Chapter 2 unit 9
The polarity and the output voltage from a dc power never
change direction
the polarity and the output voltage from an ac power source -----
changes directions
Ohm's aw demonstrates the relationship between circuit ---
dc circuits current intensity, electromotive force, and its resistance
the formulas in the formula wheel apply to
dc circuits or ac circuits with unity power factors
the Ohm's Law formula, I = E/R, states that current is ---- proportional to the voltage, and ---- proportional to the resistance
directly, inversely
the voltage applied to a resistor dramatically effects the power consumed by that resistor because power is affected in direct proportion to the voltage
doubled
Direct current is used for ---
electroplating,street trolley and railway systems, or where a smooth and wide range of speed control is required for a motor driven application
often the resistance of the power source and conductor are ---- in circuit calculations
ignored
According to the Electron Current Flow Theory, electrons leave the ---- terminal of the source, flow through the conductors and loads, and return to the --- terminal of the source
negative, positive
power in a circuit can be either "useful" or "wasted". Wasted work is still energy used: therefore it must be paid for, so we call this ----
power losses
An electrical circuit consist of the
power source, the conductors , and the load
In a dc circuit, the only opposition to current flow is the physical resistance of the material. This opposition is called ---- and is measured in ohms
resistance
Conductance or conductivity is the property of metal that permits current to flow. The best conductors in order of their conductivity are --
silver copper gold and aluminum
The best conductor in order of their conductivity is ----
silver copper gold and aluminum
the circuit resistance includes the resistance of the
the power supply, the circuit wiring, and the load
Conductance is the ---
the property of a metal that permits current flow
In an ac circuit the factors that oppose current flow are
the resistance of the material, the inductive reactance of the circuit, and the capacitive reactance of the circuit
The major advantage over ac over dc is----
the voltage can be changed through the use of a transformer
What is the resistance of the circuit conductors when the conductor voltage drop is 7.20V and the current flow is 50A? Formula: R = E/I
.144
When working any formula, the key to finding the correct answer to follow these four steps
1 - Know what the question is asking for I, E, R, or P 2 - Determine the knowns I, E, R, or P 3 - Determine which section of the forumla wheel applies: I, E, R, or P and select the formula from that section based on what you know 4 - Work out the calculation
The total circuit resistance of two AWG conductors (each 100 ft long) is 0.40 ohms. If the current of the the circuit is 16A, what is the power loss of both conductors? Formule: P = I x E
102.4
What is the power loss in watts of a conductor that carries 24A and has a voltage drop of 7.20V? Formula: P = I x E
172.8
What is the power consumed by a 10 kWh heat strip that's rated 230V, if its connected to a 115 circuit? formula: P = E^2/R
2500W or 2.5kW
What is the conductor power loss for a 120 V circuit that has a 3 percent voltage drop and carries a current flow of 12A? Formula: P = I x E
43.2
What is the voltage drop of two 12 awg conductors (total of .4 ohms for both conductors) supplying a 16A load, located 100 ft from the power supply? formula: Evd = I x R
6.4
What does it cost per year (at 8 cents per kWh) for the power loss of a 12 AWG conductor ( 100 ft long) that has a total resistance of 0.40 ohms and a current flow of 16A? Formula: Cost per Year = Power of the year in kWh x 0.08
71.7
What is the approximate power consumed by a 10kW heat strip rated 230V, when conneccted to a 208V circuit? Formula: P = E^2/R
8.2
