ANSWERS

1. (a) (i) Lenz’s Law:
Lenz's Law states that the direction of the induced current in a conductor is such that it opposes the change in magnetic flux that produced it.

(ii) Two advantages of electromagnets over permanent magnets:

1. Electromagnets can be turned on or off as needed.


2. Their magnetic strength can be varied by adjusting the electric current.



(b)
Given:

Number of turns, N = 10

Current, I = 3 A

Length of magnetic circuit, l = 25 cm = 0.25 m


Magnetic field intensity


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2. (a)
Impedance of an a.c. circuit:
Impedance is the total opposition that a circuit offers to alternating current. It includes both resistance and reactance, and is measured in ohms (Ω).

(b) Sketch a sinusoidal waveform indicating:
(i) Amplitude — the peak value above or below zero.
(ii) Cycle — one complete wave from 0 to +peak to 0 to -peak and back to 0.

(c) Given: RMS voltage = 220 V
(i) Maximum voltage (Peak value):


(ii) Average value (for a sinusoidal waveform):



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3. (a) Total power:

10 lamps × 60 W = 600 W

5 fans × 74 W = 370 W

5 ACs × 746 W = 3730 W

1 cooker = 2500 W
Total = 600 + 370 + 3730 + 2500 = 7200 W


(b) Energy consumed in 5 hours:
Energy = Power × Time = 7200 W × 5 h = 36,000 Wh = 36 kWh

(c) Using a 1.5 mm² cable for the electric cooker (2500 W):

This cable size is too small and may overheat, posing a fire hazard.

It can cause voltage drops and reduce the efficiency of the cooker.


4. (a) Sketch of a shunt motor:

Armature and field windings connected in parallel across the power supply.

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*WAEC BASIC ELECTRICITY*

*NUMBER SIX*


(6a)
Lightning Arrester:
(i) Diverts lightning surges to earth.
(ii) Protects transformers from high voltage.
(iii) Prevents damage to substation equipment.
(iv) Limits transient overvoltages.
(v) Ensures safety during lightning storms.

(6b)
Circuit Breaker:
(i) Interrupts current during faults.
(ii) Operates automatically or manually.
(iii) Protects equipment from overload.
(iv) Isolates faulted sections.
(v) Ensures system stability.

(6c)
Isolator:
(i) Disconnects parts of the system.
(ii) Used during maintenance.
(iii) Operates only under no load.
(iv) Provides visible isolation.
(v) Enhances safety for workers.

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(1a)

(i) Lenz's Law:

Lenz's Law states that the direction of an induced current is such that it opposes the change in magnetic flux that produced it. This is a consequence of the conservation of energy and is represented mathematically by the negative sign in Faraday's Law:
E = -dΦ/dt

(ii)

(a) Controllability: The magnetic field strength of an electromagnet can be easily adjusted by changing the current, whereas a permanent magnet has a fixed magnetic field.

(b) Temporary Magnetism: Electromagnets can be turned on and off, making them more versatile for applications like electric motors, relays, and magnetic cranes.

(1b)

Given:

Number of turns, N = 10
Current, I = 3 A
Length of magnetic path, l = 25cm = 0.25 m

The magnetic field intensity (H) is given by:
H = (N × I) / l

Substituting the values:

H = (10 × 3) / 0.25 = 30 / 0.25 = 120 A/m

Magnetic field intensity, H = 120 A/m

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(7ai)
(i) Reduces equipment failure: Regular maintenance helps identify and fix potential problems before they cause breakdowns.
(ii) Extends equipment lifespan: Preventive care ensures devices operate efficiently for a longer time.

(7aii)
(i) Worn-out or faulty capacitor, which can prevent the fan from starting or running at proper speed.
(ii) Damaged or loose wiring, which may lead to malfunction or electrical hazards.

(7bi)
Possible Cause of the Fault:
The possible cause is a short circuit or insulation failure in the kettle, causing the metal body to become energized.

(7bii)
Corrective Action:
Disconnect the kettle from power immediately and have it inspected and repaired or replaced by a qualified electrician.

(7biii)
Preventive Measure:
Regular inspection and maintenance of electrical appliances, including checking for worn-out insulation, damaged cords, or faulty grounding, can prevent such faults.

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