20. Heat and Temperature

A leaf of area 40cm² and mass 4.5 x 10⁻⁴ kg directly faces the Sun on a clear day. The leaf has an emissivity of 0.85 and a specific heat of 0.80 kcal/kgK. Estimate the energy absorbed per second by the leaf from the Sun.

300g of water at 40°C is in an insulated cup. What mass of copper at 500°C must be added to heat the water to 60°C?

Which of the following best describes radiation heat transfer?

Which primary type of heat transfer is used by most household clothes dryers to dry clothes?

When a metal spoon is placed in a pot of boiling water, which mechanism is primarily responsible for the transfer of heat from the water to the spoon?

Liquid helium, with a boiling point of 4.2 K, is used in ultralow-temperature experiments and also for cooling the superconducting magnets used in MRI imaging in medicine. Storing liquid helium so far below room temperature is a challenge because even a small 'heat leak' will boil the helium away. A standard helium dewar, shown in FIGURE P19.67, has an inner stainless-steel cylinder filled with liquid helium surrounded by an outer cylindrical shell filled with liquid nitrogen at –196°C. The space between is a vacuum. The small structural supports have very low thermal conductivity, so you can assume that radiation is the only heat transfer between the helium and its surroundings. Suppose the helium cylinder is 16 cm in diameter and 30 cm tall and that all walls have an emissivity of 0.25. The density of liquid helium is 125 kg/m³ and its heat of vaporization is 2.1×10⁴ J/kg. What is the mass of helium in the filled cylinder?

(I) One end of a 64-cm-long copper rod with a diameter of 2.0 cm is kept at 460°C, and the other is immersed in water at 22°C. Calculate the heat conduction rate along the rod.

Which of the following actions would decrease $Q$ (heat transfer) between two objects?

What is the primary driving force for heat transfer between two objects?

A 12-g lead bullet traveling at 220 m/s passes through a thin wall and emerges at a speed of 160 m/s. If the bullet absorbs 50% of the heat generated, If the bullet’s initial temperature was 20°C, will any of the bullet melt, and if so, how much?

Heat from the sun reaches Earth primarily through which type of heat transfer?

The process by which heat moves through air or liquid is called:

In the context of heat transfer, does convection always transfer heat more quickly than conduction in all materials and situations?

In the context of heat transfer, how does heat flow between two objects?

A mountain climber wears a goose-down jacket 3.8 cm thick with total surface area 0.95 m². The temperature at the surface of the clothing is -18°C and at the skin is 34°C. Determine the rate of heat flow by conduction through the jacket assuming (a) it is dry and the thermal conductivity k is that of goose down, and (b) the jacket is wet, so k is that of water and the jacket has matted to 0.50 cm thickness.