Free CAAS SAR-66 Module 3 Exam Flashcards

Conductors have many mobile charge carriers, insulators hold charges tightly, and semiconductors lie between them with conductivity that can be controlled by temperature, light, or added impurities.

Force is proportional to the product of the charges and inversely proportional to the square of their separation. Doubling the distance reduces the force to one quarter.

Electrons carry current in metals; positive and negative ions carry it in electrolytes. Gases require ionisation, while vacuum conduction requires emitted electrons and an electric field.

EMF is energy supplied per coulomb by a source. Potential difference is energy transferred per coulomb between two circuit points, such as across a load.

In an external DC circuit, conventional current is shown from positive to negative. Electrons physically drift from negative to positive, opposite the conventional-current arrow.

Conductance is the reciprocal of resistance: G = 1/R. Resistance is measured in ohms; conductance is measured in siemens.

The syllabus includes light, heat, friction, pressure, chemical action, magnetism, and motion. Examples include photocells, thermocouples, static charging, piezoelectric devices, batteries, and generators.

A primary cell's chemical reaction is not normally reversible, so it is not recharged. A secondary cell is designed for repeated charging and discharging.

Lead-acid cells use lead compounds and sulfuric-acid electrolyte. Nickel-cadmium cells use nickel and cadmium active materials with potassium-hydroxide electrolyte.

Series connection adds voltage and internal resistance while ampere-hour capacity remains that of one cell. Parallel connection keeps the same voltage, adds ampere-hour capacity, and lowers effective internal resistance for matched cells.

Use V = IR. Therefore I = V/R and R = V/I. For example, 24 V across 12 ohms produces 2 A.

The total current entering a junction equals the total current leaving it. This follows from conservation of electric charge.

The algebraic sum of all voltage rises and drops around a closed loop is zero. Source voltage therefore equals the combined circuit voltage drops.

R = rho times L divided by A. Resistance rises with resistivity and length and falls as cross-sectional area increases.

With a positive coefficient, resistance rises as temperature rises. With a negative coefficient, resistance falls; an NTC thermistor is a common example.

The bands identify significant digits, a multiplier, and tolerance; some codes add a temperature-coefficient band. Orient the resistor so the separated tolerance band is read last.

Series resistances add directly. For parallel branches, reciprocals add: 1/Rtotal = 1/R1 + 1/R2 + more, so the total is below the smallest branch resistance.

A potentiometer acts as a three-terminal voltage divider, a rheostat as a two-terminal variable resistance, and a balanced Wheatstone bridge measures an unknown resistance by ratio comparison.

P = VI. Combining this with Ohm's law gives P = I squared times R and P = V squared divided by R. Power is measured in watts.

Power is the rate of energy transfer. Energy equals power multiplied by time: E = Pt, measured in joules when watts and seconds are used.

Capacitance increases with plate area, number of effective plates, and dielectric constant, and decreases as plate spacing increases. C = Q/V.

Parallel capacitances add directly. Series capacitances add by reciprocals, so the series equivalent is smaller than the smallest individual capacitor.

The time constant is tau = RC. During charging, voltage reaches about 63.2% of its final value in one tau; during discharge, about 36.8% remains.

Exceeding working voltage can break down the dielectric. A polarised capacitor may overheat or fail if reversed, and capacitors must be safely discharged before testing.

In an unsaturated material, B = mu times H. Greater permeability produces more flux density for the same magnetising field; saturation limits further increase.

Point the right thumb in conventional-current direction; curled fingers show the magnetic-field direction around the conductor. For a coil, curled fingers follow current and the thumb indicates its north pole.

Use a low-coercivity magnetic material to reduce hysteresis and thin insulated laminations to interrupt eddy-current paths. Both measures reduce heat loss.

Induced-voltage magnitude equals N times the rate of change of flux through each turn: E = N times dPhi/dt. Equivalently, it is the rate of change of total flux linkage.

The induced current or voltage acts in a direction that opposes the change in magnetic flux that produced it, consistent with conservation of energy.

More turns, higher core permeability, larger linked area, and tighter magnetic coupling increase mutual inductance. Greater separation or poorer alignment reduces it.

Changing current changes the coil's magnetic flux, inducing a voltage that opposes the current change. This makes current rise and fall gradually rather than instantaneously.

Armature conductors cut magnetic flux, inducing voltage. Field strength, conductor speed, and the number of active conductors affect output; a commutator delivers unidirectional terminal voltage.

Armature rotation generates a voltage opposing the supply. As speed rises, back EMF rises and armature current falls; at starting, back EMF is nearly zero.

Series motors provide high starting torque but unsafe no-load speed. Shunt motors hold speed more nearly constant. Compound motors combine series and shunt field characteristics.

The same armature and field structure can convert electrical energy to starting torque, then convert engine-driven mechanical energy back to electrical power after start.

Frequency is cycles per second, while period T = 1/f. Phase describes angular position within a cycle, allowing two waveforms of the same frequency to be compared.

Vrms = Vpeak divided by square root of 2, or about 0.707 Vpeak. A complete sine cycle averages zero; the average over one positive half-cycle is about 0.637 Vpeak.

Three equal sinusoidal phase voltages are separated by 120 electrical degrees. Their combined power is smoother than single-phase power and naturally produces a rotating magnetic field.

In resistance they are in phase. In inductance, current lags voltage by 90 degrees. In capacitance, current leads voltage by 90 degrees.

Inductive reactance XL = 2 pi f L rises with frequency. Capacitive reactance XC = 1 divided by 2 pi f C falls with frequency.

True power P is in watts, apparent power S in volt-amperes, and reactive power Q in vars. For sinusoidal loads, S squared = P squared + Q squared, and power factor = P/S = cos phi.

Vp/Vs = Np/Ns, while Ip/Is = Ns/Np. Voltage follows turns ratio and current changes inversely. An autotransformer uses one shared tapped winding, so it does not provide galvanic isolation.

Winding resistance causes copper I-squared-R loss. Alternating core flux causes hysteresis and eddy-current losses. Leakage flux and imperfect coupling also reduce efficiency.

In star, line voltage equals square root of 3 times phase voltage and line current equals phase current. In delta, line voltage equals phase voltage and line current is square root of 3 times phase current.

Low-pass passes lower frequencies, high-pass passes higher frequencies, band-pass passes a selected band, and band-stop rejects a selected band while passing frequencies outside it.

DC excitation can reach the rotor through relatively small slip rings, while high AC output is taken from stationary windings with easier insulation, cooling, and connections.

Its permanent magnets establish the rotor flux, so rotation produces electrical output without a separately powered field winding. It can supply independent excitation or sensing power.

A synchronous rotor turns at the rotating field's synchronous speed. An induction rotor must run below synchronous speed so relative motion induces rotor current and torque.

Synchronous speed Ns = 120f/P rpm. Slip is (Ns minus rotor speed) divided by Ns, often expressed as a percentage.

Swap any two phases to reverse a three-phase rotating field. Speed depends mainly on frequency and pole count. Single-phase motors may use capacitor, inductor, split-pole, or shaded-pole methods to establish a starting field.