10.2: Transfer of Thermal Energy

• During the day, the Sun causes the land temperature to be higher than the sea causing a sea breeze. Explain how a sea breeze is formed.

  During the day, the land is hotter as it gains thermal energy faster than the sea, so the air above the land expands to become less dense relative to the surrounding air and hence it rises. The cooler and denser surrounding air above the sea is drawn in to take the place of the heated air above the land. This movement sets up a convection current, forming a sea breeze.
• Which cup will have water at a lower temp after a while, metal or plastic?

  The cup made of metal. Metal is a better conductor of thermal energy compared to plastic. Hot water molecules with higher kinetic energy, transfer the energy to the particles in the cup via collision. The particles of the cup in contact with water vibrate more vigorously, colliding with neighboring particles and transfer thermal energy to the exterior of the cup. Metals have free electrons which will help to transfer energy more quickly, and thus the water in the metal cup lose thermal energy more quickly.
• The freezer, which helps to cool the refrigerator, is located at the top of the refrigerator. Explain why.

  The freezer placed at the top cools down the warmer air at the top. Cold air being denser, will sink to keep all parts of the refrigerator cool. Warm air at the bottom being less dense, rises to take the place of the cold air. A convection current is set up, hence keeping all food cool.
• People have survived in the cold by covering themselves with snow. Explain how a layer of snow can reduce the loss of thermal energy.

  By covering themselves with snow, the air trapped in the snow, which is a poor conductor of thermal energy, will decrease the rate of thermal energy loss from the body to the surroundings. In addition, snow is white, and white is a poor emitter of infrared radiation, hence less thermal energy from the body will be radiated away. Layers of snow also prevents the body from being in contact with the surrounding air, preventing convection current from being set up.
• A girl standing behind a brick wall next to a campfire felt the warmth from the fire. Describe how the thermal energy from the campfire will reach her.

  The brick wall is a dark color, and since dark colors are a better absorber and emitter of infrared radiation, the brick wall absorbs heat from the fire and thermal energy is conducted through the wall, emitted from the wall to the girl.