McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Uncategorized topics .
| 201. |
Liquid expansion thermometers are filled with |
| A. | mercury |
| B. | amalgam |
| C. | gallium |
| D. | cesium |
| Answer» A. mercury | |
| Explanation: the liquid-expansion thermometer has the entire system filled with an organic liquid or mercury. mercury is used at a temperature range of -35 to 950 f. alcohol and creosote are used at -110 to 160 f, and 20 to 400 f respectively. | |
| 202. |
The sun shines on a 150 m2 road surface so it is at 45°C. Below the 5cm thick asphalt(average conductivity of 0.06 W/m K), is a layer of rubbles at 15°C. Find the rate of |
| A. | 5300 w |
| B. | 5400 w |
| C. | 5500 w |
| D. | 5600 w |
| Answer» B. 5400 w | |
| Explanation: there is conduction through the asphalt layer. | |
| 203. |
A water-heater is covered with insulation boards over a total surface area of 3 m2. The inside board surface is at 75°C and the outside being at 20°C and the conductivity of material being 0.08 W/m K. Find the thickness of board to limit the heat transfer loss to 200 W ? |
| A. | 0.036 m |
| B. | 0.046 m |
| C. | 0.056 m |
| D. | 0.066 m |
| Answer» D. 0.066 m | |
| Explanation: steady state conduction through board. | |
| 204. |
A large heat exchanger transfers a total of 100 MW. Assume the wall separating steam and seawater is 4 mm of steel, conductivity 15 W/m K and that a maximum of 5°C difference between the two fluids is allowed. Find the required minimum area for the heat transfer. |
| A. | 180 m2 |
| B. | 280 m2 |
| C. | 380 m2 |
| D. | 480 m2 |
| Answer» D. 480 m2 | |
| Explanation: steady conduction q = k a ∆t/∆x ⇒ Α = q ∆x / k∆Τ | |
| 205. |
The black grille on the back of a refrigerator has a surface temperature of 35°C with a surface area of 1 m2. Heat transfer to the room air at 20°C takes place with convective heat transfer coefficient of 15 W/Km^2. How much energy is removed during 15 minutes of operation? |
| A. | 202.5 kj |
| B. | 212.5 kj |
| C. | 222.5 kj |
| D. | 232.5 kj |
| Answer» A. 202.5 kj | |
| Explanation: q = ha ∆t ∆t, q = 15 × 1 × (35-20)×15×60 = 202500 j = 202.5 kj. | |
| 206. |
A small light bulb (25 W) inside a refrigerator is kept on and 50 W of energy from the outside seeps into the refrigerated space. How much of temperature difference to the ambient(at 20°C) must the refrigerator have in its heat exchanger having an area of 1 m2 and heat transfer coefficient of 15 W/Km2 to reject the leak of energy. |
| A. | 0°c |
| B. | 5°c |
| C. | 10°c |
| D. | 15°c |
| Answer» B. 5°c | |
| Explanation: total energy that goes out = 50+25 = 75 w | |
| 207. |
As the car slows down, the brake shoe and steel drum continuously absorbs 25 W. Assume a total outside surface area of 0.1 m2 with a convective heat transfer coefficient of 10 W/Km2 to the air at 20°C. How hot does the outside brake and drum surface become when steady conditions are reached? |
| A. | 25°c |
| B. | 35°c |
| C. | 45°c |
| D. | 55°c |
| Answer» C. 45°c | |
| Explanation: ∆Τ = heat / ha hence ∆t = [ Τ(brake) − 20 ] = 25/(10 × 0.1) = 25°c Τ(brake) = 20 + 25 = 45°c. | |
| 208. |
A burning wood in the fireplace has a surface temperature of 450°C. Assume the emissivity to be 1 and find the radiant emission of energy per unit area. |
| A. | 15.5 kw/m2 |
| B. | 16.5 kw/m2 |
| C. | 17.5 kw/m2 |
| D. | 18.5 kw/m2 |
| Answer» A. 15.5 kw/m2 | |
| Explanation: q /a = 1 × σ t^4 | |
| 209. |
5 cm in diameter, through which 400 W of electric energy is deposited. Assume the surface emissivity to be 0.9 and neglecting incoming radiation, find the rod surface temperature? |
| A. | 700k |
| B. | 800k |
| C. | 900k |
| D. | 1000k |
| Answer» D. 1000k | |
| Explanation: outgoing power equals electric power | |
| 210. |
A water-heater is covered up with insulation boards over a total surface area of 3 m2. The inside board surface is at 75°C and the outside surface is at 20°C and the board material has a conductivity of 0.08 W/m K. How thick a board should it be to limit the heat transfer loss to 200 W ? |
| A. | 0.066 m |
| B. | 0.166 m |
| C. | 0.266 m |
| D. | 0.366 m |
| Answer» A. 0.066 m | |
| Explanation: steady state conduction through a single layer board. | |
| 211. |
16 W/m K, with a temperature difference between the two sides of 20°C. |
| A. | 113 w/m2 |
| B. | 213 w/m2 |
| C. | 230 w/m2 |
| D. | 312 w/m2 |
| Answer» B. 213 w/m2 | |
| Explanation: . q = .q/a = k Δt/Δx = 0.16 wm /k × 20k/0.015 m = 213 w/m2. | |
| 212. |
A 2 m2 window has a surface temperature of 15°C and the outside wind is blowing air at 2°C across it with a convection heat transfer coefficient of h = 125 W/m2K. What is the total heat transfer loss? |
| A. | 2350 w |
| B. | 1250 w |
| C. | 2250 w |
| D. | 3250 w |
| Answer» D. 3250 w | |
| Explanation: .q = h a Δt = 125 w/m2k × 2 m2 × (15 – 2) k = 3250 w. | |
| 213. |
A radiant heating lamp has a surface temperature of 1000 K with ε = 0.8. How large a surface area is needed to provide 250 W of radiation heat transfer? |
| A. | 0.0035 m2 |
| B. | 0.0045 m2 |
| C. | 0.0055 m2 |
| D. | 0.0065 m2 |
| Answer» C. 0.0055 m2 | |
| Explanation: .q = εσat^4 | |
| 214. |
12 W/m K Thermal diffusivity = 5.16 * 10 -7 m2/s |
| A. | 43.07 hours |
| B. | 53.07 hours |
| C. | 63.07 hours |
| D. | 73.07 hours |
| Answer» A. 43.07 hours | |
| Explanation: t – t a/t 0 – t a = 0.86. also, α t/l 2 = 0.32. | |
| 215. |
5 W/m K |
| A. | 245 w/m2k |
| B. | 235 w/m2k |
| C. | 225 w/m2k |
| D. | 215 w/m2k |
| Answer» C. 225 w/m2k | |
| Explanation: l = volume/surface area = r/3. so, h = (0.1) (k) (3)/r. | |
| 216. |
The transient response of a solid can be determined by the equation. (Where, P is density, V is volume, c is specific heat and A is area) |
| A. | – 4 p v c = h a (t – t0) |
| B. | – 3 p v c = h a (t – t0) |
| C. | – 2 p v c = h a (t – t0) |
| D. | – p v c = h a (t – t0) |
| Answer» D. – p v c = h a (t – t0) | |
| Explanation: it can be determined by relating rate of change of internal energy with conductive heat exchange at the surface. | |
| 217. |
Transient condition means |
| A. | conduction when temperature at a point varies with time |
| B. | very little heat transfer |
| C. | heat transfer with a very little temperature difference |
| D. | heat transfer for a short time |
| Answer» A. conduction when temperature at a point varies with time | |
| Explanation: the term transient or unsteady state designates a phenomenon which is time dependent. | |
| 218. |
Which of the following is not correct in a transient flow process? |
| A. | the state of matter inside the control volume varies with time |
| B. | there can be work and heat interactions across the control volume |
| C. | there is no accumulation of energy inside the control volume |
| D. | the rate of inflow and outflow of mass are different |
| Answer» C. there is no accumulation of energy inside the control volume | |
| Explanation: in transient heat conduction there is accumulation of energy inside the control volume. | |
| 219. |
The curve for unsteady state cooling or heating of bodies is |
| A. | hyperbolic curve asymptotic both to time and temperature axis |
| B. | exponential curve asymptotic both to time and temperature axis |
| C. | parabolic curve asymptotic to time axis |
| D. | exponential curve asymptotic to time axis |
| Answer» D. exponential curve asymptotic to time axis | |
| Explanation: α/α 0 = exponential [- h a t/p c v], which represents an exponential curve. | |
| 220. |
What is the wavelength band for TV rays? |
| A. | 1 * 10 3 to 34 * 10 10 micron meter |
| B. | 1 * 10 3 to 2 * 10 10 micron meter |
| C. | 1 * 10 3 to 3 * 10 10 micron meter |
| D. | 1 * 10 3 to 56 * 10 10 micron meter |
| Answer» B. 1 * 10 3 to 2 * 10 10 micron meter | |
| Explanation: this is the maximum and minimum wavelength for tv rays. | |
| 221. |
Thermal conductivity is defined as the heat flow per unit time |
| A. | when the temperature gradient is unity |
| B. | across the wall with no temperature |
| C. | through a unit thickness of the wall |
| D. | across unit area where the temperature gradient is unity |
| Answer» D. across unit area where the temperature gradient is unity | |
| Explanation: thermal conductivity of a material is because of migration of free electrons and lattice vibrational waves. | |
| 222. |
Mark the matter with least value of thermal conductivity |
| A. | air |
| B. | water |
| C. | ash |
| D. | window glass |
| Answer» A. air | |
| Explanation: for air, it is .024 w/ m degree | |
| 223. |
Which one of the following forms of water have the highest value of thermal conductivity? |
| A. | boiling water |
| B. | steam |
| C. | solid ice |
| D. | melting ice |
| Answer» C. solid ice | |
| Explanation: for ice, it is 2.25 w/m degree | |
| 224. |
The average thermal conductivities of water and air conform to the ratio |
| A. | 50:1 |
| B. | 25:1 |
| C. | 5:1 |
| D. | 15:1 |
| Answer» B. 25:1 | |
| Explanation: for water, it is 0.55-0.7 w/m degree and for air it is .024 w/m degree. | |
| 225. |
Identify the very good insulator |
| A. | saw dust |
| B. | cork |
| C. | asbestos sheet |
| D. | glass wool |
| Answer» D. glass wool | |
| Explanation: glass wool has a lowest thermal conductivity of 0.03 w/m degree amongst given option. | |
| 226. |
Most metals are good conductor of heat because of |
| A. | transport of energy |
| B. | free electrons and frequent collision of atoms |
| C. | lattice defects |
| D. | capacity to absorb energy |
| Answer» B. free electrons and frequent collision of atoms | |
| Explanation: for good conductors, there must be electrons that are free to move. | |
| 227. |
Heat conduction in gases is due to |
| A. | elastic impact of molecules |
| B. | movement of electrons |
| C. | em waves |
| D. | mixing of gases |
| Answer» A. elastic impact of molecules | |
| Explanation: if there is elastic collision then after sometime molecules regain its natural position. | |
| 228. |
The heat energy propagation due to conduction heat transfer will be minimum for |
| A. | lead |
| B. | water |
| C. | air |
| D. | copper |
| Answer» C. air | |
| Explanation: it is because air has lowest | |
| 229. |
Cork is a good insulator because |
| A. | it is flexible |
| B. | it can be powdered |
| C. | low density |
| D. | it is porous |
| Answer» D. it is porous | |
| Explanation: cork has thermal conductivity in the range of 0.05-0.10 which is very low so it can be porous. | |
| 230. |
Choose the false statement |
| A. | for pure metal thermal conductivity is more |
| B. | thermal conductivity decreases with increase in the density of the substance |
| C. | thermal conductivity of dry material is lower than that of damp material |
| D. | heat treatment causes variation in thermal conductivity |
| Answer» B. thermal conductivity decreases with increase in the density of the substance | |
| Explanation: thermal conductivity increase with increase in the density of a substance. | |
| 231. |
A composite wall generally consists of |
| A. | one homogenous layer |
| B. | multiple heterogeneous layers |
| C. | one heterogeneous layer |
| D. | multiple homogenous layers |
| Answer» B. multiple heterogeneous layers | |
| Explanation: walls of houses where bricks are given a layer of plaster on either side. | |
| 232. |
Three metal walls of the same thickness and cross sectional area have thermal conductivities k, 2k and 3k respectively. The temperature drop across the walls (for same heat transfer) will be in the ratio |
| A. | 3:2:1 |
| B. | 1:1:1 |
| C. | 1:2:3 |
| D. | given data is insufficient |
| Answer» A. 3:2:1 | |
| Explanation: as, δ1 = δ2 = δ3 and cross sectional areas are same i.e. temperature drop varies inversely with thermal conductivity. | |
| 233. |
A composite wall is made of two layers of thickness δ1 and δ2 having thermal conductivities k and 2k and equal surface area normal to the direction of heat flow. The outer surface of composite wall are at 100 degree Celsius and 200 degree Celsius. The minimum surface temperature at the junction is 150 degree Celsius. What will be the ratio of wall thickness? |
| A. | 1:1 |
| B. | 2:1 |
| C. | 1:2 |
| D. | 2:3 |
| Answer» C. 1:2 | |
| Explanation: q = k 1 a 1 d t 1 / δ1 = k 2 a 2 d t 2 / δ2 also areas are same. | |
| 234. |
A pipe carrying steam at 215.75 degree Celsius enters a room and some heat is gained by surrounding at 27.95 degree Celsius. The major effect of heat loss to surroundings will be due to |
| A. | conduction |
| B. | convection |
| C. | radiation |
| D. | both conduction and convection |
| Answer» C. radiation | |
| Explanation: as there is temperature difference so radiation suits well. | |
| 235. |
“Radiation cannot be affected through vacuum or space devoid of any matter”. True or false |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: it can be affected only by air between molecules and vacuum of any matter. | |
| 236. |
A composite wall of a furnace has two layers of equal thickness having thermal conductivities in the ratio 2:3. What is the ratio of the temperature drop across the two layers? |
| A. | 2:3 |
| B. | 3:2 |
| C. | 1:2 |
| D. | log e 2 : log e 3 |
| Answer» B. 3:2 | |
| Explanation: we know that temperature is inversely proportional to thermal conductivity, so the ratio is 2:3. | |
| 237. |
According to Indian Standards, it is recommended that the overall thermal transmittance of a roof should not be more than kcal/m2 h deg C. |
| A. | 2.00 |
| B. | 5.00 |
| C. | 7.00 |
| D. | 9.00 |
| Answer» A. 2.00 | |
| Explanation: according to indian standards, it is recommended that the overall thermal transmittance of a roof should not be more than 2.00 kcal/m2 h deg c. it is also recommended that the thermal dampness of a roof should be less than 75%. | |
| 238. |
The process of direct transmission of heat through a material is known as |
| A. | conduction |
| B. | radiation |
| C. | thermal insulation |
| D. | thermal energy |
| Answer» A. conduction | |
| Explanation: the process of direct transmission of heat through a material is known as conduction. the amount of heat transfer by this process depends on various factors like temperature difference, the conductivity of the medium, the time for which the flow takes place etc. | |
| 239. |
Thermal insulation keeps the room cool in winters and hot in summers. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: thermal insulation keeps the room hot in winters and cool in summers which results in comfortable living. it minimises heat transfer and helps in saving | |
| 240. |
The amount of heat flow through a unit area of material of unit thickness in one hour, when the temperature difference is maintained at 1°C is known as of the material. |
| A. | thermal conductivity |
| B. | thermal resistivity |
| C. | thermal conductance |
| D. | thermal resistance |
| Answer» A. thermal conductivity | |
| Explanation: the amount of heat flow through a unit area of material of unit thickness in one hour, when the temperature difference is maintained at 1°c is known as the thermal conductivity. it is denoted by k. its units are w/(mk). | |
| 241. |
The reciprocal of thermal conductivity is known as |
| A. | thermal conductance |
| B. | surface resistance |
| C. | specific conductance |
| D. | thermal resistivity |
| Answer» D. thermal resistivity | |
| Explanation: the reciprocal of thermal conductivity is known as thermal resistivity. it is given by 1/k. its units are (mk)/w. | |
| 242. |
Which of the following is the correct relationship between thermal resistance and thermal conductivity? |
| A. | r = k/l |
| B. | r = k.l |
| C. | r = l/k |
| D. | r = l2/k |
| Answer» C. r = l/k | |
| Explanation: thermal resistance and thermal conductivity are related by the equation r = l/k. here, r is the thermal resistance, k is the thermal conductivity and l is the thickness. thermal resistance is the reciprocal of thermal conductance. | |
| 243. |
Surface resistance is the reciprocal of |
| A. | surface coefficient |
| B. | surface resistivity |
| C. | surface conductance |
| D. | surface conductivity |
| Answer» A. surface coefficient | |
| Explanation: surface resistance is the reciprocal of the surface coefficient. it is given by 1/f where f denotes the surface coefficient. its units are (m2k)/w. | |
| 244. |
Thermal damping is given by the equation D = (T-t/T) x 100. Here, T denotes |
| A. | outside temperature range |
| B. | inside temperature range |
| C. | total outside and inside temperature range |
| D. | thickness |
| Answer» A. outside temperature range | |
| Explanation: thermal damping is given by the equation d = (t-t/t) x 100. here, t denotes outside temperature range and t denotes inside temperature range. also, thermal damping is denoted by d. | |
| 245. |
Which of the following is the correct relation between thermal transmittance and thermal time constant? |
| A. | t = q x u |
| B. | t = k/u |
| C. | t = q/u |
| D. | t = u/q |
| Answer» C. t = q/u | |
| Explanation: thermal transmittance and thermal time constant are related by the equation t = q/u. here, t denotes the thermal time constant, u is the thermal transmittance and q is the quantity of heat stored. | |
| 246. |
Which of the following is not a quality of a good thermal insulating material? |
| A. | it should be durable |
| B. | it should have a low thermal resistance |
| C. | it should be readily available |
| D. | it should be fireproof |
| Answer» B. it should have a low thermal resistance | |
| Explanation: a good thermal insulating material should be durable. it should have high thermal resistance. it should be fireproof and readily available. | |
| 247. |
Reflective sheet materials used as thermal insulating material have reflectivity and emissivity. |
| A. | high, high |
| B. | high, low |
| C. | low, low |
| D. | low, high |
| Answer» B. high, low | |
| Explanation: reflective sheet materials used as thermal insulating material have high reflectivity and low emissivity. because of this property, it offers high heat resistance. | |
| 248. |
Thermal insulation of roofs can be obtained by covering the top exposed surface of the roof with |
| A. | 2.5 cm thick layer of mud mortar |
| B. | 2.5 cm thick layer of coconut pitch cement concrete |
| C. | 7.5 cm thick layer of mud mortar |
| D. | 7.5 m thick layer of coconut pitch cement concrete |
| Answer» B. 2.5 cm thick layer of coconut pitch cement concrete | |
| Explanation: thermal insulation of roofs can be obtained by covering the top exposed surface of the roof with 2.5 cm thick layer of coconut pitch cement concrete. coconut pitch cement concrete is prepared by mixing coconut pitch with water and cement. | |
| 249. |
Presence of moisture in the thermal insulating material increases thermal insulation while the presence of air spaces decreases thermal insulation. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: presence of moisture in the thermal insulating material decreases thermal insulation while the presence of air spaces increases thermal insulation. the choice of the thermal insulating material depends on various factors like cost of material, area to be covered, coat of heating or cooling, etc. | |
| 250. |
According to the Indian Standards, it is recommended that thermal damping of a wall should not be less than |
| A. | 40% |
| B. | 50% |
| C. | 60% |
| D. | 80% |
| Answer» C. 60% | |
| Explanation: according to the indian standards, it is recommended that the thermal damping of a wall should not be less than 60%. hence, it is also recommended that the thermal time constant should not be less than 16 h. | |
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