P3 Thermal Physics Flashcards
Describe qualitatively the molecular structure of solids, liquids and gases in terms of the arrangement, separation and motion of molecules.
SOLIDS
- They are closely packed together
- They keep their shapes; shapes do not change
- They have strong bonds between particles
- They can vibrate at a fixed point
LIQUIDS
- They are packed together, but not as closely as solids.
- It keeps its liquid’s volume. It moves around more freely than solids
- They are not very far away, but not very close to each other.
- They are not very strongly bonded
GASES
- They are far away from each other
- They keep the volume of their container
- They are not packed together
- They don’t have bonds with each other
Show an understanding of Brownian motion (the random motion of particles in a suspension) as evidence for the kinetic molecular model of matter.
Brownian motion is the motion of air particles colliding with other particles.
They are colliding constantly that the air molecules are causing other particles to move.
It was discovered by a Scottish botanist, Robert Brown.
Example:
smoke grains are suspended in air or in water, they will move because they collide with air molecules in the air or water molecules in water.
Describe melting and boiling in terms of energy input without a change in temperature.
Because after the molecules reach their boiling point, in order to break their intermolecular force, they absorb heat energy (input energy). Thus, even when the molecules are heated, because molecules are using heat energy to break their bonds, the temperature is constant. Only after all bonds are broken, the temperature start to increase again.
Describe qualitatively the pressure of a gas in terms of the motion of its molecules and their colliding with the walls creating a force.
The pressure is formed as gas molecules collide with the surface of the wall.
Thus:
• The more collisions, the more pressure
↳ higher temperature = more collisions / energies
↳ less volume, more concentrated molecules = more collisions
• The stronger the hit, the more pressure (note that if the volume Is not constant, the pressure doesn’t increase)
↳ higher speed, stronger force = stronger hit on walls
Show an appreciation that massive particles may be moved by light, fast-moving molecules.
Huge molecules can be caused to be moved by lighter, smaller molecules. When the small molecules are moving at a very fast rate of speed, they will carry a lot of energy. Thus, when they hit big molecules, they will collide and cause big molecules to move. Tiny molecules exert huge forces when they are fast-moving. (Each molecule that collides with the surface exerts a tiny force)
Distinguish between boiling and evaporation.
Boiling is when a liquid becomes gas at its boiling point, caused by a very high temperature.
Evaporation is when a liquid becomes gas at any temperature spontaneously.
Demonstrate an understanding of how temperature, surface area and draught over a surface influence evaporation.
TEMPERATURE
- Higher temperature causes more particles to gain energy and cause them to move fast enough to evaporate and escape from the surface of the liquid.
SURFACE AREA
- Greater surface area causes more particles to be closer to the surface of the liquid, and they can escape the liquid easily.
DRAUGHT
- A draught is moving air. When particles escape from the surface, they are blown away so that they cannot fall back into the water.
Identify and explain some of the everyday applications and consequences of thermal expansion.
• Rivets are used to hold steel plates together very tightly. A very hot rivet is pushed through the two plates and its end is hammered over. When the rivets cool
down it pulls the two plates together very tightly.
• If we find it difficult to remove the stopper from a glass bottle, we can heat the neck of the bottle. Now the neck of the bottle expands and the stopper comes out easily
• Metal bridges and railway lines expand on hot days, and there is a danger that they might buckle
Explain in terms of the motion and arrangement of molecules, the relative order of the magnitude of the expansion of solids, liquids and gases.
Fastest to slowest: Gases → Liquids → Solids
It is easier for gas molecules to gain energies, and move around freely, so take up more space. That is why it is easier for gases to expand. Whereas because solids are packed together, it is harder for them to expand.
Demonstrate understanding of sensitivity, range and linearity.
A thermometer device is said to be linear when the temperature-measuring device (like mercury) increase or expands at a steady rate. It means that it is calibrated in an even space.
Sensitivity is how sensitive the measuring can be. This means if the thermometer can measure tiny changes of temperature, it is said to be sensitive.
The range is the minimum and maximum temperature the thermometer can measure. For example, a mercury thermometer has a wide range as mercury has a wide range of temperatures.
Describe and explain the structure and action of liquid-gas thermometers.
When a liquid-gas thermometer measures the temperature of an object, its thermometer bulb absorbs heat energy from the object until it will have the same temperature as the object. Then, as the thermometer is heated, the liquid in it expands, and rise up. That’s how the temperature is read from calibration.
Describe the structure of a thermocouple and show understanding of its use as a thermometer for measuring high temperatures and those that vary rapidly.
A thermocouple gives an output voltage that depends on the temperature. It is made from pieces of wire made from two different metals. A wire of metal X is joined at each end to wires of metal Y to form two junctions.
Its ends are connected to a sensitive voltmeter. Then one junction is placed in melting ice at 0℃ while the other junction is placed in the temperature-measuring object. The voltmeter then shows a reading. It is usually used with an electronic thermometer which converts the voltage to a digital temperature reading.
They are used to measure high temp and those that vary rapidly because they are small and robust, so they don’t absorb much energy from the object. Also, they are small so they can heat up and cool down quickly. That’s why they are suitable for fast varying objects.
Recognise the need for and identify fixed points.
A fixed point is a specific temperature where a physical change of a material is recognised.
Fixed points allow us to know the temperature without having to measure it directly.
Describe and explain how the structure of a liquid-in-glass thermometer relates to its sensitivity, range and linearity.
They are linear because mercury expands at a steady rate so it is calibrated in an even space.
They can be sensitive if you make the tube longer and the mercury narrower.
They have a wide range because mercury is liquid between -39 degrees and +350 degrees.
Describe experiments to demonstrate the properties of good and bad thermal conductors
Touch one metal spoon and one plastic spoon which have the same temperature. You will feel cold when you touch the metal spoon even if it has the same temperature as the plastic spoon because metals are good conductors of heat.
You can compare different thermal conductors using a rod experiment. Place a blob of wax at one end of different metal rods (they all must have the same size). They are all heated equally at the other end. The best conductor is the metal on which the wax melts first.
