Using a tongs heat the ball over the flame of a bunsen burner for a few minutes. 3.Using the tongs again try to pass the ball through the ring. 4.Allow ball to cool, try again to pas the ball through the ring. Result: The ball will not pass through the ring when heated. When cooled the ball once again passes through the ring. Conclusion: The ball expands when heated and contracts when cooled.

Exp: To investigate the expansion of liquids when heated, and contraction when cooled Method: 1.Fill a round bottomed flask up to the brim with water. 2. Insert a cork with a glass tube into the flask and attach to a retort stand, as in the diagram. 3.Heat the flask with a bunsen burner for a few minutes and observe the glass tube. 4.Remove the heat source and observe the water in the tube. Result: The water expands and moves up the glass tube when heated. The level of the water in the glass tube drops as the water cools. Conclusion: Liquid expands when heated and contracts when cooled. Level rises

Exp: To investigate the expansion of gases when heated, and contraction when cooled Method: 1.Place a cork with a glass tube through it into a round-bottomed flask. 2.Fix the flask to a retort stand in such a way that the glass tubing is immersed in water. 3.Carefully heat the flask which contains air and observe the end of the glass tubing in the water. 4.Allow the flask to cool and observe the glass tubing again. Result: Bubbles of air are seen in the water when heated. Water enters the glass tubing and rises along the tube when cooled. Conclusion: Gas expands when heated and contracts when cooled.