Uses of Gas Laws

1.Bicycle Pump

When the piston is pushed into the cylinder, the air in the cylinder is compressed.

According to Boyle's Law, the air pressure inside the cylinder will increase.

This causes the air pressure in the cylinder to become higher than the pressure inside the tyre. Therefore, the air can flow into the tyre.

2. Hot-air balloon

When the air in a balloon is heated at atmospheric pressure, its temperature will increase.

According to Charles' Law, the volume of gas in the balloon will increase when its temperature increases.

Thus, the upward thrust on the balloon will increase when the volume of air displaced by the balloon increases.

Therefore, the balloon will climb upwards if the upward thrust exceeds the weight of the balloon.

3. Car Tyre

When a car is moving, the car tyre will experience frictional force and compression. This condition causes an increase in the temperature of the air inside the tyre.

According to the Pressure Law, the rise in the temperature  of air inside the tyre will cause the pressure inside the tyre to increase. Therefore, it is wise to pump the tyre just slightly below the recommended value in order to prevent over inflation and prevent bursting (though this rarely happens)

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Pressure Law

The Pressure Law can be clarified using the Kinetic Theory of Gases.

When gas is heated at a fixed volume, the gas molecules will move faster and with more energy.

The rate of collision of the gas molecules onto a unit area of the wall of the container will increase.

Each collision will also produce a greater force, because the change in momentum for each molecule increases when its speed is higher.

To maintain the same pressure in the container, the volume of the gas will increase so that the above effects will be balanced by the effect of an even smaller number of molecules per unit.

The Pressure Law states that:

For a fixed unit of mass and volume of a gas, the gas pressure is directly proportional to the absolute temperature of the gas.

P1/T1 = P2/T2

P1 = Initial Pressure
P2 = Final Pressure
T1 = Initial temperature in kelvin
T2 = Final temperature in Kelvin

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Boyle's Law

Boyle's Law can be explained by using the kinetic theory of gases.

When the volume of a gas in a container of gas molecules is reduced.

a) its density increased, that is, the number of gas molecules per unit volume increases.
b) the surface area of the container decreases.

The end result is that, the number of gas molecules which hit onto a surface area of the container per unit area also increases.

The increase in this rate of change of momentum in turn causes the pressure of the gas to increase.

If the volume of the container is otherwise increased:

a) density of the gas decreases, the number of gas molecules per unit volume increases.
b) the surface area of the container increases.

Therefore, the number of gas molecules hitting a unit surface area per unit time decreases.

The pressure is thus decreased.

BOYLE'S LAW states that for a fixed mass of gas at a fixed temperature, the pressure of the gas is inversely proportional to its volume.




According to Boyle's Law

P1V1 = P2V2

P1 = initial pressure

V1 = initial volume

P2 = final pressure

V2 = final volume

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Charles' Law

Charles' Law can be explained by the kinetic theory of gases.

When the temperature of a gas is raised, the gas molecules will move more actively and with more energy.

The rate of collision of the gas molecules onto a unit area of the wall of the container will increase.

Each collision will also produce a greater force because the change in momentum for each molecule increases when its speed is higher.

To maintain the same pressure in the container, the volume of the gas will increase so that the above effects will be balanced by the effect of an even smaller number of molecules per unit.

Charles' Law states that for a mass of gas held at a fixed pressure, the volume of the gas is directly proportional to the absolute temperature of the gas.

According to Charles' Law

V1/T1 = V2/T2

V1 = initial volume
v2 = final volume
T1 = initial temperature in Kelvin
T2 = final temperature in Kelvin

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Absolute zero temperature and the absolute zero scale

The absolute zero temperature of - 273 °C is the lowest possible temperature that could be attained.

The volume of the gas becomes zero at the absolute zero temperature but before this temperature is attained, all of the gas would have changed to liquid.

The Kelvin Scale is also known as the absolute zero temperature scale.

The SI Unit is Kelvin (K)

The temperature interval is 1K = 1 °C .

By referring the Celsius scale, we have the Kelvin scale, T = (θ + 273) K

With T being the temperature at the Kelvin θ being the temperature at the Celsius scale.

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Universal Gas Law

From the various Gas Laws, the relationship among the three quantities; Volume, V, Temperature, T and Pressure, P can be connected by the equation as follows:

Boyle's Law : PV = a constant with T fixed

Charles' Law: V/T =  a constant with P fixed

Pressure Law: P/T = a constant with V fixed

All three Gas Laws are connected to obtain a Universal Gas Law which is given by PV/T = a constant.

That constant is known as the Universal Gas Constant.

P1V1/T1 = P2V2/T2

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Understanding the Gas Laws: Gas Laws and Kinetic Theory of Gases

Gas theory can be explained by way of the kinetic energy.

When gas molecules hit the walls of the container and bounce back, a change in momentum occurs in a split second. This is obviously a very very fast action.

The end result of the above momentum is that the walls of the container experience a force.

Pressure is defined as the force that acts on a unit surface area. Therefore, all surfaces that are knocked by air will experience a pressure. In order for this to take effect all of the gases molecules in the container or free surface must be moving swiftly in a very short time and hit the surface repeatedly.

This pressure is called gas pressure.

Kinetic Theory of Gases

The basic assumption for the kinetic theory of gas is as follows:

Gas is composed of molecules.

Gas molecules are continually in random and independent motion in all directions at high and different speed.

The motion of gas molecules follows all of the Newton Laws of Motion.

All collisions between the gas molecules (i.e. one with another) and the walls of the container are assumed to be perfectly elastic. Therefore, momentum and kinetic energy are conserved during collision.

 The volume of the molecules can be conserved compared to the volume occupied by the gas.

The force among the gas molecules can be neglected except during collision.

The time period of a collision can be neglected when compared with the time interval between two collisions.

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Application of Specific Latent Heat

Steaming Food

The specific latent heat of vaporisation for water is large.

Plates filled with food are able to absorb heat from the hot steam.

The condensation of steam at the base of the plate releases a large quantity of heat and thus enables food such as cakes,fish, eggs and others to be steamed.


Cooling drinks with cold water and ice

A glass of hot water can be cooled faster by adding cold water or ice into it.

During the melting of ice, a large quantity of specific latent heat is absorbed from the drink and this causes the drink towards a temperature that approaches the melting limit of ice.

Ice absorbs a large quantity of latent heat during the process of melting.

Extinguishing fire by using boiling water

Water that is quickly boiled will become steam which is able to absorb a larger quantity of latent heat from the fire.

Melting Ice on the road by using Salt

It is known that the specific latent heat of fusion of salt is higher than of ice. Therefore, when salt is put on the road - having a thick layer of ice, salt will require more heat energy and absorb energy from the ice. Therefore, Ice will melt.

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Specific Latent Heat of Vaporisation

The specific latent heat of vaporisation, L of a substance is the heat quantity required to convert one unit mass of a liquid into water vapour at its boiling limit without any change in temperature.

Its unit is JKg-1.

If m Kg of liquid or water vapour is involved, the quantity of heat, Q absorbed or released is
Q = ml

Q = quantity of heat that is absorbed or released.
m = mass of the substance
l = specific latent heat of vaporization

The list below show the specific latent heat of vaporization for a few substances

Methylated spirit - 1.12 X 10^3

Ether - 3.70 X 10^2

Mercury - 2.72 X 10^2

Water - 2.26 X 10^6

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Specific Latent Heat 2

The specific latent heat of a substance is the energy which is required to change 1 Kg of a substance from a certain physical condition to another physical condition without any change in temperature.


The unit for specific latent heat is JKg-1.

Source:http://wordpress.mrreid.org/


The graphs above shows how the temperature of a quantity of substance such as water changes over time when heat is supplied to it.

As you can see above,, all along the temperature from 0 to 273 K, water is in the form of solid, that is ice.
In this phase:
- When the temperature is raised, the water molecules vibrate even faster.
- Heat energy supplied is converted to kinetic energy.

All along the straight line at 273K, a change of phase from ice to water occurs.
As:
- Even though heat is still supplied to it, the temperature does not increase all along.
- This is because the heat energy supplied is needed to separate the water molecules and not for the increasing their energy.
- The heat that is required in the change of phase from Solid to liquid is termed the latent heat of fusion.

At the end of the straight line at 273K, all of the solid (ice) has melted into liquid.

All along the graph from 273K to 373K, water only exist in the form of liquid only. Therefore, the temperature of water will increase when heat is supplied to it.

All along the graph of 373 K (the level phase), the change of phase from liquid to gas occurs.
Along the line:
- Water is boiling.
-it is observed that the temperature does not change even though heat is constantly supplied to the substance.
- Heat is required to separate the water molecules and to do the work of opposing air pressure when the liquid changes into gas.
-The heat required to convert liquid into gas is termed the latent heat of evaporation.

At the end of the level line at 373K, all of the liquid has been changed into gas.

At the graph from 373K to 473K, water is in the form of gas and the temperature rises when heat is supplied.

When there is cooling, the reverse process occurs.

Latent heat of fusion and latent heat of evaporation will be released.

Since the heat energy supplied during the change in phase cannot be detected by a thermometer, this type of heat is referred to as latent heat.

Therefore, the change of state is an 'energy change without any loss of temperature change' phenomenon.

Now I am going to discuss about the, Specific Latent heat of fusion. Specific latent heat of fusion, L of a substance is the quantity of heat which is required to change one unit mass of the substance from solid to liquid without any change of temperature at the melting limit.

Its unit is JKg-1.

Specific latent heat of fusion occurs at the melting point of the solid.

For example, 336000J of heat is required to change 1Kg of ice at 0°C.
Therefore the latent heat of fusion, L for ice is 336 000 JKg-1.

It has to be noted that when liquid solidifies, the specific latent heat of fusion will be released.

This condition occurs at the freezing limit of a liquid.

For example, when 1 Kg of water at 0°C solidifies to become 1 Kg of ice of 0°C, 336 000 J of heat are released.

If m Kg of solid or liquid is involved, the quantity, Q of heat absorbed or released is

Q = mL

where Q = quantity of heat that is absorbed or released
m = mass of substance
L = latent heat of fusion

Below are examples of substance with its specific latent heat

• Aluminum 3.96x10^5 JKg-1.
• Copper 2.05x10^5 JKg-1.
• Iron 2.67x10^5 JKg-1.
• Lead 0.23x10^5 JKg-1.
• Brass Unknown 
• Magnesium 3.7x10^5 JKg-1.
• Zinc 1.1x10^5 JKg-1.

Hope you will understand what Specific latent heat is.

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Understanding Specific Latent Heat I : Latent Heat

Before we begin, let's think about this situation.

When ice melts. There is a change of phase from solid to liquid. The ice absorbs heat from the surroundings. The heat energy absorbed by the ice does not cause the increase in temperature. The energy absorbed is not transferred to the molecules of ice as kinetic energy.

1. When a substance experiences a change of phase, it absorbs heat energy without a change in temperature. The heat absorbed is known as latent heat.

2. Heat energy needs to be supplied to change a substance from solid to liquid phase and from liquid to gaseous phase.

3. When a solid melts, heat is absorbed but the temperature remains constant.

4. When a a liquid is boiling, heat is also absorbed but the temperature remains constant.

5. From the principle of conservation of energy, we can infer that:

a) latent heat must be given out when a gas condenses to become a liquid and when the liquid solidifies to the solid phase.

b) These two processes also occur at constant temperature.

The four main changes of phase are melting, boiling, condensation and solidification.

Later, we will study the heating curve and cooling curve for a substance. That's all for now

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Application of Specific Heat capacity

As we have read (supposedly) about the concept of heat capacity and specific heat capacity, we will discuss briefly about the application of Specific Heat capacity in daily situations.

1. Substances having a small specific heat capacity can be quickly heated up, it also experience a big change in temperature even though only small amount of heat is supplied.

2. Substances having a small specific heat capacity, are very useful as material in cooking instruments such as frying pans, pots, kettles and so on, because, they can be quickly heated up even when small amount oh heat is supplied.

3. Sensitive thermometers also must be made from materials with small specific heat capacity so that it can detect  and show a change of temperature rapidly and accurately.

4. Substances that have a high specific heat capacity is suitable as a material for constructing kettle handlers, insulators and oven covers, because, a high amount of heat will cause only a small change in temperature aka the material won't get hot too fast!

5. Heat storage instruments are very useful and they are usually made of substances with a high specific heat capacity.

6. Water as a cooling agent acts excellent as a cooling agent in engines. Water is also used in houses in cold climate countries because as it is heated up (boiled) it tends to retain heat and warm the house due to its high specific heat capacity.

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Understanding Specific Heat Capacity: idea of Specific Heat Capacity

Understanding Specific Heat Capacity

Heat Capacity

1. The heat capacity,C , of a substance is the heat which is required to increase the temperature of the substance by 1°C.

2. The unit for heat capacity is J° / C.

3. For example, the heat capacity for 100 g of water is 420 J°/ C. This means that 420 J of heat energy is required to raise the temperature of 100 g water by 1°C. To increase temperature by 2°C, 840 J are needed and so on.

4. Different substance, materials or body has different specific heat capacity.

5. If a body absorbs a lot of heat but there is only a slight increase in temperature, then the body is said to posses a large heat capacity.

6. On the other hand, if a body absorbs a little amount of heat but shows a big rise in temperature, then the body is said to posses a small heat capacity.

7. The relationship between heat capacity, C and specific heat capacity, c is shown by the following equation.

C = mc

Specific Heat Capacity

1. Specific heat capacity, c, of a body is the heat that is needed to increase the heat of a unit of mass or the substance by 1°C or 1K.

2. The unit of specific heat capacity is J kg-1°C-1.

3. For example, the specific heat capacity of water is 4200 J kg-1°C-1 . This means that 4200 J of heat is needed to increase the temperature of 1 Kg of water by 1°C.

4. Therefore, when a body of a mass m and specific heat capacity, c, absorbs a quantity of Heat, H, then its heat will increase by θ.

5. Therefore H = mc θ.


6. On the contrary, when the heat of a body falls by θ, the quantity of heat that disappears or lost is also H = mc θ.


7. The specific heat capacity is dependent upon the type of substances. Different substances have different specific heat capacity.


8. By knowing the specific heat capacity, we can determine the mass and also the change of temperature of a body if we know the amount of heat that is transferred.


9. Total heat transferred H = mc θ.


10. Generally, liquid has more specific heat capacity than solids. This means that liquids need more heat energy than solids to show the same value of rise in temperature.

Hope this helps!

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Thermometers and calibration of Thermometers

The definition of temperature as a physical quantity is based on the principle of thermal equilibrium.

Let say there are Thermometer A, Liquid B and Liquid C.

We put thermometer A into liquid B and then after thermal equilibrium is achieved we record the value.

We put thermometer A again into liquid C and after thermal equilibrium is achieved we record the value of reading in the thermometer.

If the temperature in both cases are the same, then liquid B and liquid C are in thermal equilibrium with one another. Eventhough, the two liquids (B and C) are not in thermal contact, they are in thermal equilibrium because their temperatures are the same.

Therefore Temperature is a physical quantity which determines whether or not two objects are in thermal equilibrium.

We measure temperature using a thermometer. 

Thermometers must be calibrated before they can be used to measure temperatures.

The calibration of an instrument refers to the process of marking-up a scale on the instrument to be used as measurement.

To produce a scale on a thermometer, two fixed points must be determined first. Then the two points must be the temperatures which can easily and correctly reproduced in any part of the world.

On the Celsius scale, the two fixed points are the ice point (0°C) and the steam/boiling point (100°C).

The ice point (0°C), or lower fixed point is the melting temperature of pure ice at standard atmospheric pressure (760 mm Hg).

The steam point (100°C), or upper fixed point is the temperature of steam at standard atmospheric pressure (760 mm Hg).

After obtaining, the highest point and the lowest point. We divide the length between them to equal parts / scale.

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Types of Thermometer

There are several types of thermometer, here, I explain only a few of the possibly many types of thermometer.

Mercury thermometer

1. The physical quantity that is used to determine the temperature of a body by means of a mercury thermometer is the length of the thread mercury, or to be more exact, the volume of mercury.

2. When the temperature increases, the volume of the mercury increases too.

3. The sensitivity of a mercury thermometer can be increased by

a. reducing the diameter of the capillary tube.
b. increasing the size of the bulb.
c. using a thinner-walled glass bulb.

4. Normally mercury is used in a thermometer because it:

a. Expands uniformly.
b. has a higher boiling limit.
c. is opaque and therefore it is easier to read off the temperature.
d. is a good conductor of heat.
e. does not stick to the glass.

5. One weakness of the mercury thermometer in the measurement of an accurate temperature is that the glass of the capillary tube also expands when the temperature expands.

In addition to that, it is extremely dangerous if the glass tube breaks because mercury is very poisonous.

Mercury thermometer is suitable to measure temperature between -30 degree celsius to 300 degree celcius.

Resistance thermometer

1. Thermometers which use liquids inside the glass are not suitable to be used for measuring a wide range of temperature. e.g temperature ranging from -250 degree celcius to about 700 degree celsius.

2. A suitable thermometer which is used for the above range of temperatures is a resistance thermometer.

3. A resistance thermometer uses the property of the change in the platinum wire with a change in temperature.

4. The current flowing in the wire experiences more resistance when the wire becomes hot.

5. The change in the resistance of the wire is directly proportional to the change in temperature.

6. A milliammeter can and should be calibrated before hand to measure the temperature.

7. Its calibration of the melting limit of water and the boiling point of water at a pressure of 1 atmosphere is able to convert the milliameter scale to a temperature scale in degree celsius.

8. Therefore, this thermometer is very accurate.


Thermocouple thermometer

1. An electromotive force (e.m.f) will be produced in a thermocouple when there is a temperature difference between the hot junction and the cold junction. Once this happens, a current will flow.

2. This thermometer is very sensitive and responds towards slight change in temperature.

3. Since the physical quantity which is used to measure the temperature is the e.m.f, this thermometer can be connected to other electrical circuits to control or record the surrounding temperature.

4. A thermocouple thermometer is a very sensitive thermometer which is suitable for measuring temperatures ranging from -250 degree celsius to 1600 degree celsius.

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Understanding Thermal equilibrium

It has to be noted that temperature is one of the basic quantities in physics.

Temperature is a physical quantity which measures the DEGREE of HOTNESS of an object. So as you probably have inferred, hot object has a higher temperature than a cold object.

The SI unit of temperature is Kelvin, K. Other units such as degree Celcius (centigrade) and Fahrenheit is also used.

When two objects are in thermal contact, heat is transferred from one object to the other.

The temperature of the objects determines the direction of energy transfer between them. The energy transferred between objects in thermal contact is known as heat.

Let say there are two objects A and B.

Say A has higher temperature than B. When A and B is in contact with each other, heat will be transferred from A to B. That is A will give heat and B will receive heat. When this situation occurs, it is called net heat transfer from A to B.

The heat transfer will continue until a state of thermal equilibrium is achieved.

It is expected that temperature of B will rise to a certain degree (due to increased kinetic energy in the molecules), when thermal equilibrium is achieved and temperature A will fall. At the end of transfer, both A and B will have the same temperature.

Heat

Heat is a form of energy. The SI unit for heat is joule, J.

Heat is produced by mechanical energy or from the conversion of other types of energy. such as electrical energy to heat energy and so on.

It must be noted that Temperature is NOT the same as Heat.

Temperature is a measure of degree of hotness of an object, is a base quantity, SI unit is kelvin and other units are degree celcius and fahrenheit. It determines the direction of heat flow.

Heat is a form of energy, is a derived quantity and SI unit is Joule, J. (other unit is calorie, cal.) It is being transferred from a region of higher temperature to another region of lower temperature.

Hope this does not confuse you!

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