Electricity Electricity

IMPORTANT POINTS

PROPERTIES OF CHARGES• There are two types of charges positive and

negative• Like charges repel each other and unlike

charges attract each other• Charges are quantized(with fixed amount of

charge)• Magnitude of every charge is 1.6x10¯¹⁹C

Charge conservation

• electric charges can neither be created nor destroyed but can be transferred from one body to another

Insulators

• Insulators do not conduct charges, as electrons are tightly bound with the nucleus.

• Insulators can be charged easily by friction.the charges remain localized in them

Conductors

• Substance in which charges can flow freely are called conductors.

• Here electrons are loosely bound with the nucleus.

AC and DC

• Current may be direct, varying or alternating current

• Direct current (DC)

t

I

• Alternating current(AC)

t

I

Voltmeter

• Used to find potential difference between two points

• Should be connected in parallel• symbol

VV

Ammeter

• Used to find current in a closed circuits• Should be connected in series• symbol

AA

Galvanometer

• Used to detect current in a circuit• Should be connected in series• symbol

AG

Ohm’s law

• At constant temperature and pressure the current flowing through a wire is directly proportional to the potential difference across its ends.

• V ᾳ I• Or V=IR where R is the resistance

• Graphically it can be shown:

I

V

One Ohm

• Resistance offered by wire carrying 1A of current 1V is applied across its ends

• Ohm(ᾨ) is unit of resistance• It is in honour of the German physicist George

Simon Ohm.

Resistance

• The opposition caused to the flow of current • It is denoted by the symbol

Factors affecting resistance

• Resistivity• Length of the wire (R ᾳ l)• The Area of cross section (Rᾳ1/A)• So R=ῤl/A

Resistivity(ῤ)

• The resistance offered by a wire of unit ength and unit area of cross section

• Its unit is ohm-metre(Ω-m)• The reciprocal of resistivity is conductivity • Unit of conductivity mho metre(ʊ-m)

Resistor

• It is the device used to offer resistance in a circuit

Rheostat

• It is a variable resistor• It is connected in series

Combinations of resistors

• Resistors in series• Resistors in parallel

Resistors in series

• When resistors placed in series the current through them will be the same

• The total the potential difference is the algebraic sum of all P.D

• V=V1+V2+V3• The equivalent resistance is the sum of

individual resistances• R=R1+R2+R3

Resistors in parallel

• The potential difference across their ends is the same

• The sum of the current through them is the current drawn from the main source of energy

• The equivalent resistance is found by 1/R=1/R1+1/R2+1/R3

Joule’s law of heating

• When a current I flows through a conductor ,heat is produced. The produced is directly proportional to the product of square of current, resistance and the time.

• H=I²RtOther equations• H=(V²/R)t• H=IVt

Electric power

• The rate at which electric energy is consumed• Power P=W/t=H/t=I²Rt/t=I²ROther eqns• P=VI• P=V²/R• Unit is Watt

• 1kW=1000watts• 1MW=1000000Watts• 1hp (horse power)=746watts

Kilowatt hour

• It is the commercial unit for electrical energy• 1kWh=1000Wh=1000xJ/sx3600seconds=3600000J

Calculation of power in daily life situation

• Number of units of electricity consumed=No.kWh=total (wattsxhours)/1000

• Total unit of electricity consumed in a month=No.of unitsxNo.of days in month

• Total cost of electricity =total units x cost per unit of electricity

Tungsten filament in bulbs

• High resistivity• High melting point(3380˚C)

Fuse wire

• The wire which melts, breaks the circuits and prevents the damage of various appliances in house hold connections

• It is connected in series with the live wire• It is made up of Lead(Pb) and Tin(Sn)• It is generally encased in a cartridge of

porcelain material

• Material ρ (Ω•m) at 20 °CResistivity σ (S/m) at 20 °CConductivity Silver 1.59×10−8 6.30×107 Copper 1.68×10−8 5.96×107 Annealed copper 1.72×10−8 5.80×107

• Gold 2.44×10−8 4.10×107 • Aluminum 2.82×10−8 3.5×107 • Calcium 3.36×10−8 2.98×107 Tungsten 5.60×10−8 1.79×107 Zinc 5.90×10−8 1.69×107

Nickel 6.99×10−8 1.43×107 Lithium 9.28×10−8 1.08×107 Iron 1.0×10−7 1.00×107 Platinum 1.06×10−7 9.43×106 Tin 1.09×10−7 9.17×106 Carbon steel (1010) 1.43×10−7 Lead 2.2×10−7 4.55×106 Titanium 4.20×10−7 2.38×106 Grain oriented electrical steel 4.60×10−7 2.17×106 Manganin 4.82×10−7 2.07×106 Constantan 4.9×10−7 2.04×106 Stainless steel 6.9×10−7 1.45×106 Mercury 9.8×10−7 1.02×106 Nichrome 1.10×10−6 9.09×105 GaAs 5×10−7 to 10×10−3 5×10−8 to 103 Carbon (amorphous) 5×10−4 to 8×10−4 1.25 to 2×103 Carbon (graphite) 2.5×10−6 to 5.0×10−6 //basal plane 3.0×10−3 basal plane 2 to 3×10⊥ 5 //basal plane3.3×102 basal plane Carbon (diamond) 1×10⊥ 12 ~10−13