A room is heated by means of a charcoal fire, an occupants of the room standing away from the fire is warmed mainly by

Analysis

The question asks for the primary method by which heat is transferred from a charcoal fire to a person standing at a distance in a room. We need to distinguish between the three main modes of heat transfer: conduction, convection, and radiation.

Concept Explanation

• Conduction: This is the transfer of heat through direct contact between particles of a solid. It requires a medium and physical touch (e.g., touching a hot metal spoon).
• Convection: This is the transfer of heat through the actual movement of fluid particles (liquids or gases). Warm air rises and cool air sinks, creating convection currents. While this warms the air in the room, it is not the primary way a person feels the "direct" heat from a fire source at a distance.
• Radiation: This is the transfer of heat in the form of electromagnetic waves (specifically infrared radiation). Unlike conduction and convection, radiation does not require a material medium to travel. It can pass through a vacuum or air, which is why you can feel the warmth of a fire or the sun even when you are not touching the source or standing directly in the path of rising hot air.

Step-by-Step Solution

1. Identify the source: The charcoal fire emits heat energy.
2. Evaluate the distance: Because the occupant is standing away from the fire, heat cannot reach them through conduction (no physical contact).
3. Evaluate the medium: While convection currents circulate warm air throughout the room, the "instant" or "direct" warmth felt from a fire source at a distance is primarily due to infrared rays traveling through the air.
4. Conclusion: Heat transfer by electromagnetic waves is defined as radiation. This is the most efficient way for heat to travel across an open space to an occupant.

Answer

The correct option is B.

Exam Tip

Think of the sun: it warms the Earth through the vacuum of space where there are no particles for conduction or convection to occur. Whenever you see a question about heat transfer across a distance or through empty space, the answer is almost always Radiation.