Electromagnetic Induction and DC Motors Quiz

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Questions and Answers

What is the primary function of a starter resistor in a d.c. motor?

Limit the armature current to a safe starting value (correct)

Limit the field current to a safe starting value

Prevent the field current flowing through and damaging the armature

Control the speed of the machine

Which electromagnetic waves are primarily produced by heat?

Shortwaves

Microwaves

X-rays

Infrared rays (correct)

According to Faraday's first law of electromagnetic induction, what causes an electromotive force (emf) to be induced in a conductor?

The length of the conductor

The temperature of the conductor

The speed of the conductor

The rate at which the conductor cuts magnetic flux (correct)

What is the primary role of a commutator in a generator?

Convert the a.c. generated into a d.c. output Signup and view all the answers

How is reluctance in a magnetic circuit analogous to components in an electric circuit?

Resistance Signup and view all the answers

What happens to the direction of current in a conductor when it lies in a magnetic field and the flux is changing?

It is proportional to the rate of change of magnetic flux Signup and view all the answers

Which of the following is true about a three-phase induction motor?

It operates at higher efficiency compared to single-phase motors Signup and view all the answers

Which of the following represents Faraday’s first law of electromagnetic induction?

An induced emf is proportional to the rate of change of flux linkage Signup and view all the answers

What is a significant advantage of a toroidal coil over a solenoid in some situations?

The toroid is easier to tune Signup and view all the answers

What does Lenz’s Law state about induced electric currents?

An induced current flows in a direction that opposes the change that induced it Signup and view all the answers

Which statement describes the polarity of an induced voltage?

It is determined by Lenz’s Law Signup and view all the answers

How does the area enclosed by a magnetic hysteresis loop affect hysteresis loss?

A larger area increases hysteresis loss Signup and view all the answers

What is the result of having a stationary conductor in a stationary magnetic field?

No induced voltage will occur Signup and view all the answers

How does the geomagnetic field function in relation to the Earth?

It makes the Earth behave like a giant magnet Signup and view all the answers

What factor does not directly determine the induced voltage across a conductor?

The position of the conductor Signup and view all the answers

What is required for a coil to provide 800 At of magnetizing force when it has 400 turns?

0.2 A of current Signup and view all the answers

If the flux density is half that of the magnetic field strength, what happens to the absolute permeability?

Reduced Signup and view all the answers

When the north poles of two bar magnets are brought close together, what occurs?

A force of repulsion Signup and view all the answers

What induces a voltage in a closed wire loop when moved through a magnetic field?

A voltage is induced in the wire Signup and view all the answers

What is the term for the amount of magnetic field needed to remove residual magnetism from a transformer core during each half-cycle?

Demagnetizing force Signup and view all the answers

Which device can be used to limit the torque or force applied by an electromechanical device?

A back-pressure sensor Signup and view all the answers

Which materials have a permeability slightly less than that of free space?

Paramagnetic Signup and view all the answers

Magnetic flux can always be attributed to which of the following?

Motion of charged particles Signup and view all the answers

What is the polarity of induced voltage while a magnetic field is collapsing?

Opposite to the force creating the field Signup and view all the answers

How should the winding axes of solenoid inductors be arranged to minimize their mutual inductance?

At right angles Signup and view all the answers

Which of the following statements about reluctance is correct?

It is directly proportional to the length of magnetic path Signup and view all the answers

What is a primary advantage of an electromagnet over a permanent magnet?

Electromagnets can be switched on and off Signup and view all the answers

When magnetic lines of force pass through a coil perpendicular to its plane, what happens to the lines of magnetic force?

The number of lines of force is maximized Signup and view all the answers

What analogy can be drawn between magnetic and electrical quantities?

Resistance is analogous to reluctance Signup and view all the answers

What is the effect on the galvanometer deflection when a bar magnet is moved at a steady speed towards a coil?

Deflection in the opposite direction with increased magnitude Signup and view all the answers

What is the relationship between reluctance and the magnetic properties of materials?

Higher permeability results in lower reluctance Signup and view all the answers

Which of the following best describes the behavior of flux in a magnetic circuit?

Flux decreases with increased reluctance Signup and view all the answers

What direction does the force exerted on a horizontal conductor, with current flowing from left to right and a magnetic field directed from above, take?

Away from the viewer Signup and view all the answers

Which statement about induced emf is true?

The direction of an induced emf is always such as to oppose the effect producing it. Signup and view all the answers

What is the term for the amount of magnetic field needed to remove residual magnetism from a transformer core?

Demagnetizing field Signup and view all the answers

What is the unit of magnetic flux?

Weber Signup and view all the answers

Which law can be used to determine the direction of an induced emf in a circuit?

Lenz’s law Signup and view all the answers

In electromagnetism, what does permeability refer to?

The state of being permeable to a magnetic field Signup and view all the answers

Which of the following statements is false regarding induced emf?

An induced emf is only created from a static magnetic field. Signup and view all the answers

What does the Hall effect relate to?

The interaction of electric current and magnetic fields Signup and view all the answers

Study Notes

Electromagnetic Induction and Energy Principles

An electric current is induced in a conductor lying within a magnetic field, proportional to the rate of change of magnetic flux linked with the coil.
Faraday's First Law of Electromagnetic Induction states that an electromotive force (emf) is induced when a conductor is placed in a varying magnetic field.
Lenz's Law indicates that induced current flows in a direction that opposes the change generating it, affecting the polarity of induced voltage.

Direct Current (DC) Motors

Starter resistors in DC motors prevent the field current from damaging the armature and limit the armature current to safe starting levels.
Commutators convert alternating current (AC) generated by motors into direct current (DC), while preventing sparking when the load changes.

Characteristics of Electromagnetic Waves

Electromagnetic waves produced primarily by heat are known as Infrared Rays.

Magnetic Properties and Behaviors

Reluctance in a magnetic circuit is analogous to resistance in an electric circuit, directly proportional to the length of the magnetic path, but not to the permeability of the material.
The geomagnetic field affects navigational instruments and influences electromagnetic devices.
Magnetic flux is associated with the alignment of atoms and the motion of charged particles.

Electromagnetic Components and Concepts

The induced voltage across a stationary conductor in a magnetic field equals zero; it varies with movement of the conductor within the magnetic field.
The difference between magnetic flux and magnetic flux density lies in their definitions and implications in electromagnetism.
A closed wire loop moving through a magnetic field induces a voltage based on direction and motion, with current also generated.

Practical Applications of Magnetic Principles

In a three-phase induction motor, properties such as hysteresis and permeability play crucial roles in efficiency.
Materials with lower permeability than free space are characterized as paramagnetic or diamagnetic.

Electromagnetic Devices and Sensors

Various electromagnetic devices, including back-pressure sensors and optical shaft encoders, can limit torque or force in applications like robotics.
An electromagnet can be switched on and off, providing more control compared to a permanent magnet.

Key Rates and Reactions

The force exerted on a conductor in a magnetic field is determined by the relative direction of the current and magnetic field, evaluated through Fleming's left-hand rule.
The induced emf is always proportional to the rate of change of the magnetic flux and varies with the circuit's magnetic environment.

Measurement Units in Electromagnetism

Magnetic flux is measured in Webers, while magnetic field strength is essential for understanding induced currents and torque in electrical systems.

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Description

Test your knowledge on electromagnetic induction, the principles behind DC motors, and the characteristics of electromagnetic waves. This quiz covers essential laws such as Faraday's and Lenz's laws and explores the behavior of electric currents in magnetic fields.