NEB Physics exam group A question answers, for grade 12 students

 Group 'A'

1. Attempt any four questions.

Note:- You don't need to include figures for answers of Group 'A' questions(short answer questions), however figures help to illustrate answers. So, you are recommended to include simple figures to make your answer unique and illustrative. 

(a) Long-distance, electric power, transmission lines always operate at very high voltage, sometimes as much as 750 kV. What are the advantages of such high voltages ?

Ans:- When electric power is transmitted from one place to another, the resistance of the wires used for transmission becomes significant and dissipates some energy in the form of heat, given as, `P = I^2R`. In addition to this, there is a gradual fall of potential along the wires so that the potential available at the target place will be less than that in the station, `V_o = V_i - IR`. So the power loss will be minimum when the long distance electric power transmission lines are operated at very high voltage and low current. 

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(b) Does the thermo-electric effect obey the law of conservation of energy ? Explain.

Ans:- Yes, it does. In thermo-electric effect, electricity is generated in metallic conductors, called thermocouple, due to temperature differences between their two ends. Free electrons of metal take energy from hot junction of thermocouple and move towards cold junction. Due to movement of these electrons, current is generated in thermocouple. That is, heat energy taken by these electrons from hot junction is equal to the energy generated due to their movement. It is actually the conversion of heat energy into electrical energy. 

(c) A permanent magnet can be used to pick up a string of nails, tacks or  paper clips, even though these are not magnets by themselves. How can this be ?

Ans:- The molecular magnets in the unmagnetized substance are randomly oriented and form closed chain.  So, they neutralize the effect of each other and unmagnetized specimen exhibits no magnetism. But when permanent magnet is brought close to magnetic substances such as a string of nails, tacks or paper clips, the molecular magnets are aligned in the same direction. As a result, one end behaves as a strong south pole and another as strong north pole. Consequently, they are attracted by the permanent magnets.  

(d) If a permanent magnet is dropped down a vertical copper pipe, it eventually reaches a terminal velocity even if there is no air resistance. Why should this happen ?

Ans:- If a permanent magnet is dropped down a vertical copper pipe, the magnetic flux linked to the copper pipe is changed and according to Lentz law, the current induced in the pipe opposes the motion of the magnet. Consequently, it's motion is retarded and it eventually reaches a terminal velocity even if there is no air resistance.

(e) An electron beam and a proton beam are moving parallel to each other in the beginning. Do they always maintain this situation? Justify your answer.

Ans:- No, because as everyone knows that electrons carry negative charges and protons carry positive charges, both being opposite in charges, attract each other. Also motion of such charged particles produces electric current. There is repulsive force due to opposite direction of conventional current between them. So, they do not always maintain the situation of parallel motion as in the beginning. 

(f) Define r.m.s value of alternating current. 

Ans:- R.M.S value of alternating current is the steady current, when passes through a given resistance for a given time, produces the same amount of heat as produced by the alternating current when passes through the same resistance for the same time.  

2. Attempt any four questions.

(a) What property of the cathode rays indicates that they consist of electrons?

Ans:- When cathode rays are subjected to an electric field, they get deflected in a direction opposite to the direction of electric field i.e. towards the anode of the source. Additionally, when subjected to strong magnetic field perpendicular to plane of paper, the rays get deflected in the direction opposite to that given by Fleming's left hand rule. This experiment indicates that cathode rays are deflected by electric field and magnetic field and this property indicated that cathodes rays consist of electrons. Besides this, The value of charge(e) and specific charge (e/m) for cathode rays is same as that of electrons. This property also suggests that cathode rays consist of electrons.

(b) Would there be any advantage if n-type or p-type impurities are added on copper ? Why or why not?

Ans:- Copper is already an excellent conductor of electricity, since it has partially filled conduction band. Furthermore, copper forms a metallic crystal as opposed to the covalent crystal of silicon or germanium. So, the scheme of using impurities to donate or accept an electron doesn't work for copper. In fact adding impurities to copper, decreases it's conductivity because the impurity tends to scatter electrons impeding the flow of current. Hence, there would no any advantage to adding n-type or p-type impurities on copper. 

(c) What are the similarities and differences between a neutrino and a photon?

Ans:- The similarity of neutrino and photon is that both of them have no observable rest mass, no charge, but have momentum and energy.  

The differences of neutrino and photon are that neutrino belongs to leptons family and it has half integer spin (`s = \frac{1}{2}`), so it is called fermion and obey Fermi-Dirac statistics. Neutrinos exhibit no strong interactions but interact weakly and also follow Pauli Exclusion principle.

Photons belong to mediator family. They are particles each having spin 1. So, they are called bosons. They mediate the electromagnetic interaction and  do not follow Pauli Exclusion principle.

(d) Heavy unstable nuclei usually decay by emitting an `\alpha` or a `\beta` particle. Why don't they usually emit a single proton or neutrino?

Ans:-  Heavy unstable nuclei emit an `\alpha` - particle because of high value of binding energy of `\alpha` - particle. Therefore on emission of `\alpha`-particle, the binding energy per nucleon of the residual nucleus increases appreciably. 

Again, `\beta`-particle originates from a nucleus by conversion of a neutrino t proton. On the other hand, the emission of a single proton or neutrino may not be energetically possible.

(e) If energy is conserved, why is there an energy crisis?

Ans:- It is because there is increasing demand of energy throughout the world due to over population. There has been an over exploitation of non renewable source of energy. The petroleum products, coals etc. are being finished in their mines. The forests are being reduced and ground water level has decreased. In this way, useful form of energy is being converted into non useful form of energy. If the present rate of consumption of fuel energy is continued and no alternative sources of energy are brought in, the fuels like coal, petroleum etc. will be evacuated. As a result, the world is likely to face a serious problem of energy scarcity. This is an energy crisis.

(f) When X -rays are produced, only about 10% of the initial input energy appears as X-ray energy. Explain what has happened to other 90%  of the energy ?

Ans:- The X-rays are produced when fast moving electrons strike a target of atomic number like Tungsten, Molybdenum etc. The electrons emitted from the cathode are accelerated by a potential of many thousand volts to strike against the target. When such  a rapidly moving electrons strike the target, only about 10% of initial input energy appears as X-ray energy and remaining 90% of the energy is converted into heat., which heats up the cathode tube. 

3. Attempt any one question.

(a) Which has more influence on loudness of a sound wave , the displacement amplitude or the pressure amplitude Explain your reasoning.

Ans:- Loudness is related to the intensity of sound. It also depends on the sensitivity of individual ear. For a given frequency, the greater pressure amplitude of a sinusoidal wave, the greater the perceived loudness. Moreover, the sinusoidal waves of the same intensity but different frequency have different displacement amplitudes but the same pressure amplitude. Hence the pressure amplitude has a more direct influence on the loudness of a source wave than the displacement amplitude. 

(b) Is it possible to have a longitudinal wave on a stretched string? Why or why not ?

Ans:- Yes, it is possible to have a longitudinal wave on a stretched string by stroking it along it's length by a rosined cloth. However, in the wave motion of the stretched string, wave produced at one fixed end travels along the length of the string and get reflected back at the other end. Since the original wave and the reflected wave have the same frequency and amplitude. They superimpose to produce stationary transverse wave.

4. Attempt any one question.

(a) Describe what happens to the single slit diffraction pattern when the width of the slit is less than the wavelength of the wave?

Ans:- In a single slit diffraction pattern, the first minimum occurs at

`sin \theta_1 = \frac{\lambda}{d}`.

where `\lambda` is the wavelength of light and d is slit width.

When width of the slit is less than the wavelength of light or wave, the value of ratio `\frac{\lambda}{d}` increases. Hence the width of  the central maxima will increase. It in turn makes the diffraction pattern wider.

(b) What is a wave front?

Ans:- Each source of light is a center of disturbance from which waves spread in all directions. All particles equidistant from the source and vibrating in same phase lie on a surface known as wave front. 

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