Physics Form 1 Notes : CHAPTER SIX – THERMAL EXPANSION.

CHAPTER SIX

THERMAL EXPANSION

Temperature is the degree of hotness or coldness of a body. Both the Celsius scale (°C) and the Kelvin scale (thermodynamic scale) are used to measure temperature. The Kelvin scale is also known as the absolute temperature scale and is measured from absolute zero (0 K).

Expansion of solids

When solids are heated, they expand. The expansion is so small that it is not easily visible.

The following experiments demonstrate the actual expansion of solids.

Experiment 1: Ball and ring experiment

Procedure

1. Obtain a ball and ring apparatus.
2. Pass the ball through the ring at room temperature and observe that it easily slips through.
3. Heat the ball using a Bunsen burner for one minute.
4. Try to pass the ball through the ring and observe what happens.
5. Let it cool for some time and try passing the ball again.

Discussion

When the ball is heated, it expands and increases in diameter, making it unable to pass through the ring. After cooling, the ball slips through the ring easily again.

Experiment 2: The bar-breaker

Procedure

1. Try to break the cast-iron pin with your hands. Can you? (A bar-breaker is a strong iron frame which holds a steel bar fitted with a wing-nut. The other end is held by a cast-iron pin as shown below).
2. Tighten the nut but do not break the pin.
3. Heat the bar strongly using two Bunsen burners while continuing to tighten the nut.
4. Continue heating for another five minutes, then let it cool.
5. Observe what happens.

Discussion

When the bar cools, the cast-iron pin breaks. This shows that as the bar cools, it contracts and strong forces pull against the pin, causing it to break.

Experiment 3: Heating a bimetallic strip

Procedure

1. Heat a brass-iron bimetallic strip evenly using a Bunsen burner.
2. Observe what happens after a short while.

Discussion

When a brass-iron bimetallic strip is heated, it bends towards the iron side. This means that brass expands more than iron, causing the strip to bend. This demonstrates that different materials expand at different rates when heated.

Applications of the expansion of solids

1. Construction of railway lines – an expansion joint is allowed between any two rails to accommodate expansion. A fish plate is used to join two rails. Modern railway systems use the overlapping joint at the ends of rails.

2. Construction of bridges and rooftops (steel girders) – for bridges, one side has rollers while the other is fixed to allow for expansion. Concrete slabs are also laid on expansion joints to accommodate expansion.

Expansion joints for expansion.

4. Riveting – used to join two pieces of metal together, e.g., bimetallic strips, car bodies, drums, etc. Fitting rail cart wheels using heat uses the principle of rivets. Bimetallic strips are used in thermostats (to control temperature) such as in electric iron boxes, alarm systems, and car flasher units.

Expansion of liquids and gases

Expansion of liquids

Liquids expand more than solids, making their expansion easier to observe. The hot water bottle demonstrates the expansion of water. Water is placed in the bottle as shown below.

When the bottle is immersed in hot water, initially the water level in the glass tube drops, then it steadily rises. This shows that liquids expand with an increase in volume. Different liquids expand at different rates as shown below.

Expansion of gases

Gases are the easiest to observe since they expand the most.

Experiment: Expansion of air

Procedure

1. Obtain an empty 500 ml round-bottomed flask fitted with a cork and glass tubing.
2. Place a beaker with some water on a bench.
3. Rub your hands together thoroughly and place them on the flask, then place the flask in the water as shown.
4. Observe what happens.

Discussion

The heat produced by the hands makes the air inside the flask expand. This increases the volume and forces the excess air out as bubbles.

Applications of the expansion of gases and liquids

1. Land and sea breeze – during the day, the land is heated by the sun causing the air above it to expand. The air becomes less dense and rises. The space left is quickly filled by cooler air (generally from the sea since the land heats faster). This causes a cool breeze from the sea during the day. At night, the land loses heat faster than the sea. The air above the sea rises since it is less dense, and cool air from the land rushes to fill the gap. This causes a breeze blowing from the land to the sea.

Thermometers

1. Liquid-in-glass thermometer – this uses the expansion of a liquid in a thin-walled glass tube. The liquid moves up the tube when the bulb is heated. The liquid must be a good conductor, visible, and able to contract and expand quickly and uniformly over a wide temperature range. It should also not stick to the sides of the tube. Common liquids used are mercury and coloured alcohol. The scale is obtained by choosing two fixed points: 0°C (ice point) and 100°C (boiling steam). The scale is divided into 100 equal parts (calibration). The melting and boiling points of mercury and alcohol are (-39°C to 357°C) and (-112°C to 78°C) respectively.
2. Clinical thermometer – a special mercury-in-glass thermometer used to measure body temperature. Since body temperature is normally 37°C, the scale covers a few degrees below and above 37°C. It has a constriction that prevents mercury from returning after expansion for convenient reading. This thermometer has a narrow bore for greater sensitivity and accuracy.
3. Six’s maximum and minimum thermometer – used to measure the temperature of the surroundings of an area or place. It records both maximum and minimum temperatures attained. It consists of a large bulb (A) containing oil of creosote connected to a U-shaped stem which connects to a second bulb (B) containing the same liquid. The base (C) contains a thin thread of mercury. The range is -20°C to 50°C. After each reading, the indices are pulled down to the level of mercury using a magnet.
4. Bimetallic thermometer – made of a bimetallic strip with one end fixed and the other connected to a pointer. Metals used are usually brass and invar. As temperature increases, the strip unwinds and moves the pointer over a calibrated scale. It is used to measure high temperatures.
5. Thermocouple thermometer – a junction made of copper and iron looped at both ends. In practice, a sensitive millivoltmeter is used instead of a galvanometer. A cold junction is maintained in melting ice (0°C) while the other junction is heated steadily. This thermometer does not use the principle of expansion.

Unusual expansion of water

If water is heated from -15°C, it expands normally like any solid but only up to 0°C.

At 0°C, it starts to melt and contracts. This contraction continues up to 4°C. When heated further, water expands up to its boiling point. This unusual expansion causes the top of water to freeze at 0°C in temperate countries, allowing the water below to remain liquid at 4°C. This supports marine life during winter.

Molecules and heat

1. Solids – when heated, molecules in solids absorb heat energy and vibrate. They push against one another, causing expansion. Further expansion may result in collapse, as seen in melting ice.

2. Liquids – besides vibrating, particles in a liquid move short distances. As they move, they collide with each other, resulting in more expansion. For boiling to occur, molecules absorb enough energy to escape from the liquid.

3. Gases – individual particles are free from one another and move rapidly. When heated, they collide with the walls of the container, resulting in high pressure inside the container.