2.3.2 Convection (3)
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1.
A metal block is placed in a container of water at a constant temperature. Explain how thermal energy is transferred from the water to the metal block. Consider the different methods of thermal energy transfer and explain why convection is not the primary method in this situation.
Thermal energy is transferred from the water to the metal block primarily through conduction. Conduction is the transfer of heat through direct contact between particles. Since the metal block is in direct contact with the water, heat energy is transferred from the more energetic water molecules to the less energetic metal molecules.
While convection does occur in the water surrounding the metal block, it is not the primary method of heat transfer to the metal. The water's convection currents are driven by a heat source (e.g., a radiator or a heater). In this scenario, the water is at a constant temperature, so there is no significant temperature difference to drive convection currents within the water itself. The heat is transferred directly through the metal via conduction.
Other methods of thermal energy transfer considered (and why they are not primary here):
- Radiation: Heat transfer via electromagnetic waves. While some radiation will occur, it is less significant than conduction in this scenario because the metal is in direct contact with the water.
- Convection: As explained above, convection is not the primary method of heat transfer from the water to the metal when the water is at a constant temperature.
2.
Describe an experiment to illustrate convection in a gas. Include a list of materials needed, the procedure, and what observations you would expect to make. (6 marks)
Experiment: Convection in Air
Materials Needed:
- A clear, tall, transparent tube (e.g., a clear plastic tube or a glass test tube held vertically)
- A heat source (e.g., a lamp or a warm plate)
- A thin piece of paper (e.g., tissue paper or a small piece of card)
Procedure:
- Place the tube vertically, with the open end facing upwards.
- Position the heat source close to the bottom of the tube.
- Carefully hold the piece of paper just above the open end of the tube, allowing it to hang freely.
- Observe the paper for several minutes.
Observations:
As the air near the bottom of the tube is heated by the heat source, it becomes less dense and rises. This warm air will rise through the tube.
As the warm air rises, it will push the paper upwards. The paper will be lifted by the rising warm air.
The cooler, denser air from the top of the tube will sink to replace the rising warm air. This creates a convection current within the tube.
In summary: The rising warm air will visibly move the paper upwards, demonstrating the principle of convection in a gas. The cooler air sinking is not directly visible but is the driving force behind the convection current.
3.
Explain how convection currents contribute to the warmth within a room heated by a central heating system. Include a description of the temperature gradient and the role of different gases in the process.
A central heating system typically heats water in a boiler. This hot water is then circulated through pipes to radiators in different rooms. The radiators heat the air surrounding them, creating a temperature gradient – a difference in temperature between the radiator and the cooler air in the room.
The warm air directly above the radiator becomes less dense and rises. This creates a convection current: the warm air rises, and cooler air is drawn in to take its place. This continuous circulation of air ensures that the warmth is distributed throughout the room.
The role of different gases is important. The air in a room is primarily composed of nitrogen and oxygen. While these gases do not significantly affect the rate of convection, they do influence the air's density. When heated, the air molecules move faster and spread out, making the air less dense and allowing it to rise more readily. The presence of these gases also affects the specific heat capacity of the air, which influences how quickly it heats up and cools down.
In summary: The central heating system creates a temperature difference, leading to convection currents. The warm air rises, and cooler air is drawn in, distributing heat throughout the room. The gases in the air play a role in the air's density and heat capacity, influencing the efficiency of the convection process.