Written Paper Section I Question 21-23 from the 2001 HSC Examination
Summary: Q21. Explain why no resistance is required when a large d.c. motor is running at high speed, but a substantial resistance is needed when the motor is starting up. Q22. Identify the direction of the force that exists between two parallel wires, each with a current flowing in the same direction along the wires. On a set of axes, sketch a graph that shows how the force between the two wires would vary if the length of the shorter wire were increased. Explain how these results demonstrate the moto-effect. Q23. Discuss the effects of the development of electrical generators on society and the environment. Outline the methods of X-ray diffraction used by the Braggs to determine the structure of crystals.