A wire carrying current sits in a magnetic field. The force depends on the field strength (flux density), the current and the length of wire in the field. Each choice gives all three.
Pick the row where the wire feels the smallest force.
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Force = magnetic field strength × current × length. So multiply the three numbers in each row. The units are the same in every row, so the products compare directly.. First row:. 6 × 4 × 0.8 = 19.2. Second row:. 8 × 4 × 0.5 = 16. Third row:. 8 × 5 × 0.4 = 16. Last row:. 12 × 6 × 0.2 = 14.4. The last row is the smallest, which in newtons is 0.0144 N. The first row has the weakest field, but its long wire gives the biggest force, so choosing by one value alone fails. The second and third rows tie at 0.016 N, both larger than the last row.
How the marks are earned
- Choose the last row, 12 mT, 6 A, 0.2 m: force = field strength × current × length gives 12 × 6 × 0.2 = 14.4, the smallest of the four (0.0144 N).
- The right option alone earns the mark with no working needed. One answer only: two answers earn nothing, and a circled or underlined right option counts if the box is blank.