Basic Electricity 1 Flashcards
Define Electrostatic Force
Force of attraction generated between an electron and the nucleus of an atom.
Define Potential Difference
Size of electrostatic force between 2 objects
Define electromotive Force
Sum of the potential differences of all charged particles in the electrostatic field.
Define Ion Charge
Positive or negative Charge gained by an atom based on losing or gaining electrons
Define Coulombs Law
The strength of the attraction/repulsion force between 2 charged objects depends upon:
1: Amount of charge on each object.
2: Distance between the objects.
Define conductor
Materials with electrons loosely bound to their atoms. (Free motion of a large number of electrons)
Define Insulator
Materials with electrons tightly bound to their atoms and require large amounts of energy to free the electrons from the influence of the nucleus.
Define Resistor.
Made of materials that conduct electricity but offer more opposition to current flow than good conductors.
Common conductor materials
Copper, Silver, Gold
Common insulator materials
Rubber, plastics, glass and dry wood
Common Resistor Materials
Carbon, silicon, germanium, tin, lead.
Define Voltage
Potential Difference that causes one coulomb of current to do one joule of work. “E” or sometimes “V”
What is a Coulomb?
6.28x10^18 electrons
Define Current
Movement or flow of Free Electrons from one atom to the next magnitude of potential difference applied.
“I” or sometimes “A”
Describe Electron Flow
Flow of electrons from negative potential to positive potential.
Describe Conventional Current
Flow of positive charges in opposite direction of current flow due to holes left by moving electrons.
Describe Direct Current
Current flow continuously in one direction.
Describe Alternating Current
Current that periodically reverses direction.
Describe Ideal Source
Theoretical concept of current or voltage supply (like a battery) that has no losses and is a perfect voltage or current supply.
Describe a Real Source
A source that has losses and not perfect.
Describe Resistance
Opposition to current flow. Depends on amount of free electrons of a material.
“R”. Unit-Ohms.
1 ohm limits 1 amp when voltage is 1 volt.
What is Ohm’s Law?
E=IR
Describe Conductance
Opposite (reciprocal) of Resistance
“G”
G=1/R
Describe Power with respect to electricity.
Rate of performing work or heat generation in electrical components.
P=IE=IIR
P=I^2R
Watts
Describe Inductance
Ability of a coil to store energy, induce a voltage in itself, and oppose changes in current flow through it.
“L”
Unit-Henry (H)
L=delta current / delta time.
Voltage induced in a coil (VsubL)
VsubL=-L(delta current / delta time)
What is a Henry?
Unit of Inductance
1 Henry = amount of inductance that induces 1 volt when the current is 1 amp per second.
Describe Capacitance
Ability to store electric charge
“C”
Unit-Farads
Amount of charge (Q) stored divided by voltage applied.
C=Q/E
Describe Frequency
Number of alternating voltage or current cycles per second.
Hertz
Describe electron domains
Electrons spin in the same direction around atoms and form domains which produce magnetic poles.
How is Magnetism produced
As a result of electrons spinning on their axis around the nucleus of an atom.
What is the Law of Magnetism?
Like magnetic poles repel and unlike poles attract.
Define Magnetic Flux
Group of magnetic field lines emitted outward from the North Pole of a magnet.
Greek letter phi. Units of Weber (Wb)
1 Wb=1x10^8 magnetic field lines.
Describe Magnetic Flux Density (B)
B=amount of magnetic flux that passes thru a certain area perpendicular to direction of magnetic flow.
Weber/ m^2
Describe Permeability
Ability of material to form magnetic field in itself, or magnetization a material obtains when magnetic field is applied.
If high->more likely to become magnetized.
Describe Magnetomotive Force (MMF)
Strength of a magnetic field in a coil of wire.
Describe Reluctance
Opposition to production of flux in a material.
Inversely proportional to permeability.
Describe Ferromagnetic Materials
Higher Permeability. Properties of iron
Iron, steel, nickel, cobalt, Al icon and peralloy (commercial alloys). These materials keep magnetism after removal of magnetic field.
Describe Paramagnetic Materials
Materials with permeability slightly higher than 1. Lose magnetism upon removal of magnetic field.
Aluminum, platinum, manganese, chromium.
Describe Diamagnetic Materials
Relative permeability <1.
Magnetic field repels them.
Bismuth, antimony, copper, zinc, mercury, gold, and silver.
Left Hand Rule for Current Carrying Conductors
Thumb points in direction of current flow and fingers wrap around in direction of magnetic field.
Left Hand Rule for Coils
Fingers wrap around coil in direction of current flow. Thumb points to the North pole of induced field.
What is Hysteresis?
After magnetizing ferromagnetic material in one direction, the material won’t relax back to zero magnetism upon removal of magnetic field.
Examples of magnetic components that use hysteresis and electromagnetism
Contactors, starters, relays, solenoids.
Faradays Law (induced voltage, EMF)
V=-N(delta flux/delta time)
Factors
1: number of turns of a coil
2: how fast conductor cuts across magnetic lines of force, or flux.
Know electrical symbols on prints
I don’t have pictures of these.
What are schematics?
Uses symbols for components to show info about circuits. Shows relationship of components with one another. It’s possible to trace operation of the circuit with schematics.
What are one line diagrams?
Shows components using single lines with graphic symbols. Not as detailed as schematics. Doesn’t show physical connections.
What are block diagrams?
Shows relationship between component groups, or stages in a circuit. Path from circuit input to output.
What is a wiring diagram?
Simple way to show wiring connections in easy to follow manner. Show relative location of component parts and color coding.
Define Resistivity
Measure of resistance a material imposed on current flow. Depends on specific resistance of material, length of conductor, and cross sectional area of conductor.
R=p(L/A)
p=specific resistance
Define Temperature Coefficient of Resistivity (alpha).
Amount of change of resistance of material for given temperature change.
Rsubt = Ro +Ro(alpha x deltaT).
Ro=Resistance at 20C
Describe Resistance Temperature Detector (RTD)
Uses changing resistance with change in temp to correlate temperature with measured resistance change.
Uses material with linear coefficient of resistance.
4 parts of electrical circuits
1: A source of EMF
2: Conductors
3: Load or loads
5: Some means of control
Closed circuit
Unbroken path for current from EMF, thru load and back.
Short Circuit
Very little resistance to current flow and cause dangerously high flow thru circuit.
Open Circuit
Incomplete circuit, preventing current flow.
Series Corcuit
Only one path for current flow.
Current flow is the same at all points thru circuit.
Parallel Circuit
2 or more components connected across the same voltage source.
Voltage is constant across each parallel branch of the circuit. Current is divided among the branches.
What is a protective relay?
Auto device that senses abnormal condition and closes contacts.
Overcurrent, over voltage, undervoltage, underfrequency.
What are Fuses?
Melt to protect circuits from hi amps. Destroyed after melt
What are Circuit Breakers?
Opens at predetermined condition for protection and can be reset and used again.
Purpose of a Battery
Store chemical energy and to convert it to electrical energy when needed.
What is a Voltaic Cell?
Combo of materials that convert chemical energy to electrical energy.
2 electrodes of different metals in an electrolyte.
Batteries are 2 or more of these usually in series to gain desired voltage.
1 battery cell typically is 1.5 VDC
What is an Electrode?
Metallic compound, or metal, with an abundance of electrons (negative electrode) or positive charges (positive electrode).
What is an Electrolyte?
Solution capable of carrying electric current.
Dry Cell-paste electrolyte.
Wet Cell-liquid solution.
What is specific gravity?
Ratio comparing weight of liquid to weight of equal volume of pure water.
What is Ampere-hour?
1 Ampere flowing for 1 hour.
What is a Primary Cell?
Battery cells that can’t be recharged.
Most dry cells.
What are Secondary Cells?
Battery cells that can be recharged.
Most wet cells.
What is Battery Capacity?
How long the battery will operate at a certain discharge rate.
What is battery shelf life?
Time a battery may be stored and retain 90% of its original capacity.
What is Battery Charge?
Relative state of capacity of the battery, or
Actual act of applying current flow in reverse direction to return the battery to fully charged.
What is Battery Discharge?
Act of withdrawing current from a battery.
Simple Voltaic Cell operation.
Negative Zinc electrode and positive Copper electrode in acid electrolyte.
Zinc slightly dissolves making it more negative. Positive charges repelled to copper electrode creating more positive charge. When load is connected to terminals, current flows from negative terminal out and back to positive terminal.
Chemical equation for lead acid battery
(2 types of lead and sulfuric acid)
Charging:
PbO2+2H2O->PbO2+Pb+2H2SO4.
Discharging - reverse reaction.
Specific gravity of electrolyte increases while charging.
Series/Parallel connected batteries
In series-add the voltages together.
In parallel-voltage is the same across the parallel branches-raises current carrying capacity without increasing voltage.
Carbon Zinc Battery
Old type, dry cell.
Carbon is rod in center-positive terminal.
Zinc is the case-negative electrode.
Electrolyte paste.
Durable and cheap.
Alkaline Cell
Alkaline electrolyte-potassium hydroxide.
Zinc and manganese electrodes.
Extended life over carbon-zinc.
More expensive.
Nickel-Cadmium Cell
Secondary cell-potassium hydroxide electrolyte.
Nickel and Cadmium electrodes.
Dry cell that can be recharged.
Extreme conditions (temp, shock, vibration).
Good for portable comm equipment.
Battery Hazards.
Shorted cell: sediment buildup in bottom. Caused by excessive charging and discharging.
H2/O2 gas: excess energy of charging dissociates water by electrolysis. (>2.3volts)
Heat generated: (52C,125F) can do damage and cause early failure.
Calculate Total resistance of series circuit.
Rt=R1+R2+R3+….etc.
Calculate total resistance of parallel circuit.
1/Rt=1/R1+1/R2+1/R3+…etc.
Explain voltage divider.
A way to set up a circuit in order to obtain different values of voltage from a single energy source.
Explain current divider.
Setting up a circuit in parallel to obtain different current values with various different resistances.
How to determine polarity of voltage drop in a circuit.
Add up total potential sources and the net is the direction of current flow.
Flowing out the negative terminal of the battery, thru loads (from negative to positive), and back to the positive terminal of the battery.
Kirchhoff’s Voltage Law (1st law)
The sum of voltage drops in a closed loop circuit is equal to the sum of the voltage sources of that loop.
Kirchhoff’s Current Law (2nd law)
The current arriving at a junction point in a circuit is equal to the current leaving that junction.
Apply Kirchhoff’s voltage law
Esource=E1+E2+E3+etc.
sumEsource=sumIxR
Demonstrate Kirchhoff’s current law
Current in-current out=0 or
current in=current out.
Describe action of current carrying conductor in a magnetic field.
Current flow through the conductor causes a magnetic field that on one side of the conductor is in the same direction as the main field and therefore strengthens it. The field on the other side of the conductor is weakened and therefore a force is generated from the strong side to the weak side.
Right Hand Rule for Motors
Index finger points in direction of magnetic field (N to S).
Middle finger points in direction of current flow.
Thumb points in direction of movement.
How is torque developed in a DC motor?
With a constant magnetic field and current flow thru the armature, the combination of magnetic field induced by current flow thru the armature and the field causes movement. The armature is typically a rotor so that movement turns into rotation. Commutators in the armature reverse the poles so movement will continue in the same direction.
What is Counter Electromotive Force?
Generated voltage in a motor due to the motion of the conductor in a magnetic field (the conductor cuts the lines of force).
Generates current flow that opposes the armature current.
Initially zero but builds up as motor comes up to speed.
How does CEMF affect DC motors?
Initially before a motor starts there is no CEMF. Right when voltage and current is applied there is very little resistance to current flow thru the armature and current can be dangerously high. As the motor comes up to speed the CEMF also develops, resisting current flow. For this reason starting resistors are used when starting motors.
How is DC motor speed adjusted?
Field resistors change the resistance of the field and allow more or less current flow.
Higher resistance=less current=weaker field=slower speed.
Lower resistance=higher current=stronger field=faster speed.
Why starting resistors are needed for large DC motors.
No CEMF at motor start = high starting current in the armature.
I=(E-Ecemf)/R
Ecemf is initially zero, therefore high starting current.
Equation to calculate desired starting resistance.
Rs=(Et/Is)-Ra
Rs-starting resistance
Et-terminal voltage
Is-desired staring current
Ra-armature resistance.
4 Nameplate Ratings for DC Motors.
1: Voltage
2: Current
3: Speed
4: Power (continuous)
Describe Electrochemistry DC voltage production.
Certain chemicals and metals when combined cause a chemical reaction that will transfer electrons to produce electrical energy. (Voltaic cell)
Describe Chemical Cell voltage production.
Similar to Electrochemistry description.
What is a Battery?
2 or more Chemical Cells connected in series.
How Static Electricity induced a voltage.
Friction causes certain atoms to lose some valence electrons and therefore causes positive charge. Electrons then transfer to another body in close proximity to equalize charge.
Describe Magnetic Induction.
As a conductor is moved (rotated) thru a magnetic field, a voltage is induced.
Describe a DC Generator.
A simple one Consists of an armature coil with a a single turn of wire. This coil cuts across the magnetic field and produces a voltage.
Commutator reverses the polarity of the armature as it rotates in contact with the brushes.
Describe Piezoelectric Effect.
Applying pressure to certain crystals (quartz, Rochelle salts) or ceramics (barium titanate) makes it possible to force electrons out of orbit in the direction of the force.
Describe Thermoelectricity.
Some materials give/accept electrons readily. When joined they xfer electrons creating potential. At higher temps the electron xfer (potential) is greater and measurable. Basis behind Thermocouple.
What is a Thermocouple?
Converts heat energy to voltage (Thermoelectricity, Same principle of operation). Used to measure temperature.
What is Photoelectric Effect?
Photons strike material surface and transfer energy to the material. This May dislodge electrons from orbit. This electron loss causes a photosensitive material to become positively charged and an electric force is created.
(Photovoltaic cell)
Purpose of Rectifiers
Converts AC to DC.
Uses diodes that act as gates that allow current in one direction only.
Half Wave Rectifier
Forward bias diode allows electron flow when AC sine wave is in positive direction. Reverse bias diode (same diode) resists electron flow when AC sine wave is in negative direction. One diode in a circuit tends to allow “pulses” of DC thru because only half of the AC sine wave is allowed to flow thru
Full Wave Rectifier.
4 diodes in diamond orientation arranged such that positive and negative sections of the sine wave will be allowed to pass thru to the positive terminal of the load
What is Commutation?
Positioning of DC generator brushes so the commutator segments change at the same time the armature current changes direction.
DC machine components
Armature: provide energy conversion in a DC machine.
Rotor: rotating element in DC machine.
Stator: stationary part, magnetic field.
Field: produces voltage in armature.
3 conditions to induce a voltage.
1: A magnetic field
2: A conductor
3: Relative motion between them.
Left Hand Rule for Generators.
Index finger points in direction of Magnetic Field (N to S).
Thumb points in direction of motion (of conductor)
Finger points in direction of current flow (induced).
What is Terminal Voltage and how is it adjusted?
Voltage measured at output of generator.
Dependent upon:
1: Number of loops in armature (constant)
2: Armature speed (also constant)
3: magnetic field strength (variable).
DC generator ratings.
1: Voltage: depends on insulation type/design of machine.
2: Current: size of conductors in armature and field and heat dissipation capability.
3: Power: mechanical limitations of device that turns the generator.
4: Speed: upper limit is before machine will fly apart. Lower limit is max field current (lower speed-higher field needed to produce same voltage).
