Questions asked in Physics Viva Voce for Class 12 Physics Practical

Q1. Why are the resistances used in a resistance box is like 1,2,2,5,10,20,20,50,100,200,200,500,1000,2000,2000,5000 etc?

Ans: In this arrangement all natural numbers upto 10000 ohms (and above) can be realized.

1, 2, 2+1=3,2+2=4,5,5+1=6,5+2=7,5+2+1=8,5+2+2=9 etc

Try it yourself to get all numbers from 1 to 10000

Double Folded Resistance coil
Double Folded Resistance coil

Q2. Why wire is double folded in resistance box?

Ans: The resistance wires in resistance box and in standard resistances are double folded and then coiled so as to nullify the effect inductance. When double folded and coiled, in one half the current will be clockwise and in next half the current will be anticlockwise. Therefore, the magnetic induction created by one half will be cancelled by the other half.

Q3. we make coil of wire when finding resistance of wire through metre bridge ?

Ans: This is not correct. If it is to be coiled, it should be done as in Q1.

Normally the wire is stretched on a half meter scale and fixed with terminals (or arranged in some other manner)

If you feel that the wire need to be coiled, then take  care to avoid the magnetic effect.

 

Inside a Car Engine

(Internal Combustion Engine)

We cannot imagine modern life without the car. The automobile has been one of the singular most amazing inventions of the 21st century and almost defines modern times. Inside of most mechanical petroleum based vehicles is the internal combustion engine.
Today we shall have a brief look at the internal combustion engine and it’s basic working principles.
Basically the the internal combustion engine is a cyclic machine that works on the basis of polytropic expansion laws.
polywhat?!

Ok so a polytropic process is one in which the gas equations work differently than that stated in boyle’s law because temperatures need to be constant for the famous law to work. (Pressure*Volume= constant at constant temperature)
Here since during expansion of the gases the temperature does not remain constant and hence the law looks like:
Pressure*Volumex =constant

The variable x depends upon the gas used.

So as you can see pressure will have to change greatly for the equivalent volume to change and to keep the equation valid. That’s exactly what happens in an internal combustion engine where an small explosion changes the pressure greatly but the limited volume is not much and hence the equivalent change in volume remains small.

First cycle: Suction
As you can see from the diagram the camshaft being slightly oval is rotating about a central point which for a certain time pushes the lever which opens the valve connecting the gasoline tank to the combustion chamber. The engine is so tuned such that when the fuel flows from the gasoline tank to the combustion chamber the bigger rotating structure downwards pulls the piston downwards. Hence the fuel gets trapped in the chamber when the valve closes.

Second cycle:Compression
This is the part where things get interesting. With the fuel trapped inside the chamber due to valve action the wheel rotates such that it pushes the piston above and compresses the fuel. This fuel now now has to withstand high pressure as the piston has lowered the volume greatly. When it is compressed at the maximum a spark plus in the chamber is ignited which causes a small explosion to increase pressure rapidly and push the piston downwards.[remember the part where i told you that the volume is quite limited but the change in pressure is enormous]

Third cycle: Expansion
Here work is obtained from the engine as it causes the rotation in the rotating part attached to the piston to increase. The expansion is the part where the actual energy is obtained from the engine. The wheel continues to rotate by inertia. This is why sometime you need to move the car a bit before getting it to work. It is because the first two parts of the cycle need energy to function and it’s the third cycle which actually provides work.

Fourth cycle: Exhaust
Now the second camshaft is made to rotate at such a speed that by the end of the third cycle  the exhaust valve opens up and the fumes that remain after the explosion is forced out by the upward movement of the piston.
Now the second cycle starts where the piston is made to move by inertia and hence the cycle continues.

That’s it! That’s how the internal combustion engine works. Now obviously you will not get an exact replica of this engine inside your car. Rather they have as many as four piston moving up and down to form a 4-stroke internal combustion engine. Also they have a variety of cooling mechanisms and noise filters to reduce the noise from the explosion that takes place inside the machine. However this is the absolute basic and all other made upon this model. There are differences between a petrol and a diesel engine but then again they are not much different from this model.
So now the looking up the car engine  won’t seem like an engineering job at NASA anymore, would it?!

Hydrogen Economy

 

As the world’s fossil fuels are continuously depleted with increasing use we run the risk of facing a shortage soon enough in the future.  Alternatives are a must as we already face the heat here in India with capping of LPG cylinders. One of the biggest consumers of fossil fuels are petroleum-based vehicles. Whether it is a diesel car or a petrol car or a CNG vehicle it doesn’t really matter. They all use fossil fuels which are bound to run out one day or the other. We need to continuously improve current efficiencies of transportation vehicles while still improving on alternative energy sources. One of the biggest solutions to alternative energy solutions are using hydrogen instead of carbon based fuel as an energy source.

 

Hydrogen as a fuel has a lot of potential to be used as energy because it not only releases a large amount of energy when burnt but it also prevents the release of toxic fumes as pollutants.

Let’s do a little bit of math shall we?

I promise it won’t be too large.

 

 

Bond energies of hydrogen

 

The bond energy of the hydrogen bond is 103 kcal/mole. And the bond energy of the hydrogen oxygen bond is 110 kcal/mole. Also the oxygen molecule has 116 kcal/mole of energy.

Therefore the energy released on formation of the water molecule requires breaking one oxygen molecule and two hydrogen molecules first.

 

That would mean the energy required = 2 x 103 + 116 = 322 kcal/mole

 

But when water is formed the energy released is = 4 x 110 = 440 kcal/mole

 

Hence the net energy gain = 440 – 322 = 118 kcal/mole

 

And this simple math shows the huge amount of energy which is gained on burning hydrogen as a fuel. If a human being were to be powered up by a hydrogen engine he would run 52 miles on just 1 litre! Hydrogen based cars can theoretically provide mileage as good as 93 miles per gallon which is still much better than gasoline based cars providing 35 miles per gallon.

 

Yes and that’s exactly why hydrogen is a potential energy alternative.

Elements of the hydrogen economy

 

This picture shows exactly what the components of the hydrogen economy consist of.I won’t talk about all the boring points in detail but only some of the parts which are really interesting.

 

Production and delivery

Hydrogen has a huge source in the form of our vast oceans and seas which provide an ample source of hydrogen. The problem is converting the water to hydrogen which requires energy in the first place.

 

Well right now they use natural gas and other petroleum products as well as biogas to produce hydrogen. Yes production is actually the biggest hurdle to the hydrogen economy. Hydrogen requires huge amounts of energy to be produced.(It only makes sense since it produces a huge amount of energy on combustion as well.) People are actively working on getting hydrogen to be obtained from a safe source. There are many technologies like aluminium based production (still waiting to be patented) for effective hydrogen production.

And yes there are several hoaxes doing the rounds around the internet which claim to have made super efficient methods to obtain hydrogen from water and hence make cars that run miles on just plain water!!! Well NASA itself has yet to make such a breakthrough but of course there is no giving up hope as people are still trying.

 

Although I am optimistic that this  can be effectively taken care of using solar , wind , hydel power and various other renewable energy sources(like i told you before the future depends on all energy renewables working together) Then the hydrogen can be transported using storage tanks to fuel stations where the vehicles can refill their tanks!

And no I am not talking about science fiction. This is reality as industries work to implement these systems into the current economy.

 

Fuel cells

This is the talk of the town when it comes to hydrogen based economy. Fuel cells use a continuous supply of hydrogen to be directly converted to energy using a chemical reaction. This means efficiencies are quite high when using a fuel cell to power up a vehicle as compared to using internal combustion engines to power up a car.

There is huge money in the R&D for fuel cells in the automobile sector.There have been lots of hydrogen based fuel cells which are running commercially as well. Although there has been electric vehicles which have been shown to have much higher efficiencies they lack the power or speed pick up of fuel cell based cars and car maniacs out there swear by top speeds and revving pick up.

Storage

Now this is a real tricky issue hydrogen is the lightest of all elements out there and it is easily the least dense of all gases. In fact 1 gram of hydrogen would occupy as much as 22.4 litres! that too at 0 degrees celsius. Not too good when considering that we would need more than that to provide decent mileage to the vehicles. Well scientists have been looking for ways to combat this dilemma.

 

One of the ways include in-situ production of hydrogen.

It means the hydrogen would not be carried directly as a fuel. Instead hydrogen containing compounds would be used to generate the hydrogen at the time of running the car. However this method reduces efficiency significantly and renders the use of the vehicle as useless!

 

Another one of the methods include storage of hydrogen in a really efficient way

This means nanotechnology again! Yes the use of carbon nanotubes to store hydrogen has been deemed a potentially powerful method to store huge quantities of hydrogen in the vehicles. That however is a costly method and current research includes making hydrogen storage even cheaper.

 

 

Safety?

Now this is a bit of a tricky issue because hydrogen is even more susceptible to combustion than petroleum. Although that also means that fires breaking out won’t produce toxic fumes but then again they will bring out a bigger bang if they accidentally catch fire. Also current hydrogen storage techniques are not exactly 100% leak proof which means the dangers of accidents are higher! However this can be offset by using standards and protocols to enhance the safety factor of using hydrogen.

 

So what’s the real problem in implementation of hydrogen as a potential energy source?

Well this again lies in costs. Changing equipment and technology that has been built on petroleum will cost huge amounts and also current petroleum-based vehicles and fuel rates are cheaper than those of hydrogen based ones. Of course this should rapidly change in the very close future. In fact there are many companies already that provide hydrogen cars and hydrogen refuelling outlets.


Yes these are real hydrogen cars in the market out there.

 

Ocean Energy

Ocean energy
Now renewable energy sources come in various forms and make great pollution alternatives to coal and other exhaustive resources. Now scientists have been looking for all sorts of alternatives and creativity is often more valued than analytical solutions. Our great oceans store vast amounts of energy in the form of heat(anyone remember that water has the highest specific heat capacity?) mechanical (think thousands of gallons of moving water and the momentum it possesses!) as well as chemical energy (water has a lot of energy in those bonds between hydrogen and oxygen).

Some forms of utilizing tidal energy

1) Wave energy

Here the moving energy of waves is utilized. As can be seen from the figure the moving energy of the waves forces air out of the turbine when it is moving up and forcing air into the turbine when it is moving down. Hence this movement of air can be used to turn turbines . It creates a small source of power but nevertheless it is used in coastal areas for powering small lighthouses and other small electrical equipment.
Sometimes the motion of the wave itself can be used to power a piston to move up and down which in turn can turn a generator.

2)Tidal Energy
Tidal energy is the real current practical source of energy. It uses the tides to store water during high tide and then during low tide the water is released just like a hydroelectric power dam. It’s really simple but the problem is that the tides need to rise really high in order to make it work effectively.
However India’s own coastal lines are quite suitable for this sort of energy and some our IITs are working on making tidal energy a viable source of energy to provide the coastal needs of power.

3) Ocean thermal energy

There is a vast difference in the temperatures of the surface of the ocean and the bottom of the ocean. This temperature difference itself can be used to produce electricity.
Now normally one would assume this electricity produced to be of very poor voltage.(In the seebeck effect about 500 degrees celsius produces about 500 mV).
Hence it has not gained much popularity as an alternative energy source.

New research
Now don’t be so quick to cutting out Ocean Thermal Energy Conversion(OTEC). Ocean thermal energy can be used to generate electricity in new ways it seems again thanks to the potential boundless limits of nanotechnology. It seems scientists have found new ways to harness the heat to generate electricity.

Over time if this new technology could be developed to harness the temperature difference between the surface of the ocean and it’s bottom there could be potentially boundless limits since it could also use the temperatures found in factories, automobiles, deserts(think of deserts as giant energy batteries or so that’s what i think.)
Another interesting phenomenon about tidal energy is that it is not a solar based energy source(most of our energy sources are solar based directly or indirectly) but it is actually based on the lunar force of attraction. So it seems that using the moon as a potential energy source is by itself quite an innovative idea…don’t you think?

Quick Revision for Class X Physics SA1

CHAPTER -12 ELECTRICITY

GIST OF THE LESSON

  1. Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.
  2. Coulomb: It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 109 N when placed in vacuum at a distance of 1 meter from it. Charge on an electron = -1.6 x 10-19 coulomb.
  3. Static and current electricities: Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.
  4. Conductor: A substance which allows passage of electric charges through it easily is called a ‘conductor’. A conductor offers very low resistance to the flow of current. For example copper, silver, aluminium etc.
  5. Insulator: A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an ‘insulator’. For example rubber, glass, plastic, ebonite etc.
  6. Electric current: The flow of electric charges across a cross-section of a conductor constitutes an electric current. It is defined as the rate of flow of the electric charge through any section of a conductor.                                                                    

       Electric current = Charge/Time     or        I = Q/t

Electric current is a scalar quantity.

  1. Ampere: It is the S.I. unit of current. If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere.                                                                                                                         1 ampere = 1 coulomb/1 second    or      1 A = 1C/1s = 1Cs-1                                                                                                                                                                                                              1 milliampere =    1 mA = 10-3 A                                                                                                                                                                                 1 microampere = 1µA = 10-6 A
  2. Electric circuit: The closed path along which electric current flows is called an ‘electric circuit’.
  3. Conventional current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to that of the negatively charged electrons.
  4. Electric field: It is the region around a charged body within which its influence can be experienced. Continue reading “Quick Revision for Class X Physics SA1”

Five marks questions from Electronic Devices (Long Answer Type)

  1. Explain the formation of energy Bands in solids. Distinguish between metals, insulators and semiconductors on the basis of band theory.
  2. Distinguish between intrinsic and extrinsic semiconductors and the conduction in P type and N type semiconductors.
  3. Explain the formation of depletion region and barrier potential in a pn junction.
  4. Draw the circuit diagram used to study the Forward and reverse bias characteristics and draw the graph for forward bias and reverse bias.
  5. Describe the working of a half wave rectifier  with the help of a neat labeled diagram and draw the input and output wave forms.
  6. Describe the working of a full wave rectifier with the help of a neat labelled diagram and draw the input and output wave forms.
  7. Draw the symbols of npn and pnp transistor. Show the biasing of a transistor and explain transistor action. 
  8. Describe the working of an npn transistor in CE configuration as an amplifier.
  9. Explain the working of a transistor in CE configuration as oscillator.
  10. Explain the action of transistor as a switch.

(Have some more idea? Post them as comments)

NIFT Has Launched New Professional Programs In Jewelry Design and Handicraft

National Institute of Fashion Technology (NIFT) in Gandhinagar is ready to start its Continuing Education (CE) programmes. These programmes are specially designed for working professionals, who are keen to pursue professional courses. The Institute will launch a new professional one-year programme from this academic year. The new programme is “Jewelry Design and Handicraft” NIFT faculty and the courses’ coordinator Mr. Ranjeet Kumar said, "Through the programme, the participants will be taught designing and manufacturing techniques of jewellery, so that on completion of the course, they can either initiate their own enterprises, or pursue careers such as jewellery designing, merchandising, product developing and so on."