Q258 · Practice questionComplex unfamiliar6 marks
QUESTION 258 (6 marks)
A U-shaped magnet rests on a digital scale. A horizontal wire is held above it by a separate support and does not touch it. A wire segment lies perpendicular to the field. The graph shows scale reading in kg against conventional current . For positive , the magnetic force on the wire is upward. Use .
a)[1 mark]
Read the magnet mass when the current is zero.
b)[3 marks]
Determine the field strength from the graph.
c)[2 marks]
Predict the scale reading at and explain its change from the zero-current value.
WORKED SOLUTION
6 marksPractice marking scheme
ANSWER
(a) . (b) . (c) ; current reversal gives an upward reaction force on the magnet.
Worked solution
(a) The wire is separately supported. At zero current no wire-magnet magnetic force acts, so the intercept is the magnet mass.
(b) The magnet experiences a downward reaction force , so . The graph gradient is . Hence .
(c) . The force on the wire is now down; the opposite force on the magnet is up and reduces its support force.
Equivalent physically justified methods, alternative valid sketches and consistent units accepted.
Accept sensible significant figures and consistent rounding from stated constants.
Part a: Read the zero-current intercept.
Part b: Find the scale-reading gradient with its units.
Part b: Relate the reading change times g to the magnetic reaction force.
Part b: Determine B.
Part c: Calculate the reading.
Part c: Explain the reaction-force reversal.
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusHow many marks did you earn?
Compare your working with the guide above.