Previous Year Question Papers

### Calculate the threshold frequency of photon for photoelectric emission from a metal of work function .

Ans: Given is the work function $0.1 \mathrm{eV}$, $$\phi_{o}=h v_{0}$$ Class 12 Physics $\underline{\text { wuw. vedantu.com }}$ 1 $10 a-x=10 a-2.5 a=7.5 a$ from charge $9 q$

### A straight wire carries a current of . An electron moving at is at distance from the wire. Find the force acting on the electron if its velocity is directed towards the wire.

Ans: We are given the current through the straight wire to be, $I=10 \mathrm{~A}$ Speed of the electron, $\mathrm{v}=10^{7} \mathrm{~m} / \mathrm{s}$ Distance of electron from the wire,...

### Give one difference each between diamagnetic and ferromagnetic substances. Give one example of each.

Ans: Diamagnetic substances are the ones that are weakly repelled by a magnet. For example, gold. Ferromagnetic materials are the ones that are strongly attracted by a magnet. For example, iron.

### A steady current flows in the network What will be the magnetic field at the center of the network?

Ans: The magnetic field at the center of the network is zero. This is because, magnetic field at the center of the loop would just equal and opposite $i . e .$, magnetic field due PQR is equal and...

### A cyclotron is not suitable to accelerate electron. Why?

Ans: A cyclotron is not suitable to accelerate electron as its mass is known to be less due to which they gain speed and step out of the dee immediately,

### Two wires of equal lengths are bent in the form of two loops. One of the loops is square shaped whereas the other loop is circular. These are suspended in a uniform magnetic field and the same current is passed through them. Which loop will experience greater torque? Give reasons.

Ans: We know the expression for torque as, $$\begin{array}{l} \tau=\mathrm{NIAB} \\ \Rightarrow \tau \propto \mathrm{A} \end{array}$$ Since, we know that the area of circular loops is more than...

### What will be the path of a charged particle moving along the direction of a uniform magnetic field?

Ans: The path of a charged particle moving along the direction of a uniform magnetic field would be a straight line path as no force would act on the particle.

### State two properties of the material of the wire used for suspension of the coil in a moving coil galvanometer.

Ans: Two properties of the material of the wire used for suspension of the coil in a moving coil galvanometer are: i. Non-Brittle conductor ii. Restoring Torque per unit twist should be small.

### V.I graph for a metallic wire at two different temperatures Which of these two temperatures is higher and why?

Ans: From the graph: Slope $=\frac{\mathrm{i}}{\mathrm{V}}$ It is known that, $\frac{i}{V}=\frac{1}{R}$ It means that the smaller the slope, the larger the resistance.

### A stream of electrons travelling with speed at right angles to a uniform electric field is deflected in a circular path of radius Prove that

Ans: The path of the electron that is travelling with velocity $\mathrm{vm} / \mathrm{s}$ at right angle of $\bar{E}$ is of circular shape. It requires a centripetal in nature, $F=\frac{m v^{2}}{r}$...

### If the amount of electric flux entering and leaving a closed surface are and respectively. What is the electric charge inside the surface?

Ans: Net flux can be given as $=\phi_{2}-\phi_{1}$ Since $\phi=\frac{q}{\epsilon_{0}}$ $$\Rightarrow \mathrm{Q}=\left(\phi_{2}-\phi_{1}\right) \in_{0}$$ The electric charge inside the surface can...

### Draw one equipotential surfaces (1) Due to uniform electric field (2) For a point charge

Ans: The diagrams are given as: Equipoterhibil Surfaces 3 $\mathrm{~ ( 2 c ) r o m}$

### What happens to the capacitance of a capacitor when a dielectric slab is placed between its plates?

Ans: The introduction of dielectric in a capacitor will reduce the effective charge on plate and therefore will increase the capacitance.

### The plates of a charged capacitor are connected by a voltmeter. If the plates of the capacitor are moved further apart, what will be the effect on the reading of the voltmeter?

Ans: The relation between capacitance, area, distance and dielectric constant is $$C=\frac{A E_{0}}{d} \Rightarrow C \propto \frac{1}{d}$$ Hence, if distance increases, capacitance decreases....

### The distance of the field point on the equatorial plane of a small electric dipole is halved. By what factors will the electric field due to the dipole changes?

Ans: The formula $E x \frac{1}{r^{3}}$ gives the relation between electric field and distance. $$\therefore E \propto \frac{1}{\left(\frac{r}{2}\right)^{3}} \Rightarrow E \propto \frac{8}{r^{3}}$$...

### Force of attraction between two point electric charges placed at a distance d in a medium is . What distance apart should these be kept in the same medium, so that force between them becomes ?

Ans: If two point charges are $q_{1}$ and $q_{2}$ separated by distance $d$, it can be expressed as: $\mathrm{F}=\frac{\mathrm{Kq}_{1} \mathrm{q}_{2}}{\mathrm{~d}^{2}}$ Suppose if force becomes...

### What is the work done in moving a point change from corner to corner of a square when a charge exists at the centre of the square?

Ans: Point $A \& B$ are at the same distance from the point $O$. Hence,

### Why does the electric field inside a dielectric decrease when it is placed in an external electric field?

Ans: The electric field that is present inside a dielectric decreases when it is placed in an external electric field because of polarisation as it creates an internal electric field which is...

### If the net outward flux through the surface of the box were zero, could you conclude that there were no charges inside the box? Why or why not?

Ans: No. The net flux entering out through a body depends on the net charge contained in the body. If the net flux is given to be zero, then it can be inferred that the net charge inside the body is...

### Careful measurement of the electric field at the surface of a black box indicate that the net outward flux through the surface of the box is a) What is the net charge inside the box?

Ans: It is given that: Net outward flux through surface of the box, $\phi=8.0 \times 10^{3} \mathrm{Nr}^{2} / \mathrm{C}$. For a body containing net charge a, flux is given by...

### What is the net flux of the uniform electric field of exercise through a cube of side oriented so that its faces are parallel to the coordinate planes?

Ans: It is given that all the faces of the cube are parallel to the coordinate planes. Clearly, the number of field lines entering the cube is equal to the number of field lines entering out of the...

### When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge. Ans: Rubbing is a phenomenon in which there is production of charges equal in

Ans: Since unlike charges attract and like charges repel each other, the particles 1 and 2 moving towards the positively charged plate are negatively charged whereas the particle 3 that moves...

### Why can one ignore the quantization of electric charge when dealing with macroscopic i.e., large scale charge?

Ans: When dealing with macroscopic or large-scale charges, the charges used are huge in number as compared to the magnitude of electric charge. Hence, the quantization of electric charge is of no...

### Explain the meaning of the statement ‘electric charge of a body is quantized”.

Ans: The statement 'electric charge of a body is quantized' suggests that only integral $(1,2,3,4, \ldots, \mathrm{n})$ number of electrons can be transferred from one body to another. This further...

### An electric dipole when held at with respect to a uniform electric field of experiences a torque of . Calculate dipole moment of the dipole?

Ans: It is given that $$\begin{array}{l} \theta=30^{\circ} \\ \tau=9 \times 10^{-26} \mathrm{Nm} \\ E=10^{4} \mathrm{~N} / \mathrm{C} \end{array}$$ Dipole moment P needs to be calculated. It is...

### Explain: (i) Zone refining (ii) Column chromatography

Solution 4: (i) Zone refining: This method is used for production of semiconductors and other metals of very high purity, e.g., $\mathrm{Ge}, \mathrm{Si}, \mathrm{B}, \mathrm{Ca}$ and $\mathrm{In}$....

### Explain: (i) Zone refining (ii) Column chromatography

Solution 4: (i) Zone refining: This method is used for production of semiconductors and other metals of very high purity, e.g., $\mathrm{Ge}, \mathrm{Si}, \mathrm{B}, \mathrm{Ca}$ and $\mathrm{In}$....

### Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?

Solution 3: $\Delta_{f} G^{o}$ of $\mathrm{Cu}_{2} \mathrm{~S}$ is more negative than $\Delta_{f} G^{\circ}$ of $\mathrm{CS}_{2} \mathrm{H}_{2} S$. So $C_{u} 2 S$ can not be reduced by carbon or...

### What is the role of depressant in froth-floatation process?

Solution 2: The role of depressant is to prevent one type of sulphide ore particles from forming the froth with air bubbles. $\mathrm{NaCN}$ is used as a depressant to separate lead sulphide (PbS)...

### Copper can be extracted by hydrometallurgy but not zinc. Explain.

Solution Copper can be extracted by hydrometallurgy but not zinc, this is because $E_{Z n^{2}+Z n}^{o}=-0.76 \mathrm{~V}$ lower than that of $E_{\mathrm{Cu}^{2+} / C i}^{o}=-0.34 \mathrm{~V}$ Hence,...

### Is it true that under certain conditions, Mg can reduce and can reduce ? What are those conditions?

Solution Yes, below $1350^{\circ} \mathrm{C}, \mathrm{Mg}$ can reduce $\mathrm{Al}_{2} \mathrm{O}_{3}$ and above $1350^{\circ} \mathrm{C}, \mathrm{Al}$ can reduce $\mathrm{MgO}$. This can be...

### The reaction, is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?

Solution This is explained on the basis of $\mathrm{Keq}$, the equilibrium constant. In the given redox reaction, all reactants and products are solids at room temperature, so, there is no...

### What is the significance of leaching in the extraction of aluminium?

Solution 2: Aluminium contains silica $\left(\mathrm{SiO}_{2}\right)$, iron oxide $\left(\mathrm{Fe}_{2} \mathrm{O}_{3}\right)$ and titanium oxide $\left(\mathrm{TiO}_{4}\right)$ as impurities....

### Why is glass considered a super cooled liquid?

Ans: Similar to liquids, glass has a tendency to flow, though very slowly. Therefore, glass is considered as a super cooled liquid. This is the reason that glass windows and doors are slightly...

### Quality factor of series resonance circuit is given by A B c) D

Correct option is D $\frac{\mathrm{f}_{0}}{\mathrm{f}_{2}-\mathrm{f}_{1}}$ Quality factor is a relation between stored energy and energy dissipation in a device or system. As per definition of...

### What is Q-factor? Write its expression and write the conditions for its maximum value.

Q-factor: In LCR Circuit, the ratio of resonance frequency to the difference of its neighboring frequencies so that their corresponding current is $1 / \sqrt{2}$ times of the peak value, is called...

### Why do solids have a definite volume?

Ans: The intermolecular forces of attraction that are present in solids are very strong, The constituent particles of solids have fixed positions i.e., they are rigid. Hence, solids have a definite...

### Why are solids rigid?

Ans: The intermolecular forces of attraction that are present in solids are very strong. The constituent particles of solids cannot move from their positions i.e., they have fixed positions....

### Using Biot-Savart’s law, obtain an expression for the magnetic field at a distance r meter from an infinitely long wire carrying a current of i ampere.

Biot savart law is the base of all magnetism and the effects of current. It states that the magnetic field intensity due to an element of a current-carrying wire at a point near the element is...

### The principle on which transformer works is? A Mutual induction B Self induction C Electromagnetic induction D None of these

The correct option is A Mutual induction Mutual inductance is where the magnetic field generated by a coil of wire induces a voltage in an adjacent coil of wire. A transformer is a device...

### Calculate the mole fraction of benzene in solution containing by mass in carbon tetrachloride.

Solution 2: $30 \%$ by mass of $\mathrm{C} 6 \mathrm{H} 6$ in $\mathrm{CCl} 4 \Rightarrow 30 \mathrm{~g} \mathrm{C} 6 \mathrm{H} 6$ in $100 \mathrm{~g}$ solution $\therefore$ Number of moles of...

### Calculate the mass percentage of benzene and carbon tetrachloride if of benzene is dissolved in of carbon tetrachloride.

Solution Mass percentage of  \begin{array}{l} C_{6} H_{6}=\frac{\text { Mass of } \mathrm{C}_{6} \mathrm{H}_{6}}{\text { Total mass of the solution }} \times 100 \% \\ =\frac{\text { Mass of }...

### Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous.

(i) Tetra phosphorus decoxide $\left(\mathrm{P}_{4} \mathrm{O}_{20}\right)$ (ii)Ammonium phosphate $\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4}$ (iii) $\mathrm{SiC}$ (iv) $\mathrm{I}_{2}$ (v)...

### What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?

Ans: The arrangement of the constituent particles makes glass different from quartz. In glass, the constituent particles have short range order, but in quartz, the constituent particles have both...

### Define the term ‘amorphous’ give a few examples of amorphous solids.

Ans: Amorphous solids are the solids that have their constituent particles of irregular shape and have short range order. These solids are isotropic in nature and melt over a range of temperature....

### How are synthetic defergents better than soup?

Solution Soaps work in soft water. However, they are not effective in hard water. In contrast, synthetic detergents work both in soft water and hard water. Therefore, synthetic detergents are better...

### What problem arises in using alitame as artificial sweetener?

Solution Alitame is a high potency sweetener. It is difficult to control the sweetness of food while using alitame as an artificial sweetener.

### Name a sweetening agent used in the preparation of sweets for a diabetic patient

Solution Artificial sweetening agents such as saceharin, alitame, and aspartame can be used in preparing sweets for diabetie paticats. Question 19: What problem arises in using alitame as artificial...

### What are artificial sweetening agents? Give two examples.

Solution Artificial sweetening agents are chemicals that sweeten food. However, unlike natural sweeteners, they do not add calories to our body. They do not harm the human body. Some artificial...

### Why is use of aspartame limited to cold foods and drinks?

Solution Aspartame becomes unstable at cooking temperature. This is the reason why its use is limited to cold foods and drinks.

### What are food preservatives?

Solution Food preservatives are chemicals that prevent food from spoilage due to microbial growth. Table salt, sugar, vegetable oil, sodium benzoate (ChHsCOONa), and salts of propanoic acid are...

### What is tincture of iodine? What is its use?

Solution Tincture of iodine is a $2-3$ percent solution of wodine in alcohol - water mixture, It is applied to wounds as an antiseptic.

### What are the main constituents of dettol?

Solution The main constituents of dettol are chloroxylenol and a-terpineol. CC1=CC(O)=CC(C)C1Cl CC1=CCC(C(C)(C)Br)CC1 Chatisiars a-ticyanat

### Name a substance which can be used as an antiseptic as well as disinfectant.

Solution Phenol can be used as an antiseptic as well as a disinfectant. $0.2$ peroent solution of phenol is used as an antiseptic, while 1 per cent of its solution is used as a disinfectant.

### Why are cimetidine and ranitidine better antacids than sodium hydrogen carbonate or magnesium or aluminium hydroxide?

Solution Cimetidine and rantidine are better antacids as they control the root eause of acidity. These drugs prevent the interaction of histamine with the receptors present in the stomach walls....

### How do antiseptics differ from disinfectants? Give one example of each.

Solution Antiseptics and disinfectants are effective against micro-organisms. However, antiseptics are applied to the living tisswes such as wounds, cuts, ulcers, and diseased skin surfaces, while...

### What is meant by the term ‘broad spectrum antibiotics’? Explain.

Solution 9: Antibiotics that are effective against a wide range of gram-positive and gram-negative bacteria are known as broad spectrum antibiotics. Chloramphenicol is a broud spectrum antibiotic....

### Low level of noradrenaline is the cause of depression. What types of drugs are needed to cure this problem? Name two drugs

Solution \&: Anti-depressant drugs are needed to counteract the effect of depression. These drugs inhibit Enzymes catalysing the degradation of the neurotransmitter, noradrenaline. As a result, the...

### While antacids and antiallergic drugs interfere with the function of histamines, why do these not interfere with the function of each other?

Solution Specific drugs affect particular receptors. Antacids and anti-allergie drugs work on different receptors. This is the reason why antacids and anti-allergic drugs do not interfere with each...

### Which foroes are involved in holding the drugs to the active site of enzymes?

Solution Either of the following forces can be involved in holding drugs to the active sites of enzymes, (i) Ionic bonding (ii) Hydrogen bonding (iii) Dipole - dipole interaction (iv) van der Waals...

### Define the tern chemotherapy.

Solution The use of chemicals for therapeutic effect is called chemotherapy. For example: the use of chemicals in the diagnosis, prevention, and treatment of diseases

### Name the macromolecules that are chosen as drug targets.

Solution The macromolecules chosen as drug targets are carbohydrates, lipids, proteins, and nucleic acids,

### Explain the term target molecules or drug targets as used in medicinal chemistry.

Solution In medicinal chemistry, drug targets refer to the key molecules involved in certain metabolic pathways that result in specific diseases, Carbohydrates, proteins, lipids, and nucleic acids...

### Why do we need to classify drugs in different ways?

Solution The classification of drugs and the reasons for classification are as follows: (i) On the basis of pharmacological effect: This classification provides doctors the whole range of drugs...

### Write the IUPAC names of the following ketones and aldehydes. Whenever possible, give also common names.

(i) $\mathrm{CH}_{3} \mathrm{CO}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CH}_{3}$ Ans: The IUPAC name of the given compound is Heptan-2-one and its common name is Methyl n-pentyl ketone. (ii)...