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NABTEB Automobile Electrical Works study materials
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NABTEB Automobile Electrical Works practice questions

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Question 1Electrical Principles and Circuit Fundamentals

In an automobile electrical circuit, which quantity is measured in amperes?

  1. The opposition of a conductor to current
  2. The rate of flow of electric charge Correct answer.
  3. The electromotive force pushing the charge
  4. The power dissipated by a load

Explanation

Current is the rate at which electric charge passes a point in a circuit, and its unit is the ampere, equal to one coulomb per second. Electromotive force is measured in volts and resistance in ohms, so only the flow rate matches the ampere.

Question 2Electrical Principles and Circuit Fundamentals

A 12 V battery supplies a headlamp circuit whose total resistance is 3 ohms. What current flows?

  1. 0.25 A
  2. 9 A
  3. 36 A
  4. 4 A Correct answer.

Explanation

Ohm's law states that current equals voltage divided by resistance, so 12 divided by 3 gives 4 amperes. The other figures come from dividing or multiplying the wrong pair of quantities.

Question 3Electrical Principles and Circuit Fundamentals

Two 6 ohm resistors are connected in series across a 12 V supply. What is the current in the circuit?

  1. 1 A Correct answer.
  2. 4 A
  3. 2 A
  4. 0.5 A

Explanation

In a series circuit the resistances add, giving 12 ohms total. Applying Ohm's law, 12 V divided by 12 ohms gives 1 ampere, and the same current passes through both resistors.

Question 4Electrical Principles and Circuit Fundamentals

Why are motor vehicle loads such as lamps and accessories normally connected in parallel rather than in series?

  1. Series connection would reverse the polarity at each lamp
  2. Parallel connection lowers the resistance of each individual lamp filament
  3. Each load receives the full battery voltage and can operate independently Correct answer.
  4. Parallel connection reduces the total current drawn from the battery

Explanation

In a parallel arrangement every branch sees the full supply voltage, so each lamp or accessory works at its rated voltage and switching one off does not break the others' path. A series chain would divide the voltage and one open circuit would stop every load.

Question 5Electrical Principles and Circuit Fundamentals

A circuit draws 5 A from a 12 V battery. What power does it consume?

  1. 0.42 W
  2. 17 W
  3. 2.4 W
  4. 60 W Correct answer.

Explanation

Electrical power equals voltage multiplied by current, so 12 V times 5 A gives 60 watts. This relationship is the basis for choosing cable ratings and fuse sizes on a vehicle.

Question 6Electrical Principles and Circuit Fundamentals

Which statement correctly describes conventional current direction in an automobile circuit diagram?

  1. It flows from the positive terminal through the circuit to the negative terminal Correct answer.
  2. It alternates direction each time a switch is operated
  3. It flows only inside the battery and not through the cables
  4. It flows from the negative terminal through the circuit to the positive terminal

Explanation

Conventional current is defined as flowing from the positive terminal, through the external circuit, back to the negative terminal. Electron flow is opposite to this convention, which is why diagrams and arrow markings follow the positive-to-negative rule.

Question 7Electrical Principles and Circuit Fundamentals

What is the total resistance of a 4 ohm resistor and a 12 ohm resistor connected in parallel?

  1. 8 ohms
  2. 3 ohms Correct answer.
  3. 16 ohms
  4. 48 ohms

Explanation

The reciprocal formula gives 1/R = 1/4 + 1/12 = 4/12, so the total is 3 ohms. The parallel result is always smaller than the smallest branch resistance, which immediately rules out the larger figures.

Question 8Electrical Principles and Circuit Fundamentals

A 12 V battery is connected to a 6 ohm resistor. What power is dissipated in the resistor?

  1. 72 W
  2. 2 W
  3. 0.5 W
  4. 24 W Correct answer.

Explanation

Using P = V squared divided by R, 144 divided by 6 gives 24 watts. Alternatively the current is 2 A and 12 V times 2 A also gives 24 watts.

Question 9Electrical Principles and Circuit Fundamentals

Which material property makes copper preferred over steel for automotive wiring conductors?

  1. Higher resistivity with greater mechanical strength
  2. Higher magnetic permeability
  3. Lower melting point
  4. Lower resistivity with good ductility Correct answer.

Explanation

Copper has low resistivity, so it carries current with small voltage drop, and it is ductile enough to be stranded and routed through a vehicle. Steel has higher resistivity and would waste energy as heat in the same conductor size.

Question 10Electrical Principles and Circuit Fundamentals

What happens to the resistance of a typical metallic conductor as its temperature rises?

  1. It stays exactly constant
  2. It increases Correct answer.
  3. It decreases steadily
  4. It falls to zero

Explanation

In metals, higher temperature increases the vibration of the lattice, which scatters moving electrons more and raises resistance. This is why a lamp filament has a much higher resistance when hot than when cold.

Question 11Electrical Principles and Circuit Fundamentals

A 12 V battery has an internal resistance of 0.5 ohm and is connected to a 5.5 ohm load. What current flows?

  1. 24 A
  2. 2.18 A
  3. 2 A Correct answer.
  4. 1.83 A

Explanation

The internal resistance is in series with the load, giving 6 ohms total. Dividing 12 V by 6 ohms gives 2 amperes, which is less than the 2.18 A that would flow if the battery were ideal.

Question 12Electrical Principles and Circuit Fundamentals

What is the voltage drop across a 0.02 ohm cable carrying 150 A during engine cranking?

  1. 0.3 V
  2. 7.5 V
  3. 3 V Correct answer.
  4. 0.003 V

Explanation

Ohm's law gives 150 A multiplied by 0.02 ohm, which is 3 volts. Such a drop is significant on a 12 V system and explains why cranking cables must be short and of large cross-section.

Question 13Electrical Principles and Circuit Fundamentals

Which unit correctly expresses electrical resistance?

  1. The watt
  2. The volt
  3. The coulomb
  4. The ohm Correct answer.

Explanation

Resistance is measured in ohms, where one ohm allows one ampere to flow when one volt is applied. The volt measures potential difference, the watt measures power and the coulomb measures charge.

Question 14Electrical Principles and Circuit Fundamentals

A 60 W headlamp and a 24 W tail lamp are both connected to the same 12 V supply in parallel. What total current do they draw?

  1. 84 A
  2. 3 A
  3. 7 A Correct answer.
  4. 5 A

Explanation

The headlamp draws 60/12 = 5 A and the tail lamp draws 24/12 = 2 A. In parallel the branch currents add, giving a total of 7 amperes.

Question 15Electrical Principles and Circuit Fundamentals

Which relationship correctly states the voltage divider rule for two resistors in series?

  1. The voltage appears only across the larger resistor
  2. The voltage across each resistor is proportional to its own resistance Correct answer.
  3. Each resistor always drops exactly half the supply voltage
  4. The voltage across each resistor is inversely proportional to its own resistance

Explanation

In a series circuit the current is common, so the voltage across each resistor equals that current times its resistance. The larger resistor therefore drops the larger share of the supply voltage.

Question 16Electrical Principles and Circuit Fundamentals

A technician measures 0 V across a lamp while the switch is closed and the battery reads 12 V. Which fault best explains this reading?

  1. The lamp is short-circuited by a zero-ohm path across it Correct answer.
  2. The lamp is rated at 12 V
  3. The lamp filament has an open circuit
  4. The battery is fully charged

Explanation

A short circuit across the lamp provides a near-zero resistance path, so almost no voltage appears across the lamp and the current bypasses it. An open filament would normally show the full supply voltage across the break.

Question 17Electrical Principles and Circuit Fundamentals

What is the effective resistance of three 12 ohm resistors connected in parallel?

  1. 12 ohms
  2. 36 ohms
  3. 4 ohms Correct answer.
  4. 0.25 ohm

Explanation

For equal resistors in parallel the total is one resistance value divided by the number of branches, so 12 divided by 3 gives 4 ohms. The result must be smaller than any single branch, which confirms the answer.

Question 18Electrical Principles and Circuit Fundamentals

A 12 V battery is connected to a circuit of 2 ohms. Which formula gives the power dissipated?

  1. P = V squared divided by R Correct answer.
  2. P = V multiplied by R
  3. P = R divided by V
  4. P = V divided by R

Explanation

Combining Ohm's law with P = VI gives P = V squared divided by R, which for this circuit is 144 divided by 2, or 72 watts. The other expressions do not yield units of power.

Question 19Electrical Principles and Circuit Fundamentals

Which statement about a series circuit is correct?

  1. The voltage is the same across every component
  2. The total resistance is less than the smallest resistance
  3. The current is the same at every point in the circuit Correct answer.
  4. Each component can be switched independently

Explanation

A series circuit provides only one path for charge, so the same current passes through every component. The supply voltage divides among the components and the total resistance is the sum of the parts.

Question 20Electrical Principles and Circuit Fundamentals

A 24 W lamp is rated for 12 V. What is its resistance when operating at rated voltage?

  1. 0.5 ohm
  2. 6 ohms Correct answer.
  3. 288 ohms
  4. 2 ohms

Explanation

The lamp current is 24/12 = 2 A, and resistance is 12 V divided by 2 A, giving 6 ohms. The same result comes from R = V squared divided by P.

Question 21Electrical Principles and Circuit Fundamentals

Why does a short circuit in an unfused automotive circuit present a fire risk?

  1. It creates a very low resistance path that allows excessive current and heat Correct answer.
  2. It raises the circuit resistance and lowers the current
  3. It reduces current to near zero and causes the insulation to harden
  4. It reverses the polarity of the battery

Explanation

A short circuit bypasses the intended load, leaving only the small resistance of the cables, so current rises sharply. The resulting heat in the cable and insulation can ignite nearby material, which is why fuses and circuit breakers are fitted.

Question 22Electrical Principles and Circuit Fundamentals

What is the purpose of a fuse in an automobile electrical circuit?

  1. To store charge for later use by the load
  2. To convert alternating current to direct current
  3. To increase the voltage available to the load
  4. To limit current by melting when the rated value is exceeded Correct answer.

Explanation

A fuse contains a calibrated element that melts and opens the circuit when current exceeds its rating, protecting cables and components from overheating. It does not regulate voltage or store energy.

Question 23Electrical Principles and Circuit Fundamentals

A circuit has two resistors of 3 ohms and 6 ohms in parallel, connected in series with a 4 ohm resistor. What is the total resistance?

  1. 9 ohms
  2. 13 ohms
  3. 6 ohms Correct answer.
  4. 2 ohms

Explanation

The parallel pair combines to 2 ohms because 3 times 6 divided by 3 plus 6 equals 2. Adding the series 4 ohm resistor gives a total of 6 ohms.

Question 24Electrical Principles and Circuit Fundamentals

Which instrument connection is correct when measuring current in an automotive circuit?

  1. A voltmeter connected in series with the load
  2. An ammeter connected in series with the load Correct answer.
  3. An ammeter connected in parallel with the load
  4. An ohmmeter connected across the live battery terminals

Explanation

An ammeter must carry the circuit current, so it is connected in series with the load and has very low internal resistance. A voltmeter is placed in parallel, and an ohmmeter must only be used on an isolated, unpowered circuit.

Question 25Batteries and Charging Systems

In a lead-acid battery, what is the active material of the positive plate after formation?

  1. Lead dioxide Correct answer.
  2. Antimony alloy
  3. Lead sulphate
  4. Sponge lead

Explanation

After formation charging, the positive plate is converted to lead dioxide (PbO2), a dark brown material, while the negative plate becomes spongy lead. This difference in active material is what allows the cell to develop an electromotive force. Lead sulphate is the discharged state of both plates.

Question 26Batteries and Charging Systems

Which acid is used as the electrolyte in a conventional automotive lead-acid battery?

  1. Sulphuric acid Correct answer.
  2. Nitric acid
  3. Phosphoric acid
  4. Hydrochloric acid

Explanation

Automotive lead-acid batteries use dilute sulphuric acid, typically of specific gravity around 1.260 when fully charged. Sulphuric acid participates directly in the plate reactions during charge and discharge. The other acids listed do not support the reversible lead/lead dioxide chemistry.

Question 27Batteries and Charging Systems

What is the nominal voltage of one lead-acid cell?

  1. 1.2 V
  2. 2.4 V
  3. 1.5 V
  4. 2.0 V Correct answer.

Explanation

Each lead-acid cell produces approximately 2.1 V on open circuit when fully charged, and this is treated as a nominal 2 V per cell. A 12 V battery therefore contains six cells connected in series. The other values correspond to different chemistries or to charging voltages.

Question 28Batteries and Charging Systems

How are the six cells of a 12 V automotive battery connected?

  1. In star formation
  2. In parallel
  3. In a series-parallel bridge
  4. In series Correct answer.

Explanation

The six cells are connected in series so that their individual voltages add, giving roughly 12 V at the terminals. The current capacity is set by plate area within each cell, not by the interconnection. Parallel connection would hold voltage at about 2 V while increasing capacity.

Question 29Batteries and Charging Systems

Which instrument is used to check the specific gravity of battery electrolyte?

  1. Multimeter
  2. Hydrometer Correct answer.
  3. Timing light
  4. Feeler gauge

Explanation

A hydrometer measures electrolyte density, which falls as the battery discharges because sulphuric acid is consumed and water is produced. The reading is corrected for temperature and compared with the manufacturer's state-of-charge chart. A multimeter measures voltage, not density.

Question 30Batteries and Charging Systems

What is the main function of the alternator in a vehicle charging system?

  1. To store charge chemically
  2. To limit current to the lamps
  3. To convert mechanical energy into electrical energy Correct answer.
  4. To crank the engine

Explanation

The alternator is belt-driven by the engine and converts mechanical rotation into electrical energy by electromagnetic induction. Its rectifier converts the alternating output to direct current for the battery and loads. Cranking is the job of the starter motor and chemical storage is the battery's role.

Question 31Batteries and Charging Systems

Which component converts the alternating output of the alternator into direct current?

  1. Rectifier Correct answer.
  2. Slip ring
  3. Stator
  4. Voltage regulator

Explanation

The rectifier uses a bridge of six diodes to convert the three-phase alternating current from the stator into direct current. The voltage regulator controls field current to hold output voltage steady, but does not rectify. Slip rings and the stator are parts of the generating section.

Question 32Batteries and Charging Systems

A fully charged battery reads 12.6 V at rest, but drops to 9.4 V while cranking the engine. What does this most likely indicate?

  1. The electrolyte level is too high
  2. The battery is overcharged
  3. The battery has high internal resistance or a weak cell Correct answer.
  4. The alternator belt is slipping

Explanation

A healthy battery should hold roughly 10 V or more during cranking. A fall to about 9.4 V under load points to high internal resistance, sulphation or a failing cell. Open-circuit voltage alone can look normal while the battery still cannot deliver heavy current.

Question 33Batteries and Charging Systems

During charging, a battery begins to gas freely and the electrolyte specific gravity stops rising. What does this indicate?

  1. The plates are short-circuited
  2. The charger polarity is reversed
  3. The battery is deeply discharged
  4. The battery is fully charged Correct answer.

Explanation

When the active material has been fully converted, further charge current decomposes water and produces gas, while specific gravity remains constant. These two signs together confirm a full state of charge. Continuing to charge at high rate then only causes water loss and heating.

Question 34Batteries and Charging Systems

What is the purpose of the voltage regulator in an alternator charging system?

  1. To limit charging current to the battery
  2. To synchronise the diode bridge
  3. To keep the charging voltage within a set range Correct answer.
  4. To prevent reverse current at night

Explanation

The regulator varies the field current so that alternator output stays near a preset value, commonly about 13.8 to 14.4 V. This protects the battery from overcharge while ensuring adequate charging. Current limiting and reverse-current blocking are separate functions of the system design.

Question 35Batteries and Charging Systems

A warning lamp stays on with the ignition on but the engine stopped, and goes out when the engine starts. What is the lamp actually indicating?

  1. That the regulator has failed open
  2. That the battery is fully charged
  3. That the alternator is not yet producing output Correct answer.
  4. That the diode bridge is shorted

Explanation

The charge warning lamp is connected so that it receives supply through the ignition switch and earths through the alternator field circuit when the engine is stationary. Once the alternator produces voltage, equal voltage on both sides extinguishes the lamp. This proves the field circuit and lamp are intact.

Question 36Batteries and Charging Systems

Why is a lead-acid battery never allowed to stand in a discharged condition for long periods?

  1. The separators dissolve in weak acid
  2. The electrolyte freezes only when charged
  3. Sulphation of the plates reduces capacity permanently Correct answer.
  4. The terminal voltage rises dangerously

Explanation

In a discharged battery both plates are covered with lead sulphate. If left, this sulphate hardens into a coarse crystalline form that is difficult to reconvert, permanently reducing capacity. A discharged battery is also more prone to freezing in cold weather.

Question 37Batteries and Charging Systems

What is the effect of adding too much distilled water to a battery cell before charging?

  1. The plates become permanently polarised
  2. The cell voltage rises above 2.4 V
  3. The specific gravity reading becomes falsely high
  4. The electrolyte may overflow and dilute during gassing Correct answer.

Explanation

Electrolyte expands and gasses during charging, so overfilling causes acid to overflow, which corrodes terminals and lowers the acid concentration. Correct practice is to fill only to the indicated level, preferably before charging so the mixture stirs. Dilution lowers specific gravity rather than raising it.

Question 38Batteries and Charging Systems

A vehicle's battery keeps going flat overnight. A test shows a current drain of 0.9 A with all accessories switched off. What is the most appropriate next step?

  1. Fit a larger capacity battery
  2. Top up the electrolyte with acid
  3. Check circuits with an ammeter by pulling fuses one at a time Correct answer.
  4. Increase the alternator belt tension

Explanation

A quiescent drain of about 0.9 A is far above the normal figure of a few tens of milliamperes. Pulling fuses one at a time while watching the ammeter isolates the faulty circuit. Fitting a larger battery would only mask the fault temporarily.

Question 39Batteries and Charging Systems

Which statement correctly describes the charging of a lead-acid battery from the alternator?

  1. The regulator discharges the battery at idle
  2. The battery charges only when its voltage exceeds the alternator output
  3. Charging current flows regardless of the voltage difference
  4. The alternator output voltage must exceed battery EMF for current to flow into the battery Correct answer.

Explanation

Charging current flows only when the alternator terminal voltage is higher than the battery's electromotive force, so current is forced into the battery. The regulator holds the alternator voltage above battery voltage across the normal speed range. If the difference disappears, no charging occurs.

Question 40Batteries and Charging Systems

What is the function of the diode trio or auxiliary diodes in some alternator designs?

  1. To rectify the battery charging current
  2. To suppress radio interference
  3. To protect the regulator from reverse polarity
  4. To supply field current after the engine starts Correct answer.

Explanation

The auxiliary diodes tap the stator output and feed the field winding once the alternator is generating, so field current no longer has to come through the warning lamp. This gives stronger excitation at low speed. The main rectifier handles battery charging current.

Question 41Batteries and Charging Systems

A customer reports that the charge warning lamp glows dimly whenever the headlamps are switched on, and brightens with engine speed. Which fault is most likely?

  1. A blown main fuse
  2. A faulty diode in the rectifier Correct answer.
  3. A worn starter brush
  4. A loose alternator mounting bolt

Explanation

A dim glow that varies with electrical load and engine speed usually means one diode of the rectifier is open or shorted, so output is reduced and ripple appears. The lamp sees a small potential difference through the faulty bridge. A blown main fuse would stop charging completely and light the lamp brightly.

Question 42Batteries and Charging Systems

An alternator is bench-tested and produces about 14.2 V at 2000 rpm, but the same unit fitted to the vehicle reads only 12.8 V at the battery terminals at the same speed. What should be checked first?

  1. The rotor bearing clearance
  2. The stator winding resistance
  3. The regulator set point on the bench
  4. The voltage drop in the charging circuit and earth return Correct answer.

Explanation

A large difference between alternator output terminal voltage and battery terminal voltage points to resistance in the charging cable, connections or earth return. Measuring voltage drop across each leg under load locates the faulty joint. The alternator itself has already proven capable of the correct output.

Question 43Batteries and Charging Systems

Why does a battery's available capacity fall sharply at low temperatures even though its open-circuit voltage remains close to normal?

  1. The separators become conductors
  2. The electrolyte becomes more concentrated
  3. The plate area physically shrinks
  4. Chemical reaction rates and acid diffusion slow down Correct answer.

Explanation

Cold electrolyte increases internal resistance and slows the chemical reactions and diffusion of acid into the plates. The battery can therefore deliver much less current under heavy load such as cranking, even though the resting voltage looks normal. This is why cold cranking ratings are specified at low temperature.

Question 44Batteries and Charging Systems

During a charging system test, the technician notes that alternator output voltage rises with engine speed and is not held steady, reaching over 15.5 V at high rpm. What is the most probable cause?

  1. Defective voltage regulator Correct answer.
  2. Open field winding
  3. High resistance in the battery earth strap
  4. Shorted stator winding

Explanation

A regulator that fails to control field current allows output to rise with speed, risking overcharging, gassing and lamp failure. Normal regulated output should stay within a narrow band regardless of engine speed. An open field winding would give no output at all.

Question 45Batteries and Charging Systems

Which condition would cause a battery to accept very little charge current even though the alternator output voltage is correct?

  1. Heavy sulphation of the plates Correct answer.
  2. A blown charge warning lamp bulb
  3. A loose fan belt
  4. A missing alternator suppressor

Explanation

Heavy sulphation raises internal resistance so the battery resists charging current and heats or gasses early. Voltage at the terminals may still appear normal because the alternator is regulating correctly. The battery must be tested and usually replaced.

Question 46Batteries and Charging Systems

Which safety precaution is essential when connecting a charger to a battery that is still fitted in the vehicle?

  1. Connect and disconnect with the charger switched on
  2. Remove the battery caps and leave them off during charging
  3. Place the charger on top of the battery
  4. Ensure correct polarity and switch the charger off before making or breaking connections Correct answer.

Explanation

Observing correct polarity prevents damage to the alternator diodes and the charger, and switching off before connecting or disconnecting avoids sparking at the terminals. Sparks near a gassing battery can ignite hydrogen. Caps should be loosened but not left fully off where acid spray can escape.

Question 47Batteries and Charging Systems

What is the purpose of the separators between plates in a lead-acid battery?

  1. To insulate plates of opposite polarity while allowing electrolyte contact Correct answer.
  2. To hold the plates rigid against vibration only
  3. To conduct current between plates
  4. To store sulphuric acid permanently

Explanation

Separators are porous insulating sheets that prevent positive and negative plates from touching and shorting, while allowing electrolyte and ions to pass. Modern separators are often envelopes around the plates. They are not conductors and do not store acid.

Question 48Batteries and Charging Systems

A battery is being charged at a constant current and the electrolyte temperature rises above 45 degrees Celsius. What is the correct action?

  1. Seal the vent caps to retain vapour
  2. Increase the charging current to finish sooner
  3. Add cold water to the cells immediately
  4. Reduce or stop the charge until the battery cools Correct answer.

Explanation

Excessive temperature during charging accelerates grid corrosion and can warp plates or damage separators, and it can indicate a failing cell. The charge rate should be reduced or stopped to let the battery cool. Never add water to a hot battery or seal vents, as pressure could build.

Question 49Starting Systems

In a 12 V automotive starting system, which component converts electrical energy from the battery into mechanical rotation of the engine?

  1. The voltage regulator
  2. The alternator
  3. The starter motor Correct answer.
  4. The ignition coil

Explanation

The starter motor is a heavy-duty series-wound DC motor that draws high current from the battery and produces the torque needed to crank the engine. The alternator and voltage regulator belong to the charging system, while the ignition coil belongs to the ignition system.

Question 50Starting Systems

Which component in a pre-engaged starter circuit closes the heavy-current contacts that feed the starter motor windings?

  1. The solenoid Correct answer.
  2. The commutator
  3. The brush assembly
  4. The field winding

Explanation

The solenoid is an electromagnetic relay built into the starter. When the ignition switch energises its pull-in winding, the plunger moves the drive pinion into mesh and closes the main contacts that pass battery current to the motor.