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What is a coulomb?
A coulomb (C) is a quantity of measurement for electrons. It is a unit of charge. One coulomb is equal to 6.25x1018 (6 250 000 000 000 000 000) electrons.
What is an ampere?
An Ampere (A) is a measurement of the amount of electrical current (I) flowing through a circuit. 1 Ampere is said to be flowing when 1 coulomb (6.25x1018 electrons) moves past a certain point in a conductor over a period of 1 second.
Voltage (E) is referred to as electric pressure, potential difference or electromotive force (EMF). It is the force that pushes electrons through a wire. 1 Volt is the amount of pressure necessary to cause 1 coulomb to produce 1 joule of work.
An ohm (Ω) is the unit of resistance to the flow of current. The letter “R” is used to represent resistance in algebraic formulae. 1 ohm is the amount of resistance that will allow 1 ampere to flow when 1 volt is applied to a material.
A watt (W) is the unit of measurement for the amount of power being used by a circuit. The letter “P” is used to represent resistance in algebraic formulae. The measurement of power is proportional to the amount of voltage and current on and through a material.
What range of electrical current generally causes death and how?
- • As little as 50mA, but typically 100-200mA can cause fibrillation of the heart and result in death.
- • Fibrillation of the heart occurs when the heart vibrates at a fast rate like a “quiver” and ceases to pump blood to the rest of the body.
From an electrical system perspective, what is a fault?
- • A ground fault occurs when a path to ground other than the intended path is established.
- • A fault can also be an abnormal, low-resistance connection between two nodes on an electrical circuit with different potentials.
Define Ohm’s Law.
- • All electric quantities are proportional to each other and can therefore be expressed as mathematical formulas.
- • In a DC circuit, the current is directly proportional to the voltage and inversely proportional to the resistance.
With the aid of a drawing, explain a series circuit.
A series circuit is a circuit that has only one path for current to flow and the current therefore is the same at any point in the circuit.
With the aid of a drawing, explain a parallel circuit.
A parallel circuit can be characterized as a circuit that has more than one path for current flow.
In a series circuit with resistances of 5 ohms and 3 ohms, the total resistance is what?
In a parallel circuit with resistances of 5 ohms and 3 ohms, the total resistance is what?
State Kirchhoff’s voltage law
The algebraic sum of the voltage sources and voltage drops in a closed circuit must equal zero.
State Kirchhoff’s current law.
The algebraic sum of the currents entering and leaving a point must equal zero.
How is an ammeter connected into a circuit?
- • The ammeter is used to measure current and must be connected in series with the load to permit the load to limit the current flow
- • The ammeter, unlike the voltmeter, is a very low impedance device.
- • A Current Transformer is used to measure large amounts of AC current. This Type of meter is connected around one of the conductors supplying power to the load. They are designed to be operated with a 5-ampere AC ammeter connected directly to their secondary winding.
How is a voltmeter connected into a circuit?
A voltmeter is connected directly across (in parallel with) the load or power source.
An electric heating element has a resistance of 16 ohms and is connected to a voltage of 120 V. How much current will flow?
An electric motor has an apparent resistance of 15 ohms. If 8 A of current are flowing through the motor, what is the connected voltage?
The south pole of one magnet is brought close to the south pole of another magnet. Will the magnets repel or attract each other?
A 0.5 W, 2000 ohm resistor has a current of 0.01 A flowing through it. Is this resistor operating within its power rating?
0.2 watts are being dissipated. It is operating within its power rating.
What is a potentiometer?
A resistor with a sliding contact that can used to vary its resistive value.
An ammeter has a voltage drop of 50 mV when 50 A of current flow through it. What is the resistance of the shunt?
How much current does a 60 W light bulb draw when operating at 120 V?
A series circuit has individual resistor values of 200 ohm, 86 ohm, 91 ohm, 180 ohm, and 150 ohm. What is the total resistance of the circuit?
A series circuit contains four resistors. The total resistance of the circuit is 360 ohm. Three of the resistors have values of 56 ohm, 110 ohm, and 75 ohm. What is the value of the fourth resistor?
A series circuit contains five resistors. The total voltage applied to the circuit is 120 V. Four resistors have voltage drops of 35 V, 28 V, 22 V, and 15 V. What is the voltage drop of the fifth resistor?
A circuit has three resistors connected in series. Resistor R2 has a resistance of 220 ohm and a voltage drop of 44 V. What is the current flow through R3?
A circuit has four resistors connected in series. If each resistor has a voltage drop of 60 V, what is the voltage applied to the circuit?
Define a series circuit.
A series circuit is a circuit that has only one path for current to flow and the current therefore is the same at any point in the circuit.
State the three rules for series circuits.
- 1. Current is the same at any point in a series circuit.
- 2. The voltage drops along a series circuit must add to total the applied voltage.
- 3. The total amount of resistance to current flow in a series circuit is equal to the sum of the resistances in the circuit.
A series circuit has resistance values of 160 ohm, 100 ohm, 82 ohm, and 120 ohm. What is the total resistance of this circuit?
If a voltage of 24 V is applied to 462 ohms what will be the total amount of current flow in the circuit?
Referring to the circuit described in Questions 8 and 9 (29 and 30?), determine the voltage drop across each of the resistors.
A series circuit contains the following values of resistors: R1 = 510 ohm, R2 = 680 ohm, R3 = 390 ohm, R4 = 750 ohm. Assume a source voltage of 48 V. Use the general voltage divider formula to compute the voltage drop across each of the resistors.
- V1= 10.5V
- V2 = 14V
- V3 = 8.0V
- V4 = 15.5V
A parallel circuit contains the following values of resistor, each in parallel with the other: R1 = 510 ohm, R2 = 680 ohm, R3 = 390 ohm, R4 = 750 ohm. What is the total resistance of the circuit?
State the three rules for parallel circuits.
- 1. The total current flow in the circuit is equal to the sum of the currents through all the branches
- 2. The voltage drop across any branch in parallel with the source is the same as the source voltage. The voltage across any branch directly in parallel is the same.
- 3. The total resistance of a parallel circuit is the reciprocal of the sum of the reciprocals of the individual branches
Refer to Unit 8—Review Question 5 and solve for RT. If ET is 240 V, what is IT?
What determines the polarity of magnetism when current flows through a conductor?
The direction of the current.
What determines the strength of the magnetic field when current flows through a conductor?
The intensity or strength of the current.
What is a vector?
A quantity having direction as well as magnitude
Which trigonometric function is used for find the angle if the length of the hypotenuse and of the adjacent side are known?
If side A has a length of 18.5 ft and side B has a length of 28 ft, what is the length of the hypotenuse?
Side A has a length of 12 m, and angle Y is 12 deg. What is the length of side B?
Side A has a length of 6 inches and angle Y is 45 degrees. What is the length of side B?
The hypotenuse has a length of 65 inches and side A has a length of 31 inches. What is angle X?
The hypotenuse has a length of 83 feet. And side B has a length of 22 feet. What is the length of side A?
Side A has a length of 1.25 inches and side B has a length of 2 inches. What is angle Y?
Side A has a length of 14 feet and angle X is 61 degrees. What is the length of the hypotenuse?
What is the most common type of AC waveform?
How many degrees are there in one complete sine wave?
At what angle does the voltage reach its maximum negative value in a sine wave?
What is frequency?
- • Frequency is the number of occurrences of a repeating event per unit of time.
- • Frequency is often measured in Hertz (Hz);
- • 1 Hertz is equal to 1 complete cycle in 1 second. If 60 cycles occur in 1 second, we can say that the frequency of the phenomenon is 60 Hz.
- • Calculation: Time in seconds (T) / number of cycles (n)
A sine wave has a maximum value of 230V. What is the voltage after 38 degrees of rotation?
- E(INST) = E(MAX) x sin(angle in degrees)
How many degrees are the current and voltage out of phase with each other in a pure resistive circuit?
How many degrees are the current and voltage out of phase with each other in a pure inductive circuit?
How many degrees are the current and voltage out of phase with each other in a pure capacitive circuit?
Define inductive reactance.
- Inductive reactance (XL) can be described as the current-limiting property in an AC circuit caused by its inductive properties.
- Inductive properties of an AC circuit oppose changes in current.
- The unit of measurement for inductance is the Henry (H), and is represented by the letter L.
Define capacitive reactance.
- Capacitive reactance (XC) can be described as the current-limiting property in an AC circuit caused by its capacitive properties.
- Capacitive properties of an AC circuit oppose changes in voltage.
- The unit of measurement for capacitance is the farad (F), and is represented by the letter C.
What is a dielectric?
A dielectric is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields.
List three factors that determine the capacitance of a capacitor.
- • The surface area of the plates
- • The distance between the plates
- • The type of dielectric used
In what form is energy of a capacitor stored?
- • A capacitor stores energy in an electrostatic field.
- Electrostatic refers to electric charges that are stationary, or not moving. They are very similar to the static electric charges that form on objects that are good insulators. The electrostatic field is formed when electrons are removed from one plate and deposited on the other.
In what form is energy of an inductor stored?
A magnetic field.
Does the current in a pure capacitive circuit lead or lag the applied voltage?
Draw and label the power triangle including Real, Reactive and Apparent power.
The hypotenuse is the Apparent Power which is the complex vector sum of the 2 other sides which at are the Real Power and Reactive Power values.
Define Real Power.
- • Real power is a function of a circuit's dissipative elements, usually resistances (R).
- • Quantified in Watts (W), algebraically represented by P.
Define Reactive Power.
- • Reactive power is a function of a circuit's cumulative reactance’s (X).
- • Quantified in Volt Amperes-Reactive (VARs), algebraically represented by Q.
- • VARs are needed in an alternating-current transmission system to support the transfer of real power over the network.
Define Apparent Power.
- • The complex vector sum of Real Power and Reactive Power.
- • Quantified as Volt-Amperes (VA), algebraically represented by S.
- • Apparent Power is the actual measured value of power measured in a circuit.
Define power factor. What is the calculation for power factor if both Watts and VA are known?
- Power Factor (PF) is a ratio of the true power to the apparent power, typically expressed as a percentage.
- Formula: PF = Real Power (P) / Reactive Power (Q)
- PF = P / Q
A load with a resistance of 5 ohms and inductive reactance of 3 ohms is connected to a circuit being fed at 125V. What is the overall impedance of the circuit? What is the total current flow? What is the apparent power?
- VA = 2680VA
- IT = 21.44A
- Z = 5.83 ohms
- How many degrees out of phase with each other are the voltages of a three phase system?
- 120 degrees.
- What are the 2 main types of 3 phase connections?
- Delta and wye.
A wye-connected load has a voltage of 480V applied to it. What is the voltage across each phase?
A wye-connected load has a phase current of 25A. How much current is flowing through the lines supplying power to the load?
A delta connection has voltage of 560V connected to it. How much voltage is dropped across each phase?
A delta connection has 30A of current flowing through each phase winding. How much current is flowing through each of the lines supplying power to the load?
A three phase load has a phase voltage of 240V and a phase current of 18A. What is the apparent power of this load?
A three phase wye connected load has a phase voltage of 240V and a phase current of 18A what would be the line voltage and line current supplying the load?
- 416V and 18A
- In a wye circuit: ELine = EPhase x 1.73 and ILine = IPhase
An alternator with a line voltage of 2400V supplies a delta-connected load. The line current supplied to the load is a 40A. Assuming the load is a balanced three-phase load, what is the impedance of each phase?
An alternator with a line voltage of 2400V supplies a delta-connected load. The line current supplied to the load is a 40A. What is the apparent power?
166 277 VA
What is a transformer?
- • A transformer is a magnetically operated machine that can change values of voltage, current, and impedance without a change in frequency.
- • A transformer can be defined as a static electrical device that transfers energy by inductive coupling between its winding circuits. A varying current in the primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic flux through the secondary winding. This varying magnetic flux induces a varying electromotive force (emf) or voltage in the secondary winding. Transformers can be used to vary the relative voltage of circuits or isolate them, or both.
What the common efficiencies for transformers?
90 to 99 percent.
What is an isolation transformer?
A type of transformer where there is no physical or electrical connection between the primary and secondary windings. The transformation is accomplished through magnetic coupling only.
All values of a transformer are proportional to its _______ _______.
Explain the difference between a step-up and a step-down transformer.
- In a step-up transformer the output voltage is higher than the input voltage.
- In a step-down transformer the output voltage is lower than the input voltage.
A transformer has primary voltage of 240V and a secondary voltage of 48V. What is the turns ratio of the transformer?
A transformer has an output of 750VA. The primary voltage is 120V. What is the primary current?
A transformer has a turns ratio of 1:6. The primary current is 18A. What is the secondary current?