Chapter-12
Electricity
• Charge is a fundamental partice in an atom, It may be positive or negative.
• Like charges repel each other.
• Unlike charges attract each other.
Coulomb (C): S.I. unit of charge
• Charge on 1 electorn = Negative charge of 1.6 x 10-19 C.
Ammeter has low resistance and always connected in serries.
Direction of current is taken oppostie to flow of electrons as electrons were not known at the time when the phenomenon of electricity was discovered first and current was considered to be flow of positive charge.
Potential Difference (V): Work done to move a unit charge from one point to another.
1 Volte: When 1 joule work is done in carrying one Coulomb charge then potential difference is called 1 volt.`
Voltmeter: Instrument to measure potential difference.
• It has high resistance and always connected in parallel. Symbol is + V -.
• Cell is the simplest device to maintain potential difference.
• Current always flow from higher potential to lower potential.
Symbol of Some Commonly Used Components in Circuit:
| Electric cell | : | |
| Battery | : | |
| Key (open) | : | |
| Key (closed) | : | |
| Wire joint | : | |
| Wire Crossing (without join) | : | |
| Electric bulb | : | |
| Resistance | : | |
| Rheostat | : | |
| Ammeter | : | |
| Voltmeter | : |
Ohm's Law: Potential difference across the two points of a metallic conductor is directly proportional to current passing through the circuit provided that temperature remains constant.
• Mathematical expression for Ohm's law:
V I
V = IR
R is a constant called resistance for a given metal.
• V-I graph for Ohm's law:
• Ohm (): S.I. unit of resistance.
• 1 ohm = 1 volt
1 empere When potential difference is 1 V and current through the circuit is 1 A, then resistance is 1 ohm.
Resistance (R): Variable resistance is a component used to regulate current without changing the source of voltage
Factors on which the Resistance of a Conductor depends :
Resistance of uniform metallic conductor is
(i) directly proportional to the length of conductor,
(ii) inversely proportional to the area of cross-section,
(iii) directly proportional to the temperature and
(iv) depend on nature of material.
Resistivity : It is defined as the resistance offered by a cube of a material of side 1 m when current flows perpendicular to its oppostie faces.
• Its S.I. unit is ohm-metre ().
• Resistivity does not change with change in length or area of cross-section but it changes with change in temperature.
• Range of resistivity of metals and alloys is 10-8 to 10-6 .
• Range resistivity of insulators is 1012 to 1017 .
• Resistivity of alloy is generally higher than that of its constituent metals.
• Alloys do not oxidized (burn) readily at high temperature, so they are commonly used in electrical heating devices.
• Copper and aluminium are used for electrical transmission lines as they have low resistivity.
Resistors in Series :
When two or more resistors are connected end to end, the arrangement is called series combination.
• Total/resultant/overall/effective resistance in series
Rs = R1 + R2 + R3
• Current through each resistor is same.
• Equivalent resistance is larger than the largest individual resistance.
• Total voltge = Sum of voltage drops
V = V1 + V2 + V3
• Voltage across each resistor :
| V1 = IR1 V2 = IR2 V3 = IR3 V = IR1 + IR2 + IR3 IR = I(R1 + R2 + R3) R = R1 + R2 + R3 |
Resistors in Series :
• Voltage across each resistor is same and equal to teh applied voltage.
• Total current is equal to sum of current through the individual resistances.
I I1 + I2 + I3
V = V + V + V
R R1 R2 R3
• Reciprocal of equivalent resistance is equal to sum of reciprocal of individual resistance.
1 = 1 + 1 + 1
Rp R1 R2 R3
• Equivalent resistance is less than the value of the smallest individual resistance om the combination.
Advantages of Parallel Combination over Series Combination
(i) In series circuit, when out component fails, the circuit is broken and none of the component works.
(ii) Different appliances have different requirement of current. This cannot be satisfied in series as current remains same.
(iii) The total resistance in a parallel circuit is decreased.
Heating Effect of Electric Circuit
If an electric circuit is purely resistive, the source of energy continually get dissipated entirely in form of heat. This is known as heating effect of electric current.
| As | E = P x T --> t VI | { E = H } |
| Heat produced, H = VIt | { V = IR } | |
| Or | Heat produced, H = I2Rt |
Joule's Law of Heating Effect of Electric Current
It states that the heat produced in a resistor is
(i) directly proportional to square of current, H I2
(ii) directly proportional to resistance for a given current, H R
(iii) directly proportional to time for which current flows through thte conductor, H t.
So,H = I2Rt
| • Heating effect is desirable in devices like electric heater, electric iron, electric bulb, electric fuse, etc. |
| • Heating effect is undersirable in devices like computers, computers monitors (CRT), TV, refrigerators etc. |
| • In electric bulb, most of the power consumed by the filament appears a heat and a small part of it is radiated in form of light. |
| • Filament of electric bulb is made up of tungsten as (i) it does not oxidise readily at high temperature. (ii) it has high melting point (3380o C). |
| • The bulbs are filled with chemically inactive gases like nitrogen and argon to prolong the life of filament. |
Electric Fuse : It is a safety device that protects our electrical appliances in case of short circuit or overloading.
| • Fuse is made up of pure tin or alloy of copper and tin. |
| • Fuse is always connected in series with live wire. |
| • Fuse has low melting point. |
| • Current capacity of fuse is slightly higher than that of the appliance. |
Electric Power : The rate at which electric energy is consumed or dissipated in an electric circuit.
P = VIP = I2R = V2
R
| • S.I. unit of power = Watt (W) | |
| 1 Watt = 1 vlot x 1 ampere | |
| • Commercial unit of electric energy | = Kilo Watt hour (KWh) |
| 1 KWh = 3.6 x 106 J | |
| 1 KWh = 1 unit of electric energy |
