CIE iGCSE Co-Ordinated Science P4.5.5 The d.c. motor Exam Style Questions Paper 4
Question
Fig. 9.1 shows a simple d.c. motor with a coil of wire containing 100 turns.
(a) The current in the coil causes forces to act on the coil, which make it turn about its axis.
(i) Fig. 9.1 shows a force of 1.2N acting at 90° to the coil, at a distance of 3.5cm from the axis.
Calculate the moment of the force on the coil.
moment = …………………………………………… Nm
(ii) Suggest how the magnitude of the force in (a)(i) changes when both the number of turns
on the coil is doubled and the current is doubled.
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…………………………………………………………………………………………………………………….
(b) Fig. 9.2 shows a toy boat. The toy boat uses a motor similar to that shown in Fig. 9.1 to propel the toy boat across a pond.
The toy boat has a mass of 0.60kg and travels at a maximum speed of 3.0m/s.
Calculate the maximum kinetic energy of the toy boat.
State the unit for your answer.
kinetic energy = …………………… unit …………………….
(c) Fig. 9.3 shows a speed-time graph for part of the toy boat’s journey.
(i) Use Fig. 9.3 to describe the motion of the toy boat for this part of the journey.
………………………………………………………………………………………………………………………….
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(ii) Suggest why the shape of this graph is not a realistic description of the motion of the toy boat at 1.5 minutes.
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▶️Answer/Explanation
Ans: 9(a)(i) (3.5 cm =) 0.035 (m) ;
(moment =) f × d / 1.2 × 0.035 ;
(moment =) 0.042 (N m) ;
or
(35 cm =) 0.35 (m) ;
(moment =) f × d / 1.2 ×0.35 ;
(moment =) 0.42 (N m);
9(a)(ii) increases ;
by a factor of 4;
9(b) (kinetic energy =) 1⁄2 mv$^{2}$ or 1⁄2 × 0.60 × 3.0$^{2}$;
(kinetic energy = ) 2.7 ;
J / joules ;
9(c)(i) initially / in first 1.5 mins, constant acceleration ;
then / after 1.5 min, acceleration is zero / constant speed ;
9(c)(ii) (idea that) change in acceleration would take some time / change more gradually / graph would be a curve at 1.5 mins ;