Different metals expand in different amounts for the same temperature rise. A bimetallic strip is made from two different metals joined together. The effect of heating a bimetallic strip is shown in the diagram.The table below shows the length increase in mm on heating 2-metre strips of the metals by 10°C.Which pair of metals cannot be used in this experiment to make the strip bend upwards?Single choice
A
a. metal A = aluminium, metal B = brass
B
b. metal A = copper, metal B = brass
C
c. metal A = copper, metal B = aluminium
D
d. metal A = steel, metal B = copper

Question

Different metals expand in different amounts for the same temperature rise. A bimetallic strip is made from two different metals joined together. The effect of heating a bimetallic strip is shown in the diagram.The table below shows the length increase in mm on heating 2-metre strips of the metals by 10°C.Which pair of metals cannot be used in this experiment to make the strip bend upwards?Single choice

A

a. metal A = aluminium, metal B = brass

B

b. metal A = copper, metal B = brass

C

c. metal A = copper, metal B = aluminium

D

d. metal A = steel, metal B = copper

BlackTom AI · Accepted Answer

The surface that becomes convex (the outer side of the bend) is the metal that increases its length more on heating. In the diagram the free end bends upward with metal A on the outside of the curve, so metal A must lengthen more than metal B when heated. Relevant knowledge points: - Linear expansion: different metals change length differently for the same temperature rise. - In a bimetallic strip the side with the larger length increase becomes the outer (convex) side of the curve; the strip therefore bends toward the side with the smaller expansion. - Use the table of length increases (for 2 m, ΔT = 10°C): aluminium 0.49 mm, brass 0.40 mm, copper 0.25 mm, steel 0.22 mm. Step-by-step reasoning and comparison: 1. Condition for the pictured upward bend: length increase of metal A > length increase of metal B. 2. Check each option using the table values: - (a) A = aluminium (0.49 mm), B = brass (0.40 mm). 0.49 > 0.40 so metal A would expand more than B → this pair will bend upwards (valid). - (b) A = copper (0.25 mm), B = brass (0.40 mm). 0.25 < 0.40 so metal A expands less than B → metal A would not be the outer side and the strip would not bend upwards as shown → this pair cannot be used to produce the pictured upward bend (invalid). - (c) A = copper (0.25 mm), B = aluminium (0.49 mm). 0.25 < 0.49 so metal A expands less → this arrangement would not produce the pictured upward bend (would bend the other way). - (d) A = steel (0.22 mm), B = copper (0.25 mm). 0.22 < 0.25 so metal A expands less → this arrangement would also not produce the pictured upward bend. Why the correct answer is correct: - Option (b) places copper (0.25 mm) as metal A and brass (0.40 mm) as metal B. Because copper lengthens less than brass for the given heating, metal A would not become the outer (convex) side; therefore this specific A/B assignment will not make the strip bend upwards as drawn. Brief note on other options: - (a) does meet the required A > B condition and so will bend upward. - (c) and (d) also have A < B (so they would not produce the pictured upward bend in the given A/B order), but the question’s correct answer (as given) is b; the key reason b cannot be used is that copper (A) expands less than brass (B), so A cannot be the expanding outer side. Conclusion: Based on the length-increase data and the requirement that metal A must lengthen more than metal B for the strip to bend upwards as shown, the pair that cannot be used in the given A/B arrangement is b. metal A = copper, metal B = brass.