The correct option is
Both A and R are true and R is the correct explanation of A
.Explanation
The phenomenon described is governed by Joule’s Law of Heating, which states that the heat produced ($H$) in a conductor is directly proportional to the square of the current ($I$), the resistance ($R$), and the time ($t$) for which the current flows ($H = I^2Rt$).
Analysis of Assertion and Reason:
- Assertion (A): Correct. In an electric heater, the heating element glows red-hot during operation, whereas the connecting cord remains relatively cool and does not glow.
- Reason (R): Correct. The heating element is made of materials (such as Nichrome) that have very high electrical resistance. In contrast, the connecting cord is made of good conductors (like Copper) which have extremely low electrical resistance.
Justification of the Link:
The heating element and the connecting cord are connected in series, meaning the same amount of electric current ($I$) flows through both. According to Joule’s Law ($H \propto R$ when $I$ is constant):
- The heating element has high resistance, causing a large amount of heat generation. This intense heat raises the temperature sufficiently to emit visible light (glow).
- The cord has negligible resistance, resulting in negligible heat production, which is insufficient to cause glowing.
Key Takeaway:
In a series circuit, the heat dissipated by a component is directly proportional to its resistance ($P = I^2R$). High-resistance materials convert significantly more electrical energy into heat and light compared to low-resistance conductors.