Monday, 1 September 2014


How to make a good thermometer?
1. Sensitive thermometer
-Can read visibly small temperature changes.
-More noticeable expansion of the mercury in it.
Unit for sensitivity is cm or mm/degree Celsius 
Length of mercury tread/change in temperature.
Every degree increase of temperature, the length of mercury expand...

2. Responsive thermometer 
-How fast the thermometer can respond to the temperature changes and register the new reading.
The thermometer does not measure the temperature of the object but only measure the temperature of mercury inside it and the mercury always has the same temperature as the object. Responsive refers to the time taken for the mercury to become the same temperature as the object.

3. Thermometer with a wide Range
-The scope of temperature it can measure.
-The extent of readings between the maximum and minimum that can be read using the thermometer with a fixed length of stem.

Size of the bulb (Which one of the thermometer is more sensitive? More responsive? Wider rage?) Bulb B will be more sensitive less responsive and has a shorter range than A as bulb B has bigger bulb so it will have more mercury so it can expand more so it is more sensitive.  Bulb B has more mercury so it takes a longer time for the mercury to expand. Bulb B has a smaller range as there is more mercury expansion in one degree.                         
Size of the capillary bore (Which one of the thermometer is more sensitive? More responsive? Wider rage?) Bulb A is less sensitive as it has more space for the mercury to take up so with the same mercury, the length of the mercury expansion is lesser, the mercury is more in the same change in temperature for the thermometer with a thinner bore. There will me no change in responsiveness as the both have same mercury so they have the same rate of expansion in the mercury. A have a bigger range, when the mercury expands for the same length, the expansion of mercury is shorter, it has a bigger range.
Thickness of the glass around the bulb (Which one of the thermometer is more sensitive? More responsive? Wider rage?) Sensitivity and range won't be affected. Thickness affects responsiveness as it affects the rate of thermal energy transfer so thicker glass will be less responsive.
Different liquids in the bulb (Which one of the thermometer is more sensitive? More responsive? Wider rage?) ---Expansivity of the liquid. Greater expansivity of liquid, more sensitive.       ---Rate of expansion of liquid. Greater specific heat capacity, more responsive. ---Greater sensitivity, shorter range.
When sensitivity goes up, range goes down.

What is the physical quantity that is changing with temperature in the laboratory thermometer?
Volume of a fixed mass of mercury is the physical quantity that varies proportionally or linearly or uniformly with temperature.

Physical quantities
Mercury and alcohol

Different substances have different expansivity they show different increase in volume for the same rise in temperature. Ethanol has a higher expansivity than water and water has a higher expansivity than mercury.

Steps to calibrate thermometer
Mark out the fixed points
Divide the intervals
Mark each interval one degree Celsius. 

Use crushed ice
More surface area in contact with the tip of thermometer.
Ice point is the temperature of pure melting ice at standard atmospheric pressure(sea level)

Why are this two points chosen as the point to calibrate the thermometer?
This two fixed points will always be the same under given conditions.

Impurities like salt added to ice will lower melting point below 0 degree Celsius means that the ice will melt at a temperature lower than 0 degree Celsius.

On the road during winter, there is snow on the road which is not safe for cars to drive on. In order to melt the snow, people sprinkle impurities on the snow to make the snow melt at a lower temperature. And water will also freeze into ice slower.

Increase pressure on ice lowers melting point below 0 degree Celsius. 

Ways to take out ice
Put the ice tray under a running tap.
Use a sharp object to poke the ice.
Twist and turn the tray.
*All three ways applies pressure to the ice.

The physics behind ice skating.
As the sharp blade of the ice skating shoes has a small surface area in contact with the ice and there is pressure applied to the ice and ice melt at a lower temperature and becomes water thus the skating shoes enables you to slide on the ice. When you leave the ice, the water will freeze back into ice as pressure is no longer applied. 

Adding impurities like salt increases boiling point of water.
Applying pressure increases boiling point of water.

What is the physics behind pressure cooker?

Pressure Cooker

Food can be cooked faster in a pressure cooker since the boiling point of water increases with pressure. The pressure cooker consists of a pressurized pot which has controlled vent at a specified pressure above atmospheric pressure. It is typical for the pressure of venting to be about 15 pounds per square inch above atmospheric pressure. That raises the boiling point to about 121°C or 257 °F.

Since atmospheric pressure is about 14.7 lb/in2, then a gauge pressure of 15 lb/in2 amounts to about a doubling of absolute pressure. The nature of the variation of saturation vapor pressure with temperature places the boiling point at about 121°C.

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