Thermal Energy Flashcards
What is the rule for transfer of thermal energy
Thermal energy travels from a region of higher temperature to a region of lower temperature (until thermal equilibrium is reached).
What happens when an object gains or loses thermal energy?
- Heated-> expansion
- Cooled-> contraction
What material expands/ contracts more in a bimetallic strip? (Brass or steel)
Brass expands or contracts more than steel
What material expands/ contracts more in a bimetallic strip? (Brass or steel)
Brass expands or contracts more than steel
How does a bimetallic strip help to automatically turn off an electric iron when it turns too hot?
- Turning the knob of the electric iron raises or lowers the contacts and sets the temperature of the electric iron.
- The bimetallic strip is straight when the contacts touch, and it forms a closed circuit such that electricity can pass through and the iron can be used.
- When the bimetallic strip gains thermal energy, bras expands more than steel, and the bimetallic strip bends downwards
- This creates an open circuit and thus electricity cannot pass through. The electric iron is turned off.
- When the bimetallic strip cools down, the strip becomes straight and a closed circuit is formed again
How do liquid-in-glass thermometers work?
- The liquid in the bulb of the thermometer gains thermal energy, and expands.
- This causes it to rise up the narrow glass tube
- When the temperature around the bulb decreases, the liquid in the glass tube cooled down and falls back down the narrow tube and into the bulb of the thermometer
How do hot-air balloons work?
- When the air in the hot air balloon is heated, the air particles gain thermal energy and expand
- The mass of the air particles remain constant, and the density decreases
- The air inside the hot air balloon becomes less dense than the air outside the hot-air balloon, and the balloon rises
What is the definition of conduction?
Process by which thermal energy is transferred through a medium from one particle to another
Explain how conduction helps to heat up a metal rod from one end (where the flame is) to the other end
- When one end of the metal rod is heated up the particles gain thermal energy which is converted to kinetic energy f the particles
- The particles vibrate faster and collide with their less energetic neighbours which passes some of their thermal energy to those particles
- The transfer of thermal energy from one particle to another occurs down the rod until the cold end is the same temperature as the heated end
Explain why solids are better conductors of thermal energy than fluids
Solids are better conductors of thermal energy than liquids or gases as the particles are packed more closely together, which allows the kinetic energy to be transferred quicker
Is there a net movement of particles during conduction?
No
Explain why metals are the best conductors for thermal energy
Metals contain free electrons, which also vibrate and gain energy, meaning that thermal energy can be transferred even quicker
Define convection
The process by which thermal energy is transferred from one place to another through the movement of fluids
How is a convection current set up in a liquid?
- Liquid particles at the bottom of the containers/ closest to the heating element gets heated up, expands, and the volume increases
- The mass remains constant, and the density of the particles decreases
- The cooler, denser particles near the top of the container sink, while the less dense particles rise.
- This process repeats itself until a convection current is formed, and the liquid is uniformly heated
Define radiation
Process by which thermal energy is transferred by infra-red rays
Factors affecting the rate of radiation
- Temperature of the object
- Surface area of the object
- Texture+colour of the surface
What are the best emitters or absorbers of infra-red rays
Black/ dull and rough surfaces
How does a vacuum flask help to keep hot water hot for a long period of time?
- Plastic stopper: reduces conduction as plastic is a poor conductor or thermal energy, stops convection as a convection current cannot be set up, reduces evaporation
- Vacuum: stops conduction and convection as both processes can only occur through a medium.
- Outer silvered surface: poor absorber but good reflector of infra-red rays
- Inner silvered surface: poor emitter of thermal energy
