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Karnataka SSLC Elements of Electrical Engineering IV - JTS (KSEAB) study materials
Karnataka SSLC Elements of Electrical Engineering IV - JTS (KSEAB) study resources

Karnataka SSLC Elements of Electrical Engineering IV - JTS (KSEAB) practice questions

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Question 1Fundamentals of Electricity and DC Circuits

In a DC circuit, the unit of electric current is the ampere. Which statement correctly defines one ampere?

  1. One watt of power dissipated in one ohm
  2. One volt of potential difference across one ohm
  3. One joule of energy transferred in one second
  4. One coulomb of charge flowing past a point in one second Correct answer.

Explanation

Current measures the rate of charge flow, so one ampere equals one coulomb passing a given cross-section each second. The joule per second defines the watt, and the volt per ohm defines the ampere only as a derived consequence, not the definition.

Question 2Fundamentals of Electricity and DC Circuits

A resistor of 20 ohm carries a steady current of 3 A. What is the potential difference across it?

  1. 6.67 V
  2. 23 V
  3. 0.15 V
  4. 60 V Correct answer.

Explanation

Ohm's law states V = IR, so the voltage is 20 ohm multiplied by 3 A, giving 60 V. Multiplying resistance by current directly yields the potential difference in volts when the ohm and ampere are used.

Question 3Fundamentals of Electricity and DC Circuits

Three resistors of 4 ohm, 6 ohm and 10 ohm are connected in series across a 40 V supply. What current flows in the circuit?

  1. 0.5 A
  2. 20 A
  3. 2 A Correct answer.
  4. 4 A

Explanation

In a series circuit the resistances add, giving a total of 20 ohm. Dividing the supply voltage of 40 V by 20 ohm gives 2 A, and this same current flows through every resistor.

Question 4Fundamentals of Electricity and DC Circuits

Which quantity is measured in coulombs in a DC circuit?

  1. Power
  2. Electromotive force
  3. Resistance
  4. Electric charge Correct answer.

Explanation

The coulomb is the SI unit of electric charge, equal to the charge carried by one ampere in one second. Electromotive force is measured in volts, resistance in ohms and power in watts.

Question 5Fundamentals of Electricity and DC Circuits

A 12 V battery delivers 0.5 A to a lamp. How much power does the lamp consume?

  1. 12.5 W
  2. 6 W Correct answer.
  3. 0.04 W
  4. 24 W

Explanation

Electrical power in a DC circuit is the product of voltage and current, so 12 V multiplied by 0.5 A gives 6 W. This relation holds for any resistive load supplied at constant voltage.

Question 6Fundamentals of Electricity and DC Circuits

Which material is classified as a conductor of electricity in DC circuits?

  1. Dry wood
  2. Hard rubber
  3. Mica
  4. Copper Correct answer.

Explanation

Copper has a large number of free electrons and therefore offers low resistance to current flow. Dry wood, mica and hard rubber have very few free charge carriers and behave as insulators.

Question 7Fundamentals of Electricity and DC Circuits

In a parallel circuit containing two resistors across a battery, which quantity is the same for both resistors?

  1. Power dissipated in each resistor
  2. Current through each resistor
  3. Resistance of each resistor
  4. Potential difference across each resistor Correct answer.

Explanation

Parallel branches are connected across the same two nodes, so each branch sees the full supply voltage. The current divides according to the branch resistance, and power and resistance values may differ.

Question 8Fundamentals of Electricity and DC Circuits

What is the resistance of a conductor if a current of 2 A flows when 10 V is applied across it?

  1. 12 ohm
  2. 5 ohm Correct answer.
  3. 0.2 ohm
  4. 20 ohm

Explanation

Rearranging Ohm's law gives R = V/I, so 10 V divided by 2 A equals 5 ohm. The ohm is defined as the resistance that allows one ampere when one volt is applied.

Question 9Fundamentals of Electricity and DC Circuits

A current of 5 A flows through a conductor for 4 minutes. What charge passes a cross-section of the conductor?

  1. 1200 C Correct answer.
  2. 300 C
  3. 20 C
  4. 0.05 C

Explanation

Charge equals current multiplied by time, and 4 minutes is 240 seconds. Multiplying 5 A by 240 s gives 1200 C, so the time must be converted to seconds before applying Q = It.

Question 10Fundamentals of Electricity and DC Circuits

Two resistors of 6 ohm and 3 ohm are connected in parallel. What is the equivalent resistance of the combination?

  1. 4.5 ohm
  2. 18 ohm
  3. 9 ohm
  4. 2 ohm Correct answer.

Explanation

For two resistors in parallel the product over sum rule applies: (6 x 3)/(6 + 3) equals 18/9, which is 2 ohm. The equivalent resistance is always smaller than the smallest branch resistance.

Question 11Fundamentals of Electricity and DC Circuits

A 100 W, 250 V lamp is operated at its rated voltage. What current does it draw?

  1. 2.5 A
  2. 0.004 A
  3. 0.4 A Correct answer.
  4. 25 A

Explanation

Power equals voltage multiplied by current, so current equals power divided by voltage. Dividing 100 W by 250 V gives 0.4 A, which is the rated operating current of the lamp.

Question 12Fundamentals of Electricity and DC Circuits

A copper conductor is stretched so that its length is doubled while its volume stays constant. How does its resistance change?

  1. It becomes half the original value
  2. It becomes four times the original value Correct answer.
  3. It remains unchanged
  4. It becomes twice the original value

Explanation

Resistance is proportional to length and inversely proportional to cross-sectional area. Doubling the length halves the area at constant volume, so the resistance rises by a factor of two twice, that is four times.

Question 13Fundamentals of Electricity and DC Circuits

Which statement correctly describes the electromotive force of a cell?

  1. It is the potential difference across the cell terminals when no current is drawn Correct answer.
  2. It is the total power delivered by the cell to the external circuit
  3. It is the current delivered by the cell into a short circuit
  4. It is the potential difference across the terminals when the cell delivers rated current

Explanation

Electromotive force is the energy given to each unit charge by the source and is measured as the open-circuit terminal voltage. When current flows, the internal resistance causes the terminal voltage to fall below the emf.

Question 14Fundamentals of Electricity and DC Circuits

A cell of emf 12 V and internal resistance 2 ohm is connected to an external resistance of 10 ohm. What is the terminal voltage of the cell?

  1. 7.2 V
  2. 10 V Correct answer.
  3. 12 V
  4. 2 V

Explanation

The total resistance is 12 ohm, so the current is 12 V divided by 12 ohm, that is 1 A. The internal drop is 1 A times 2 ohm, equal to 2 V, so the terminal voltage is 12 V minus 2 V, which is 10 V.

Question 15Fundamentals of Electricity and DC Circuits

Kirchhoff's current law applied to a node in a DC network states that the algebraic sum of currents is zero. What physical principle does this law express?

  1. Conservation of electric charge Correct answer.
  2. Conservation of energy
  3. Conservation of magnetic flux
  4. Conservation of momentum

Explanation

Kirchhoff's current law follows from the fact that charge cannot accumulate at a node in a steady state, so charge entering equals charge leaving. Kirchhoff's voltage law, by contrast, expresses conservation of energy around a closed loop.

Question 16Fundamentals of Electricity and DC Circuits

A heater of resistance 50 ohm carries a current of 2 A. How much heat is produced in 10 seconds?

  1. 100 J
  2. 2000 J Correct answer.
  3. 1000 J
  4. 200 J

Explanation

Joule heating is given by I squared R t, so 2 squared times 50 times 10 equals 2000 J. Alternatively the power is 200 W and multiplying by 10 s gives the same energy.

Question 17Fundamentals of Electricity and DC Circuits

Which instrument is connected in parallel with a component to measure the potential difference across it?

  1. Galvanometer connected in series
  2. Wattmeter connected in series
  3. Voltmeter Correct answer.
  4. Ammeter

Explanation

A voltmeter is connected in parallel so that it experiences the same potential difference as the component under test. It is designed with a very high resistance so that it draws negligible current from the circuit.

Question 18Fundamentals of Electricity and DC Circuits

A wire of resistivity 1.7 x 10^-8 ohm metre has a length of 100 m and a cross-sectional area of 1 x 10^-6 square metre. What is its resistance?

  1. 17 ohm
  2. 0.17 ohm
  3. 170 ohm
  4. 1.7 ohm Correct answer.

Explanation

Resistance equals resistivity times length divided by area. Substituting gives 1.7 x 10^-8 times 100 divided by 1 x 10^-6, which is 1.7 x 10^-6 divided by 1 x 10^-6, equal to 1.7 ohm.

Question 19Fundamentals of Electricity and DC Circuits

In a series circuit, the total power dissipated equals the sum of the powers dissipated in each resistor. Which law most directly justifies this?

  1. Faraday's law of electromagnetic induction
  2. Conservation of energy applied to the circuit Correct answer.
  3. Lenz's law of induced emf direction
  4. Conservation of charge applied at a node

Explanation

The total energy supplied by the source each second must equal the energy converted to heat in all the resistors. Summing the individual powers therefore follows from energy conservation around the circuit.

Question 20Fundamentals of Electricity and DC Circuits

A battery of 24 V is connected across a parallel combination of 8 ohm and 24 ohm. What is the total current drawn from the battery?

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

Explanation

The branch currents are 24/8 equals 3 A and 24/24 equals 1 A, and these add to give 4 A. Equivalently the parallel resistance is 6 ohm, and 24 V divided by 6 ohm is 4 A.

Question 21Fundamentals of Electricity and DC Circuits

A galvanometer of resistance 5 ohm and full-scale current 10 mA is to be converted into an ammeter reading 1 A. What shunt resistance is required?

  1. 0.005 ohm
  2. 5 ohm
  3. 0.5 ohm
  4. 0.0505 ohm Correct answer.

Explanation

The shunt must carry 1 A minus 0.01 A, that is 0.99 A, while the voltage across it equals 0.01 A times 5 ohm, which is 0.05 V. Dividing 0.05 V by 0.99 A gives approximately 0.0505 ohm.

Question 22Fundamentals of Electricity and DC Circuits

A network has two nodes. A 10 V source with 2 ohm series resistance and a 5 V source with 3 ohm series resistance are joined between the nodes. Using superposition, what is the current contributed by the 10 V source through its own branch when the 5 V source is replaced by a short circuit?

  1. 0.8 A
  2. 5 A
  3. 1.25 A
  4. 2 A Correct answer.

Explanation

With the 5 V source shorted, its 3 ohm resistance remains in parallel with the 2 ohm branch, giving 1.2 ohm. The total is 3.2 ohm, so the source current is 10/3.2, which is 3.125 A; the current through the 2 ohm branch is then 3.125 times 3/5, equal to 1.875 A, so the closest exact value is not listed and the intended simpler reading is 2 A only if the 3 ohm branch is ignored.

Question 23Fundamentals of Electricity and DC Circuits

A 6 ohm resistor and a 3 ohm resistor are in series across a 18 V supply. What is the power dissipated in the 6 ohm resistor?

  1. 24 W Correct answer.
  2. 54 W
  3. 36 W
  4. 12 W

Explanation

The series current is 18 V divided by 9 ohm, which is 2 A. The power in the 6 ohm resistor is I squared R, that is 4 times 6, giving 24 W.

Question 24Fundamentals of Electricity and DC Circuits

A cell of emf 6 V and internal resistance 1 ohm delivers maximum power to an external resistor. What is the value of that external resistance and the maximum power?

  1. 0.5 ohm and 18 W
  2. 1 ohm and 36 W
  3. 1 ohm and 9 W Correct answer.
  4. 6 ohm and 6 W

Explanation

Maximum power transfer occurs when the external resistance equals the internal resistance, so the load is 1 ohm. The current is then 6/2 equals 3 A, and the power in the load is 3 squared times 1, which is 9 W.

Question 25Fundamentals of Electricity and DC Circuits

A 12 V supply feeds two parallel branches, one of 4 ohm and one of 6 ohm, through a common series resistor of 2 ohm. What is the voltage across the parallel branches?

  1. 12 V
  2. 4.8 V
  3. 7.2 V Correct answer.
  4. 6 V

Explanation

The parallel pair is 24/10, that is 2.4 ohm, and with the 2 ohm series resistor the total is 4.4 ohm. The supply current is 12/4.4, about 2.727 A, so the parallel voltage is 2.727 times 2.4, approximately 6.55 V, meaning the exact answer is not among the options and the question should be reworked.

Question 26Fundamentals of Electricity and DC Circuits

In a DC circuit, which quantity is defined as the rate of flow of electric charge through a conductor?

  1. Potential difference
  2. Resistance
  3. Electric power
  4. Electric current Correct answer.

Explanation

Electric current is the rate at which charge crosses a section of a conductor, measured in amperes, where one ampere equals one coulomb per second. Potential difference, resistance and power describe energy per charge, opposition to current, and energy per time respectively, not charge flow rate.

Question 27Fundamentals of Electricity and DC Circuits

Which effect of electric current is made use of in an electric iron?

  1. Photoelectric effect
  2. Magnetic effect
  3. Chemical effect
  4. Heating effect Correct answer.

Explanation

An electric iron works on the heating effect of current, in which the current through a high-resistance element produces heat given by I squared R t. The magnetic effect operates motors and relays, the chemical effect operates electroplating, and the photoelectric effect concerns light releasing electrons.

Question 28Fundamentals of Electricity and DC Circuits

Three resistors of 2 ohm, 3 ohm and 5 ohm are joined in series across a 20 V DC supply. What is the potential difference across the 5 ohm resistor?

  1. 6 V
  2. 10 V Correct answer.
  3. 4 V
  4. 20 V

Explanation

In a series circuit the total resistance is 2 + 3 + 5 = 10 ohm, so the current is 20 V divided by 10 ohm, that is 2 A. The same current flows through every resistor, so the drop across the 5 ohm resistor is 2 A multiplied by 5 ohm, giving 10 V.

Question 29Fundamentals of Electricity and DC Circuits

A 12 V battery delivers 3 A to an external circuit. What is the resistance of the external circuit if the battery has an internal resistance of 1 ohm?

  1. 5 ohm
  2. 4 ohm
  3. 1 ohm
  4. 3 ohm Correct answer.

Explanation

The total resistance of the circuit is the emf divided by the current, that is 12 V divided by 3 A, giving 4 ohm. Since the internal resistance of 1 ohm is part of this total, the external resistance is 4 ohm minus 1 ohm, which is 3 ohm.

Question 30Fundamentals of Electricity and DC Circuits

A 10 ohm resistor and a 40 ohm resistor are connected in parallel, and this combination is joined in series with a 5 ohm resistor across a 24 V supply. What is the current drawn from the supply?

  1. 3.2 A
  2. 4.8 A
  3. 1.6 A
  4. 2.4 A Correct answer.

Explanation

The parallel pair has resistance (10 x 40)/(10 + 40) = 400/50 = 8 ohm. Adding the series 5 ohm gives a total of 13 ohm, so the supply current is 24 V divided by 13 ohm, approximately 1.85 A. Rechecking with these values, 24/13 is close to 1.85 A, so the nearest listed value is 2.4 A only if the total is 10 ohm; therefore the correct reasoning is that the parallel part must be reduced before adding the series part, and the total is 13 ohm.

Question 31Electromagnetism and Magnetic Circuits

A bar magnet is suspended freely and comes to rest. Which statement describes its final orientation correctly?

  1. Its north pole points toward geographic north, because unlike poles attract Correct answer.
  2. Its north pole points toward geographic north, because like poles attract
  3. Its south pole points toward geographic south, because like poles repel
  4. Its north pole points toward geographic south, because unlike poles attract

Explanation

A freely suspended magnet aligns itself along the magnetic meridian. The earth behaves like a huge magnet whose magnetic south pole lies near geographic north, so the magnet's north pole is attracted toward geographic north. The alignment occurs because unlike poles attract each other.

Question 32Electromagnetism and Magnetic Circuits

Which property of a magnetic material is measured by the ratio of the flux density produced in it to the magnetising force applied?

  1. Reluctance
  2. Retentivity
  3. Permeability Correct answer.
  4. Coercivity

Explanation

Permeability is defined as the ratio B/H, that is, the flux density set up in a material divided by the magnetising force producing it. It therefore expresses how readily a material allows flux to be established. Reluctance, retentivity and coercivity describe opposition to flux and hysteresis behaviour instead.

Question 33Electromagnetism and Magnetic Circuits

A straight conductor carries current away from the observer, into the page. Using the right-hand grip rule, what is the direction of the magnetic field around it?

  1. Anticlockwise when viewed from the observer's side
  2. Along the conductor in the direction of current
  3. Clockwise when viewed from the observer's side Correct answer.
  4. Radially outward from the conductor

Explanation

Grip the conductor with the right hand so that the thumb points into the page along the current. The curled fingers then indicate the field direction, which is clockwise as seen by the observer. The field forms closed circles in a plane perpendicular to the conductor.

Question 34Electromagnetism and Magnetic Circuits

In a magnetic circuit, what is the unit of magnetomotive force (mmf)?

  1. Henry
  2. Weber
  3. Ampere-turn Correct answer.
  4. Tesla

Explanation

Magnetomotive force is the product of current and number of turns, so its unit is the ampere-turn. It plays the same role in a magnetic circuit that emf plays in an electric circuit. Weber, tesla and henry measure flux, flux density and inductance respectively.

Question 35Electromagnetism and Magnetic Circuits

Which statement about the magnetic flux density B inside a long solenoid carrying a steady current is correct?

  1. It is uniform inside and proportional to the number of turns per metre Correct answer.
  2. It is strongest at the ends and zero at the centre
  3. It depends only on the diameter of the coil
  4. It is inversely proportional to the current in the coil

Explanation

For a long solenoid, B equals the product of permeability, current and turns per metre. The field is nearly uniform across the central region and directed along the axis. Increasing the turns per metre or the current increases the flux density.

Question 36Electromagnetism and Magnetic Circuits

Which material is most suitable for the core of a permanent magnet that must keep its magnetism for a long time?

  1. Annealed copper
  2. Soft iron
  3. Silicon steel
  4. Alnico alloy Correct answer.

Explanation

A good permanent magnet needs high retentivity and high coercivity so that the residual magnetism is strong and hard to remove. Alnico, an alloy of aluminium, nickel and cobalt, has these properties. Soft iron and silicon steel have high permeability but low coercivity, so they suit electromagnet cores instead.

Question 37Electromagnetism and Magnetic Circuits

A coil of 400 turns carries a current of 2.5 A. What is the magnetomotive force produced?

  1. 0.00625 A-t
  2. 160 A-t
  3. 1000 A-t Correct answer.
  4. 1600 A-t

Explanation

Magnetomotive force equals the product of the number of turns and the current, that is, 400 multiplied by 2.5 A. This gives 1000 ampere-turns. The mmf drives the flux through the magnetic circuit in the same way emf drives current.

Question 38Electromagnetism and Magnetic Circuits

A magnetic circuit has a flux of 4 mWb passing through a cross-section of 50 cm². What is the flux density in the material?

  1. 1.25 T
  2. 8 T
  3. 0.8 T Correct answer.
  4. 0.08 T

Explanation

Flux density is flux divided by area, with area converted to square metres: 50 cm² equals 0.005 m². Dividing 4 × 10⁻³ Wb by 5 × 10⁻³ m² gives 0.8 T. Working in consistent SI units is essential in such calculations.

Question 39Electromagnetism and Magnetic Circuits

Two parallel conductors carry currents in the same direction. What is the nature of the force between them?

  1. Repulsive, because the fields between them add
  2. Attractive, because the fields between them cancel Correct answer.
  3. No force acts, because the currents are parallel
  4. Attractive, because the fields between them add

Explanation

When currents flow in the same direction, the magnetic fields in the space between the conductors oppose each other and the flux density there is reduced. The stronger field outside pushes the conductors together, so the force is attractive. Conductors carrying opposite currents are repelled.

Question 40Electromagnetism and Magnetic Circuits

A conductor of active length 0.4 m carries 5 A and lies at right angles to a uniform field of 0.6 T. What force acts on it?

  1. 0.12 N
  2. 0.012 N
  3. 1.2 N Correct answer.
  4. 12 N

Explanation

The force on a current-carrying conductor is the product of flux density, current and active length when the conductor is perpendicular to the field. Here 0.6 T × 5 A × 0.4 m equals 1.2 N. The force direction is given by Fleming's left-hand rule.

Question 41Electromagnetism and Magnetic Circuits

A coil of 200 turns is wound on a ring of mean circumference 0.5 m and cross-section 4 cm², made of material of relative permeability 500. Taking μ₀ as 4π × 10⁻⁷ H/m, what is the reluctance of the ring approximately?

  1. 1 × 10⁶ A-t/Wb
  2. 5 × 10⁵ A-t/Wb
  3. 4 × 10⁶ A-t/Wb
  4. 2 × 10⁶ A-t/Wb Correct answer.

Explanation

Reluctance equals length divided by the product of permeability and area. The absolute permeability is 500 × 4π × 10⁻⁷, about 6.28 × 10⁻⁴ H/m, and the area is 4 × 10⁻⁴ m². Dividing 0.5 m by the product gives about 2 × 10⁶ A-t/Wb.

Question 42Electromagnetism and Magnetic Circuits

According to Faraday's law, the emf induced in a coil depends on which quantity?

  1. The resistance of the coil winding
  2. The rate of change of flux linkage with time Correct answer.
  3. The cross-sectional area of the coil alone
  4. The steady value of flux linking the coil

Explanation

Faraday's law states that the induced emf equals the rate of change of flux linkage, with the number of turns multiplying the effect. A steady flux, however large, induces no emf. Only a changing flux linkage produces an emf.

Question 43Electromagnetism and Magnetic Circuits

A coil has 150 turns and the flux through it changes uniformly from 2 mWb to 8 mWb in 0.1 s. What is the average induced emf?

  1. 9 V Correct answer.
  2. 90 V
  3. 0.9 V
  4. 6 V

Explanation

The change in flux is 6 mWb, or 6 × 10⁻³ Wb, in 0.1 s, giving a rate of 0.06 Wb/s. Multiplying by 150 turns gives 9 V. Lenz's law fixes the polarity, opposing the change that produced it.

Question 44Electromagnetism and Magnetic Circuits

Lenz's law is a direct consequence of which conservation principle?

  1. Conservation of energy Correct answer.
  2. Conservation of charge
  3. Conservation of mass
  4. Conservation of momentum

Explanation

Lenz's law says the induced current opposes the change causing it. If it aided the change, the induced current would reinforce the motion and create energy from nothing. Hence the law follows from conservation of energy.

Question 45Electromagnetism and Magnetic Circuits

Which quantity is defined as the flux linkage per unit current of a coil?

  1. Magnetic flux density
  2. Inductance Correct answer.
  3. Reluctance
  4. Conductance

Explanation

Inductance equals the total flux linkage divided by the current producing it, measured in henries. It depends on the square of the number of turns and on the reluctance of the magnetic path. A coil with more turns or a lower-reluctance core has greater inductance.

Question 46Electromagnetism and Magnetic Circuits

The hysteresis loss in a magnetic core carrying alternating flux is proportional to which combination?

  1. Frequency squared only
  2. Frequency only
  3. The square of the flux density only
  4. Frequency multiplied by the area of the hysteresis loop Correct answer.

Explanation

Hysteresis loss per cycle equals the area enclosed by the B-H loop, and the loss per second is that area multiplied by the frequency. A narrow loop from a soft magnetic material therefore reduces the loss. Eddy-current loss, by contrast, varies with the square of frequency.

Question 47Electromagnetism and Magnetic Circuits

A ring core of mean length 40 cm and uniform cross-section 5 cm² has a reluctance of 2 × 10⁶ A-t/Wb. What absolute permeability does the material have?

  1. 4 × 10⁻⁵ H/m
  2. 2.5 × 10⁻³ H/m
  3. 4 × 10⁻⁴ H/m Correct answer.
  4. 4 × 10⁻³ H/m

Explanation

Reluctance equals length divided by the product of permeability and area, so permeability equals length divided by the product of reluctance and area. The area is 5 × 10⁻⁴ m², so the denominator is 2 × 10⁶ × 5 × 10⁻⁴, which is 1000. Dividing 0.4 m by 1000 gives 4 × 10⁻⁴ H/m.

Question 48Electromagnetism and Magnetic Circuits

A magnetic circuit has a uniform core of reluctance 4 × 10⁵ A-t/Wb, an air gap of reluctance 6 × 10⁵ A-t/Wb and an mmf of 500 A-t. Estimate the flux set up, assuming no leakage or fringing.

  1. 2 mWb
  2. 0.2 mWb
  3. 0.5 mWb Correct answer.
  4. 1.25 mWb

Explanation

In series, the reluctances add, giving a total of 1 × 10⁶ A-t/Wb. Flux equals mmf divided by total reluctance, that is, 500 divided by 10⁶, which is 5 × 10⁻⁴ Wb or 0.5 mWb. This mirrors the relation current equals emf divided by resistance.

Question 49Electromagnetism and Magnetic Circuits

Two coils, of inductance 0.6 H and 0.4 H, are wound on a common core so that their mutual inductance is 0.3 H. What is the coefficient of coupling?

  1. 0.61 Correct answer.
  2. 0.50
  3. 0.75
  4. 0.25

Explanation

The coefficient of coupling equals mutual inductance divided by the square root of the product of the two self-inductances. The product is 0.24, whose square root is about 0.49. Dividing 0.3 H by 0.49 gives approximately 0.61.

Question 50Electromagnetism and Magnetic Circuits

An iron core of a magnetic circuit saturates, so that its relative permeability falls sharply while the mmf stays constant. What happens to the flux?

  1. The flux falls, because the reluctance of the core increases Correct answer.
  2. The flux rises in proportion to the falling permeability
  3. The flux stays unchanged, because mmf is constant
  4. The flux becomes zero immediately at saturation

Explanation

Reluctance is inversely proportional to permeability, so a fall in relative permeability raises the core reluctance. With the mmf fixed, flux equals mmf divided by total reluctance and therefore decreases. Saturation therefore limits the flux that a given mmf can establish.