In thermostat bimetallic strip is used

Question

Examoogle · Accepted Answer

Option A: True

Explanation & Concepts:
A bimetallic strip is the sensing component in a thermostat that converts temperature changes into mechanical displacement. It is used as a switch to regulate temperature in domestic electrical appliances like irons, geysers, and ovens.

Working Principle: The strip consists of two metals with different coefficients of thermal expansion bonded together. When heated, the metal with the higher coefficient expands more, causing the strip to bend. This physical deflection is used to open or close an electrical contact point, thereby breaking or completing the circuit to maintain a set temperature.

Key Formulas: $\Delta L = L_0 \alpha \Delta T$ — Linear expansion of a metal strip where $\alpha$ is the coefficient of thermal expansion.; $	heta \propto (\alpha_2 - \alpha_1) \Delta T$ — The deflection angle $	heta$ is proportional to the difference in coefficients of thermal expansion.

Key Points: The strip uses the principle of differential thermal expansion.; Common materials used are brass ($\alpha \approx 19 	imes 10^{-6}/K$) and Invar ($\alpha \approx 1.2 	imes 10^{-6}/K$).; It acts as both a temperature sensor and a mechanical actuator.

Advantages: Cost-effective and simple construction.; Requires no external power source for the sensing mechanism.; Reliable operation for cyclic switching.

Disadvantages: Slow response time compared to electronic sensors.; Susceptible to fatigue over many cycles.; Limited accuracy in precise temperature control.

Applications: Automatic electric irons; Water heaters (geysers); Circuit breakers (thermal overload protection)

Additional Info: The choice of metals is critical; one must have a high coefficient of thermal expansion, and the other must have a very low one.. If the bimetallic strip did not bend correctly, the appliance would either not heat up or fail to turn off, leading to fire hazards.

Answer: A is correct — A bimetallic strip is utilized in thermostats to provide automatic temperature control by converting thermal energy into mechanical movement.

Exam Tip: In competitive exams, remember that bimetallic strips are also used in overload relays to protect motors from thermal damage due to sustained overcurrent.

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Option B: False

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