DC Fundamentals 2 Flashcards
Describe the construction of a permanent magnet?
- In ferromagnetic materials (Iron, Nickle, and Cobalt), its atoms are grouped into ‘magnetic domains’
- Each ‘magnetic domain’ acts like its own tiny magnet with a North and South pole
Define the term unmagnetised?
- Domains are arranged randomly
- The magnetic domains cancel each other out
- Zero magnetic charge
Define the term magnetised?
- An unmagnetised material is under the influence of another magnet
- The magnetic domains begin to align as the magnet gets closer
Define the term fully magnetised?
- The magnetic domains are fully aligned once put under the influence of another magnet
Define the term temporary magnet?
- Becomes a magnet only once its under the influence of another magnet
E.g. White board in classroom
Define the term permanent magnet?
- Retains its magnetic properties
- Made from magnetically hard materials, like steel
- Requires a strong magnetic force to align the magnetic domains
E.g. White board eraser block
Define the term magnetic axis?
- Imaginary line drawn through the middle of a magnetic field
Define the term magnetic field?
- Force field that surrounds a magnet
- The distance between the lines represents the strength of the magnetic field
E.g. Closer the lines - stronger the magnetic field
Define the term magnetising force?
- When a suitable material is placed under the influence of a strong magnetic field
- Represented by the symbol letter ‘H’
Define the term permeablility?
- Measure of how easy it is for a magnetic force to be established within a material
Define the term retentivity?
- Ability to retain magnetic properties after the magnetising force has been removed
Define the term reluctance?
- How reluctant (resilient) a material is to becoming a magnet
Define the term saturation?
- The maganetic domains are already aligned so they cannot be used to magnetise - Even with a bigger magnet, there are no available magnetic domains
Define the term magnetic pole?
- The alignment of the magnetic domains creates points on the magnet where the magnetic field is concentrated
- North pole and South pole
Define the term flux density?
- Magnetic field strength is described in terms of ‘flux density’
- The measure of the number of flux lines per square meter
2 operating characteristics of a magnet?
- Attraction and repulsion
- Direction of magnetic lines
Describe attraction and repulsion in terms of magnets?
- Opposites attract
E.g. - Pole of magnet 1 connects with similar pole of magnet 2 - they will repel each other
- Pole of magnet 1 connects with opposing pole of magnet 2 - they will attract each other
What direction do the magnetic lines travel in:
1) Outside the magnet
2) Inside the magnet
Outside: North to South
Inside: South to North
Example of a permanent magnet precaution?
- Keeper
Define the term keeper?
- Device used to protect a magnet
- A suitable shaped piece of low reluctance metal, placed between North and South pole magnets
- Used to protect the magnet
What does a keeper prevent?
- Prevents external magnetic fields from weakening the magnet
- Prevents the magnet from accidentally influencing magnetically sensitive devices
What is the purpose of a magnetic force?
- A magnetic force is used to align the magnetic domains within a material in order to turn it into a magnet
What is Flemming’s left hand rule?
- Grasp conductor in left hand with thumb pointing int he direction of the current flow
- The fingers wrap around the conductor in the same direction of the lines of the magnetic field
Define dart notiation?
- Simple method of indicating the direction of current flow using the cross section of a conductor
Dot: Current is flowing towards you and out of the conductor
Cross: Current is flowing away from you and into the conductor
Define the term solenoid?
- A wire which is wound into a series of loops to forma coil, similar to a spring
- The lines of flux do not cross each other, instead they add together to create a bigger magnetic field
How do the magnetic lines of force travel in a solenoid magnetic field?
- Similar to a bar magnet
Outside / Externally: North to South
Inside / Internally: South to North
How do you determine magnetic poles in a solenoid?
- Using Flemming’s left hand rule
- Wrap your fingers in the direction of the current
- Your thumb will then be pointing the North pole
What 2 things is Flemming’s left hand rule used for?
- Determining the direction of current flow
- Determining the magnetic pole
3 factors that affect the strength of a inductor?
- Number of turns the wire that make up the coil
- Amount of current flow through the coil
- Distance between adjacent wires in a coil
As the strength of a magnetic field increases in a solenoid, what happens to the current flowing through the coil?
Strength increases = Current flow increases
(Directly proportional)
As the strength of a magnetic field increases in a solenoid, what happens to the number of turns that the wire makes?
Strength increases = Number of turns increases
(Directly proportional)
As the strength of the magnetic field increases in a solenoid, what happens to the distance between the wires?
Strength increases = Distance between the wires decreases
(Inversely proportional)
In other words, the closer together the wires are - the stronger the magnetic field is
Describe electromagnetic induction?
- Inducting EMF into something that is not a magnet
- When current flows through a conductor and a magnetic field is created
What happens to the magnetic force when there is no longer relative movement between the lines of flux and the coil of wire?
- Magnetic field will stop
- EMF will no longer be induced
Define Lenz’s law?
‘The polarity of the induced EMF is such that any current resulting from it produces magnetic flux that opposes the motion or change producing the EMF’
- In simple terms, anytime there is a rate of change (movement) in the magnetic lines of force, there is an EMF induced in the solenoid which produces a current which in turn creates magnetic lines of force to oppose the original change
- The crazy toxic ex law
Define Faraday’s law?
‘The voltage induced in a conductor by a reaction between the conductor or coil and a magnetic field is directly proportional to the time rate of change of flux linkage s or flux cutting’
4 factors affecting the induced EMF in a conductor?
- Magnetic field strength
- Active length of the conductor
- Velocity of the cutting effect
- Angle at which the field is cutting
Induced voltage = Magnetic field x active length x velocity x sin (angle)
Describe self induction?
- It does it itself
Characteristics of inductors?
Symbol: L
Unit: H (Henry’s)
Core types: Iron, air and ferrite
3 types of inductor cores?
- Air
- Iron: Used for high power application
- Ferrite: Used for low power circuits
When will back EMF be at its greatest when powering an inductor circuit?
- The instant that power is applied to the circuit
What is self induced voltage with regards to inductors?
- The voltage which is induced into the inductor as a result of a change in current through the conductor
Define the term back EMF?
- As circuit current flows, a magnetic field builds in each loop of the inductor which induces EMF in adjacent loops providing an opposing current
Factors that affect the self induction of an inductor?
- Linkages
- Cross sectional area
- Number of turns
- Type of core the inductor is wound on
Define the term linkages?
- Think of it as ‘How many buddies does he have close by?’
- A coil with more lays (wound more times) has a much greater inductance
Define the term core?
- Refers to the type of centre a coil is wound on
- High permeability of an iron core would produce less reluctance than an air core
Tolerance codes for inductors?
M: 20%
K or silver: 10%
J or gold: 5%
Applications for back EMF?
- Soft starting
- Protecting electrical equipment
- Stabilising the circuit to avoid dips in voltage
The time taken for an inductor to reach full charge depends on 2 things, these are?
- Resistance in the circuit
- Inductance of the inductor
T = L/R
2 things to look for in Diodes?
- Is it installed correctly (forward or backwards bias)?
- Do we have enough voltage?
Define diode?
‘diode’ is short hand for ‘di-electrode’ with ‘di’ meaning 2
- PN junction diode is a two-electrode device
What does a diode do?
- Only allows current to flow in 1 direction
- There could be a fault in the circuit but it wont let the current back feed through
- Protects the circuit
- Controls the flow
How is a serviceability check carried out on a discrete component tester?
- Check the physical condition of the unit, wires and test probes
- Turn device on to allow the discrete component
tester to run through its power-on built in test - If no errors are displayed on
the LCD screen – the serviceability check has passed
What are the terms given to the two logic states used in DC circuits?
- Logic 1
- Logic 0
What two conditions must be met for a diode to become forward biased?
- Anode must be connected closer to the positive connection of the
power supply - Cathode must be connected closer to the negative
connection of the power supply - Power supply must be great
enough to overcome the barrier potential of the diode (0.6V)
What are Anode and Cathode, regarding their charge?
Anode: positive
Cathode: Negative
Define magnetism?
The ability of matter to attract other matter to itself
