How is a cooker useful while cooking food?

Why does food cook faster in a cooker?

In a pressure cooker the food is cooked in pressure.

We have used this word pressure.

A force can change shape and size of the object.After cooking has the quality of food has changed.What about its size and shape?

So, There is a force present to make this change.

Force is the reason for producing pressure in the cooker.In order to cook food faster we have to ________ the

pressureHow will we increase this pressure?

The pressure can be increased simply by increasing the force in the cooker.Similarly when we reduce this force the pressure decreases.

PRESSURE IS > WHEN FORCE IS MORE.PRESSURE IS < WHEN FORCE IS LESS.Pressure gives us the effect of force.

Which of the two size cooker should we use to cook less quantity?

The smaller cooker is chosen not because of its size but because food will cook quickly in it than the bigger one. Hence effect of force is more in smaller cooker.But force in a smaller cooker is lesser.How will the effect become more?

Look at the space of the cooker. When the space (surface) is less the effect is more.

Hence the effect of force PRESSURE depends not only on the magnitude of force but the area (surface) as well.

• In order to measure pressure we need to know the magnitude of force and area over which the force is distributed.

The effect of magnitude of force applied to an object depends upon the area of the surface on which it acts.

This effect is less when the force acts on a larger area while it is more if it acts on a smaller area.

Demonstrate : Effect of force and inflated balloon.

So, when a force gets distributed over large area its effect is less.When a force gets distributed over small area its effect is more.When we measure the Force acting on unit area of a surface, we have calculated PRESSURE.

Find the Pressure on the body due to the force of 100 N when distributed over its

surface area of 8 sq m.

Pressure =

Or P =

Thus effect of force depends on

1. Magnitude of the force applied. Greater the magnitude, the more is the effect of force.

2. Area over which the force is applied. Bigger the area on which the force is applied, lesser will be the effect of force and lesser is the pressure exerted by it.

3. For fixed area of contact, the pressure exerted increases with increase in force.

4. For a given force, smaller is the area of contact, greater is the pressure exerted by it.

What should be done to increase pressure on a body?a) Increase force for a fixed area of contact.b) Reduce the area of contact for a given force.

Thus pressure depends on two factors – area and force.

Units of Pressure

Unit of Pressure

1 N/= 1 Pascal or 1 Pa.

1 kPa = 1000 Pascals

Blaise Pascal a physicist and mathematician did a lot of work in the field of atmospheric pressure.

Commonly the unit 1 atm is taken as the unit of pressure1 atm = 76 cm of mercury column = 760 mm of mercury column

The standard atmosphere (symbol: atm) is a unit of pressure equal to 101325 Pa[1]

or 1013.25 millibars or hectopascals. It is equivalent to 760 mmHg (torr), 29.92 inHg, 14.696 psi.

Examples related to Area and Pressure

1. Cutting instruments like blades, knife, axe etc., have very sharp edges and smaller area of contact, to increase the pressure and penetrate easily through the given surface.

2. Foundation of high rise buildings is kept wide so that they may exert less pressure on the ground and do not sink in due to high pressure.

3. School bags and shopping bags have broad straps or belts so that the area of contact increases and thus, the pressure on the hand is reduced.

4. A man swimming under water, feels a pressing sensation upon his ear drums, due to the weight of the water above him. The deeper he goes, the greater is the pressure as the weight of water above him increases causing the force on him and his eardrum to increase. And increase in force without any increase in area increases the pressure.

5. Submarines and underwater tunnels are specially made to withstand such pressure exerted on them.

6. The sledges are not provide with wheels, because wheels have lesser area of cross-section. Hence it will exert high pressure on the snow and sink deeper into it.

7. Drawing pins, alpins have a larger top and highly pointed and sharpened lower end that is why these can be easily pressed into soft wood.

Animals like camels can walk easily in a desert as compared to a horse because camels have broader feet. The broader feet exert less pressure on the sandy ground and it becomes easier for the camel to walk while the horse feet show a tendency to sink in the sand, making it difficult to walk.

Heavy trucks have 6 – 8 tyres instead of the conventional 4. More number of tyres are uses to increase the area of contact and thereby reduce the pressure on the ground.

1. Skiers use long flat skies to slide over the snow. The larger the area of contact, the lesser is the pressure on the snow. This helps the skier to slide comfortably without sinking in the snow.

1. Army tanks are usually very heavy and they apply a heavy pressure on the ground. Hence, they are made to move over broad steel tracks called caterpillar wheels of tanks rather than on ordinary wheels. The steel tracks increase the surface area which reduces the pressure on the ground and hence avoid sinking of the wheels in the ground

• Porters wear turbans on their heads when they have to carry heavy loads. This helps them in increasing the area of contact. This increase in the area of contact reduces the pressure of the load on their heads.

Pointed heels of footwear exert more pressure on the grounds than regular flat heels.

Pressure at the bottom of the sea/ocean is much greater than that at its surface.Example : at a depth of 1000 m, pressure due to water is about 100 times greater than atmospheric pressure. That is why deep sea divers wear specially designed suit to counter such a high pressure.

An air filled balloon is pressed with a force of your hand, the balloon does not burst, it will finally burst when the force applied by the hands is more, but when it is pricked lightly with a pin it burst. Hence effect of more force of your hand is same as the effect of less force by the pin – bursting of balloon.

End of lesson

A pressure of 1 atm can also be stated as:≡1.01325 bar≡ 101325 pascal (Pa) or 101.325 kilopascal (kPa)≡ 1013.25 millibars (mbar, also mb)≡ 760 torr [B]

≈ 760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available [B, C]

Examples to understand how a decrease in area increases the pressure or conversely how an increase in area decreases the

pressure.

1. A nail or a board pin has one end pointed and sharp while the other end is blunt or flat. When we apply a particular force with our thumb on the flat surface of the pin, the pressure on the thumb will be less and the pin will easily penetrate into the board. When we apply the same force on the sharp end of the pin, the pressure applied will be more and the pin will not easily penetrate. Thus, more area needs less pressure and vice-versa.

2. Cutting instruments like the blade, knife, the axe have the cutting edge very sharp. This helps in decreasing the area of contact thereby increasing the pressure. Hence, they can easily penetrate through given surface.

3. Heavy trucks have 6 – 8 tyres instead of the conventional 4. This is done to increase the area of contact between the tyres and the road because of which the pressure on the ground is less.

4. Animals like camels can walk easily in a desert as compared to a horse because camels have broader feet. The broader feet exert less pressure on the sandy ground.

5. Skiers use long flat skies to slide over the snow. The larger the area of contact, the lesser is the pressure on the snow. This helps the skier to slide comfortably without sinking in the snow.

6. The foundations of a high-rise building are kept wide so that they may exert less pressure on the ground and do not sink in due to extremely high pressure.