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Conduction and convection
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Thermal energy can be transferred by conduction, convection and radiation. Conduction and convection both need particles.
Conduction
In a solid, particles at the hot end vibrate more and pass energy to their neighbours.
Metals are the best conductors because they also have free electrons that carry energy quickly through the metal.
Non-metals, liquids and especially gases are poor conductors (insulators).
Practical: rods of different metals with wax at one end are heated at the other; the wax melts first on the best conductor.
Metals are the best conductors because they also have free electrons that carry energy quickly through the metal.
Non-metals, liquids and especially gases are poor conductors (insulators).
Practical: rods of different metals with wax at one end are heated at the other; the wax melts first on the best conductor.
Convection
Convection happens in fluids (liquids and gases).
The heated fluid expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place. This sets up a convection current.
Practical: a crystal of potassium manganate(VII) at the bottom of a beaker of water, heated gently on one side. The purple colour shows the water circulating.
The heated fluid expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place. This sets up a convection current.
Practical: a crystal of potassium manganate(VII) at the bottom of a beaker of water, heated gently on one side. The purple colour shows the water circulating.
Convection in everyday life
Heaters are placed low in a room: warm air rises and circulates.
Freezer compartments are at the top of a fridge: cold air sinks.
Kettles have the heating element at the bottom.
Sea breezes: by day, land warms faster than the sea; warm air over the land rises and cooler air moves in from the sea.
Freezer compartments are at the top of a fridge: cold air sinks.
Kettles have the heating element at the bottom.
Sea breezes: by day, land warms faster than the sea; warm air over the land rises and cooler air moves in from the sea.
Explain why the heating element in a kettle is at the bottom.
- Water near the element is heated, expands and becomes less dense.
- It rises and cooler, denser water sinks to the element.
- A convection current heats all the water.
Answer: So convection currents carry heat through all the water.
Conduction: vibrating particles pass on energy; metals conduct best because of free electrons. Convection: heated fluid expands, becomes less dense and rises, cooler fluid sinks, forming a convection current. Used in heaters, fridges, kettles and explains sea breezes.
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