A bimetal strip is made of copper and iron welded side by side. It is straight at room temperature and held vertically with the iron strip on the left and the copper strip on the right. When the strip is heated, in which direction will it bend?

Introduction / Context:
Bimetal strips are widely used in thermostats and temperature sensing devices because two different metals bonded together expand by different amounts when heated. This difference in expansion causes the strip to bend in a predictable direction as temperature changes. The question describes a copper iron bimetal strip and asks you to determine which way it bends when heated.

Given Data / Assumptions:

• The strip is made of two metals, copper and iron, welded along their length.
• At room temperature the strip is straight.
• The strip is held vertically, with iron on the left side and copper on the right side.
• The strip is heated uniformly along its length.

Concept / Approach:
Different metals have different coefficients of linear expansion. Copper has a higher coefficient of expansion than iron, which means that for the same rise in temperature and the same original length, copper expands more than iron. In a bimetal strip, the two metals are rigidly bonded, so they must stay attached along their common surface. When heated, the metal that tries to expand more becomes the outer, longer side of the bend, while the metal that expands less becomes the inner, shorter side. Therefore, when a copper iron strip is heated, the copper side, which expands more, will be on the convex (outer) side, and the strip will bend towards the iron side.

Step-by-Step Solution:
Step 1: Recall that the coefficient of linear expansion of copper is greater than that of iron. Step 2: When the strip is heated, copper attempts to increase its length more than iron does. Step 3: Because the two metals are welded together, they cannot separate and must bend to accommodate the different expansions. Step 4: The metal that expands more will form the outer side of the curve, while the one that expands less will form the inner side. Step 5: In this case the copper strip is on the right and expands more, so it becomes the outer, longer side of the bend. Step 6: Therefore, the strip bends towards the left, where the iron strip is located and acts as the inner side of the curve.

Verification / Alternative check:
Thermostats in some appliances use bimetal strips specifically because they bend predictably when heated. If you look up tables of coefficients of thermal expansion, you will see that copper has a value around 16 to 17 * 10^-6 per kelvin, while iron is around 11 to 12 * 10^-6 per kelvin. This confirms that copper length increases more for each degree of temperature rise. Experimental demonstrations with bimetal strips taped or clamped at one end show that the strip curves toward the metal with the smaller expansion coefficient when heated.

Why Other Options Are Wrong:
It will remain straight: If the strip remained straight, it would imply that both metals expanded by exactly the same amount, which is not true for copper and iron.

It will bend towards the right (copper side): This would require iron to expand more than copper, opposite to the actual material properties.

Its bending direction is completely random: The direction is determined by the relative expansion coefficients and is entirely predictable.

Common Pitfalls:
Students sometimes remember that one side expands more but then incorrectly assume that the strip bends towards that side. It is important to visualise the geometry: the longer side must lie on the outside of the curve, so the strip bends toward the shorter side. Another mistake is to confuse which material has the higher expansion coefficient. Consulting standard material property tables helps keep these facts clear.

Final Answer:
When heated, the copper iron bimetal strip will bend towards the left, that is, towards the iron side.