Presentation on theme: "Why should crushed ice be used in the experiment? To increase the surface area of ice and keep good contact with the heater. Crushed ice should be used."— Presentation transcript:
Why should crushed ice be used in the experiment? To increase the surface area of ice and keep good contact with the heater. Crushed ice should be used because this enhances the thermal contact between the ice and the heater. Therefore, less heat is wasted to heat up the air between the ice cubes. Would you expect the value of the specific latent heat of fusion of ice obtained in the experiment to be higher than, equal to, or lower than the accepted value? Explain briefly. The value obtained is higher than the accepted value. This is because some energy supplied by the heater is lost to the surroundings. The actual energy absorbed by the ice should be lesser.
Describe, with the aid of a diagram, how a control set can be set up to improve the accuracy of the experiment. An immersion heater is set up without connecting to the power supply. The actual mass of ice melted by the energy supplied by the heater is the mass difference between the masses of water in the two beakers. Why should the melting ice be used? Melting ice is used to make sure that the temperature of the ice is at 0 °C. Hence, no energy is absorbed as the heat capacity to raise the temperature of the ice to 0 °C.
Why is the ‘control’ necessary? This is to account for the ice melted at room temperature. After switching off the heater, the cups are not removed until the drip rates become steady. Why? This is to allow time for the energy from the heater to be conducted to the ice to melt it (water and ice are poor conductors of heat).
The immersion heater should not be switched on unless it is fully immersed in water. Why? Otherwise the heater may overheat and gets damaged. What are the possible sources of error in the experiment? Sources of Error Calculated result higher / lower than accepted value? Steam condensing on the top of the heater drips back into the beaker. Higher Some water spills out from the beaker. Lower Energy is lost to the surroundings. Higher Energy from the non-immersed part of the heater is lost to the surrounding and not transferred to the water. Higher
A student suggests adding a lid on the beaker so as to improve the accuracy of the experiment. If the beaker is covered with a lid, the steam will not be able to escape from the beaker. As a result, water vapour will condense on the lid and return back to the beaker. This will lead to a serious experimental error. Is the value of specific latent heat of vaporization obtained in the experiment usually higher or lower than the accepted value? Explain briefly. Due to the heat loss to the surroundings, the energy recorded by the kilowatt-hour meter is higher than the energy absorbed by the water. The value obtained is usually higher than the accepted value.
Comment on the statement: The calculated value of the specific latent heat of vaporization of water should be lower than the accepted value because some boiling spills out from the cup. Although water may spill out from the beaker, the main source of error is due to the heat lost to the surroundings. As the energy supplied is always greater than the energy absorbed, the calculated value would be greater than the accepted value. Suggest a method to improve the accuracy of the experiment. Use a polystyrene cup instead of a beaker because polystyrene is a poor conductor of heat. Wrapping the beaker with cotton wool.
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