Abhishek Verma 12110003

Experiment 7: Absorptivity of different materials

AIM: To determine the absorptivity of different materials.

APPARATUS: Thermometer, measuring cylinder, black, blue, white and silver

coloured surface of same surface area, stopwatch.

THEORETICAL BACKGROUND:

Electromagnetic waves are synchronized oscillations of electric and magnetic field

that propagate at speed of light. The oscillations of two fields are perpendicular to

each other and also they both are perpendicular to the direction of wave.

Electromagnetic waves has its own spectrum which includes gamma rays, x rays,

ultraviolet radiation, visible light, infrared radiation, microwave, radio wave. Our

naked eyes can see visible lights and we are able to see the colour which is reflected

by the surface. Red coloured surface reflects red light etc. Black surface is considered

that it doesn’t reflect any light hence it appear black. So absorptivity of black coloured

surface is 1.

Absorptivity, α =Heat absorbed by surface, Q

Heat absorbed by black surface, Qb

PROCEDURE:

- Apparatus was setup by keeping different colored surfaces aligned side by side.

- Water was filled into the tank situated above the setup.

- Thermometer was fixed at both inlet and outlet of each surfaces.

- Each valve was opened and a uniform flowrate was maintained through each

valves.

- After 10 minutes reading of each thermometer was taken down.

- Repeat the above step to take 4-5 readings.

OBSERVATION:

Density of water = 1 gm/ml

Specific heat capacity of water = 4.186 J/g-oC

Abhishek Verma 12110003

Experiment 7: Absorptivity of different materials

Table 1.1:

Black Blue White Silver

Flow rates

(ml/s)

1.03 0.98 0.94 1.02

Inlet Temp (oC) 41.5 41 43.25 43.5

Outlet Temp

(oC)

48 46.85 46.8 46.75

Inlet Temp (oC) 42 42 43.25 44.5

Outlet Temp

(oC)

48.5 48 47.75 47.25

Inlet Temp (oC) 42.5 42.25 43 43.25

Outlet Temp

(oC)

49.25 48 47 46.2

Inlet Temp (oC) 43.25 43 44 44.25

Outlet Temp

(oC)

50 49.5 49 48.75

Table 1.2:

Sr. No. Temperature

Difference, ΔT

(oC)

Heat absorbed in

10 min, Q (Joules)

Absorptivity, α

Black 6.63 17151.47 1

Blue 6.02 14817.44 0.86

White 4.26 10057.45 0.59

Silver 3.36 8607.76 0.50

SAMPLE CALCULATION:

For Black,

- Average temperature difference ΔT = (50-43.25+49.25-42.5+48.5-42+48-41.5)/4

= 6.63 oC

- Mass flowrate of liquid, m = 1.03 * 1 = 1.03 gm/s

- Heat absorbed by the surface in 10 minutes, Qb = m*C* ΔT*60*10 =

1.03*4.186*6.63*600 = 17151.47 J

- Absorptivity of blue surface, α = Heat absorbed by surface, Q

Heat absorbed by black surface, Qb =

14817.44/17151.47 = 0.86

Abhishek Verma 12110003

Experiment 7: Absorptivity of different materials

RESULTS:

From Table 1.2,

Absorptivity of Black surface = 1

Absorptivity of Blue surface = 0.86

Absorptivity of White surface = 0.59

Absorptivity of Silver surface = 0.5

CONCLUSION:

From Table 1.1 we can say that black body absorbs the maximum heat and silver coloured

body is a good reflector. We can get more accurate results of absorptivity by taking care of

some issues,

- Temperature of surrounding should not be extremely high because it causes large

change in temperature which results in less number of readings.

- Flowrate in each valve should be constant. The more it deviates in each valve the more

error it will cause.

PRECAUTIONS:

- Check for all kind of leakage.

- Make sure the tank doesn’t become empty in between the experiment.