Portfolio Part A Flashcards
The Function of a magnetic shield is to
Divert the magnetic lines of force around the shielded object
If an electromagnet had 200 turns on the coil with a resistance of 5 Ohms, calculate the Magneto-motive force it would produce if connected to a 25v supply?
1000 ampere turns
Name the unit for magnetising force
(H) Ampere turns per meter
Describe the difference between magnetic leakage and magnetic fringing
Leakage: strays from the desired path
Fringing: when the area of the flux path is greater in the air gap than in the material of the magnetic circuit
Lenz’s law states that?
The direction of induced current will always oppose the changing magnetic field that produced it
List 3 practical applications for electromagnets
Motors, magnabend, headphones
Name 3 factors that affect the strength of a solenoid
Number of turns in the coil, length of core, cross sectional area
In what direction is the magnetic field around a conductor when the current flow Is away from the observer
Clockwise
State faraday’s law of electromagnetic induction
Placing a conductor in a changing magnetic field produces an EMF in that conductor
Which of the following devices are examples of electro magnetic induction
A: Electromagnets
B: Contactor Coil
C: Relay Coils
D: All of the above
D: All of the above
The current through an inductor is reduced from 5A to 1A in 0.75 seconds. If the inductance of the coil is 1.5 H, determine the induced voltage
8V
Magnetic sensors can be used to detect changes in magnetic fields
True or False
True
An electro magnet has 600 turns and the total reluctance of the core is 800 IN/Wb. Calculate the flux produced when 10A flows through the coil
7.5Wb
A coil of 800 turns has a flux of 90 micro Wb passing through it. If the flux is reduced to 40 micro Wb in 20ms, find the average induced voltage.
2V
When using the right hand rule to determine the direction of the magnetic field around a current carrying conductor, why are our fingers placed at a right angle around the conductor
Your fingers point at the direction of the magnetic field
A conductor placed at a right angle to a magnetic field with a density of 0.66T over a length of 0.1m. if a current of 25A is passed through the conductor, calculate the force exerted on the conductor
1.65 newtons
Define the term self-Inductance
Inductance occurs when a conductor had a voltage induced in it by its own magnetic field when in a changing field
If two conductors carry current in the same direction, the electromagnetic force will cause the conductors to repel each other
True or false
True
Which of the following is an example of mutual inductance
A: Multi-Coil transformer
B: Fluorescent Ballast
C: Solenoid Coil
D: All of the above
A
Which of the following is assigned the permeability of unity or 1
A: Actual Permeability
B: Absolute Permeability
C: Relative Permeability of air
D: Permeability of iron
C
If the rate at which a conductor cuts the magnetic field is reduced then
A: The magnitude of the voltage increases
B: The hotter the conductors become
C: The stronger the magnetic field around it
D: The magnitude of the induced EMF decreases
D
Which four components are used in the construction of a Electromagnetic relay
A: Overload, Stator, Rotor, Fuses
B: Fuses, Overload, Stator, Permanent Magnet
C: Coils, Contacts, Overloads, Permanent Magnets
D: fixed iron stator, Moving iron armature, Coil, Contacts
D
When applying Flemings right hand rule for conductors carrying current, your thumb points to
A: Direction of rotation
B: Direction of magnetic flux
C: Direction of current flow
D: Points of resistance
B
Which two factors determine iron losses in magnetic materials
A: Voltage and Eddy current losses
B: Iron and Hysteresis losses
C: Hysteresis and Eddy current losses
D: Copper and Brass losses
C
Which two Solutions help reduce Hysteresis and Eddy current losses?
A: Reduce the turns on the coil and change the voltage
B: Create an air gap in the core and change the voltage
C: Increase the turns on the coil and Use a solid core
D: Increase the permeability of the core material and laminate the core
D
Select the correct answer that defines Permeability.
A: The opposition to the magnetising force.
B: The lines of magnetism that extend from north pole to south pole
C: The ease at which a flux can be created in a material.
D: The number of lines of force per m²
C
Select the correct answer that defines magnetic flux.
A: the number of lines of force per m²
B: The ease at which flux can be created in a material
C: The opposition to magnetising force
D: The lines of magnetism that extend from North pole to South
D
Select the correct answer that defines flux density
A: The ease at which flux can be created in a material
B: The lines of magnetism that extend from North pole to South pole
C: The number of lines of force per m²
D: The ease at which flux can be created in a material
C
Calculate the flux density of an air-cored former that has a cross sectional area of 200mm² and a total magnetic flux of 0.0000025 Webers. (Note: 1mm² = 1 x 10-⁶ m²)
0.00125 Wb/m²
Name the common magnetic circuit types.
A: paramagnetic and diamagnetic
B: permanent magnet and electromagnetic
C: permanent and paramagnetic
D: Diamagnetic and electromagnetic
B
What is the common name for the opposition to the establishment of magnetic flux
Reluctance
A conductor 0.2m in length moving at right angles to a magnetic field of flux density 0.005T has a velocity of 20 m/s. Calculate the maximum induced voltage.
0.02V
A bifilar winding inductor is constructed by winding 2 conductors in parallel
True of false
True
Calculate the force exerted on two parallel conductors each carrying 50 amps, if the distance between them is 50mm.
0.01 newtons per metre
What four factors determine the value of inductance in a coil
A: size of conductors and number of turns in the coil, length of the core, cross sectional area, Permeability of core material
B: Permeability of core, resistance of the coil, Flemings right hand rule, length of the core
C: right hand rule for conductors, number of turns, material core made from, resistance of coil.
D: capacitance in coil, amount of flux produced, length of core, type of material coil made from
A
Three common types of inductor cores are.
A: air, iron, ferrite
B: Copper, iron, ferrite
C: Iron, air, brass
D: Iron, Aluminium, Ferrite
A
Name three applications for inductors in electro-technology field…
A: RF applications, Anodes, Toroidal coils
B: RF applications, Diodes, DC connections
C: RF Applications, Kettering ignition coils, AC coils
D: RF application, Capacitors, Anodes
C
Mutual induction occurs between coils when
A: One energized coil induces an EMF into a second coil
B: The coil is saturated
C: The second coil is at maximum resistance
D: The second coil has no load connected
A
Snubbing circuits are fitted with components such as diodes, resistors, inductors and capacitors across the coil. These components are used to short out high self-induced voltages that try to keep the current flowing
True or False
True
Define a time constant for an inductor connected to D.C
A time constant is the length of time during which the charge or discharge undergoes a 63.2% change
The series field windings of a 120V DC generator have an ohmic resistance of 60 ohms and an inductance of 10H determine:
A: the time constant
B: the time it will take for the current to attain it’s full value
C: the final value of currant
A: 0.17s
B: 0.85s
C: 2 amps
Name three possible causes of damage to electrical instruments
Overload, wrong connections, DC meter to AC
When using an ohm meter name two safety precautions that should be taken
Make sure the circuit is off
*
When choosing a meter to use 6000V which category of meter would be safest to use
A: Cat I
B: Cat II
C: Cat III
D: Cat IV
D
Which material is used to make the core of a Toroidal Coil
Ceramic materials
What is the effect of an air gap in a magnetic circuit
A: decrease the reluctance of that magnetic circuit
B: Increase the flux density of that magnetic circuit
C: Increase the reluctance of that magnetic circuit
D: Increase Magneto-motive force
C
Like poles attract each other
True or false
False
Name the three classifications of magnetic substances
Ferromagnetic, paramagnetic, diamagnetic
Give two situations where reed switches are used
Security installations and domestic goods
List 8 practical applications for permanent magnets
Magnetic shield, speaker, credit card, fridge door, alarms, microphones, small generators, electrical motors
Calculate the values of a shunt resistor for a current range of 2A for a digital module that has a full scale input voltage of 200mV
0.1 ohms
Name one method of extinguishing an arc across the open contacts of a D.C relay
Diode connected across the coil
What is the difference in the construction of a D.C contactors and relays compared to A.C
DC has solid magnetic Core and A.C has a laminated iron
There are 3 ways hall effect devices monitor motor rotation and speed
Small light emiting diodes, magnetic sensors, hall effect sum conductors
In 1 time constant the current will increase to 63% of the value of the maximum current in an LR circuit. How many time constants will take the current to full load?
5 constants
What is a dynamo meter used for and what is the practical name for it
Used to measure power consumed and it practical name is a watt-meter
Give one practical application for each of the following meters
Moving coil meter
Moving iron meter
Dynometer
Moving coil meter - DC current test
Moving iron meter - DC and AC testing resistance
Dynamo meter - testing power consumption
When selecting a meter for use in the electrical trade name three things that must be considered
Accuracy of the instrument needed
Sensitivity of the instrument needed
Cat rating for safety
Name the practical applications where magneto-striction material is used
Stepper motors, ultrasonic cleaner, stress and pressure
