Electromagnetism, Optics, and Mechanics:Sample Exam Questions

Electromagnetism, Optics, and Mechanics: Sample Exam Questions 1. Resistors are connected in series and parallel in a circuit. (a) Which type of connection reduces the effective resistance of the circuit? (b) Calculate the heat generated if two 100 Ω resistors are connected in series and operated for 2s on a 250 V supply. (c) If these resistors are connected in parallel with a 250 V supply, will the amount of heat produced increase or decrease? Validate your answer. 2. An object of height 6 cm is placed 30 cm from a concave mirror. If a real image of the object is formed at a distance of 15 cm from the mirror,What is the focal length of the mirror? 3. (a) Are the lines of magnetic force around points X and Y in the same direction? Or in a different direction? (b) Which law helped you to draw the magnetic field lines? State that law. 4. The number of coils in the secondary of a transformer is twice the number of coils in the primary. (a) Identify what type of transformer this is. (b) If the voltage in the primary is 100 V, calculate the voltage in the secondary. (c) If the current in the primary is 2A, calculate the current in the secondary. 5. When an object is placed 20 cm in front of a convex lens, the real image is formed at 30 cm from the lens. Calculate the focal length of the lens. 6. The critical angle of glass is 420. (a) What is meant by critical angle? (b) What is the angle of refraction when the angle of incidence in the glass is 42°? (c) Which phenomenon of light occurs when the angle of incidence is 40°? 7. (a) What does A25 written on the top of a gas cylinder mean? b) What is BLEVE? 8. Observe the picture. (a) Identify the device in the picture. (b) What energy change takes place when the device operates? (c) What do R₁ and R₂ in the figure indicate, and what is their purpose? Answers 1. a) Parallel connection reduces the effective resistance of the circuit. In a parallel connection, the reciprocal of the equivalent resistance is the sum of the reciprocals of individual resistances, leading to a smaller overall resistance than any individual resistor. b) H=I^2 R_(eq)t H=1.5625 * 200 * 2 H=625J The heat generated is 625J. c) The amount of heat produced will increase. The heat produced will increase (from 625 J in series to 2500 J in parallel) 2. The mirror formula is 1/f=1/v+1/u Focal length of mirror is -10cm 3. a) The direction of the lines of magnetic force around points X and Y would be the same if the current is flowing in the same direction through the wire segment at both points. b) The law that helps to draw the magnetic field lines around a current-carrying conductor is the Right-Hand Rule 4. a) This is a step-up transformer. b) For an ideal transformer, the ratio of voltages is equal to the ratio of the number of turns: V_s=200V The voltage in the secondary is 200 V. c) The current in the secondary is 1 A. 5. 6. (a) The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°. Beyond this angle, total internal reflection occurs. (b) When the angle of incidence in the glass is 42° (which is the critical angle), the angle of refraction in the rarer medium is 90°. (c) When the angle of incidence is 40° (less than the critical angle of 42°), the phenomenon of light that occurs is refraction. 7. a) A25 likely refers to the test date of the gas cylinder, indicating it was tested in January 2025 (A representing January, and 25 representing the year 2025) b) BLEVE stands for Boiling Liquid Expanded Vapor Explosion. It is a type of explosion that occurs when a vessel containing a liquid above its atmospheric boiling point ruptures, leading to the rapid expansion of the superheated liquid into a large volume of vapor. This can be caused by external heating, such as a fire, which weakens the vessel and increases the internal pressure 8. (a) The device in the picture is a simple electric motor. (b) When the device operates, electrical energy is converted into mechanical energy. This is the fundamental principle of an electric motor, where the interaction between a magnetic field and an electric current produces a force that causes rotation. (c) R1 and R2 in the figure indicate the split-ring commutator (or split ring). Their purpose is to reverse the direction of the current in the coil every half rotation, ensuring that the torque on the coil always acts in the same direction, thus enabling continuous rotation of the moto