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Stefan's law All objects radiate energy continuously in the form of electromagnetic waves. The rate at which an object radiates energy is proportional to the fourth power of its absolute temperature. This is known as the Stefan's law and is expressed in equation form as - [   I = σeT⁴   ] Here I is the intensity of radiation in W/m² radiated by the object, T is the absolute temperature in kelvin (K), e is a fraction between 0 and 1 called the emissivity of the object and σ is a universal constant called Stefan's constant, which has the value - σ = 5.67 × 10-⁸ W/m²/K⁴ This law is also known as Stefan-Boltzmann law. Confusion in emissivity (e) ? I can explain- Emissivity The ratio of emissive power (E) of a body to the emissive power (ε) of a black body (a body that absorbs all the radiations falling on it) at same temperature is called emissivity. It is denoted by e. Thus, emissivity -            [ e = E/ε ] It is a dimensionless quantity. (i) For highly polished body, e

Best of Luck Q1. Hairs of shaving brush cling together when it is removed from water due to- (a) force of attraction between hair (b) surface tension (c) viscosity of water (d) characteristic property of hair Q2. Coatings used on raincoat are waterproof because they - (a) increases angle of contact (b) decreases angle of contact (c) does not alters angle of contact (d) forms a smooth surface Q3. A body measures 5 N in air and 2 N when put in water. The buoyant force is- (a) 7 N (b) 9 N (c) 3 N (d) None of these Q4. A hole is made at the bottom of the tank filled with water (density = 1000 kg/m³). If the total pressure at the bottom of the tank is three atmospheres (1 atmosphere = 10⁵ N/m²), then the velocity of efflux is nearest to- (a) 20 m/s (b) 10√2 m/s (c) 20√2 m/s (d) √500 m/s Q5. When the temperature is increased the angle of contact of a liquid? (a) increases (b) decreases (c) remains the same (d) first increases and then decreases Q6. The surface tension of a soap solution is 2

Electricity - Extra questions Q1. Find the heat generated in a wire of resistance 100Ω connected to a constant voltage supply of 80 volts for 15 seconds. Q2. An electric iron of rating 200V, 4A is connected to a suitable 200V voltage source. If a fan with power consumption 100W is connected to the iron with an independent switch. Calculate - (i) Current for the entire circuit. (ii) Resistance of the entire circuit. Q3. Find the resistivity of a wire with resistance 200Ω, length 2 m and having cross-sectional area 2.5 mm². Q4. Which has more resistivity : a thin copper wire at 30°C or a thick copper wire at 30°C? Q5. Which has more resistivity : a copper wire at 30°C or an aluminium wire at 30°C? Q6. Which has more resistivity : a copper wire at 30°C or a copper wire at 42°C with same length and diameter? Q7. Calculate the resistance of a wire with resistivity 2 × 10-⁶ Ωm, length 5 m and diameter 2 mm. Take, π = 3.14 Q8. How many 10 Ohms resistors are required to make an equivale

Before the beginning, first discuss about aliens. Aliens are those creatures which do not belong to our world. Now a question comes to your mind. Can we contact aliens? The answer is most probably NO. But why? Because first we have to find that type of planets (or natural satellites) which can support life. This is not an easy task. Also, a planet which can support life is not enough. Why? Because life can exist in various forms. One form is unicellular organisms. For example, in our solar system it is believed that Titan (Saturn's moon) can support unicellular life. But the disadvantage is we cannot contact them by our communication methods. But yes, there is a high possibility in future that a rover mission can take some samples of the unicellular or may be small multicellular organisms and send the data to Earth. Now comes the interesting part. If a planet is supporting complex life like animals, plants or even species like us (humans), then also there are very very

Living on Mars... worse than we think From last two decades Mars has been a prominent candidate to settle humans in our solar system other than our home planet Earth. But there lot of things about Mars and complex life that an average person hardly knows. Now, lets dig deep in to this topic and discuss what would be the big problems in future when humans want to live on Mars permanently. 1. Low Gravity : The red planet has about 38% surface gravity compared to Earth's surface. What's a big deal in this? The answer is that humans will face heavy muscle and bone density loss because their bodies will tell their brains that they require very less bone density and muscles to walk, jump or to do any other physical activity. So those will live on Mars have to do very heavy weight training sessions on a regular basis, just only to survive on Mars. 2. Helplessness : From  our current technology, it takes about seven to eight months to reach Mars from Earth.  If we upgrade

Moving air is called wind. Wind has energy. The energy possessed by wind is due to its high speed. So we can say the wind possesses kinetic energy . It is the kinetic energy of wind which is utilized for doing work. Solar energy or sun's energy is responsible for the blowing of wind, due to the uneven heating of earth by the sun in different regions. Now let's discuss about what is a wind mill. Wind mill The energy of wind is harnessed by using a windmill. A windmill consists of big sized table fan like blades which are fixed over the top of a tall pole in such a way that they are free to rotate when the fast moving wind strikes on the blades of the windmill. It makes the blades rotate continuously. The rotatory motion of the windmill is then used to do Mechanical work through a shaft connected to the rotating blades. The traditional use of wind energy has now been modified by the improvement in technology to generate electricity through wind powered generators. Wind Generator

Capacitors Numericals Q1. Two metal plates having charges Q , -Q face each other at some separation and are dipped into an oil tank, if the oil is pumped out, the electric field between the plates will _____. Q2. Three capacitors of capacitance 6 μF each are available. The minimum and maximum capacitances, which may be obtained are respectively _____. Q3. The plates of a parallel plate capacitor are made of circular discs of radii 5 cm each. If the separation between the plates is 1 mm . What is the capacitance of the capacitor? Ans. 6.95 × 10-⁵ μF Q4. Three capacitors having capacitances 20 μF, 30 μF and 40 μF are connected in series with a 12V battery. Find the charge on each of the capacitors. How much work has been done by the battery in charging the capacitors? Ans. 110 μC on each approximately, 1.32 milli Joule Q5. The distance between the plates of a parallel plate capacitor is 4 cm . A field of 5,000 volt per metre is established between the plates and an uncharged metal pl

Q1. How much energy is needed to relocate an electron from first excited state to fourth excited state in an Hydrogen atom? Also, find the type of light needed in this process. Q2. Find the wavelength of light released when an electron jumped from 4th excited state to ground state in an Hydrogen atom. Also, name the series. Q3. Find the wavenumber of light released when an electron jumped from 5th excited state in bracket series (in H - atom). www.advguruji.blogspot.com Q4. An electron absorbed 2.55 eV energy and jumped from first excited state to nth excited state. Find n . Q5. when an electron jumped from second excited state to some different state, UV ray was released. Name the state and find the frequency of the UV ray. --------------------------------------------------------------------------------------------------- ANSWERS Ans1. 2.86 eV Ans2. 99 nm Ans3. 0.375 × 10⁶ m-¹ Ans4. n = 3 Ans5. 2.90 × 10¹⁵ Hz by    www.advguruji.blogspot.com

  Q1. Melting and boiling are examples of - A. adiabatic process B. isobaric process C. isothermal process D. cyclic process Q2. For an isothermal process the change in internal energy is - A. positive B. negative C. zero D. infinite Q3. Specific heat capacity of any gas for isothermal process is - A. zero B. infinite C. 1 D. log10⁵ Q4. For a cyclic process the change in internal energy is - A. positive B. negative C. Zero D. infinite Q5. Which is not an extensive variable ? A. Pressure B. mass C. volume D. internal energy Q6. First law of thermodynamics is based on - A. Law of conservation of momentum B. Law of conservation of angular momentum C. Law of conservation of mass D. None of these Q7. The slope of pressure volume curve in an adiabatic process is - A. Equal to the slope of isothermal process B. 'γ' times the slope of isothermal process C. "1/γ" times the slope of isothermal process D. Negative to the slope of isothermal process Q8. An ideal gas of volume 0.

Q1. Calculate the percentage decrease in the weight of a body when it is taken to a height of 40 km above the Earth's surface. Radius of Earth = 6,400 km Ans. 1.25% Q2. Calculate the depth below the surface of Earth where the acceleration due to gravity is 25% of its value at the Earth's surface. Take, radius of Earth = 6,400 km. Ans. 4.8 × 10⁶ m Q3. A body weighs 100N on Earth's surface. Calculate the gravitational force on the body at a height equal to one-fourth the radius of Earth. Ans. 64 N Q4. Calculate the percentage decrease in weight of a body when taken to a tunnel 38.4 km below the Earth's surface. Ans. 0.6% Q5. Calculate the height from the Earth's surface at which the acceleration due to gravity becomes 75% compared at Earth's surface. Take, radius of Earth = 6,400 km. Ans. 990 km ---------------------------------------------------------------------------------------------- EXTRA QUESTIONS Q6. Find the percentage increase in acceleration due to grav

Electric Current It is defined as the rate of flow of positive charge. It has the direction towards the motion of positive charge and direction opposite to the motion of negative charge. It is denoted by I. Its SI unit is ampere(A) or C/s. It is of two types :- Direct current [DC] Alternating current [AC] Direct Current ( DC) The current which has a same direction of flow. It is of two types : 1. fluctuating current The current whose magnitude changes again and again is called fluctuating current. 2. Steady current or ideal current The current whose magnitude do not changes with time is called steady current. Alternating current (AC) The current which changes its direction of flow periodically. (In detail, chapter 7) On the basis of calculation Current is of two types : 1. Average current [ I = q/t ] It is defined as the ratio of total charge flow to the total time of flow. 2. Instantaneous current [ I = dq/dt ] It is defined as the ratio of small change in c

Q1. Two isolated spherical conductors having radii 5 cm and 10 cm. They have charges of 12nC and -3nC respectively. Find the charges after they are connected by a conducting wire. Also, find the common potential after redistribution. Ans. 3 nC,  6 nC,  540V Q2. Two parallel plate capacitors A and B having capacitance of 1μF and 5 μF are charged separately to the same potential of 100V. Now, the positive plate of A is connected to the negative plate of B, and the negative plate of A is connected to the positive plate of B. Find the final charges on each capacitor. Ans. (200/3 μC),  (1000/3) μC This is the video for the Lovely students CLICK HERE by    www.advguruji.blogspot.com

Extra information on capacitors Q:1 Why there is low potential difference between the two conductors of a capacitor? Ans. As we know,              C = q/V So, for large capacitance of a capacitor and large amount of charge storing in a capacitor, V must be low. Also, in practicality if 'V' is large it will be result to a strong electric field between the conductors which can ionize the medium between the conductors resulting to acceleration in the charges towards the opposite charges and the charges inside the capacitor can fully or partially leaks away. Q:2 What is a capacitor? Ans. A capacitor is an energy storing device which stores energy in the form of charge hold inside the capacitor. Q:3 What is dielectric strength? Ans. It is the maximum amount of electric field of a dielectric medium so that the medium will not ionize. For example, dielectric strength of air = 3 × 10⁶ N/C. Pyaare bacchon, tumhari help   k liye mene video daali hai Q:4 List some uses of

Cross multiplication method This can be generally written as - For previous class questions CLICK HERE by      www.advguruji.blogspot.com

Q1. A fraction is such that if the numerator is multiplied by 3 and the denominator is reduced by 3, we get 18/11, but if the numerator is increased by 8 and the denominator is doubled, we get 2/5. Find the fraction. Ans.   12/25 Q2. The numerator of a fraction is 4 less than the denominator. If the numerator is decreased by 2 and denominator is increased by 1, then the denominator is eight times the numerator. Find the fraction. Ans.  3/7 Q3. A fraction becomes 1/3 if 1 is subtracted from both its numerator and denominator. If 1 is added to both the numerator and denominator, it becomes 1/2. Find the fraction. Ans.  3/7 BEST OF LUCK.. to my dear students.. by           www.advguruji.blogspot.com

Consider, an electric dipole of length 2a having charges +q and -q . A point P is at a distance r from the centre of the dipole such that OP is making an angle θ with dipole moment p . CONSTRUCTION :- Extend OP to BP such that BC is perpendicular to BP, Draw AD perpendicular to BP. PROOF :- Now, β = r + aCosθ      .........(1)          α = r + aCosθ      .........(2) Let, 'V1' and 'V2' be the potential due to +q and -q respectively. Net electric potential at 'P' is -               V = V1 + V2             V = kq/α + k(-q)/β             From equation (1) and (2)            Also,       p = q.2a Therefore,               [  V = kpCosθ/(r² - a²Cos²θ)   ] Note :- For axial line, θ = 0           For equatorial line, θ = π/2 Pyaare bacchon, tumhari help k liye video daali hai by  www.advguruji.blogspot.com

Best of Luck STUDENTS Que1. Two small conducting spheres of equal radius have charges 10μC & -20μC respectively placed at a distance R from each other. They experience force F1 . If they are brought in contact and separated to the same distance, they experience force F2 .   Ratio of F1 to F2 is ___ a) 1:2 b) 1:1 c) 2:1 d) 8:1 Pyaare bacchon, agar online study se headache hota hai toh dekh lena, aaram milega Que2. Electric field intensity at a point in between two parallel sheets with like charges of same surface charge densities σ is ___ a) σ/εo b) zero c) σ/2εo d) none of these Que3. There exists an electric field of 2 N/C along y direction. The flux passing through the square of side 1 m placed in xy plane inside the electric field is - a) Zero b) 2 Nm²/C c) 1 Nm²/C d) √3 Nm²/C Que4. A mass of 20g has a charge of 3mC . It moves with a velocity of 20m/s and enters a region of electric field of 80N/C in the same direction as the velocity of the mass. The velocity of t

Consider, an electric dipole of length '2a' having charges '+q' and '-q' placed in an uniform electric field 'E'. The dipole moment 'p' is making an angle 'θ' with the electric field. There are two equal amount of electrostatic forces acting in opposite direction on the dipole which resulting zero net force but a torque 'τ' on the dipole. As, torque = force × perpendicular distance          τ = qE × 2aSinθ For "potential energy of an electric dipole rotated in an uniform electric field" CLICK HERE by      www.advguruji.blogspot.com