TyroCity: Physics XI Notes

Units in different systems

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

Generally we can use any convenient unit to measure a physical quantity depending on how much magnitude we are measuring or in which system of units we want to measure it.
What kind of unit we should use?
The unit i) must be accepted internationally.
ii) Should be reproducible.
iii) Should be invariable.
iv) Should be easily available.
v) Should be consistent.
vi) Should be large, if the physical quantity to be measured is a big quantity.
Ex: To measure larger lengths we use units like Km, mt etc, to measure large magnitude of time we use units like hour , day ,week, month , year etc.
vii) Should be small if the physical quantity to be measured is small.
Ex: To measure small time we use units like millisecond, microsecond etc
To measure small lengths we use units like millimeter, centimeter etc.

Types of physical Quantities.:
We can broadly divide the physical quantities in to two types

i)Fundamental Physical quantities
ii)Derived physical quantities.

Fundamental physical quantities: A physical quantity which can exist independently is called Fundamental physical quantity.
Ex: Length, mass and time etc.

Derived physical quantities: A physical quantity which can not exist independently is called derived physical quantity. (Or) A physical quantity which is dependent or derived from any other physical quantity is called derived physical quantity.
Ex : Area, volume, density, speed, acceleration, force, energy etc.
Like the physical quantities we can divide the units in to two types. I)Fundamental units ii)derived units.

Fundamental units : The units of fundamental physical quantities are called fundamental units, (or) The units which are independent or can not derived from any other unit is called fundamental unit.
Ex:Every unit of length is fundamental unit (irrespective of the system to which it belongs);millimeter, centimeter, meter, kilometer etc.
Every unit of time is a fundamental physical quantity ; microsecond, millisecond, second, minute, hour, day etc.

Derived units: The units of derived physical quantities are called derived units. Units of area, volume, speed, density, energy etc are derived units.
Ex: Every unit of speed is a derived unit ; m/sec, cm/sec, km/hr etc.
Every unit of density is a derived unit; kg/m³, gr/cm³ etc.
Every unit of acceleration is a derived unit; m/sec², cm/sec², km/hr² etc.

Systems of units:To measure the fundamental physical quantities Length, Mass and time we have three systems of units, they are i) C.G.S System (Metric system)ii)F.P.S System (British system) and iii)M.K.S System. In all these three systems only three physical quantities length, mass and time are considered to be fundamental quantities.

But, in systems International (S.I) system there are seven fundamental physical quantities. Which are i)Length ii)Mass iii)Time iv)Electric current v)Thermo dynamic temperature vi)Luminous intensity vii)Quantity of substance.
In addition to these two more quantities were added as supplementary physical quantities. They are i)Plane angle ii)Solid angle.

Systems,Fundamental physical quantities and their units:In
C.G.S system: Length (centimeter); Mass (gram); Time (second).
F.P.S system :Length (foot);Mass(pound);Time (second).
M.K.S system: Length (meter); Mass (kilogram); Time (second).
S.I System:Length (meter); Mass (kilogram); Time (second); Electric current (ampere); Thermodynamic temperature (kelvin); Intensity of light (candela); Quantity of matter (mole). The units of suplimentary quantities are Plane angle( radian); Solid angle(Steradian).

Why is a cow more stable than a man?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The C.G. of the cow is lowered to the base of the body and the base of the foot is greater than the man, so the cow is more stable than the man.

Why a wrench of longer arm is preferred in comparison to a wrench of short arm?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

We have,
Torque = F x r
So, the wrench of longer arm is used then the moment of the force (torque) is greater than the wrench of shorter arm. So, the wrench of longer arm is preferred in comparison to the shorter arm.

Second Law of Thermodynamics- Old is Gold

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

State second law of thermodynamics.
Can the thermal efficiency of an engine ever be 100%? Give reason.
State second law of thermodynamics.
Petrol engine is less efficient than diesel engine. Explain?
Why do diesel engines need no spark plugs?
Distinguish between petrol and diesel engine.
Can a room be cooled by leaving the doors of an electric refrigerator open in a closed room?
Write down the statement of second law of thermodynamics.
Is it possible to construct a heat engine that creates no thermal pollution?
Explain why temperature of gas drops in adiabatic expansion.

Is friction a necessary evil? Explain.

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

Yes. Friction is the necessary evil because,

(a) Without friction between the feet and the ground, it would not have been possible to walk.

(b) The tyre of the motor cars and cycles are rough to increase the friction.

What do you mean by geo-stationary satellite? Explain?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The satellite which appears to be at rest from the surface of the earth is called geo-stationary satellite. The time period of geo-stationary satellite is equal to the time period of the earth. The orbit of geo-stationary satellite is called parking orbit.

How does K.E. of an object change if its momentum is doubled?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

We have,

(K.E.)1= P²/2m

Where ‘P’ is momentum and ‘m’ is the mass of the object.
If the momentum is doubled then,

(K.E.)2 = (2P)²/2m

Or, (K.E.)2 = 4(K.E.)1

Therefore, on doubling the momentum, the K.E. of the body increases by four times its initial K.E.

A helium filled balloon rises to a certain height and then halts. Why?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The density of the atmosphere air is different at different heights. At the certain height, the density of the atmospheric air becomes equal to the density of the helium then the balloon halts.

Distinguish between conservative and non-conservative force.

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

Conservative force	Non-conservative force
A force is said to be conservative if the work done by or against force is dependent only on the initial and the final position of the body and not on the path followed by the body.	A force is said to be non-conservative if the work done by or against the force is dependent on the path followed by the body.
Work done by the conservative force in a closed path is zero.	Work done by the non-conservative force in a closed path is not zero.
Eg: Gravitational force, magnetic force, electrostatic force, etc	Eg: friction, viscous force, induction force in cyclotron, etc.

A rigid, lighter than air balloon filled with Helium cannot continue to rise indefinitely. Why?

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The density of the atmospheric air is different at different level. The density of the air at upper level is less in comparison to the lower level. When the balloon rises up in air, the weight of the displaced air decreases as the density of the air decreases with height. Hence the upthrust decreases and becomes equal to zero then the balloon stops to rise.

How does ‘g’ at a point vary with distance from the centre of the earth? Where is the highest value of ‘g’? Explain.

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

The value of ‘g’ is zero at the centre of the earth. Its value increases from centre to the surface of the earth and becomes maximum at the surface of the earth. On increasing the height, its value decreases. This can be shown by the following graph.

Transfer of heat- Old is Gold

Physics XI Notes — Sun, 18 Aug 2013 05:41:42 +0000

Although aluminum is good conductor of heat, how can an aluminum foil with shiny surface be used to keep food hot for longer time?
Hot water pipes used in the room are painted black. Why?
What is a black body? How is it realized in practice?
Why are good absorbers always good emitters?
Why are two thin blankets warmer than a single blanker of double the thickness?
During the winter, the animals curl into a ball. Explain why?