BWC 131 - Extra Low Voltage, Magnetism and Meters Flashcards
What are the elastic lines that make up the magnetic field
Lines of Flux
The ability for a material to carry a magnetic field is called
Permeability
The inverse effect of permeability is called
Reluctance
How do Lines of Flux behave
Exit the north pole re-enter the south pole.
Like Poles
repel
unlike poles
attract
What is Flux Density
Measure of Field Concentration
What is the driving force for all magnetic Fields
Magneto motive force (mmf
Where is the Magnetic field the strongest
close to the conductor
If two conductors are in parallel and current is in the same direction what happens to the conductor
the are drawn together
If two conductors are in parallel and current is in different direction what happens to the conductor
The are pushed apart
What is a newton
Force of Gravity
When a conductor is wound into a coil the magneto-motive force that surrounds the conductor becomes
much more concentrated
What type of core do most magnets have?
Iron
What is a core made of
Air or Iron
breaks in a magnetic circuit are dalled
air gaps
what is the function for magnetic strength
size of the poles and flux density
how do magnetic fileds flow
they expand or contract in concentric loops
what is it called when the reluctance of the core is infinite and
the field through the core of the electromagnet will not increase regardless of the increase in the current through the coil.
saturation
the way a response lags behind its cause is
Hysteresis
how well a magnetic material retains its magnetism after the
magnetizing force is removed is called
Retentivity
the magnetic material retained after the
magnetizing force is removed is called
Residual Magnetism
the magnetizing force it takes to reduce the residual magnetism to zero
coercive force
a localized electric current induced in a conductor by a varying magnetic field is called
eddy currents
A ________ contains one or more contacts that are operated electromagnetically.
relay
When there is a change in the amount of magnetic flux linked by a circuit, an electromotive force (emf) is induced into that circuit. The magnitude of the emf is proportional to the rate at which the flux is changing is whos law
Faraday
the direction (polarity) of the induced emf will be such that the
magnetic effect of any current resulting from that emf will oppose the
change in flux that produced it is whos law
Lenz
The ____________ consists of several turns of fine copper wire wound on an aluminum frame.
Moving Coil
The two methods used to damping a moving-coil meter are
air damping and eddy current damping
meter scales are not always
Linear
what is he apparent shift in an object’s position as it is viewed from different angles.
parallax error
________ are misleading voltage readings that are caused by such things as stray induction, capacitive coupling, and insulation leakage
Ghost readings
What do the Letters “OL” stand for on a meter
Over Range
Where do the black leads go on a meter
COM
Where do red leads go on a meter
V/Ohms
the voltage range can be increased dramatically by adding a _________ to the meter circuit.
Multiplier resistor
The Ω/V rating found on analog meters is referred to as the meter’s
Sensitivity
The intensity of a magnetic field is called
Flux Density
A ________ is surrounded by an invisible magnetic field.
Permanent Magnet
What is the driving force for all magnetic fields
Magnetomotive Force
If current flows in the same direction the fields will ________ and be drawn together
Combine
if current flows in opposite directions fields will become _________ forcing conductors apart
compressed
winding conductors into a coil creates a ?
Electromagnet
which three factors determine strength of the magnetic field
- Current Flow
- number of coil wraps
- core material
what is potential difference
Voltage
How is a shunt connected?
in parallel
a typical shunt has very low ________
resistance
why are shunts made of material with low temp coefficients
min resistance change
why is it important to have a shunt with low resistance `
doesn’t change normal circuit
never connect/disconnect a ammeter from the circuit when its ________
energized
what does the term meter loading mean?
when a meter alters the current flow
under what conditions does meter loading become a problem
when the meter has a higher resistance then the circuit
shunts reduce high current to _________
Milivolt Outputs
what does Instrument is used for measuring very low levels of resistance
Wheatstone bridge
what test is used to determine if a circuit/ device is continuous.
Continuity test
to find the resistance of a single component, the component must first be _____
isolated
Never assume that a failed circuit has been ________
De-energized
why is it important to keep fingers off the probes when measuring resistance
you will measure resistance of body and circuit
with the probes shorted together you can
zero the meter
An analog meter should be ________ before each use
zeroed
An analog Ohm meter moves _______ proportional to the current
Inversely
Typical ranges for an analog meter are
Rx1
Rx100
Rx10,000
For max accuracy bring the pointer to the _______ of the scale
Right side
An OL on a meter means
Over Range
0 Ohms in a meter means that _____
The meter is incapable of measuring the resistance as its to low
typical meter leads have a resistance of
0.2-0.4 Ohm
__________ are charge storage devices
Capacitors
A Capacitor must be __________ before testing
Discharged
When voltage in a capacitor are equal to that in the battery what happens the the current.
Current will stop
Diodes are electrical Equivalent of a
Check Valve
When a Diode fails it becomes
Open / Short
Functional Diodes may test @
Several Kilohms
What are the 7 Ohmeter Precautions and Procedures
- Make sure the circuit/component are De-energized
- Make sure the circuit/components are isolated from the capacitor
- Zero out the analog meter
- For Digital meters measure the initial lead resistance.
- completely isolate the component to be tested.
- Don’t touch the tips of the probes while testing.
- Don’t leave the meter on ohms when finished
What makes up a Wheastone Bridge
- Battery
- Zero Center Galvanometer
- Three adjustable precision resistors
Two resistors on a Wheatstone Bridge are called
Ratio Arms
Ratio Arms have precise multiples of
1:1000 to 1000:1
The Third resistor of a Wheastone Bridge is controlled by
4 individual switches
What is the Equation used to calculate resistance of a Wheatstone Bridge
(R1/R(Rx/R3)
Magohm Meter provide a DC Test Voltage from _________
100V - 5000V
What are precautions using a Megohm Meter
- Don’t Connect to a energized circuit
- They have there own internal power supply
- Don’t make contact with Megohm meter leads during operation
- don’t use to test low voltage
- discharge the equipment before considering it safe to touch.
The electrons that leak through insulation to ground or to other conductors is ________
Leakage Current
