Expansion and Thermometers Flashcards
What happens when a material is heated ?
It expands
Examples of thermometers (5)
Lab thermometer Infrared thermometer Clinical thermometer Constant volume gas thermometer Thermocouple
Define thermometer
an instrument used to measure temperature
Fixed points on the celcius scale
Define upper fixed point
Upper fixed point (100*C) is the temperature of steam from pure boiling water at standard atmospheric pressure
Fixed points on the celcius scale
Define lower fixed point
Lower fixed point - (0*C) is the temperature of pure melting ice at standard atmospheric pressure
TRUE/FALSE
The freezing point of water is the same as the melting point of ice: (0°C).
TRUE
Temperature range for a liquid/mercury-in-glass/lab thermometer.
-10C to 110C
OR
0C to 100C
Temperature range for clinical thermometer
34C/35C to 43*C
Temperature range for a thermocouple
-250C to about 1500C
Most thermometers used in school are a ______
liquid in glass lab thermometer
Explain the design of a lab thermometer which makes it suitable for its particular task (4)
Give at least 2
1) Bulb that holds mercury has a thin wall so that heat can easily transfer through it.
2) The scale is positioned very close to the bore to reduce parallax error
3) The bore is very narrow so that any change in volume of the mercury will result in a noticeable change in the length of the mercury thread.
4) The bulb is relatively large so that the corresponding expansion or contraction of the mercury it contains is noticeable for small changes in temperature.
State one use of lab thermometers
In school for lab experiments
Explain the design of a clinical (mercury) thermometer which makes it suitable for its particular tasks
The scale ranges only from 34C or 35C to 43C because the normal body temperature of a human is 37C
2) The interval between markings on the scale is 0.1*C so that a very precise reading is obtain.
3) There is a narrow constriction in the bore. When the thermometer is removed from the patient, the body heat causes the mercury in the bulb to heat up, expand and rise up the capillary tube, through the constriction up to a certain level to indicate the body temperature. The constriction breaks the liquid thread (to prevent the mercury from flowing back into the bulb) and allows the temperature to be read.
Explain the design of a thermocouple which makes it suitable for its particular task
It is electrical - can be connected to digital displays and computer systems
A property of a material which varies with temperature. ______ property
Thermometric
State how a thermometer calibrated
The values of the thermometric property at upper and lower fixed points are found and marked on the thermometer and the interval between the fixed points is then calibrated.
Expansion in terms of kinetic theory
Explain expansion in solids
When a solid is heated, the heat energy supplied converts into kinetic energy of its particles(thermal energy). The molecules of the solid vibrate faster with greater amplitude and therefore occupies more space
Expansion in liquid and gases
When a liquid or gas is heated, the heat energy supplied converts to kinetic energy of its particles. The molecules of liquid and gas translate faster and therefore occupy more space.
Application of thermal expansion
for example,
power lines
opening jars
creaking roofs.
Power lines must be laid slack in summer so that strong tension forces are not produced when they contract in winter
Roofs shrink at night as temperature falls. Contraction of the wood causes the joints to move relative to each other producing a creaking sound
A tight metal lid on a glass jar is easily removed by running hot water unto it. The lid rapidly expands due to its high thermal conductivity and becomes loose.
Bimetallic strip can be used in
simple fire alarm
The two metal strips in a bimetallic strip are ____ and ____
brass
invar
____ expands more than ____ when heated and so this combination forms a good bimetallic strip. It will also contract more on cooling.
Brass
invar
Explain thermal expansion
When a material is heated the particles gain thermal energy. In a solid this increase in energy causes the particles to vibrate more while staying in the same position. This increase in vibration will force the particles slightly further apart and as a the material expands
