Basic Electricity - 1 Flashcards
Describe a wiring diagram
Simple method to show wiring connections in an easy-to-follow manner. Shows the component parts in pictorial form. Components identified by name. Also, shows the relative location of component parts and color coding of conductors or leads.
Describe the law of magnetism
Like magnetic poles repel and unlike poles attract
Define batter discharge
The act of drawing current from a battery
Describe conductance, including the unit of measurement and the relationship to other parameters
- The ability to conduct current
- Measured in siemens
- G=1/R
Define conventional current flow
Convention that states current flows from the positive charge to the negative charge in a circuit
Describe how the speed of a DC motor is adjusted
The speed of a DC motor is adjusted by varying the field using external devices, usually field resistors
State the relationship between specific gravity and state of charge
The higher the specific gravity the higher the charge
Using the left hand rule for generators, determine the direction of the field, the motion of the conductor, or the direction of the induced current.
Define temperature coefficient of resistivity
The amount of change of resistance of a material for a given change in temperature
Define battery charge
May refer to either:
- The relative state of capacity of a battery
- The actual act of applying current flow in the reverse direction to return the battery to a fully charged state.
Describe how thermoelectricity (thermocouple) produces a DC voltage
Two dissimilar metals are joined together. When heat energy is applied more electrons are released from one metal and transferred to the other causing a DC voltage.
How is terminal voltage for a DC generator adjusted
Varying the current through the field winding to adjust the field strength (can’t adjust # of windings, and changing speeds isn’t practical)
Define specific gravity
Defined as the density (weight) ratio of any liquid to pure water of an equal volume. Density of liquid/Density of water Specific gravity of pure water = 1
Define permeability
The measure of a material’s ability to form a magnetic field within itself The degree of magnetization that a material obtains in response to applied magnetic field
Describe how magnetic induction produces a DC voltage
Produces a voltage by rotating coils of wire through a stationary magnetic field or rotating a magnetic field through stationary coils of wire
Define parallel circuit
Two or more components connected across the same voltage source. Current through each branch can be different but voltage across each branch is equal.
State the purpose of a battery
Batteries are used to store chemical energy and to convert this chemical energy into electrical energy when the need arises.
What purpose does the armature serve in a DC machine
The current carrying conductor that rotates in a magnetic field. Converts mechanical energy into electrical energy.
Define resistance temperature detector (RTD)
Uses the phenomenon of changing resistance with changes in temperature to correlate temperature with a measured resistance change.
Name three common battery hazards
- Shorted cell
- Hydrogen and Oxygen gas generation
- Heat generation
Describe how the piezoelectric effect produces a DC voltage
A mechanical motion applied to a crystal generates a voltage. Mechanical energy is converted to DC voltage through the material properties of the crystal.
Apply the left hand rule for current carrying conductors to determine the direction of lines of magnetic flux
Define electromotive force
The sum of the potential differences of all charged particles in the electrostatic field
Describe the principles of protective relays
An automatic device which senses an abnormal condition and closes contacts
Define battery shelf life
The time that battery may be stored and not lose more than 10 percent of its original capacity
Describe voltage, including the unit of measurement and the relationship to other parameters
- The electromotive force that causes electrons to move in a conductor.
- Measured in volts.
- V=IR
Describe how nameplate speed is determined (i.e., what is the bases)
- Upper limit is determined by the speed at which mechanical damage will be done.
- Lower limit is based on the maximum limit for field current (as speed decreases field current needs to increase to maintain constant voltage).
Define real source
Current or voltage supply that has some losses associated with it such as a battery or electrical generator
Define voltaic cell
A combination of materials used to convert chemical energy into electrical energy
Describe electron domains
In a magnetic material, atoms have areas called domains. Domains are aligned such that their electrons tend to spin in the same direction. When domains are aligned, the object develops magnetic poles.
Describe the principles of fuses
Circuit protection device where the wire is smaller (i.e., capacity) than the conductor it protects and melts before the conductor is harmed
Define magnetomotive force
The strength of a magnetic field in a coil of wire
Describe hazards associated with heat generation and how they are managed
- Current flow during charging and discharging causes heat to be generated which can reduce the capacity of the battery.
- Managed by using HVAC
Describe hazards associated with hydrogen and oxygen gas generation and how they are managed
- An explosive mixture of hydrogen and oxygen can be produced during battery charging.
- Managed by ensuring the area is well ventilated and free of any open flames
Describe a block diagram
Less detail than a one-line diagram. Blocks or rectangles connected by singles lines w/ arrowheads at terminal end. Shows relationship between component group, or stages in a circuit, including flow path through a circuit from input to output.
Define battery capacity
How long a battery will operate at a certain discharge rate. Usually given in ampere-hours.
Define voltage
Potential difference between charged objects
Describe the composition of an atom
Atoms are comprised of protons (positive charge), electrons (negative charge), and neutrons (neutral). Protons and Neutrons make up the nucleus of an atom. Atoms can either gain or lose electrons and become either positively or negatively charged.
Describe the chemical reaction that occurs when a lead-acid battery is being discharged
- The electrolyte separates into H2 and SO4.
- H2 combines with some of the oxygen formed on the positive plate to produce water (reduces the amount of acid in the electrolyte)
- Sulfate (SO4) combines with the lead (Pb) of both plates, forming lead sulfate
Describe resistance, including the unit of measurement and the relationship to other parameters
- Opposition to current flow
- Measured in ohms
- R=V/I
Define current
The movement of electrons within a material
Define direct current
Current that flows continuously in the same direction
Describe frequency, including the unit of measurement and the relationship to other parameters
- The number of alternating voltage or current cycles completed per second
- Measured in Hertz
Define flux density
The amount of magnetic flux that passes through a certain area, perpendicular to the direction of magnetic flow
Describe the concept of electron flow
Electricity is electron flow through materials and devices. Electrons in motion try to pull away from the nucleus of an atom. Free electrons move around within an object. Movement of free electrons provide electric current flow in a material.
Describe hazards associated with a shorted battery cell and how they are managed
Can be caused by:
- Faulty separators
- Lead particles or other metals forming a circuit between the positive and negative plates
- Buckling of the plates (primary cause - typically due to overcharging or over discharging).
- Excessive sediments in the bottom of the jar (primary cause - typically due to overcharging or over discharging).
Results in great reduction in battery capacity.
Describe the relationship between total battery voltage, cell voltage, and current for a series connected battery
- Number of cells connected increases total voltage of the battery.
- Total voltage equals the sum of the individual cell voltages.
- The positive terminal of one cell is connected to the negative terminal of the next cell
Describe the operation of a simple magnetic circuit including effects of hysteresis
When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetism when the imposed magnetic field is removed. The energy to realign the magnetic domains is called hysteresis
Describe how electrochemistry (chemical cell/battery) produces a DC voltage and provide an example
When combined with certain metals, chemicals cause a reaction that will transfer electrons which produces electrical energy. Two electrodes made of different metals are immersed in an electrolyte solution. The combination of materials converts chemical energy into electrical energy. Example - Voltaic Cell
Define closed circuit
An uninterrupted or unbroken path for current from the source, through the load, and back to the source
Describe the principles of circuit breakers
A device that breaks (i.e. opens) the circuit when pre-determined conditions are sensed. Can also be manually operated to open the circuit.
Define resistivity
measure of the resistance a material imposes on current flow
Define conductor
Materials with electrons that are loosely bound to their atoms. Materials that permit free motion of a large number of electrons.
List three conditions that are necessary to induce a voltage
- A magnetic field
- A conductor
- Relative motion between the conductor and the magnetic field
Describe power, including the unit of measurement and the relationship to other parameters
- Rate at which work is done, or the rate heat is generated
- Measured in watts
- P=I2R
Define open circuit
A break in a circuit preventing a complete path for current flow
Describe magnetic materials
Certain metals and metallic oxides have the ability to attract other metals. This property is called magnetism. Some magnets occur naturally; others are manufactured.
Define ideal source
Theoretical concept of current or voltage supply (such as a battery) that has no losses and is a perfect voltage or current supply
Why are starting resistors necessary for large DC motors
To reduce high starting currents due to the lack of CEMF present during motor startup.
Define electrode
A metallic compound, or metal, which has an abundance of electrons (negative electrode) or positive charges (positive electrode).
Describe how nameplate voltage is determined (i.e., what is the bases)
Determined based on the type of insulation.
Describe the action of a current carrying conductor in a magnetic field
When a conductor is carrying a current in a magnetic field a force is applied to the conductor which attempts to move the conductor in a direction perpendicular to the magnetic field.
Define ion charge
Atoms that gain or lose one or more electrons become ionized (i.e., they have an ionic charge).
Define primary cells
Cells that can’t be returned to a good condition, or recharged after their voltage output has dropped to a value that is not useable. Dry cells used in flashlights or radios (e.g., AA cells, etc.) are examples of primary cells.
Define electrostatic force
The force that holds electrons in their orbits
What is commutation in a DC generator
The positioning of the brushes in a DC generator so that commutator segments change brushes at the same time the armature current changes direction
Define electron flow
Electrons flow (i.e., current) from the negative charge to the positive charge in a circuit
State the advantages and any disadvantages of a Carbon-Zinc cell battery
Advantages:
- Durable
- Inexpensive to produce
Describe how nameplate power is determined (i.e., what is the bases)
Determined based on mechanical limitations of the prime mover and thermal limits
Apply the left hand rule for coils to determine the polarity of a coil
Describe how DC generator produces a DC voltage
Armature coil rotates in a magnetic field to produce DC voltage. Brushes in contact with the commutator cause the charge of the commutator to switch polarity every half turn of the armature. Voltage is always positive.
Describe inductance, including the unit of measurement and the relationship to other parameters
- Ability of a coil to store energy, induce a voltage in itself, and oppose changes in current flow
- Measured in Henrys
- VL=-L(dI/dt)
Describe capacitance, including the unit of measurement and the relationship to other parameters
- Ability to store an electric charge
- Measured in Farads
- C=Q/V
What are the four nameplate ratings for DC motors
- Current
- Voltage
- Speed
- Power
Describe how a photoelectric effect produces a DC voltage
When photons strike the surface of a material they release energy and transfer it to the atomic electrons of the material. Any electrons dislodged cause a DC voltage.
What purpose does the stator serve in a DC machine
Stationary component that provides the magnetic field
What is the relationship between field current and induced voltage and armature current and torque in DC motors
- As the field current increases the voltage and current induced in the armature increases which increases CEMF and slows the armature and increases torque.
- As the field current decreases voltage and current induced in the armature decreases which decreases CEMF and speeds up the armature lowers torque.
Describe how counter-electromotive force (CEMF) is developed and the function it performs in a DC motor
- Current is applied to the armature which starts the armature rotating.
- After rotation starts the current induced in the armature by the motor’s magnetic field is in the opposite direction as the applied current.
- CEMF opposes applied voltage and functions to lower armature current (preventing prolonged overcurrent during motor start).
Define alternating current
Current that periodically reverses direction
Describe a schematic diagram
The standard way to show information about electrical and electronic circuits. Components are represented by graphic symbols. Symbols and associated lines show how circuit components are connected and relationship of components with one another.
Define electrical circuit
Four basic parts:
- Source of electromotive force
- Conductors
- Load(s)
- Some means of control
Define series circuit
A circuit where there is only one path for current to flow. The current will be the same through the entire circuit.
Describe the relationship between total battery voltage, cell voltage, and current for a parallel connected battery
- Number of cells increases the current capacity of the battery.
- All positive terminals are connected together.
- All negative terminals are connected together.
- Total voltage output is the same as a single cell
What purpose does the rotor serve in a DC machine
Also known as the armature.
What is the purpose of a rectifier
The purpose of a rectifier is to convert AC power to DC power
Define Coulomb’s Law
The strength of the attractive or repulsive force between two charged objects is dependent upon:
- The amount of charge on each object
- The distance between the objects
Describe the operation of a simple voltaic cell
Consists of two electrodes made of different types of metals placed in an electrolyte solution.
What purpose does the field serve in a DC machine
Permanent magnet or an electromagnet provides field in a DC machine
Define secondary cells
Cells that can be recharged to nearly their original condition are called secondary cells. A lead-acid automobile batteries are an example of a secondary cell.
Describe current, including the unit of measurement and the relationship to other parameters
- Movement of free electrons through a conductor.
- Measured in amps
- I=V/R
Define terminal voltage as it applies to DC generators
The voltage that can be measured at the output of the generator
State the function of torque in a DC motor and how it is developed
Torque is the force which produces and maintains rotation. It is developed due to the current carrying conductor (i.e., armature) being in the presence of the magnetic field.
Define potential difference
How large the electrostatic force is between two charged objects
Define reluctance
Opposition to the production of flux in a material
Describe how nameplate current is determined (i.e., what is the bases)
Determined based on the size of the conductors
State the advantages and any disadvantages of a Nickel-Cadmium cell battery
Advantages:
- A dry cell that is a true storage battery with reversible chemical action (i.e. it can be recharged).
- Rugged, dependable battery under extreme conditions of temperature, shock, and vibration
Define ampere-hour
A current of one ampere flowing for one hour. Normally used to indicate the amount of energy stored in a battery.
Define insulator
Materials with electrons that are tightly bound to their atoms and require large amounts of energy to free them from the influence of the nucleus.
Define resistor
Materials that conduct electricity but offer opposition to current flow
Use the right-hand rule for motors to determine the direction of the magnetic filed, direction of current flow, or force on a conductor.
Define flux
Group of magnetic field lines emitted outward from the north pole of a magnet
State the advantages and any disadvantages of an Alkaline cell battery
Advantages:
- Extended life when compared to carbon-zinc cell of the same size
Disadvantages:
- Usually more expensive to produce
Describe how static electricity produces a DC voltage
Electrons are removed (e.g., by friction) from neutral atoms to cause a charge.
Describe a one-line diagram
Less detail than a schematic diagram. Components of a circuit shown using single lines and graphic symbols. Information about the sequence of the circuit.
Define electrolyte
A solution which is capable of conducting an electric current. May be a liquid (wet cell) or a paste (dry cell).
Name three types of materials as they relate to permeability and give an example of each type.
-
Ferromagnetic -
- Permeability in the range of 50-200.
- Strongly attracted by magnetic fields.
- Examples include iron, steel, nickel, cobalt
-
Paramagnetic -
- Permeability of slightly more than 1.
- Lose magnetism when field is removed.
- Weakly attracted by magnetic fields.
- Examples include aluminum, platinum, manganese, and chromium
-
Diamagnetic -
- Permeability of less than 1.
- Repelled by magnetic fields.
- Examples include bismuth, antimony, copper, zinc, mercury, gold, and silver.
Describe the chemical reaction that occurs when a lead-acid battery is being charged
- Lead sulfate (PbSO4) is driven off the plates and back into the electrolyte (H2SO4) reducing the sulfate on plates
- Increases specific gravity of electrolyte
Define short circuit
A circuit that offers very little resistance to current flow and can cause dangerously high current flow through the circuit
Define battery
A battery is made up of two or more voltaic cells, usually connected in series, in order to obtain a desired voltage.
