Physics

Thermal Properties and Thermodynamics

380 Questions

Thermal properties and thermodynamics questions evaluate concepts of heat transfer, thermal efficiency, and temperature variations. Problems involve calculating heat content, conductivity, and the performance of heat engines. This subject is regularly tested in physics sections across multiple competitive platforms.

Heat transfer calculationsThermal efficiencyBlack body radiationTemperature variationsRefrigeration performance

Thermal Properties and Thermodynamics Questions

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

An ideal heat engine has an efficiency $ \eta$ . The co-efficient of performance of the engine when driven backward is 

  1. $1-\dfrac{1}{\eta}$
  2. $\dfrac{\eta}{1- \eta}$
  3. $\dfrac{1}{\eta}-1$
  4. $\dfrac{1}{1- \eta}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Efficiency n = W/Q1 = (Q1-Q2)/Q1. Coefficient of performance (COP) for a refrigerator is Q2/W. Since W = Q1 - Q2, COP = Q2/(Q1-Q2). Dividing by Q1 gives (Q2/Q1) / (1 - Q2/Q1). Since n = 1 - Q2/Q1, Q2/Q1 = 1 - n. Substituting gives (1-n)/n = (1/n) - 1.

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

A heat engine absorbs $Q _1$ heat from hot reservoir and work produced by engine is $W$, then:

  1. $Q _1$ is always $= W$
  2. only in some special cases $Q _1 = W$ otherwise $Q _1$ is greater than $W$
  3. $Q _1$ is always less than $W$
  4. $Q _1$ is always greater than $W$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In a heat engine , if $Q _{1}$ is the heat absorbed from reservoir , $W$ be the work done by engine and let $Q _{2}$ be the heat given to sink , then by the first law of thermodynamics 

       $Q _{1}-Q _{2}=W$    (internal energy$dU=0$ for cyclic process)
this equation gives that $Q _{1}$ is always greater than $W$ .

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

Which of the following is correct for the efficiency of a heat engine:

  1. $\eta=\dfrac{W}{Q _1}$
  2. $\eta=\dfrac{W}{Q _2-Q _1}$
  3. $\eta=\dfrac{W}{Q _2}$
  4. $\eta=\dfrac{Q _2}{Q _1}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Efficiency of a heat engine is defined as the ratio of net work done per cycle by the working substance ($W$) to the heat absorbed per cycle from the source ($Q _{1}$) ,

          $\eta=\frac{W}{Q _{1}}$ 

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

A household refrigerator with a coefficient of performance $1.2$ removes heat from the refrigerated space at the rate of $60kJ/min$. What would be cost of running this fridge for one month (30 days) (assuming each day it is used for $4$ hours and cost of one electrical unit is $6$ Rs.)

  1. $180$ Rs.
  2. $300$ Rs.
  3. $480$ Rs.
  4. $600$ Rs.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Heat removed = 60 kJ/min. Work input = Heat / COP = 60 / 1.2 = 50 kJ/min. Total time = 30 days * 4 hours/day * 60 min/hour = 7200 min. Total work = 50 kJ/min * 7200 min = 360,000 kJ. 1 unit = 1 kWh = 3600 kJ. Units = 360,000 / 3600 = 100 units. Cost = 100 units * 6 Rs/unit = 600 Rs.

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

An inventor claims to have developed an engine that takes in $1000\  J$ of heat and produces $1500\  J$ of work during each cycle. Comment on the validity of this claim.

  1. The proposed engine claims to produce more work in a cyclic process than the amount of heat that is supplied, so it is in violation of the first law of thermodynamics.

  2. The statement is completely valid as it increases the efficiency by increasing the work output.

  3. The proposed engine claims to produce more work in a cyclic process than the amount of heat that is supplied, so it is in violation of the zeroth law of thermodynamics.

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

An inventor claims to have developed an engine that takes in 1000 J of heat and produces 1500 J of work during each cycle.The proposed engine claims to produce more work in a cyclic process than the amount of heat that is supplied, so it is in violation of the first law of thermodynamics.

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems. The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed. The first law is often formulated.
${\displaystyle \Delta U=Q-W.}$

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

A heat engine takes in $700\  J$ of heat from high-temperaturere reservoir and rejects $500\  J$ of heat to a lower temperature reservoir. How much work does the engine do in each cycle?

  1. $100\ J$
  2. $20\ J$
  3. $200\ J$
  4. $10\ J$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given, $Q _h=700 J$ and $Q _c=500 J$

The work done by a heat engine is given by $W=Q _h=Q _c=700-500=200 J$

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

Calculate the efficiency of a Carnot engine operating between temperatures of 900 K and 300 K.

  1. $87$ %
  2. $67$ %
  3. $100$ %
  4. $45$ %
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given, the temperature of hot reservoir, $T _h=900 K$ and temperature of cold reservoir, $T _c=300 K$

Thus, the efficiency of the Carnot engine is $\eta=(1-T _c/T _h)\times 100=(1-300/900)\times 100=66.67 $ % $\sim 67$ % 

Multiple choice physics isothermal and adiabatic processes work done by an ideal gas in isothermal expansion thermodynamic processes heat and thermodynamics

The slope of adiabatic curve is ________ than the slope of an isothermal curve.

  1. Greater.

  2. lesser

  3. data insufficient

  4. can be both a and b
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The slope of an adiabatic process is given by -gamma * P/V, while the slope of an isothermal process is -P/V. Since gamma (Cp/Cv) is always greater than 1 for an ideal gas, the adiabatic slope is steeper (greater in magnitude).

Multiple choice physics transfer of heat applications of heat conduction applications of insulation clothes - our necessity

Four rods of same material but with different radii and lengths are used to connect two reservoirs of heat with the same temperature difference. Which one will conduct more heat

  1. r=1 cm, 1=1 cm

  2. r=1 cm, 1= 2 cm
  3. r= 1 cm, $1=\frac{1}{2}m$
  4. $r=\frac{1}{2}cm$, $1=\frac{1}{2}m$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Heat conduction rate H = k*A*(delta T)/L. Since material and delta T are constant, H is proportional to A/L, or (r^2)/L. Calculating r^2/L for each: A) 1/1=1, B) 1/2=0.5, C) 1/0.5=2, D) 0.25/0.5=0.5. Option C gives the highest value.

Multiple choice physics transfer of heat applications of heat conduction applications of insulation clothes - our necessity

In a steady state of thermal conduction, temperature of the ends $A$ and $B$ of a $20cm$ long rod are $100^oC$ and $0^oC$, respectively. What will be the temperature of the rod at a distance of $6cm$ from the end $A$ of the rod?

  1. $-30^oC$
  2. $70^oC$
  3. $5^oC$

  4. None of the above
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In steady state, the temperature gradient is linear. Temperature T(x) = T_A - (T_A - T_B) * (x / L). T(6) = 100 - (100 - 0) * (6 / 20) = 100 - 100 * 0.3 = 100 - 30 = 70 degrees Celsius.

Multiple choice physics heat - measurement introduction to temperature application of various thermometric scales different types of thermometers

The following three objects (1) a metal tray, (2) a block of wood,and (3) a wooden cap are left in a closed room overnight. Next day the temperature of each is recorded as $T _1, T _2$ and $T _3$ respectively. The likely situation is 

  1. $ T _1 = T _2 = T _3 $
  2. $ T _3 > T _2 > T _1 $
  3. $ T _3 = T _2 > T _1 $
  4. $ T _3 > T _2 = T _1 $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

All three will be inthermal equilibrium with air of room. so temprature of the three will be same

Multiple choice physics heat - measurement introduction to temperature application of various thermometric scales different types of thermometers

An iron ball at $40^oC$ is dropped in a mug containing water at $40^o$C.

The heat will

  1. flow from iron ball to water

  2. not flow from iron ball to water or from water to iron ball.

  3. flow from water to iron ball.

  4. increase the temperature of both.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Thermal Equilibrium - A system is said to be in thermal equilibrium when there is no temperature difference between system and surroundings.

An iron ball at $40^oC$ is dropped in a mug containing water at $40^oC.$

The heat will not flow from iron ball to water or from water to iron ball.

Multiple choice physics heat - measurement introduction to temperature application of various thermometric scales different types of thermometers

A piece of lead falls from height of $100m$ on a fixed non-conducting slab which brings it to rest. if the specific heat of lead is $30.6 cal/kg ^ \circ C $ the increase in temperature of the slab immediately after collision is  

  1. ${6.72^ \circ }C$
  2. ${7.62^ \circ }C$
  3. ${5.62^ \circ }C$
  4. ${8.72^ \circ }C$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Potential energy lost (mgh) is converted into heat (ms deltaT). Using g = 9.8 m/s^2, mgh = m * 30.6 * 4.184 * deltaT (converting cal to Joules). The mass cancels out, leading to deltaT = (9.8 * 100) / (30.6 * 4.184) approx 7.65 C.

Multiple choice physics heat - measurement introduction to temperature application of various thermometric scales different types of thermometers

Two identical rods of a mental are welded in series then 20 cal of heat flows through them in  4 minute. If the rods are welded in parallel then same amount of heat will flow in 

  1. 1 minute

  2. 2 minute

  3. 4 minute

  4. 15 minute
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$dq/dt = [(AK)/2x][T2-T1] = 20/4 = 5.$

 $Therefore ( AK/x)(T2-T1) = 10$

When they are one above the other,

$dq/dt = 2Ak/x][T2-T1] =20/t$

Therefore$t = 2\times10  = 20/t$

Therefore$ t = 1 min$