Electricity 1-7 Ch 3 Review Flashcards
AC current ____________ change direction.
Periodically
Why is AC used more than DC?
Cheaper, easier to distribute to customers, and more versatile
Why is DC not used in transmission?
Large power losses
Power/heat loss is directly proportional to ____________ and to the square of ____________.
Resistance; current
AC power can be converted, via transformer, to:
Various combinations of voltage and current
What device converts AC to various combos of voltage and current?
Transformers
Name 2 AC power sources
Generators and alternators
AC generators combine ____________ and _________ to produce electricity.
Physical motion; magnetism
What is angular motion?
Motion in a circle
What does a waveform show?
The magnitude and direction of current and voltage at every instance of time
Fluctuating DC waveforms can be the same shape as AC, except…
They never go below the horizontal axis
when AV voltage or current goes through 360°, it is said to have completed one __________.
Cycle
Current and voltages are _____________ when both reach their min and max values at the same time
In-Phase
The peak value of an AC sine wave is equal to ___________.
The amplitude or max value of I or E
Equation for effective value of pure sine wave?
.707 x peak value
What is used to indicate the time relationship between I and E
Phase
The number of cycles generated in one second is the_____________.
Frequency
In AC, the magnitude of I is directly proportional to _______________.
The magnitude of E.
The skin effect results in:
Higher concentration of current near the surface of the conductor. This reduces the cross-sectional area of the conductor and increases the resistance.
What is electric current made up of?
Free electrons moving in the same direction
The magnitude of induced EMF is directly proportional to 3 things. Name them:
- Strength of the magnetic field
- Length of conductor
- Speed the conductor moves through the magnetic field
Strength and direction of magnetic field around the conductor through which AC is flowing depends on:
Magnitude and direction of current
What determines the magnitude of the self induced EMF?
Frequency of AC current and its amplitude
When current flowing una circuit, the magnetic field holds all of the energy it has taken from the circuit until current begins ____________.
Decreasing
Why is a greater back EMF produced by self induction if a conductor is coiled?
Because flux lines cut the conductor at more than one point.
If either inductance or capacitance is present in AC, current flow is __________.
Limited
______________ oppose all current in a circuit.
Resistors
Inductive reactance is directly proportional to:
Inductance and frequency
______________ is the property of an electrical current that tends to oppose any change of current through a circuit.
Inductance
Inductors are sometimes called:
Chokes or coils
What is the unit of inductance?
Henry
A coil is said to be saturated when even large changes in current cannot:
Increase the flux
Sometimes, on transformer symbols, dots are used to indicate:
The in-phase terminal of the two windings
If you increase current in a transformer, the current in the primary will ___________.
Increase
Secondary current is out of phase with primary current by ____°.
180
A charged capacitor has an electric field that exists between:
The plates, and therefore stores energy
With all other things held at constant, the capacitance of a capacitor is directly proportional to:
The surface area of the plates
Voltage across a capacitor lags the current through the capacitor by ___°
90
In a purely resistive circuit, the power factor equals:
1
In a purely resistive circuit, the apparent power is equal to:
True power
