Heat and Thermodynamics - SS2 Physics Past Questions and Answers - page 2

Conduction is the heat transfer mechanism that occurs through direct contact between particles within a substance. In solids, heat is transferred through the vibration and collision of atoms or molecules. Good conductors, such as metals, have high thermal conductivity, enabling efficient heat transfer. Examples of conduction include touching a hot object and feeling the heat transfer from the object to your hand, or heating a metal spoon in a hot cup of tea.

Convection is the heat transfer mechanism that occurs in fluids (liquids or gases) through the movement of particles. It involves the transfer of heat by the bulk movement of the fluid. Natural convection occurs when heat is transferred due to the density differences within the fluid. For example, hot air rising and cool air sinking in a room is an example of natural convection. Forced convection, on the other hand, is when the fluid movement is induced by an external force, such as a fan or a pump. Cooling a computer with a fan or circulating heated water in a radiator are examples of forced convection.

Radiation is the heat transfer mechanism that occurs through electromagnetic waves without the need for a medium. Unlike conduction and convection, radiation can occur in a vacuum. Heat energy is emitted in the form of electromagnetic waves, typically in the infrared region. Radiation does not require direct contact between objects. It can travel through space and is responsible for the transfer of heat from the Sun to the Earth. Other examples include feeling the warmth of a campfire or the heat emitted by a glowing filament in an incandescent bulb. Radiation also plays a crucial role in applications such as infrared heating, solar panels, and thermal imaging.

Conduction, convection, and radiation are all heat transfer mechanisms, but they differ in how heat is transferred. Conduction occurs through direct contact between particles, primarily in solids. It is efficient in materials with high thermal conductivity and finds applications in cooking, metalworking, and insulation materials.

Convection involves the movement of fluids and is more effective in liquids and gases. Natural convection occurs due to density differences, while forced convection requires external forces. It is used in heating, cooling, and ventilation systems. Radiation is the transfer of heat through electromagnetic waves and is the only mechanism that can occur in a vacuum. It is essential for heat transfer from the Sun, and its applications include heating, cooking, and thermal imaging.

To calculate the heat energy, we use the formula: Q = m c ΔT

   Q = 50 g x 0.897 J/g°C x (80°C - 20°C)

   Q = 50 g x 0.897 J/g°C x 60°C

   Q = 2691 J

In this case, we can use the principle of conservation of energy. The heat lost by the copper is equal to the heat gained by the water.

   Q(copper) = Q(water)

   m(copper) x c(copper) x ΔT(copper) = m(water) x c(water) x ΔT(water)

   100 g x c(copper) x (25°C - 100°C) = 200 g x 4.18 J/g°C x (25°C - 20°C)

   -75 c(copper) = 200 g x 4.18 J/g°C x 5°C

   c(copper) = - (200 g x 4.18 J/g°C x 5°C) / 75

   c(copper) ≈ - 55.73 J/g°C

  Note: The negative sign indicates that heat was lost by the copper.

We can rearrange the formula  Q = m c ΔT to solve for ΔT:

   ΔT = Q / (m x c)

   ΔT = 4,000 J / (500 g x 4.18 J/g°C)

   ΔT ≈ 1.91°C