Heat and Energy — ICSE Class 9 Physics Important Questions

Answer: (b) joule.

Heat is a form of energy, so its SI unit is the joule $(\text{J})$(J). The older unit was the calorie, where $1\ \text{cal}=4.186\ \text{J}$1 cal=4.186 J.

Answer: (b) $4^\circ\text{C}$4° C.

Due to the anomalous expansion of water, its volume is least (and density greatest) at $4^\circ\text{C}$4° C.

Answer: (b) Energy is never destroyed, but no transfer is $100\%$100% efficient: part of it always spreads out as heat that cannot easily be used again. Example: a bulb turns electrical energy into light and a lot of heat.

Answer: (a) In Hope's experiment the lower thermometer settles at $4^\circ\text{C}$4° C while the upper one falls to $0^\circ\text{C}$0° C, because water at $4^\circ\text{C}$4° C is densest and sinks to the bottom.

Answer: (a) Because of anomalous expansion, water expands as it turns to ice, so the ice occupies more volume and exerts pressure that bursts the pipe. R correctly explains A.

Heat is the total thermal energy possessed by a body due to the motion of its molecules; it is a form of energy measured in joules $(\text{J})$(J) and flows from a hotter to a colder body.

Temperature is the degree of hotness or coldness of a body that decides the direction of heat flow; it is measured in $^\circ\text{C}$° C or kelvin $(\text{K})$(K). Heat is the cause, temperature is the effect.

(i) The density decreases: below $4^\circ\text{C}$4° C water expands on cooling (anomalous expansion), so the same mass takes up more volume.

(ii) The density decreases: above $4^\circ\text{C}$4° C water expands normally on heating.

Apparatus: a tall metal cylinder filled with water, with a trough around its middle holding a freezing mixture (ice and salt). One thermometer is fitted near the top and one near the bottom.

Observation: at first the lower thermometer falls and settles at $4^\circ\text{C}$4° C; the upper thermometer stays steady for a while and then falls to $0^\circ\text{C}$0° C, and ice may form at the top.

Conclusion: water cooled to $4^\circ\text{C}$4° C is densest and sinks; below $4^\circ\text{C}$4° C it expands, becomes lighter and rises. So water has its maximum density at $4^\circ\text{C}$4° C (anomalous expansion).

Most liquids expand continuously on heating, but water behaves anomalously between $0^\circ\text{C}$0° C and $4^\circ\text{C}$4° C. When water is heated from $0^\circ\text{C}$0° C to $4^\circ\text{C}$4° C it contracts instead of expanding; only above $4^\circ\text{C}$4° C does it expand normally. Hence water has maximum density at $4^\circ\text{C}$4° C.

Importance: In a freezing pond, the coldest water at the surface turns to ice while the denser water at $4^\circ\text{C}$4° C sinks to the bottom. Ice, being a poor conductor, floats and insulates the water below, which stays at about $4^\circ\text{C}$4° C. Fish and other aquatic animals survive in this liquid water beneath the ice.

(i) Energy degradation is the change of energy into forms that are less useful, mainly heat that spreads into the surroundings and cannot easily be used again. Energy is not destroyed; it becomes less useful.

(ii)

(a)

The density rises from $0^\circ\text{C}$0° C to a maximum at $4^\circ\text{C}$4° C and then falls as the temperature rises to $10^\circ\text{C}$10° C.

(b) As the surface cools to $4^\circ\text{C}$4° C, this densest water sinks and warmer water rises to be cooled. Below $4^\circ\text{C}$4° C the cooler water is less dense, so it stays at the top. The surface reaches $0^\circ\text{C}$0° C first and freezes, while the water at the bottom stays at about $4^\circ\text{C}$4° C.

(c) The bottle cracks. Water expands as it cools below $4^\circ\text{C}$4° C and expands further on freezing, so the ice needs more volume than the bottle gives it and pushes the glass apart.

(a) The greenhouse effect is the trapping of the Sun's infrared (heat) radiation near the Earth's surface by certain atmospheric gases, which keeps the Earth warm. Greenhouse gases: carbon dioxide $(CO_2)$(CO₂) and methane $(CH_4)$(CH₄) (also water vapour, nitrous oxide, CFCs).

(b) Harmful effects: (i) melting of polar ice caps and glaciers causing a rise in sea level and flooding of coastal areas; (ii) disturbance of climate and rainfall patterns, harming crops and wildlife.

(c) Use more renewable energy (solar, wind) instead of coal and oil; plant more trees (social forestry) and stop cutting forests, since trees absorb carbon dioxide. (Also: save electricity and use public transport.)

(i) At $4^\circ\text{C}$4° C.

(ii) At $4^\circ\text{C}$4° C: the same mass occupies the least volume there, and density $=\dfrac{\text{mass}}{\text{volume}}$=mass/volume.

(iii) The volume increases (water expands on cooling below $4^\circ\text{C}$4° C): this is the anomalous expansion of water.

(iv) The densest water, at $4^\circ\text{C}$4° C, sinks to the bottom, and ice forms only at the surface. The ice layer insulates the water below, which stays at about $4^\circ\text{C}$4° C, so fish can live there.