Portfolio Part A

Question 1

The Function of a magnetic shield is to

Answer 1

Divert the magnetic lines of force around the shielded object

Question 2

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?

Answer 2

1000 ampere turns

Question 3

Name the unit for magnetising force

Answer 3

(H) Ampere turns per meter

Question 4

Describe the difference between magnetic leakage and magnetic fringing

Answer 4

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

Question 5

Lenz’s law states that?

Answer 5

The direction of induced current will always oppose the changing magnetic field that produced it

Question 6

List 3 practical applications for electromagnets

Answer 6

Motors, magnabend, headphones

Question 7

Name 3 factors that affect the strength of a solenoid

Answer 7

Number of turns in the coil, length of core, cross sectional area

Question 8

In what direction is the magnetic field around a conductor when the current flow Is away from the observer

Answer 8

Clockwise

Question 9

State faraday’s law of electromagnetic induction

Answer 9

Placing a conductor in a changing magnetic field produces an EMF in that conductor

Question 10

Which of the following devices are examples of electro magnetic induction

A: Electromagnets
B: Contactor Coil
C: Relay Coils
D: All of the above

Answer 10

D: All of the above

Question 11

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

Answer 11

8V

Question 12

Magnetic sensors can be used to detect changes in magnetic fields

True or False

Answer 12

True

Question 13

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

Answer 13

7.5Wb

Question 14

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.

Answer 14

2V

Question 15

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

Answer 15

Your fingers point at the direction of the magnetic field

Question 16

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

Answer 16

1.65 newtons

Question 17

Define the term self-Inductance

Answer 17

Inductance occurs when a conductor had a voltage induced in it by its own magnetic field when in a changing field

Question 18

If two conductors carry current in the same direction, the electromagnetic force will cause the conductors to repel each other

True or false

Answer 18

True

Question 19

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

Answer 19

A

Question 20

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

Answer 20

C

Question 21

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

Answer 21

D

Question 22

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

Answer 22

D

Question 23

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

Answer 23

B

Question 24

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

Answer 24

C

Question 25

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

Answer 25

D

Question 26

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

Answer 26

C

Question 27

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

Answer 27

D

Question 28

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

Answer 28

C

Question 29

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)

Answer 29

0.00125 Wb/m²

Question 30

Name the common magnetic circuit types. A: paramagnetic and diamagnetic B: permanent magnet and electromagnetic C: permanent and paramagnetic D: Diamagnetic and electromagnetic

Answer 30

B

Question 31

What is the common name for the opposition to the establishment of magnetic flux

Answer 31

Reluctance

Question 32

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.

Answer 32

0.02V

Question 33

A bifilar winding inductor is constructed by winding 2 conductors in parallel True of false

Answer 33

True

Question 34

Calculate the force exerted on two parallel conductors each carrying 50 amps, if the distance between them is 50mm.

Answer 34

0.01 newtons per metre

Question 35

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

Answer 35

A

Question 36

Three common types of inductor cores are. A: air, iron, ferrite B: Copper, iron, ferrite C: Iron, air, brass D: Iron, Aluminium, Ferrite

Answer 36

A

Question 37

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

Answer 37

C

Question 38

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

Answer 38

A

Question 39

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

Answer 39

True

Question 40

Define a time constant for an inductor connected to D.C

Answer 40

A time constant is the length of time during which the charge or discharge undergoes a 63.2% change

Question 41

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

Answer 41

A: 0.17s B: 0.85s C: 2 amps

Question 42

Name three possible causes of damage to electrical instruments

Answer 42

Overload, wrong connections, DC meter to AC

Question 43

When using an ohm meter name two safety precautions that should be taken

Answer 43

Make sure the circuit is off | *

Question 44

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

Answer 44

D

Question 45

Which material is used to make the core of a Toroidal Coil

Answer 45

Ceramic materials

Question 46

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

Answer 46

C

Question 47

Like poles attract each other True or false

Answer 47

False

Question 48

Name the three classifications of magnetic substances

Answer 48

Ferromagnetic, paramagnetic, diamagnetic

Question 49

Give two situations where reed switches are used

Answer 49

Security installations and domestic goods

Question 50

List 8 practical applications for permanent magnets

Answer 50

Magnetic shield, speaker, credit card, fridge door, alarms, microphones, small generators, electrical motors

Question 51

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

Answer 51

0.1 ohms

Question 52

Name one method of extinguishing an arc across the open contacts of a D.C relay

Answer 52

Diode connected across the coil

Question 53

What is the difference in the construction of a D.C contactors and relays compared to A.C

Answer 53

DC has solid magnetic Core and A.C has a laminated iron

Question 54

There are 3 ways hall effect devices monitor motor rotation and speed

Answer 54

Small light emiting diodes, magnetic sensors, hall effect sum conductors

Question 55

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?

Answer 55

5 constants

Question 56

What is a dynamo meter used for and what is the practical name for it

Answer 56

Used to measure power consumed and it practical name is a watt-meter

Question 57

Give one practical application for each of the following meters Moving coil meter Moving iron meter Dynometer

Answer 57

Moving coil meter - DC current test Moving iron meter - DC and AC testing resistance Dynamo meter - testing power consumption

Question 58

When selecting a meter for use in the electrical trade name three things that must be considered

Answer 58

Accuracy of the instrument needed Sensitivity of the instrument needed Cat rating for safety

Question 59

Name the practical applications where magneto-striction material is used

Answer 59

Stepper motors, ultrasonic cleaner, stress and pressure