Karnataka SSLC Elements of Electrical Engineering IV - JTS (KSEAB) flashcards

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

Ohm's law

For a metallic conductor at constant temperature, current is directly proportional to applied voltage: V = IR, where R is resistance in ohms.

Specific resistance

Resistivity ρ is the resistance of a unit cube of material: R = ρL/A. Copper has very low ρ, so it makes efficient conductors.

Kirchhoff's current law

At any junction, the algebraic sum of currents entering and leaving is zero, because charge cannot accumulate at a node.

Electrical energy unit

One kilowatt-hour is the energy used by a 1000 W load running for one hour, equal to 3.6 million joules.

Magnetic flux density

B = Φ/A measured in tesla, where Φ is flux in weber and A is the cross-sectional area in square metres perpendicular to the flux.

Magnetic Ohm's law

Flux equals magnetomotive force divided by reluctance: Φ = NI/S, the magnetic analogue of V = IR in electric circuits.

Faraday's law of induction

The emf induced in a coil equals the negative rate of change of flux linkage: e = −N dΦ/dt, with the sign given by Lenz's law.

Fleming's left-hand rule

With thumb, forefinger and middle finger mutually perpendicular, they give force, flux density and current direction for a conductor in a magnetic field.

Inductor stored energy

A coil of inductance L carrying current I stores magnetic energy equal to half L times I squared, released when current falls.

RMS value

For a sine wave the root-mean-square value is peak divided by root two, about 0.707 of peak; it equals the equivalent DC heating value.

Capacitive reactance

X_C = 1/(2πfC), measured in ohms; it falls as frequency or capacitance rises, so a capacitor blocks DC and passes AC.

Power factor

Cosine of the angle between voltage and current; it equals R/Z in a series circuit and determines real power P = VI cos φ.

Series resonance

When X_L equals X_C in a series RLC circuit, impedance is minimum and equals R, current is maximum, at f = 1/(2π√(LC)).

Star and delta relation

In star, line voltage is root three times phase voltage and line current equals phase current; in delta, line current is root three times phase current.

Three-phase power

Balanced load power equals root three times line voltage times line current times power factor, whether the load is star or delta connected.

Transformer emf equation

E = 4.44 f N Φm gives the rms induced emf, where f is frequency, N turns and Φm the maximum core flux in weber.

Transformer efficiency maximum

Efficiency peaks when copper loss equals core loss; core loss stays constant while copper loss varies with the square of load current.

DC motor back emf

The rotating armature induces a back emf E_b = V − I_aR_a that opposes supply voltage and controls armature current and motor speed.

Synchronous speed

N_s = 120f/P revolutions per minute, where f is supply frequency and P the number of poles; an induction motor runs slightly slower.

Slip

Slip s = (N_s − N)/N_s expresses how far rotor speed falls below synchronous speed; rotor frequency equals slip times supply frequency.

DC series motor caution

A series motor must never be started without load, because its flux is set by armature current and speed would rise dangerously.

Two-way switch

A two-way switch has one common terminal and two strappers, allowing a lamp to be controlled from either of two positions.

RCCB function

A residual current device compares phase and neutral current and trips when the difference, an earth leakage, exceeds its rated sensitivity.

Earthing purpose

Earthing connects exposed metalwork to the general mass of earth, giving a low-resistance path so protective devices operate and touch voltage stays safe.

Fuse selection

The fuse rating is chosen above the normal load current but below the current-carrying capacity of the cable it protects.

PMMC instrument

A permanent-magnet moving-coil meter deflects with torque proportional to current, reads average value, and needs a rectifier for alternating current.

Energy meter constant

Meter constant is the number of disc revolutions per kilowatt-hour; energy consumed equals revolutions counted divided by that constant.

Two-part tariff

A tariff with a fixed charge based on connected load or maximum demand plus a running charge for each unit of energy consumed.

Moving-iron instrument

An instrument where a soft-iron vane is attracted or repelled by a coil's field; it reads RMS values and works on both AC and DC.

Electric shock first aid

Switch off the supply first, separate the victim using dry insulating material, and begin artificial respiration if breathing has stopped.

Lux and lumen

Lumen measures luminous flux emitted; lux is illuminance, one lumen falling on one square metre of surface.

Star-delta starter

A starter that runs the motor in star during acceleration, reducing phase voltage and starting current to one third, then switches to delta.