Radiation is direct heat transfer from a higher-temperature energy source to a cooler target. Which statement is true?

Radiation is direct heat transfer from a higher-temperature energy source to a cooler target. Which statement is true?

• A. There must be a difference in temperature between the energy source and the target area.
• B. There must be direct contact between the energy source and the target area.
• C. The rate is influenced only by the energy source temperature.
• D. A medium is required to transmit energy.

Answer: (A)

Radiation transfers heat by electromagnetic waves and can occur without direct contact or a medium; heat reaches through space just as sunlight warms the Earth. For there to be net heat transfer by radiation, there must be a temperature difference between the source and the target—the hotter object will transfer energy to the cooler one. That makes the statement true: there must be a difference in temperature between the energy source and the target area. If the temperatures were the same, there would be no net radiative transfer, even though all objects emit some radiation. The rate of this transfer isn’t determined solely by the source’s temperature; it also depends on the target’s temperature, surface area, emissivity, and the distance between them. The other points—needing direct contact or a medium, or the rate depending only on the source temperature—aren’t accurate for radiation.

Radiation transfers heat by electromagnetic waves and can occur without direct contact or a medium; heat reaches through space just as sunlight warms the Earth. For there to be net heat transfer by radiation, there must be a temperature difference between the source and the target—the hotter object will transfer energy to the cooler one.

That makes the statement true: there must be a difference in temperature between the energy source and the target area. If the temperatures were the same, there would be no net radiative transfer, even though all objects emit some radiation. The rate of this transfer isn’t determined solely by the source’s temperature; it also depends on the target’s temperature, surface area, emissivity, and the distance between them.

The other points—needing direct contact or a medium, or the rate depending only on the source temperature—aren’t accurate for radiation.