Electromagnetism

Physics, HSSC-KPK Second Year. This chapter is split into 32 topics and has 797 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Cyclotron motion, e/m determination and the velocity selector 26 questions
2. Galvanometer construction and sensitivity 29 questions
3. Conversion of galvanometer into ammeter and voltmeter 28 questions
4. Applications of magnetic field and motion of a charged particle in a magnetic field 27 questions
5. Torque on a current-carrying loop and MRI 26 questions
6. Magnetic field and force on a current-carrying conductor 30 questions
7. Magnetic flux and flux density 27 questions
8. AVO meter and multimeter 26 questions
9. Ampere's law and the magnetic field of straight wires and solenoids 29 questions
10. Force on a current-carrying wire: numerical problems 30 questions
11. Direction of force on a current-carrying wire (Fleming's left-hand rule) 24 questions
12. Magnetic field of a long straight wire: numerical problems 26 questions
13. Magnetic field inside a solenoid: numerical problems 27 questions
14. Magnetic flux: numerical problems 28 questions
15. Charged particle in a magnetic field: force and radius numerical problems 25 questions
16. Cyclotron period, frequency and charge-to-mass ratio: numerical problems 25 questions
17. Velocity selector and e/m measurement: numerical problems 25 questions
18. Torque on a current-carrying coil: numerical problems 25 questions
19. Galvanometer deflection and current sensitivity: numerical problems 24 questions
20. Conversion of a galvanometer into an ammeter: numerical problems 25 questions
21. Conversion of a galvanometer into a voltmeter: numerical problems 26 questions
22. Meter loading and ohmmeter calculations 21 questions
23. Magnetic field and field lines: conceptual reasoning 22 questions
24. Force on a current-carrying conductor: reasoning and applications 23 questions
25. Magnetic flux and flux density: units and reasoning 22 questions
26. Ampere's law and the solenoid field: derivation and reasoning 21 questions
27. Charged particles in magnetic fields: conceptual reasoning 21 questions
28. Torque on a coil and MRI: conceptual reasoning 22 questions
29. Moving-coil galvanometer design and working: reasoning 21 questions
30. Ammeters, voltmeters and AVO meters: conceptual reasoning 21 questions
31. Electromagnetism: assertion and reason questions 25 questions
32. Electromagnetism: sequences and ordering 20 questions

Sample questions: Electromagnetism

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. If a charged particle enters a uniform magnetic field at an arbitrary angle (neither exactly parallel nor exactly perpendicular to B) rather than perpendicularly, what path does it follow, as shown in Figure 13.10?

From your book: Figure 13.10 (page 121) Full page Open book

• A. A perfect straight line, identical to the perpendicular case
• B. A perfect circle, identical to the perpendicular case
• C. A helical (spiral) path, since the velocity component along B is unaffected while the perpendicular component causes circular motion(correct answer)
• D. The particle simply stops

Answer: C.As shown in Figure 13.10, when the charge enters the field at an angle other than 0° or 90°, the velocity component parallel to B is unaffected (constant), while the perpendicular component produces circular motion — the combination is a helical (spiral) path, as described in the text ('no longer parallel to B').

Q2. In the lamp and scale method of observing a galvanometer's deflection, how is the (typically small) angle of deflection made easier to read precisely?

• A. The observer directly reads the angle off the coil with a protractor
• B. A beam of light is directed at a small mirror attached to the coil; after reflection, it produces a spot on a translucent scale placed a set distance away, and the spot's displacement is proportional to the (small) angle of deflection(correct answer)
• C. There is no way to make small deflections easier to read
• D. The coil is photographed at high speed and measured digitally

Answer: B.In sensitive galvanometers, a beam of light is directed at a small mirror on the coil, and after reflection produces a spot on a translucent scale placed at a distance (e.g., one metre); the displacement of the spot is proportional to the angle of deflection (for small angles), effectively magnifying the readable displacement.

Q3. Describe the use of an avometer/multimeter (analogue and digital) — a stated chapter learning objective. Which key structural feature is shared by both analog AVO meters and digital multimeters?

• A. Digital multimeters cannot measure resistance at all, unlike analog AVO meters
• B. Both use a selector mechanism to choose between measuring current, voltage, or resistance, and both require the meter to convert or condition the signal before display (mechanically via shunt/series resistors for analog, electronically via amplification for digital)(correct answer)
• C. There is no meaningful similarity between analog AVO meters and digital multimeters
• D. Both use exactly the same mechanical moving-coil mechanism with no electronic components whatsoever

Answer: B.Both analog AVO meters and digital multimeters share the fundamental structure of a selectable measurement mode (current/voltage/resistance) combined with appropriate signal conditioning — mechanical resistor networks for the analog galvanometer-based AVO meter, and electronic amplification/conversion for the digital multimeter.

Q4. Solve problems involving the use of F=BILsinθ (a stated chapter learning objective, closely tied to flux concepts). If sinθ in this force formula is replaced by cosθ in the related flux formula Φ=BAcosθ, why the difference?

• A. Force depends on the angle between the current direction (along the wire) and B, which is maximum at 90° (sin90°=1); flux depends on the angle between the area's NORMAL vector and B, which is maximum at 0° (cos0°=1) — the two formulas use different reference angles by convention(correct answer)
• B. The difference is arbitrary and has no physical basis
• C. Force and flux are actually the same physical quantity with different names
• D. This is a printing error; both formulas should use the identical trigonometric function

Answer: A.F=BILsinθ uses θ as the angle between the wire (current direction) and B (force is max when wire⊥B, i.e., θ=90°), while Φ=BAcosθ uses θ as the angle between the area's normal vector and B (flux is max when the area is ⊥B, i.e., normal ∥B, θ=0°) — different angle conventions for physically related but distinct concepts.

Q5. Apply Ampere's law to find magnetic flux density around a wire and inside a solenoid (a stated chapter learning objective); by contrast, in the e/m and cyclotron formulas, does Ampere's law itself play any direct role?

• A. Ampere's law has no application anywhere in this chapter
• B. No — the e/m determination and cyclotron-motion formulas are derived from Newton's second law applied to circular motion (mv²/r=qvB) combined with the Lorentz force, not from Ampere's law, which instead relates field to source current, a separate concept(correct answer)
• C. Yes, Ampere's law is the sole basis for deriving all of the cyclotron and e/m formulas
• D. Ampere's law and the cyclotron formulas are exactly the same equation written differently

Answer: B.Ampere's law (ΣB·ΔL=μ0I) relates a magnetic field to the current that PRODUCES it (used for straight wires and solenoids); the e/m and cyclotron formulas instead come from applying Newton's second law (centripetal force) to a charge already MOVING THROUGH a given external field — two distinct applications of magnetism.

Q6. How does the AVO meter's use of a single galvanometer for three different measurement functions (current, voltage, resistance) exemplify an efficient instrument design?

• A. Rather than requiring three separate meters, the AVO meter reuses one sensitive galvanometer, switching in different resistor networks (shunt for current, series resistor for voltage, internal battery + resistor for resistance) via a single selector switch(correct answer)
• B. There is no shared component; each function uses entirely independent, unrelated hardware
• C. The AVO meter actually contains three completely separate galvanometers, one for each function
• D. The AVO meter cannot measure more than one quantity; the name is purely historical

Answer: A.As described in 13.11, the AVO meter's core is a single galvanometer, with a selector switch connecting different resistor networks (a shunt combination for current, a series resistance for voltage, or the battery-powered resistance circuit for ohms) to reuse that one sensitive movement for three distinct measurement functions.

Q7. What practical trade-off exists between a highly sensitive (lamp-and-scale) galvanometer and a less sensitive (pivoted pointer) galvanometer?

• A. There is no meaningful trade-off; the two types are functionally and practically identical in every respect
• B. The pivoted type is always more sensitive than the lamp-and-scale type
• C. The lamp-and-scale type requires no calibration at all, unlike the pivoted type
• D. The lamp-and-scale type offers much higher sensitivity/precision for detecting tiny currents but is bulkier, more delicate, and requires careful setup (light source, distant scale); the pivoted type is more robust, compact, and portable but less sensitive(correct answer)

Answer: D.The lamp-and-scale (suspended coil, mirror) design achieves much finer sensitivity by optically magnifying small angular deflections over a set distance, but is bulkier and more delicate; the pivoted-coil (hairspring, pointer) design is sturdier and more compact/portable for everyday use, at the cost of lower sensitivity.

Q8. For an AVO meter's ohmmeter scale, why does the scale read in the opposite direction compared to the current/voltage scales (i.e., zero ohms is at one end where full-scale current/voltage would be, and infinite ohms at the other)?

• A. The scale direction is arbitrary and has no relationship to how the circuit works
• B. Ohmmeters cannot have a numbered scale at all, only a pass/fail indicator
• C. Because a LOWER external resistance allows MORE current to flow from the internal battery through the galvanometer (larger deflection), while a HIGHER external resistance allows LESS current (smaller deflection) — the relationship between resistance and deflection is inverse, unlike the direct relationship for current/voltage measurement(correct answer)
• D. The ohmmeter scale is identical in direction to the current/voltage scales, with no difference at all

Answer: C.Since a lower external resistance allows more current to flow from the ohmmeter's internal battery (larger galvanometer deflection), while a higher resistance restricts current (smaller deflection), the ohms scale is inherently inverted relative to the deflection compared to a direct current/voltage scale — hence 0Ω corresponds to full-scale deflection and ∞Ω corresponds to zero deflection.

Q9. What is the fundamental reason an AVO meter (or any multimeter) cannot measure resistance while the component being tested is still connected to a live/powered circuit?

• A. Resistance measurement requires the component to be physically removed and destroyed first
• B. AVO meters cannot measure resistance under any circumstances, live or not
• C. There is no such restriction; resistance can always be measured safely on a fully powered, live circuit
• D. The meter's own internal battery supplies the test current for resistance measurement; any additional external voltage/current from a live circuit would interfere with or overwhelm this internal test current, giving an inaccurate or even dangerous reading(correct answer)

Answer: D.Since the ohmmeter function relies on the meter's own internal battery to supply a small, known test current through the component, any external voltage present (from a live circuit) would add to or interfere with this internal current, producing an inaccurate (or unsafe) reading — hence resistance measurements must be made on de-energized components.

Q10. An AVO meter's ohmmeter uses an internal battery of 1.5V and has a 'mid-scale' resistance setting of 1000Ω (meaning a 1000Ω external resistor gives half-scale deflection, corresponding to half the maximum current). What is the full-scale current of the galvanometer used, given that the meter's own internal resistance (excluding the external test resistor) is also 1000Ω at this range?

• A. 0.375 mA
• B. 0.75 mA(correct answer)
• C. 3 mA
• D. 1.5 mA

Answer: B.At mid-scale (half deflection = half of full-scale current Ig), the external 1000Ω plus internal 1000Ω gives total 2000Ω; full-scale current occurs when external R=0, giving Ig=1.5V/1000Ω=1.5mA. Half-scale current (at external=1000Ω, total=2000Ω) = 1.5V/2000Ω=0.75mA — confirming Ig(full-scale)=1.5mA is consistent, and 0.75mA is the half-scale value being asked about here.