Electric Current

1. An electric generator primarily converts

An electric generator primarily converts

electrical energy to heat energy.
electrical energy to sound energy.
electrical energy to mechanical energy.
mechanical energy to electrical energy.
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UPSC CISF-AC-EXE – 2021
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An electric generator is a device that converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction, where relative motion between a conductor and a magnetic field induces an electric current.
Generators transform mechanical work (like rotation) into electrical output.
Electric motors perform the opposite function, converting electrical energy into mechanical energy.
Examples of mechanical energy sources for generators include turbines driven by steam, water, wind, or internal combustion engines. The induced voltage and current can then be used to power electrical devices.

2. A 100 W electric bulb is used for 8 hrs/day. What would be the units o

A 100 W electric bulb is used for 8 hrs/day. What would be the units of energy consumed in the month of April ?

24 units
16 units
8 units
0·8 units
This question was previously asked in
UPSC CISF-AC-EXE – 2021
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To calculate the energy consumed, we use the formula: Energy = Power × Time.
Power = 100 W = 0.1 kW (kilowatt).
The bulb is used for 8 hours per day.
The month of April has 30 days.
Total usage time in April = 8 hours/day × 30 days = 240 hours.
Energy consumed in kWh = Power (kW) × Total time (hours) = 0.1 kW × 240 hours = 24 kWh.
One unit of electrical energy is equivalent to 1 kWh.
Therefore, the energy consumed is 24 units.
– Convert power from Watts to Kilowatts (1 kW = 1000 W).
– Calculate the total usage time in hours for the month.
– Use the formula: Energy (kWh) = Power (kW) × Time (hours).
– 1 unit of energy = 1 kWh.
Electrical energy consumption is typically measured in kilowatt-hours (kWh) by electricity meters. This unit is often referred to as a “unit” in common parlance for billing purposes.

3. An electric refrigerator rated 400 W operates 10 hours/day. What is th

An electric refrigerator rated 400 W operates 10 hours/day. What is the cost of the energy to operate it for 30 days at ₹ 3.00 per kWh?

₹ 360
₹ 3,600
₹ 36
₹ 400
This question was previously asked in
UPSC CISF-AC-EXE – 2020
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The power rating of the refrigerator is 400 W, which is equal to 0.4 kW (since 1 kW = 1000 W). The refrigerator operates for 10 hours per day. Over 30 days, the total operation time is 10 hours/day * 30 days = 300 hours. The total energy consumed is the power multiplied by the time: Energy (kWh) = Power (kW) * Time (hours) = 0.4 kW * 300 hours = 120 kWh. The cost of energy is ₹ 3.00 per kWh. Total cost = Energy consumed * Cost per kWh = 120 kWh * ₹ 3.00/kWh = ₹ 360.
Energy consumed is calculated as Power (in kW) multiplied by Time (in hours). The total cost is the energy consumed multiplied by the rate per unit of energy (kWh).
The unit of energy used for billing is typically the kilowatt-hour (kWh), often called a ‘unit’ of electricity. Power in watts needs to be converted to kilowatts before calculating energy in kWh if time is in hours.

4. Which one of the following determines the direction of induced current

Which one of the following determines the direction of induced current ?

Fleming's left hand rule
Fleming's right hand rule
Feynman's left hand rule
Right hand thumb rule
This question was previously asked in
UPSC CISF-AC-EXE – 2018
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Fleming’s Right-Hand Rule is used to determine the direction of the induced electric current in a conductor when it is moved in a magnetic field or when the magnetic field around it changes. The thumb, forefinger, and middle finger are held mutually perpendicular: the thumb points in the direction of motion of the conductor, the forefinger points in the direction of the magnetic field, and the middle finger points in the direction of the induced current.
Fleming’s Right-Hand Rule determines the direction of induced current, while Fleming’s Left-Hand Rule determines the direction of force on a current-carrying conductor in a magnetic field.
The phenomenon of induced current is described by Faraday’s Law of electromagnetic induction, and its direction is governed by Lenz’s Law, which is encapsulated by Fleming’s Right-Hand Rule.

5. With reference to ‘fuel cells’ in which hydrogen-rich fuel and oxygen

With reference to ‘fuel cells’ in which hydrogen-rich fuel and oxygen are used to generate electricity, consider the following statements :

  • 1. If pure hydrogen is used as a fuel, the fuel cell emits heat and water as by-products.
  • 2. Fuel cells can be used for powering buildings and not for small devices like laptop computers.
  • 3. Fuel cells produce electricity in the form of Alternating Current (AC).

Which of the statements given above is/are correct?

1 only
2 and 3 only
1 and 3 only
1, 2 and 3
This question was previously asked in
UPSC IAS – 2015
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Only statement 1 is correct.
In a hydrogen fuel cell, hydrogen reacts with oxygen to produce electricity, water, and heat. This process is efficient and produces only water and heat as by-products when pure hydrogen is used.
Statement 2 is incorrect because fuel cells are highly scalable and can power everything from large buildings, vehicles, and grid-level energy storage to small devices like laptops and mobile phones. Statement 3 is incorrect because fuel cells generate Direct Current (DC) electricity. An inverter is required to convert the DC output to Alternating Current (AC) if needed for applications requiring AC.

6. Which one among the following does NOT have any linkage with the pheno

Which one among the following does NOT have any linkage with the phenomenon of electromagnetic induction ?

Electric transformer
Induction cooker
Galvanometer
Electron microscope
This question was previously asked in
UPSC CAPF – 2024
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Electromagnetic induction is the phenomenon where a change in magnetic flux through a circuit induces an electromotive force (EMF) or voltage, which can drive a current. This is described by Faraday’s Law of Induction. An electric transformer works entirely on the principle of mutual induction between coils. An induction cooker heats a metal pan by inducing eddy currents within it using a changing magnetic field, which is a direct application of electromagnetic induction. A galvanometer is a device used to detect and measure electric current. Its operation is typically based on the motor principle: a current-carrying coil placed in a magnetic field experiences a torque, causing it to deflect. This principle is derived from the Lorentz force on moving charges in a magnetic field, and while related to electromagnetism, it is distinct from electromagnetic *induction* (generating voltage/current from changing magnetic fields). An electron microscope uses magnetic lenses to focus beams of electrons. This focusing action is achieved by the Lorentz force exerted by magnetic fields on the moving electrons, not electromagnetic induction. However, considering the options, the galvanometer’s operating principle (motor effect) is most clearly and fundamentally distinct from electromagnetic induction, which is the basis of the transformer and induction cooker. The electron microscope uses magnetic fields to steer charges, a direct application of Lorentz force. Out of C and D, C (Galvanometer) is the most conventional example of a device whose core principle is the motor effect rather than induction.
Electromagnetic induction is the process of generating voltage/current through changing magnetic fields (Faraday’s Law). The motor principle (force on a current in a magnetic field) and the Lorentz force (force on a moving charge in a magnetic field) are related but distinct principles of electromagnetism. Transformers and induction cookers directly rely on electromagnetic induction. A galvanometer primarily relies on the motor principle.
The relationship between the motor principle and electromagnetic induction is linked by Lenz’s Law and energy conservation. However, the fundamental operational principle of a galvanometer is the torque on a current loop, not the generation of current by changing flux.

7. In experiment #1, a bar magnet is moved towards a conducting wire loop

In experiment #1, a bar magnet is moved towards a conducting wire loop axially, with the magnet’s north pole facing the loop. In experiment #2, the same process as in experiment #1 is repeated except that the south pole of the magnet faces the loop. Which one of the following statements is true in this context?

The direction of current in the loop will be of opposite nature in both the experiments.
The direction of current in the loop will be the same in both the experiments.
No current will flow in either of the two experiments.
More current will flow in the loop in experiment #1.
This question was previously asked in
UPSC CAPF – 2023
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The direction of current in the loop will be of opposite nature in both the experiments.
This question is based on Faraday’s Law of electromagnetic induction and Lenz’s Law. Faraday’s Law states that a changing magnetic flux through a loop induces an electromotive force (EMF), which drives a current in a conducting loop. Lenz’s Law provides the direction of the induced current: it flows in such a direction as to oppose the change in magnetic flux that produced it.
In experiment #1, the North pole of the bar magnet is moved towards the loop. This increases the magnetic flux through the loop in the direction of the magnet’s approaching field lines (which emerge from the North pole). To oppose this increase, the induced current in the loop creates a magnetic field pointing away from the magnet. By the right-hand rule, this corresponds to a specific direction of current flow (e.g., counter-clockwise when viewed from the magnet).
In experiment #2, the South pole of the same magnet is moved towards the loop. This increases the magnetic flux through the loop in the direction of the magnet’s approaching field lines (which enter the South pole). To oppose this increase, the induced current in the loop creates a magnetic field pointing away from the magnet’s approaching South pole (i.e., in the direction of the field lines leaving a South pole). By the right-hand rule, this corresponds to the opposite direction of current flow compared to experiment #1 (e.g., clockwise when viewed from the magnet). Therefore, the direction of the induced current will be opposite in the two experiments.
Lenz’s law is a consequence of the conservation of energy. If the induced current’s magnetic field reinforced the change in flux, the process would accelerate, producing energy indefinitely, which violates the law of conservation of energy. The strength of the induced current depends on the speed of the magnet and the strength of its magnetic field.

8. A wire of resistance R is cut into four equal parts. These parts are t

A wire of resistance R is cut into four equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R’, then the ratio $\frac{\text{R’}}{\text{R}}$ is :

$rac{1}{16}$
$rac{1}{4}$
4
16
This question was previously asked in
UPSC CAPF – 2023
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The ratio $\frac{\text{R’}}{\text{R}}$ is $\frac{1}{16}$.
Let the original resistance of the wire be R. When the wire is cut into four equal parts, the resistance of each part becomes $\frac{R}{4}$ (assuming uniform material and cross-section). Let these four parts be $R_1, R_2, R_3, R_4$, where $R_1 = R_2 = R_3 = R_4 = \frac{R}{4}$. These parts are then connected in parallel. The equivalent resistance R’ of resistances connected in parallel is given by the formula: $\frac{1}{R’} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \frac{1}{R_4}$. Substituting the values, we get: $\frac{1}{R’} = \frac{1}{R/4} + \frac{1}{R/4} + \frac{1}{R/4} + \frac{1}{R/4} = \frac{4}{R} + \frac{4}{R} + \frac{4}{R} + \frac{4}{R} = \frac{4+4+4+4}{R} = \frac{16}{R}$. Therefore, $R’ = \frac{R}{16}$. The required ratio $\frac{\text{R’}}{\text{R}}$ is $\frac{R/16}{R} = \frac{1}{16}$.
The resistance of a wire is directly proportional to its length. Cutting a wire into four equal parts reduces the length of each part to one-fourth of the original length, thus reducing the resistance of each part to R/4. Connecting resistors in parallel decreases the total equivalent resistance compared to the individual resistances. This principle is used in electrical circuits to control current flow.

9. At the time of short circuit, the current in an electric circuit

At the time of short circuit, the current in an electric circuit

becomes zero
remains same
increases sharply
decreases sharply
This question was previously asked in
UPSC CAPF – 2022
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The correct answer is C, increases sharply.
A short circuit occurs when a low-resistance path is created between two points in an electric circuit that are normally at different potentials. This bypasses the intended load (which usually has significant resistance).
According to Ohm’s Law (V = IR), the current (I) in a circuit is directly proportional to the voltage (V) across it and inversely proportional to its resistance (R).
During a short circuit, the resistance (R) in the path becomes very low, ideally approaching zero. If the voltage (V) of the source remains relatively constant, the current (I = V/R) will increase dramatically or sharply.
This sudden surge in current can generate excessive heat, potentially causing damage to the circuit, components, or leading to fires.
Safety devices like fuses or circuit breakers are used in electrical circuits to detect this dangerously high current during a short circuit and quickly interrupt the circuit to prevent damage and hazards.

10. The electrical device used for converting mechanical energy into elect

The electrical device used for converting mechanical energy into electrical energy is called

voltmeter
ammeter
motor
generator
This question was previously asked in
UPSC CAPF – 2022
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The correct answer is D, generator.
A generator is a device that converts mechanical energy into electrical energy. This conversion is based on the principle of electromagnetic induction, where a relative motion between a conductor and a magnetic field induces an electric current in the conductor. Mechanical work is done to cause this relative motion.
A motor is a device that converts electrical energy into mechanical energy.
A voltmeter is an instrument used to measure electric potential difference (voltage).
An ammeter is an instrument used to measure electric current.
Generators are essential components in power plants, converting mechanical energy from sources like turbines (driven by steam, water, or wind) into electrical energy for distribution.
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