⚡ Physics — Electricity, Magnetism and Modern Physics

1. According to Coulomb's law, the electrostatic force between two point charges is...

1. directly proportional to the sum of the charges and inversely proportional to the distance between them
2. inversely proportional to the product of the charges and directly proportional to the square of the distance between them
3. directly proportional to the product of the charges and inversely proportional to the square of the distance between them
4. directly proportional to the product of the charges and inversely proportional to the distance between them

Coulomb's law states F = kq1q2/r^2, so the force is proportional to the product of the charges and inversely proportional to the square of their separation. (JAMB UTME Physics Syllabus, Electrostatics (Coulomb's law))

2. In Coulomb's law, F = kq1q2/r^2, the value of the constant k in a vacuum is approximately...

1. 6.67 x 10^-11 N m^2 kg^-2
2. 9.0 x 10^9 N m^2 C^-2
3. 1.602 x 10^-19 N m^2 C^-2
4. 3.0 x 10^8 N m^2 C^-2

The Coulomb constant k = 1/(4 pi epsilon0) has the value 9.0 x 10^9 N m^2 C^-2, the standard value used in JAMB electrostatics calculations. (CODATA/NIST recommended values; JAMB UTME Physics Syllabus, Electrostatics)

3. Two point charges of +2 x 10^-6 C and +3 x 10^-6 C are placed 0.3 m apart in air. Using k = 9.0 x 10^9 N m^2 C^-2, the force between them is approximately...

1. 6.0 N
2. 0.06 N
3. 60 N
4. 0.6 N

F = kq1q2/r^2 = (9.0 x 10^9 x 2 x 10^-6 x 3 x 10^-6)/(0.3)^2 = 0.6 N. (JAMB UTME Physics Syllabus, Electrostatics (application of Coulomb's law))

4. If the distance between two point charges is tripled while the charges remain unchanged, the electrostatic force between them becomes...

1. three times the original value
2. one-third of the original value
3. one-ninth of the original value
4. nine times the original value

Since force is inversely proportional to r^2, tripling the distance reduces the force by a factor of 3^2 = 9, so the new force is one-ninth of the original. (JAMB UTME Physics Syllabus, Electrostatics (inverse square law in Coulomb's law))

5. The magnitude of the charge on a single electron is approximately...

1. 9.0 x 10^9 C
2. 1.602 x 10^-19 C
3. 6.626 x 10^-34 C
4. 3.0 x 10^8 C

The elementary charge, the magnitude of charge carried by an electron or proton, is 1.602 x 10^-19 coulomb. (CODATA 2018 (NIST); JAMB UTME Physics Syllabus, Structure of the Atom)

6. Given that the charge on an electron is 1.602 x 10^-19 C, the number of electrons required to make up a total charge of 1 coulomb is approximately...

1. 1.602 x 10^19
2. 9.0 x 10^9
3. 6.25 x 10^18
4. 1.0 x 10^-19

Number of electrons = Q/e = 1/(1.602 x 10^-19) is approximately 6.25 x 10^18 electrons. (JAMB UTME Physics Syllabus, Electrostatics/Structure of the Atom (elementary charge))

7. Capacitance of a capacitor is defined as...

1. the potential difference per unit charge stored
2. the product of charge and potential difference
3. the energy stored per unit charge
4. the charge stored per unit potential difference across it

Capacitance C = Q/V, the charge stored per unit potential difference, and is measured in farads. (JAMB UTME Physics Syllabus, Capacitors)

8. When two capacitors are connected in series, the combined capacitance is...

1. equal to the sum of the two capacitances
2. less than the smaller of the two individual capacitances
3. greater than the larger of the two individual capacitances
4. equal to the product of the two capacitances

For capacitors in series, 1/C = 1/C1 + 1/C2, so the combined capacitance is always smaller than the smallest individual capacitance, unlike resistors in series. (JAMB UTME Physics Syllabus, Capacitors (series and parallel arrangement))

9. A capacitor of capacitance 4 microfarads is charged to a potential difference of 100 V. The energy stored in the capacitor is...

1. 0.2 J
2. 2.0 J
3. 0.02 J
4. 0.002 J

Energy stored W = (1/2)CV^2 = 0.5 x 4 x 10^-6 x (100)^2 = 0.02 J. (JAMB UTME Physics Syllabus, Capacitors (energy stored in a charged capacitor))

10. Faraday's law of electromagnetic induction states that the magnitude of the induced e.m.f. in a circuit is...

1. directly proportional to the magnetic flux only
2. directly proportional to the rate of change of magnetic flux linkage
3. inversely proportional to the rate of change of magnetic flux linkage
4. independent of the rate of change of magnetic flux

Faraday's law states that induced e.m.f. is proportional to the rate of change of flux linkage through the circuit. (JAMB UTME Physics Syllabus, Electromagnetic Induction (Faraday's law))

11. Lenz's law states that the direction of an induced current in a conductor is always such that it...

1. supports the change in magnetic flux that produces it
2. is independent of the change in magnetic flux
3. flows only when the conductor is stationary
4. opposes the change in magnetic flux that produces it

Lenz's law, a consequence of energy conservation, states that the induced current opposes the change producing it. (JAMB UTME Physics Syllabus, Electromagnetic Induction (Lenz's law))

12. In an a.c. generator, if the thumb, first finger and second finger of the right hand are held mutually at right angles to represent motion, field and induced current respectively, the rule used to find the direction of the induced current is called...

1. Fleming's right-hand rule
2. Fleming's left-hand rule
3. the right-hand grip rule
4. Lenz's law

Fleming's right-hand rule (thumb = motion, first finger = field, second finger = induced current) gives the direction of induced current in generators. (JAMB UTME Physics Syllabus, Electromagnetic Induction (Fleming's right-hand rule for generators))

13. For an ideal transformer, the ratio of secondary voltage to primary voltage, Vs/Vp, is equal to...

1. the ratio of primary turns to secondary turns, Np/Ns
2. the ratio of secondary current to primary current, Is/Ip
3. the ratio of secondary turns to primary turns, Ns/Np
4. the product of the turns on both coils

For an ideal transformer, Vs/Vp = Ns/Np, the voltage ratio equals the turns ratio. (JAMB UTME Physics Syllabus, Electromagnetic Induction (the transformer))

14. A transformer has 200 turns on its primary coil and 1000 turns on its secondary coil. If the primary voltage is 12 V, the secondary voltage is...

1. 12 V
2. 60 V
3. 2.4 V
4. 600 V

Vs = Vp x (Ns/Np) = 12 x (1000/200) = 60 V; since Ns is greater than Np, this is a step-up transformer. (JAMB UTME Physics Syllabus, Electromagnetic Induction (transformer turns ratio))

15. Transformers can only operate on alternating current and not direct current because...

1. direct current damages the transformer core
2. direct current produces too high a voltage
3. only a changing current produces the changing magnetic flux needed to induce an e.m.f.
4. alternating current does not require a magnetic core

Transformers rely on electromagnetic induction, which requires a continuously changing flux; a steady direct current produces a constant flux with no induced e.m.f. (JAMB UTME Physics Syllabus, Electromagnetic Induction / A.C. circuits (the transformer))

16. At the magnetic equator, the angle of dip of the Earth's magnetic field is...

1. 90 degrees, since the needle rests vertically
2. 0 degrees, since a freely suspended magnetic needle rests horizontally
3. 45 degrees, since the needle rests at an incline
4. 180 degrees, since the needle points downward

The angle of dip is 0 degrees at the magnetic equator, where a freely suspended needle stays horizontal, and 90 degrees at the magnetic poles. (JAMB UTME Physics Syllabus, Magnets and Magnetic Fields (angle of dip))

17. An ideal step-down transformer has a primary voltage of 240 V and a secondary voltage of 12 V. If the primary current is 0.5 A, the secondary current is...

1. 0.5 A
2. 0.025 A
3. 20 A
4. 10 A

For an ideal transformer, input power equals output power (Ip Vp = Is Vs), so Is = (Ip x Vp)/Vs = (0.5 x 240)/12 = 10 A. (JAMB UTME Physics Syllabus, Electromagnetic Induction (transformer; conservation of power in an ideal transformer))

18. Which of the following is a commonly used semiconductor material in the manufacture of diodes and transistors?

1. Copper
2. Rubber
3. Silicon
4. Sodium chloride

Silicon (and germanium) are the most widely used semiconductor materials because their conductivity lies between that of conductors and insulators and can be controlled by doping. (JAMB UTME Physics Syllabus, Basic Electronics (semiconductor materials))

19. The process of adding a small, controlled amount of impurity atoms to a pure semiconductor crystal in order to increase its conductivity is called...

1. annealing
2. doping
3. rectification
4. induction

Doping introduces impurity atoms into a pure (intrinsic) semiconductor to increase the number of charge carriers and hence its conductivity. (JAMB UTME Physics Syllabus, Basic Electronics (semiconductors and doping))

20. An n-type semiconductor is formed when a pure semiconductor crystal is doped with a pentavalent (5-valence-electron) impurity. Its majority charge carriers are...

1. holes
2. protons
3. positive ions
4. free electrons

A pentavalent dopant donates a spare electron to the crystal lattice, making free electrons the majority carriers in an n-type semiconductor. (JAMB UTME Physics Syllabus, Basic Electronics (n-type and p-type semiconductors))

21. A p-type semiconductor is produced by doping a pure semiconductor with a trivalent (3-valence-electron) impurity. The majority charge carriers in this material are...

1. holes
2. free electrons
3. neutrons
4. ions

A trivalent dopant creates a deficiency of electrons, called holes, which act as the majority charge carriers in a p-type semiconductor. (JAMB UTME Physics Syllabus, Basic Electronics (n-type and p-type semiconductors))

22. The main function of a p-n junction diode in an electronic circuit is to...

1. allow current to flow equally in both directions
2. allow current to flow easily in one direction only
3. store electrical energy in an electric field
4. amplify a weak electrical signal

A p-n junction diode conducts readily when forward biased but blocks current when reverse biased, making it useful for rectification. (JAMB UTME Physics Syllabus, Basic Electronics (the p-n junction diode))

23. A p-n junction diode is said to be forward biased when...

1. the positive terminal of the battery is connected to the n-side and the negative terminal to the p-side
2. both terminals of the battery are connected to the p-side
3. the positive terminal of the battery is connected to the p-side and the negative terminal to the n-side
4. no battery is connected across the junction

Forward bias connects the battery's positive terminal to the p-side and negative terminal to the n-side, narrowing the depletion region and allowing current to flow easily. (JAMB UTME Physics Syllabus, Basic Electronics (forward and reverse bias of a diode))

24. Compared with a single diode used for half-wave rectification, a bridge rectifier made of four diodes used for full-wave rectification...

1. converts only the positive half of the a.c. cycle, giving a rougher output
2. blocks all current flow in both halves of the a.c. cycle
3. converts alternating current into a higher-frequency alternating current
4. converts both halves of the a.c. cycle into direct current, giving a smoother output

A full-wave (bridge) rectifier uses both halves of the a.c. cycle, producing a more continuous and smoother direct current output than a half-wave rectifier, which uses only one diode and one half-cycle. (JAMB UTME Physics Syllabus, Basic Electronics (rectification: half-wave and full-wave rectifiers))

25. A junction transistor has three terminals known as the emitter, base and collector, and it is commonly used in electronic circuits as a...

1. source of magnetic field only
2. current amplifier or electronic switch
3. device for storing charge
4. source of electromotive force

The transistor's three terminals allow it to control a larger collector current with a small base current, so it is widely used as an amplifier or as a switch in electronic circuits. (JAMB UTME Physics Syllabus, Basic Electronics (the transistor: structure and uses))

26. According to Ohm's law, the current flowing through a metallic conductor is directly proportional to the potential difference across its ends provided that:

1. alternating current is used
2. the conductor is non-metallic
3. the temperature and other physical conditions remain constant
4. the resistance varies with current

Ohm's law (V = IR) holds only when the temperature and other physical conditions of the conductor remain unchanged. (JAMB UTME Physics Syllabus, Current Electricity)

27. A metallic conductor has a potential difference of 12 V across its ends and carries a current of 2.5 A. Calculate the resistance of the conductor.

1. 30 Ω
2. 4.8 Ω
3. 0.208 Ω
4. 9.6 Ω

By Ohm's law, R = V/I = 12/2.5 = 4.8 Ω. (JAMB UTME Physics Syllabus, Current Electricity)

28. Three resistors R1, R2 and R3 are connected in series. The total (equivalent) resistance of the combination is given by:

1. 1/R = 1/R1 + 1/R2 + 1/R3
2. R = (R1R2R3)/(R1+R2+R3)
3. R = R1R2R3
4. R = R1 + R2 + R3

For resistors in series, the equivalent resistance is simply the sum of the individual resistances. (JAMB UTME Physics Syllabus, Current Electricity (series and parallel arrangement of cells and resistors))

29. When two resistors are connected in parallel, the equivalent resistance of the combination is:

1. always greater than the largest individual resistance
2. always less than the smallest individual resistance
3. equal to the sum of the resistances
4. equal to the average of the two resistances

Since 1/R = 1/R1 + 1/R2 for a parallel combination, the equivalent resistance is always smaller than the smallest individual resistance. (JAMB UTME Physics Syllabus, Current Electricity (series and parallel arrangement of cells and resistors))

30. A 6 Ω resistor is connected in parallel with a 3 Ω resistor. Calculate the equivalent resistance of the combination.

1. 9 Ω
2. 4.5 Ω
3. 2 Ω
4. 3 Ω

1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. (JAMB UTME Physics Syllabus, Current Electricity (series and parallel arrangement of cells and resistors))

31. A uniform wire of resistivity ρ and cross-sectional area A has its length doubled while ρ and A remain unchanged. The resistance of the wire will:

1. double
2. halve
3. remain unchanged
4. quadruple

Since R = ρL/A, resistance is directly proportional to length, so doubling L doubles R. (JAMB UTME Physics Syllabus, Current Electricity (resistivity and conductivity))

32. The resistivity of a conducting material is measured in:

1. ohm per metre (Ω/m)
2. ohm (Ω)
3. siemens per metre (S/m)
4. ohm-metre (Ω m)

Resistivity is defined by R = ρL/A, so ρ has units of ohm-metre (Ω m). (JAMB UTME Physics Syllabus, Current Electricity (resistivity and conductivity))

33. The resistivity of a given wire depends on:

1. the length of the wire only
2. the material and its temperature only
3. the cross-sectional area only
4. both the length and cross-sectional area

Unlike resistance, resistivity is a property of the material and temperature, independent of the wire's dimensions. (JAMB UTME Physics Syllabus, Current Electricity (resistivity and conductivity))

34. A cell of e.m.f. E and internal resistance r drives a current I through an external resistance R. Which equation correctly relates these quantities?

1. E = IR - r
2. E = Ir only
3. E = I(R + r)
4. E = I(R - r)

The e.m.f. of a cell equals the sum of the potential drops across the external and internal resistances, so E = I(R + r). (JAMB UTME Physics Syllabus, Electric Cells / Current Electricity (e.m.f. and internal resistance of a cell))

35. A cell of e.m.f. 6 V and internal resistance 0.5 Ω is connected to an external resistor of 2.5 Ω. Calculate the current flowing in the circuit.

1. 2.0 A
2. 2.4 A
3. 12 A
4. 3.0 A

I = E/(R + r) = 6/(2.5 + 0.5) = 6/3 = 2.0 A. (JAMB UTME Physics Syllabus, Electric Cells / Current Electricity (e.m.f. and internal resistance of a cell))