CHAPTER 13 Heat & Its Effects Expansion and Contraction

What happens when states change When a material changes from one state to another, there is a dramatic change in the look at shape of the material. Most materials contract when they are cooled. Also, the temperature stays constant while the material changes states. Ice is one of the few materials that expands when cooled. Unfortunately, this is the cause of much of the damage to road in the winter. Water seeps into a small crack in the road, and then when it freezes, the ice expands and makes the crack bigger. Cars drive over the crack and loosen the material, so the water and ice can work again to make a pothole in the road. (You can demonstrate this with a balloon filled with air. See Experiment with Balloons.)

Effects of Heat Expansion : Occurs when things are heated. Heat energy is gained by the body and results in an increase in size.

Expansion and Contraction All metals expand when heated, contract when cooled. Put a steel bar into a furnace and heat it up to a temperature of 5000oC. It will get longer. Take it out and cool it. It will return to its original length. Further, if you can find a way to precisely measure the width of the bar while it is hot, and again while it is cold, you’ll find that width also increased during the heating process. To put it another way, expansion and contraction are ”three-dimensional”.

Expansion and Contraction If the length of the bar increased one per cent, both dimensions of the cross-section increased one per cent. Now suppose that this steel bar, instead of being placed in a furnace so that it could expand in all directions, is mounted between two immovable objects, such as two five-ton blocks of granite, and then heated with a torch until the center of the bar reaches a temperature of 5000oC.

The granite blocks will effectively keep it from getting longer as it heats up. As a result, it will get ”fatter” than it would had it been free to increase in length. In fact, if the bar is now allowed to cool down to its original temperature, it may wind up a bit shorter than it was at the start, and also a bit ”fatter”. Let’s try one more example to show what can happen if normal three-dimensional expansion or contraction is restrained.

Take a steel rod exactly five feet long, and heat it up to 50000C or so with no restraint on its movement. Then, while it is hot, clamp the ends in some way so that the bar can’t get shorter as it cools. Let it cool down.Then measure it. You’ll find that it’s a bit longer than five feet, and, somewhere along its length, a bit ”skinnier”.

Effects of Heat Contraction: Occurs when things are cooled. Heat energy is lost by the body and results in a decrease in size.

Expansion and contraction Most substances expand when heated and contract when cooled. This continues even after they have changed their states. For example: Water will contract until it starts to freeze and turn into ice. It then starts to expand and get larger. Ice will expand until the temperature reaches about -3° C, after which is will start to contract again with lowering temperatures. Metal contracts means the temperature is lowered.

Expansion of Solids Solid expands when heated.

Expansion of Solids Different solids expand at a different rate. Invar 0.1 mm/m Pyrex 0.3 mm/m Platinum alloy 0.9 mm/m Glass 0.9 mm/m 1m length solid bars when heated by 1000C Concrete 1 mm/m Steel 1 mm/m Brass 2 mm/m Aluminium 3 mm/m

Expansion of Liquids Liquid expands when heated. When heating water in a beaker, you will initially notice a dip in the water level before the water level rises.

Expansion of Liquids Different liquids expand at a different rate.

Expansion of Gases Gases expands when heated.

Expansion in Solids, Liquids and Gases For the same rise in temperature, gas expands more than liquid, and liquid expands more than solid.

Expansion Problem in Solids Small Gaps Railway – small expansion of gaps are left along the rails to allow for expansion.

Expansion Problem in Solids Bridge – one end on the bridge is fixed while a gap is left at the other end supported by rollers.

Expansion Problem in Solids Concrete Ground Gaps filled with bitumen Pavement – small gaps between cement sections are filled with soft black bitumen.

Expansion Problem in Liquids Bottles containing liquids are never completely filled. This allow expansion when the surrounding temperature is higher.

Aerosol cans carry a warning - to be stored in cool places. Expansion Problem in Gases Aerosol cans carry a warning - to be stored in cool places.

Thermal expansion of solids and liquids Most (but not all) solids and liquids expand when heated. Thermal expansion is a consequence of the average change of separation between the constituent atoms in an object. One of the exceptions is CaCO3. It expands along one direction and contract along another as its temperature is increased.

Unusual behavior of water Liquids generally increase in volume with increasing temperature. Water is an exception to this rule in the temperature range 0 – 4oC. 0 2 4 6 8 10 1.0004 1.0003 1.0002 1.0001 1.0000 Temperature oC Volume of 1g of water Let’s investigate this further!

Expansion and Contraction of Water As temperature increases from 4OC to 100 OC, water expands. But from 0OC to 4OC, water contracts, i.e. its volume decreases. At 4OC, water has the lowest volume and therefore it has the highest density (since density = Mass/Volume and mass is constant). Ice Temperature/oC Density Volume 1g/cm3 4oC

How marine life survives during cold weather?

Application of Expansion and Contraction - Riveting Rivets are applied when heated. Once cooled they contract and bind the metal sheets together tightly.

Application of Expansion and Contraction – Bimetallic Strip Bimetallic strips works on the principle that different metals expand / contract at different rates for the same temperature increase /drop.

Application of Bimetallic Strip – Fire Alarm

Application of Bimetallic Strip – Heating Element Control

Hot air balloons depend on the low density of the air to float.. Application of Expansion and Contraction – Hot air balloons Hot air balloons depend on the low density of the air to float.. Heat is applied to expand the air in the balloon. With the air expanded, its volume will increase causing its density to decrease. This results in the hot air in the balloon to rise, thereby floating the hot air balloon into the air

Temperature Thermometers make use of a measurable physical property that varies with temperature. A Thermometer measured temperature in degree Celsius (oC).

Temperature Ice point is the temperature at which pure ice melts at 0OC

Temperature Steam point is the temperature at which pure water boils under normal conditions at 100OC.

Liquid in Glass – Lab thermometer Purpose – For Laboratory Use Range : -10oC to 110oC

Liquid in Glass – Clinical thermometer Purpose – Measure body temp Range : 35oC to 42oC A constriction serves to prevent the mercury thread from going back into the bulb after the thermometer is removed from the mouth.

Thermocouple thermometer Purpose – Lab use, strength of current varies with temperature

Resistance thermometer Purpose – Lab use, resistance of wire changes with changes in temperature

Bimetallic thermometer Purpose – Measure hot oven/ molten metal Ranges: 0 to 400OC

Constant Volume Gas Thermometer Pressure of the gas at constant volume varies with changes in temperature

Assembly and fitting of metal fittings?

Flexible expansion loops Rubber Pipe Joint Duct Joint                                                                                                    BRAIDED METAL CONNECTORS It is constructed from lengths of corrugated stainless steel bellows which are covered with flexible stainless steel braiding to add strength. Flexible expansion loops