Kirchoff's Law

Q. The emissive power of a black body at $T=300\text{K}$ is $100{\text{W/m}}^2$. Consider a body $B$ of area $A=10{\text{m}}^2$, coefficient of reflectivity $r=0.3$ and coefficient of transmission $t=0.5$. Its temperature is $300\text{K}$. Then choose the correct statement.

1. The emissive power of $B$ is $20{\text{W/m}}^2$.
2. The emissive power of $B$ is $200{\text{W/m}}^2$.
3. The power emitted by $B$ is $100\text{watts}$.
4. The emissivity of $B$ is $0.3$.

Q.

What is emissivity formula?

Q. Statement 1 : Emissivity of any body $\left(e\right)$ is equal to its absorptivity power $\left(a\right)$ at thermal equilibrium.
Statement 2 : The ratio of emissive power to the absorptive power is different for different bodies at a given temperature.

1. Statement 1 is true, statement 2 is false
2. Statement 1 is false, statement 2 is true
3. Both Statement 1 and statement 2 are true
4. Both Statement 1 and statement 2 are false

Q. The graph shown in the figure, represents the fall of temperature $\left(T\right)$ of two bodies $x$ and $y$ having the same surface area, with time $\left(t\right)$ due to radiation. Choose the correct relationship between emissive power $\left(E\right)$ and absorptive power $\left(a\right)$ of the two bodies.

1. $E_x>E_y;a_x<a_y$
2. $E_x>E_y;a_x>a_y$
3. $E_x<E_y;a_x<a_y$
4. $E_x<E_y;a_x>a_y$

Q.

Heat always flows from higher temperature to lower temperature.

1. True
2. False

Q. A device in which heat loss due to conduction, convection and radiation is minimized is :

1. solar cooker
2. thermos flask
3. thermometer
4. electric heater

Q.

4. The amount of heat energy required to increase the temperature of X g of water by 10⁰C is found to be 15 cal. Calculate X.

1. 0.5 g

2. 15 g

3. 1.5 g

4. 7.5 g

Q. Statement 1 : Emissivity of any body $\left(e\right)$ is equal to its absorptivity $\left(a\right)$ at thermal equilibrium.
Statement 2 : The ratio of emissive power to the absorptive power is different for different bodies at a given temperature.

1. Statement 1 is true, statement 2 is false
2. Statement 1 is false, statement 2 is true
3. Both Statement 1 and statement 2 are true
4. Both Statement 1 and statement 2 are false

Q. A person with black clothes experiencing more heat and cold as compared to a person with white clothes is an application of

1. Kirchhoff’s law
2. Lambert’s law
3. Planck’s law

Q. A metal ball of mass 1 kg is heated by means of a 20 W heater in a room at 20°C. The temperature of the ball becomes steady at 50°C. (a) Find the rate of loss of heat to the surrounding when the ball is at 50°C. (b) Assuming Newton's law of cooling, calculate the rate of loss of heat to the surrounding when the ball rises 30°C. (c) Assume that the temperature of the ball rises uniformly from 20°C to 30°C in 5 minutes. Find the total loss of heat to the surrounding during this period. (d) Calculate the specific heat capacity of the metal.

Q. The emissive power of a blackbody at $T=300K$ is $100W/m^2$. Consider a body B of area $A=10m^2$ coefficient of reflectivity $r=0.3$ and coefficient of transmission $t=0.5$. Its temperature is $300K$. Then which of the following is correct?

1. The emissive power of B is $20$ $W/m^2$
2. The emissive power of B is $200$ $W/m^2$
3. The power emitted by B is $100$ Watts
4. The emissivity of B is $0.3$

Q. The emissive power of a black body at $T=300\text{K}$ is $100{\text{W/m}}^2$. Consider a body $B$ of area $A=10{\text{m}}^2$, coefficient of reflectivity $r=0.3$ and coefficient of transmission $t=0.5$. Its temperature is $300\text{K}$. Then choose the correct statement.

1. The emissive power of $B$ is $20{\text{W/m}}^2$.
2. The emissive power of $B$ is $200{\text{W/m}}^2$.
3. The power emitted by $B$ is $100\text{W}$.
4. The emissivity of $B$ is $0.2$.

Q. What is the ratio of 1 KWh (kilowatt hour) to 1 MeV (million electron volt)?

1. $2.25\times {10}^{19}$
2. $2.25\times {10}^{17}$
3. $2.25\times {10}^{23}$
4. $2.25\times {10}^{12}$

Q.

The graph shown in the adjacent diagram, represents the variation of temperature (T) of two bodies, x and y having same surface area, with time (t) due to the emission of radiation. Find the correct relation between the emissivity ($e$) and absorptivity ($a$) power of the two bodies. (IIT JEE 2003)

1. $e_x>e_y,a_x>a_y$

2. $e_x>e_y,a_y>a_x$

3. $e_y>e_x,a_y>a_x$

4. $e_y>e_x,a_x>a_y$

Q. State whether given statement is True or False
Shining bodies are good absorbers and good radiators of heat

1. True
2. False

Q. Good absorbers of heat are :

1. poor emitters
2. non-emitters
3. good emitters
4. highly polished

Q. The graph shown in the figure, represents the fall of temperature $\left(T\right)$ of two bodies $x$ and $y$ having the same surface area, with time $\left(t\right)$ due to radiation. Choose the correct relationship between emissive power $\left(E\right)$ and absorptive power $\left(a\right)$ of the two bodies.

1. $E_x>E_y;a_x<a_y$
2. $E_x<E_y;a_x>a_y$
3. $E_x>E_y;a_x>a_y$
4. $E_x<E_y;a_x<a_y$

Q. A heated body emits radiation which has maximum intensity at frequency $v_m$. If the temperature of the body is doubled:

1. the maximum intensity radiation will be at frequency $2v_m$
2. the maximum intensity radiation will be at frequency $\frac{1}{2}{v}_m$
3. the total emitted energy will increase by a factor 16
4. the total emitted energy will increase by a factor 2

Q.

The graph shown in the adjacent diagram, represents the variation of temperature (T) of two bodies, x and y having same surface area, with time (t) due to the emission of radiation. Find the correct relation between the emissivity ($e$) and absorptivity ($a$) power of the two bodies. (IIT JEE 2003)

1. e_y>e_x, a_x> a_y

2. e_x>e_y, a_x> a_y

3. e_x>e_y, a_y> a_x

4. e_y>e_x, a_y> a_x

Q. The graph shown in the figure, represents the fall of temperature $\left(T\right)$ of two bodies $x$ and $y$ having the same surface area, with time $\left(t\right)$ due to radiation. Choose the correct relationship between emissive power $\left(E\right)$ and absorptive power $\left(a\right)$ of the two bodies.

1. $E_x>E_y;a_x<a_y$
2. $E_x<E_y;a_x>a_y$
3. $E_x>E_y;a_x>a_y$
4. $E_x<E_y;a_x<a_y$

Q. What is the relation between emissive power of a real body $\left(E\right)$ and emissive power of black body $\left(E_b\right)$ when both are at same temperature? ; $\alpha =\text{absorptivity}$

1. $\frac{E_b}{E}=1$
2. $\frac{E_b}{E}=\alpha$
3. $\frac{E}{E_b}=1$
4. $\frac{E}{E_b}=\alpha$

Q. Heat and work are equivalent. This means,

(a) when we supply heat to a body we do work on it
(b) when we do work on a body we supply heat to it
(c) the temperature of a body can be increased by doing work on it
(d) a body kept at rest may be set into motion along a line by supplying heat to it

Q.

Which one of the following statements is not true about thermal radiations?

1. Thermal radiations travel in free space with a velocity of

2. All bodies emit thermal radiations at all temperatures

3. Thermal radiations are electromagnetic waves

4. Thermal radiations are not reflected from a mirror

Q. For an opaque body coefficient of transmission is:

1. 1
2. 0.5
3. zero
4. $\infty$

Q. State whether given statement is True or False
The bottom part of cooking utensils are polished

1. True
2. False

Q. The window shades of rooms are pulled down so that a layer of still air is formed near the window to reduce the heat loss by

1. Conduction
2. Convection
3. Radiation
4. All

Q.

The graph shown in the adjacent diagram, represents the variation of temperature (T) of two bodies, x and y having same surface area, with time (t) due to the emission of radiation. Find the correct relation between the emissivity ($e$) and absorptivity ($a$) power of the two bodies. (IIT JEE 2003)

1. $e_x>e_y,a_x>a_y$

2. $e_x>e_y,a_y>a_x$

3. $e_y>e_x,a_y>a_x$

4. $e_y>e_x,a_x>a_y$

Q.

name the heat energy given out by hot bodies in the form of infrared rays