RBSE SS Voc Electrical and Elect flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

Current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, written V = I × R at constant temperature.

Power in a resistor

Power dissipated equals voltage times current, P = VI, and can also be written I²R or V²/R using Ohm's law. The unit is the watt.

Parallel resistance rule

The reciprocal of total resistance equals the sum of the reciprocals of the branch resistances, so total resistance is always less than the smallest branch value.

RMS value

The root-mean-square value of a sine wave equals the peak value divided by the square root of two, about 0.707 of peak, and is the value a voltmeter displays.

Kirchhoff's voltage law

Around any closed loop in a circuit, the algebraic sum of the source electromotive forces equals the sum of the resistive voltage drops, so the net loop voltage is zero.

Earthing purpose

Bonding exposed metalwork to earth gives fault current a low-resistance path, so the protective device trips quickly and touch voltage stays at a safe level.

MCB tripping curves

Type B, C and D miniature circuit breakers trip magnetically at roughly 3–5, 5–10 and 10–20 times rated current, chosen to suit the load inrush.

RCD 30 mA

A residual current device rated 30 mA trips on small earth leakage and is used on socket circuits to reduce the risk of fatal electric shock.

Cable core colours

Under IEC convention the line conductor is brown or red, the neutral is blue or black, and the protective earth is green with yellow stripes.

Transformer EMF equation

The induced winding voltage equals 4.44 times frequency, turns and peak core flux. It shows voltage per turn depends on flux and frequency.

Transformer losses

Copper loss varies as the square of load current, while core loss from hysteresis and eddy currents stays nearly constant at rated voltage and frequency.

Induction motor slip

Slip is the difference between synchronous speed and actual rotor speed expressed as a fraction of synchronous speed; it is typically a few percent at full load.

Synchronous speed formula

Synchronous speed in rpm equals 120 times supply frequency divided by the number of poles, giving 3000 rpm at 50 Hz for two poles.

Star-delta starting

The motor starts with windings in star, reducing phase voltage and starting torque to one third, then switches to delta for normal running.

Three-phase power formula

For a balanced three-phase load, total power equals the square root of three times line voltage times line current times power factor.

Resistor colour code

Colour bands give digit values, with black zero through white nine, then a multiplier band and a tolerance band such as gold for five percent.

Capacitor energy

A capacitor stores energy equal to half the capacitance times the square of the voltage, and its reactance falls as frequency rises.

Silicon diode drop

A forward-biased silicon junction drops about 0.7 volts before conducting appreciable current, while a germanium junction drops about 0.3 volts.

Transistor current gain

In the active region the collector current equals beta times the base current, with beta typically between 50 and 300 for small-signal bipolar transistors.

Zener diode

Operated in reverse breakdown, a Zener diode holds a nearly constant voltage across itself and is used for regulation and voltage reference.

Bridge rectifier

Four diodes in a bridge give full-wave rectification without a centre-tapped transformer; two diodes conduct on each half cycle of the input.

Ripple frequency

After full-wave rectification the ripple repeats at twice the supply frequency, so a 50 Hz supply produces 100 Hz ripple at the filter capacitor.

NAND gate

A NAND gate outputs zero only when all its inputs are one; it is called a universal gate because any logic function can be built from it.

De Morgan's theorem

The complement of a product equals the sum of the complements, and the complement of a sum equals the product of the complements.

Op-amp inverting gain

With the non-inverting input grounded, the closed-loop gain equals negative feedback resistance divided by input resistance, and the output is inverted.

Wheatstone bridge balance

The bridge is balanced when the ratio of the two arms on one side equals the ratio on the other, and no current flows through the galvanometer.

Megger insulation test

An insulation tester applies a high DC voltage, commonly 500 volts, and reads resistance in megohms; readings above one megohm are generally acceptable.

Voltmeter connection

A voltmeter is connected in parallel with the component and must have very high internal resistance so it draws almost no current from the circuit.

Oscilloscope timebase

The timebase sets the horizontal scale in time per division, so period is read by counting divisions for one cycle and frequency is its reciprocal.

Cold solder joint

A dull, rounded joint where solder did not properly wet the surfaces, giving high resistance and intermittent contact; it must be reheated and reworked.

Soldering iron temperature

For tin-lead solder the tip is set around 300 to 350 degrees Celsius; lead-free solder needs a higher setting of roughly 350 to 380 degrees.

Lockout-tagout

Isolate the supply, lock the isolating device, attach a warning tag and verify dead before working, so the circuit cannot be re-energised accidentally.

Job costing formula

Total job cost combines material, labour, overhead and contingency; adding the profit margin to that total gives the quoted price to the customer.