D8. Electricity - Notes from MM

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Resistance (R)

Potential (V)

Current (I)

Conductor

Non-Conductor

Ohm's Law (V = I x R)

Electrical Energy (W)

Ohm = 1 V/A

Obstruction offered to the flow of current by the conductor

Conductance = 1/Resistance

Ohmic Resistors

Non-Ohmic Resistors

Depends on Material, Length, Thickness, Temp of conductor. (R=p l/A)

Resistivity = p (Ohm m)

Series Connection

Parallel Connection

Conductivity = 1/p

Obeys Ohms law, V/I Graph - Straight line passing through origin & constant slope E.g. All metallic conductors like Ag, Cu, Fe, electrolytes with electrodes etc

Does not obey Ohm's law, V/I graph - Curve and Not a straight line through origin, slope not constant E.g. Junction Diode, LED, transistor, bulb filament

Low Resistivity materials E.g. Metals like Cu, Ag

High Resistivity Materials

Materials with High melting point E.g. Tungsten

Electrical Connections

Power Transmission

Resistance does not depend of temp E.g. Alloys like Manganin, Constantan

Low melting point E.g. Alloy of Pb & Sn

Resistance increases with temp E.g. Nichrome

Standard Resistors

Fuse Wires

Heating element in heaters, toasters, ovens etc

Filament of an electric bulb

Increases effective resistance

Reduces effective resistance

Volt (V) = 1 J/C

Work done in moving a unit +ve charge from infinity to that point

Potential Difference

Work done in moving a unit +ve charge from one point to the other

Voltmeter

Current flows only when there is a potential difference, unless it is a superconductor

e.m.f (E)

In Parallel

Terminal Voltage (V)

Work done in taking a unit +ve charge around the complete circuit (i.e. inside & outside circuits)

Depends on material of the electrode, electrolytes

Does not depend on shape of electrodes, distance between electrodes, amount of electrolyte

Volt (V) = W/q

Voltage Drop (v) in the cell

e.m.f (E) = Terminal Voltage (V) + Voltage drop (v)

Characteristic of the cell, does not depend on current drawn

Equals e.m.f. when circuit is open

Work done in moving a unit +ve charge around the external/outside circuit

Volt (V) = W'/q

Depends on current drawn from cell. Greater the current, lesser the Terminal Voltage

Equals e.m.f when circuit is open

Volt (V) = w/q

Work done in moving a unit +ve charge around the internal circuit/electrolyte

Ampere (A) = 1 C/s

Charge

rate of flow of charge (I=Q/t)

Ammeter

Coulomb (C)

electron

no. of electrons x charge per electron i.e. Q = n.e

-1.6 x (10^-19) C

In Series

Charges flow due to the large no. of free electrons that can flow through the conductor

Superconductors E,g, Mercury, Niobium

Materials that exhibit zero resistance (or infinite conductance) at very low temp (absolute zero or 0 K)

Current flows through a superconductor even when there is no potential difference

Charge is static, resides at a point where there is an excess or deficit of electrons. Does not flow

Current flowing through a conductor is directly proportional to the potential difference across its ends (@ constant physical conditions and temperature)

Law of Conservation of Energy

Joule (J) = QV = VIt = I^2Rt = V^2t/R

Eletrical Power (P)

Commercial Unit = Power x Time (Watt Sec, Wh, kWh)

Heating Effect

Energy cannot be created or destroyed. It can only be transformed from one form to another

Heat Energy

Heat & Light Energy

Mechanical & Heat Energy

Chemical Energy

Sound Energy

Magnetic Energy

Electric Motor (Fans, Mixers, Juicers etc)

Current through an electrolyte...Chemical reaction

Loudspeaker

E;lectro-magnet

Watt (W) = W/t = VI = V^2/R = I^2R

Rate at which electrical energy is supplied by the source

Power Rating

Resistance of element

Safe Current Limit

R = Voltage Rating ^ 2/ Power Rating

I = Power Rating/Voltage Rating

Proportional to Square of Current, Resistance of wire, Time for which current is passed (H = I^2Rt Joules )

1 cal = 4.18 J 1 J = 0.24 cal