Chapter-1BASICS OF ELECTRICAL ENGINEERING

byGAGAN GOELAssistant Manager (Electrical)Ch.1.2 ELECTRIC PRESSURE & ELECTROMOTIVE FORCE

ELECTRIC FIELD

The space surrounding an electric charge has a property called an electric field. This electric field exerts a force on other electrically charged objects.

The strength of the field at a given point is defined as the force that would be exerted on a positive test charge of +1 coulomb placed at that point; the direction of the field is given by the direction of that force.

The electric field can be visualized by field lines, which point in the direction which a positive test charge would accelerate if placed upon the line. The lines are therefore directed away from positively-charged source charges and towards negative charges.

ELECTRIC FIELD STRENGTH OR INTENSITY

The magnitude of the electric field is simply defined as the force per charge on the test charge.

If the electric field strength is denoted by the symbol E, then the equation can be rewritten in symbolic form as

Field Intensity at any point in an electric field is equal to potential gradient at that point. If in an electric field potential falls by V over a distance d then Field strength

E = V /d volt/meter

Electromotive ForceTo maintain a steady current in an external circuit we require the use of a source that supplies electrical energyWhereas in the external circuit the current flows from higher potential to lower potential, in this source the current must flow from lower potential to higher potential, even though the electrostatic force within the source is in fact trying to do the oppositeIn order to do this we must have an electromotive force, emf, within such a sourceThe unit for emf is also Volt

Electromotive ForceIdeally, such a source would have a constant potential difference, e, between its terminals regardless of currentReal sources of emf have an internal resistance which has to be taken into accountThe potential difference across the terminals of the source is then given by

ELECTROMOTIVE FORCE (EMF)

Electromotive force, or EMF, is used to indicate electrical pressure or voltage that causes current to flow.The volt is unit of electrical pressure, similar to pressure used to make gases and liquids flow through pipes. The unit of measurement volt was chosen to honor Alessandro Volta One volt is the force required to push one amp through one ohm of resistance.A potential difference of 1 volt (V) exists between two points if 1 joule (J) of energy is exchanged in moving 1 coulomb (C) of charge between the two points

Potential difference: The algebraic difference in potential (or voltage) between two points of a network.Voltage: When isolated, like potential, the voltage at a point with respect to some reference such as ground. Voltage difference: The algebraic difference in voltage (or potential) between two points of a system. A voltage drop or rise is as the terminology would suggest.Electromotive force (emf): The force that establishes the flow of charge (or current) in a system due to the application of a difference in potential. DEMISTIFY TERMD RELATED TO EMF

Ohms Law:E = I * RWhere:E = Voltage (Volts)I = Current (Amps)R = Resistance (Ohms)

Voltage Drop (IR)

Voltage Drop (IR)The sum of the voltage drop in an electrical circuit always equals the applied voltage.The voltage drop across any part of a circuit is equal to I x R.

Sources of Electromotive Forces (a) Chemical sources e.g. Voltaic cells, Fuel CellEmf Cell chemistry1.2 V Nickel-cadmium1.2 V Nickel-metal hydride1.5 V Zinc-carbon2.1 V Lead-acid3.7 V Lithium-ion

(b) Mechanical Source (by Electromagnetic induction) e.g. Synchronous generator, DC Generator(c) Light Source e.g. Solar Cell, Photodiode(d) Heat Source e.g. Thermocouple(e) Pressure Source e.g. Piezo Electric Crystal

MEASUREMENT OF EMF BY VOLTMETERNotations for sources of voltage and loss of potential E - Voltage sources (volts) V - Voltage drops (volts)Voltmeters measure the electromotive force of a power source.There are DC voltmeters that measure DC voltage and AC voltmeters that measure AC voltage.One kilovolt (kV) is 1,000 voltsOne millivolt (mv) is 1/1000 (.001) of a voltOne microvolt (mV is 1/1,000,000 (.000001) of a volt.

Moving plate (electrostatic) type.Digital type (uses electronic circuitry instead of electromagnetic effects).Types of Voltmeter

Digital meters use solid-state semiconductors and can withstand shock and vibration.Digital meters have a numeric (number) display instead of a pointer. Some designs offer auto-ranging, which automatically selects the voltage range and the proper scale.VOLTMETERS ARE ALWAYS CONNECTED IN PARALLEL WITH THE CIRCUIT. Advantages of digital meters include:No moving mechanical parts.Easy readability.Smaller size.Does not affect the circuit you are measuringA brief look into a Digital Voltmeter

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