While teaching my students, I found them to fumble in this topic, so I am writing this lesson to tell you all in simple terms what the topic is:
Consider two cases in both cases; a block moves from A to B point and back.
Let's say a block is pushed from point A on a rough surface with a velocity V and the block goes to point B. While this is happening, let's say only frictional force acts on the block towards A. We can see that the block is being slowed down by friction, or in other words, the force is reducing the kinetic energy of the block. Now when this block reaches B, it stops, and friction stops acting. Now you need to think about where did the kinetic energy of the block GO?
Now take case 2; let's say the block has a spring attached, and now it is pushed from A to B but on a smooth surface with no friction. So only the spring force acts and pulls the block backwards towards A. So the spring force is reducing the speed of the block like friction is reducing the kinetic energy. The block reaches B and stops. At this moment, where did the kinetic energy GO?
If you compare both the cases and think about the kinetic energy disappearing, we find something interesting happening. We know that when we rub our palms, heat is produced because of friction. Similarly, in the first case, heat is produced, which might be very small in amount but is produced, and it is transferred on the block and on the floor, which eventually cools. The energy is dissipated or lost in the surrounding, never to come back. So the KE is lost in the form of heat due to friction. Hence the block stops since it has no more energy and cannot return to point A unless someone pushes it again.
If you consider the spring and what happens to the block after reaching point B, we see the block is pulled by the spring back towards A. This happens when the block stops just for an Instant. Now, where did the spring get the energy to pull the block? Wolla! It's the same Kinetic Energy that the block had. So as the block. Slowed down, its kinetic energy started to get stored in the molecular bonds of the spring in the form of elastic energy. And this energy was again given back to the block as kinetic energy. So you might be thinking, when the block reaches A, does it get all of its KE back, and the answer is Yes.
Such force like friction which removes energy and doesn't give it back is a Non-conservative force; as the name suggests, it doesn't Conserve the initial energy, but a force like spring force keeps on exchanging energy unless someone stops the block. Hence this force is called Conservative Force since it is conserving or not losing any energy.
You can apply the same logic of moving the block and checking the behaviour of the forces.
Now you try on the following forces:
1) Electrostatic Force.
2) Viscous Force.
3) Gravitational Force.
Answer:
Conservative Force: 1,3.
Non-Conservative Force: 2.

Microbiology is the study of organisms that can only be seen through the microscope. These living beings are called microorganisms as they can not be seen with eyes alone and they are both friends and enemies of human beings. The major groups of these small living things are-

1. Bacteria

2. Fungi

3. Algae

4. Virus

5. Protozoa

Some microorganisms are useful to us while some are infectious causing harm to the plants, animals and humans. They can reduce the harvest and infect the livestock.

Pathogens are the disease causing microorganisms. They can enter the body though air, water or food. The other media of communication are physical contact with an infected person or animal. The diseases caused due to this contact are called communicable diseases. 

The insects and animals that carry the micobes are called carriers. 

Q. Have you seen a rotten potato?

Q. Can you eat food cooked a few days back that was not refrigerated?

Q. Why do the elders advise to keep a handkerchief on mouth while sneezing?

Many of my students make a widespread mistake of taking friction value as f= μ N. Where μ = coefficient of friction and N =Normal Reaction.

So is it incorrect? The answer is not exactly. With that, I mean to say that the above formula gives you maximum friction.

Still, confused? Read below

Consider a block on a rough surface with μ= 0.5. Now let's say this block has a mass of 10kg, and you are pushing the block in the horizontal direction towards the right with a force of 150N. If we take acceleration due to gravity g as 10m/s², what we can calculate is as follows

The Normal Reaction on the block will be

N=mg = 10 x10= 100N.

So the Max Friction Value here is

f= μ N = 0.5 x 100 = 50N

The block is acted upon by two forces, one on the right, which you applied (150N) and max friction on the left (50N), so the block moves rightwards with 150 - 50 = 100N.

But!!!!!!!!!!!!!!

Consider the same scenario, instead of pushing with 150N, what if you would have pushed with, say, a force of 30N.

If you take the friction value as 

f= μ N = 0.5 x 100 = 50N

With that logic, you pushed the block rightwards with 30N and friction acting on the block leftwards with 50N, so basically, the block moves towards the left with 50-30 =20N.

Common! Think is it possible?? You are pushing something on the right, and it moves on the left??

So what mistake are we making?

See the frictional force which u take as f= μ N is maximum, and the friction force can take any value between 0 and f= μ N. So when you apply a force less than f= μ N, friction force will be the same value as that force if no other forces are acting.

In the above example, when you apply 30N, the friction acting is also 30N; this cancels the external force, and the block remains Stationary.

Now that makes sense.

Cheers!

Cathode Rays

Cathode rays are the stream of fast moving electrons. These rays are produced in a discharge tube at a pressure below 0.01 rom of mercury.

Properties of Cathode Rays

• Cathode rays are not electromagnetic rays.
• Cathode rays are deflected by electric field and magnetic field.
• Cathode rays produce heat in metals when they fall on them.

Specific charge of cathode rays means ratio of charge and mass. Specific charge of electron was determined by J J Thomson .Specific charge of electron  = (e / m)  . The value of specific charge of an electron is 1.7589 * 10¹¹ C / kg.

Millikan measured the charge of an electron through his popular oil drop experiment.

The charge of the electron as determined by Millikan was found to be 1.602 * 10^-19 C.

Electron Emission

It is the phenomenon of emission of electron from the surface of a metal. Electron emission can obtained from the following process

• Thermionic – By suitable heating, provide sufficient thermal energy.
• Photoelectric emission - When light of suitable frequency illuminates a metal surface, electrons are emitted from the metal surface. These photo(light)-generated electrons are called photoelectrons.
• Field emission - By applying a very strong electric field to a metal.

Photoelectric Effect

The phenomena of emission of electrons from a metal surface, when radiations of suitable frequency is incident on it, is called photoelectric effect.

Terms Related to Photoelectric Effect –

• Work Function( φ0) The minimum amount of energy required to eject one electron from a metal surface, is called its work function.
• Threshold Frequency (ʋ_o) The minimum frequency of light which can eject photo electron from a metal surface is called threshold frequency of that metal.
• Threshold Wavelength (λ_max) The maximum wavelength of light which can eject photo electron from a metal surface is called threshold wavelength of that metal.
• Relation between work function, threshold frequency and threshold wavelength

φ0 = hʋ_o = (hc / λmax )

Laws of Photoelectric Effect - (Basic features)

1. For a given metal and frequency of incident light, the photo electric current ( rate of emission of photoelectrons) is directly proportional to the intensity of incident light. Means number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation.
2. For a given metal, there is a certain minimum frequency, called threshold frequency, below which there is no emission of photoelectrons takes place.
3. For above threshold frequency, the maximum kinetic energy of photoelectrons depends upon the frequency of incident light.
4. The photoelectric emission is an instantaneous process.

Effect of potential on photoelectric current-

• Saturation current- Maximum value of the photoelectric current is called saturation current.
• Stopping Potential (Vo). The minimum negative potential at which photoelectric current becomes zero.
• Maximum kinetic energy of photo electrons

 (K.E.)max = ( ½) mv²_max = e.Vo

• For a given frequency of the incident radiation, thestopping potential is independent of its intensity. Means maximum kinetic energy of photoelectrons depends on the light sourceand the emitter plate material, but is independent of intensity of incident

Effect of frequency of incident radiation on stopping potential –

• greater thefrequency of incident light, greater is themaximum kinetic energy of the photoelectrons.
• there exists certain minimum cut-off (threshold) frequency ν0 for which stopping potential is zero.
• maximum kinetic energy of the photoelectrons and stopping potential V0  varies linearlywith the frequency of incident radiation, but is independent of its
• frequency ν of incident radiation, lower than threshold frequency (ν0 ) , no photoelectric emission is possible even if theintensity is large.

For a given photosensitive material & frequency ( ʋ > ʋ_o ) of incident radiation - photoelectric current is directly proportional to intensity of incident light. Stopping potential or equivalent maximum kinetic energy of the emitted photoelectrons increases linearly with the frequency of the incident radiation, but is independent of its intensity.

PHOTOELECTRIC EFFECT AND WAVE THEORY OF LIGHT—

Phenomena of interference, diffraction and polarization explained in a natural and satisfactory way by the wave picture of light— light is an electromagnetic wave consisting of electric and magnetic fields with continuous distribution of energy over the region of space over which the wave is extended.

Wave theory is unable to explain the most basic features of photoelectric emission ?

• According to the wave theory of light, the free electrons at the surfaceof the metal absorb the radiantenergy continuously, when beam of radiation falls on it. So greater the intensity of radiation, the greater arethe amplitude of electric and magnetic fields.Greaterthe intensity, the greater should be the energy absorbed by each electron.maximum kinetic energy of the photoelectrons is then expected to increase with increase in intensity. So it’s not depend on frequency of radiation. So threshold frequency does not exist in wave theory. So it contradict with basic feature of photoelectric emission.
• Photoelectric emission is instantaneous process. But in the wave picture, the absorption ofenergy by electron takes place continuously over the entirewavefront of the radiation.Energy absorbed per electron per unit time turns out to be small. So it take much time to absorb sufficient energy to come from surface, so it contradict that photoelectric process is instantaneous process.

EINSTEIN’S PHOTOELECTRIC EQUATION : ENERGY QUANTUM OF RADIATION—

• Einstein proposed a radically new theory of electromagnetic radiation to explain photoelectric effect. Radiation energy is buildup of discrete units called quanta of energy of radiation.Each quantum of radiant energy has energy = hʋ, where h is Planck’s constant and ʋ is thefrequency of light.
• If radiation absorbed by photoelectron having quantum of radiation energy hʋ exceeding the work function then then photoelectron come out with maximum kinetic energy
• (K)­_MAX = hʋ - φ₀     this equation called as Einstein’s photoelectric equation.

 Observation from equation-

• Kmax depends linearly on frequency ʋ , and is independent of intensity of radiation
• Kmax must be positive, impliesthat photoelectric emission is possible only if

               hʋ > φ0     or  ʋ > ʋ_o  where  ʋ_o = φ₀ /h           

• Threshold frequency is directly proportional to work function.
• In this picture, intensity of radiation is proportional to number of energy quanta perunit area per unit time. so greater the number ofenergy quanta available, the greater is the number ofelectrons absorbing the energy quanta andtherefore , greater number of electrons coming out ofthe metal (for ν > ν₀). This explains why, for ν > ν₀, photoelectric current is proportional to intensity.
• In Einstein’s picture, basic elementary process (absorption of a light quantum by a single
  electron) involved in photoelectric effect. This process is instantaneous. Thus, whatever may be intensity (the number of quanta of radiation per unit area per unit time) photoelectric emission is instantaneous.

• Intensity only determines how many electrons are able to participatein elementary process means photoelectric current.

How to use kinematic equations and when to use them?

We all know the 3 kinematic equations which are

S= ut + 1/2 at²

V = u + at

V²= u² + 2as

Many students find it difficult to understand when to use them and how to use them.

Just keep a watch for the following points:

1) The equations are used only when acceleration is constant or zero. So while reading a question check whether a body has constant acceleration which will be directly mentioned in the question or will be indirectly given by saying something like " a constant force acts on the body " (which means constant acceleration since F=ma) or "velocity is constant" ( this means acceleration is zero.

2) If you observe these equations have 5 terms u,v,a,t and S. So many a times when quantities like velocity or acceleration are required for calculating quantities like kinetic energy ( = 1/2 mv²) or Force (=ma) they will give you the other terms. For example lets say you are asked final kinetic energy, to calculate that you will require mass and final velocity (V) but instead of velocity the question has stated the body starts from rest which means initial velocity (u)  is zero and time t is given along with acceleration a. By using v=u+at you can easily calculate v and hence Kinetic Energy.

So no matter what the chapter is find atleast 3 terms out of the 5 terms of the equations if you spot them that is the biggest hint for using kinematic equations.

Note : These won't be used in cases where acceleration changes.

Krishna, you need to get a tutor for you.  Who would help you out and make understand better with a lot of examples and with the sessions it would run smooth to speak.  

Self study is also important, but until you get a guide, you will not be able to proceed fully with self study.  You need alot of dedication too.  Wishing you all the best.

Regards,

Helen Francis

(English Trainer)

Firstly, don’t hesitate to continue if you make mistakes while speaking or writing. Learn from your mistakes. Try to talk with persons in English whom you are comfortable with. Read English newspapers and watch English news on television. Try to write essays of your own and find the mistakes. Be confident, and you will become fluent in English.