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Questions and Answers
What is the primary function of a starter resistor in a d.c. motor?
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?
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?
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?
What is the primary role of a commutator in a generator?
How is reluctance in a magnetic circuit analogous to components in an electric circuit?
How is reluctance in a magnetic circuit analogous to components in an electric circuit?
What happens to the direction of current in a conductor when it lies in a magnetic field and the flux is changing?
What happens to the direction of current in a conductor when it lies in a magnetic field and the flux is changing?
Which of the following is true about a three-phase induction motor?
Which of the following is true about a three-phase induction motor?
Which of the following represents Faraday’s first law of electromagnetic induction?
Which of the following represents Faraday’s first law of electromagnetic induction?
What is a significant advantage of a toroidal coil over a solenoid in some situations?
What is a significant advantage of a toroidal coil over a solenoid in some situations?
What does Lenz’s Law state about induced electric currents?
What does Lenz’s Law state about induced electric currents?
Which statement describes the polarity of an induced voltage?
Which statement describes the polarity of an induced voltage?
How does the area enclosed by a magnetic hysteresis loop affect hysteresis loss?
How does the area enclosed by a magnetic hysteresis loop affect hysteresis loss?
What is the result of having a stationary conductor in a stationary magnetic field?
What is the result of having a stationary conductor in a stationary magnetic field?
How does the geomagnetic field function in relation to the Earth?
How does the geomagnetic field function in relation to the Earth?
What factor does not directly determine the induced voltage across a conductor?
What factor does not directly determine the induced voltage across a conductor?
What is required for a coil to provide 800 At of magnetizing force when it has 400 turns?
What is required for a coil to provide 800 At of magnetizing force when it has 400 turns?
If the flux density is half that of the magnetic field strength, what happens to the absolute permeability?
If the flux density is half that of the magnetic field strength, what happens to the absolute permeability?
When the north poles of two bar magnets are brought close together, what occurs?
When the north poles of two bar magnets are brought close together, what occurs?
What induces a voltage in a closed wire loop when moved through a magnetic field?
What induces a voltage in a closed wire loop when moved through a magnetic field?
What is the term for the amount of magnetic field needed to remove residual magnetism from a transformer core during each half-cycle?
What is the term for the amount of magnetic field needed to remove residual magnetism from a transformer core during each half-cycle?
Which device can be used to limit the torque or force applied by an electromechanical device?
Which device can be used to limit the torque or force applied by an electromechanical device?
Which materials have a permeability slightly less than that of free space?
Which materials have a permeability slightly less than that of free space?
Magnetic flux can always be attributed to which of the following?
Magnetic flux can always be attributed to which of the following?
What is the polarity of induced voltage while a magnetic field is collapsing?
What is the polarity of induced voltage while a magnetic field is collapsing?
How should the winding axes of solenoid inductors be arranged to minimize their mutual inductance?
How should the winding axes of solenoid inductors be arranged to minimize their mutual inductance?
Which of the following statements about reluctance is correct?
Which of the following statements about reluctance is correct?
What is a primary advantage of an electromagnet over a permanent magnet?
What is a primary advantage of an electromagnet over a permanent magnet?
When magnetic lines of force pass through a coil perpendicular to its plane, what happens to the lines of magnetic force?
When magnetic lines of force pass through a coil perpendicular to its plane, what happens to the lines of magnetic force?
What analogy can be drawn between magnetic and electrical quantities?
What analogy can be drawn between magnetic and electrical quantities?
What is the effect on the galvanometer deflection when a bar magnet is moved at a steady speed towards a coil?
What is the effect on the galvanometer deflection when a bar magnet is moved at a steady speed towards a coil?
What is the relationship between reluctance and the magnetic properties of materials?
What is the relationship between reluctance and the magnetic properties of materials?
Which of the following best describes the behavior of flux in a magnetic circuit?
Which of the following best describes the behavior of flux in a magnetic circuit?
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?
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?
Which statement about induced emf is true?
Which statement about induced emf is true?
What is the term for the amount of magnetic field needed to remove residual magnetism from a transformer core?
What is the term for the amount of magnetic field needed to remove residual magnetism from a transformer core?
What is the unit of magnetic flux?
What is the unit of magnetic flux?
Which law can be used to determine the direction of an induced emf in a circuit?
Which law can be used to determine the direction of an induced emf in a circuit?
In electromagnetism, what does permeability refer to?
In electromagnetism, what does permeability refer to?
Which of the following statements is false regarding induced emf?
Which of the following statements is false regarding induced emf?
What does the Hall effect relate to?
What does the Hall effect relate to?
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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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