Electricity

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Electricity

Electric Current  Electric current is expressed as the amount of charge flowing through a particular area in unit time.  Quantitatively, electric current is defined as the rate of flow of electric charge. Charge flowing (Q) Current, I Time taken (t)   The S.I. unit of current is ampere (A), where 1 ampere = 1 coulomb/second.  1 mA = 10−3 A, 1 μA = 10−6 A  The conventional direction of electric current is the one in which positive charges move orderly. Electric Potential Different  Electric potential difference (pd) between two points in an electric circuit, carrying some current, is the amount of work done to move a unit charge from one point to another. Work done (W) Potential difference, pd Quanity of charge moved (Q)   The S.I. unit of pd is volt (V), where 1 volt = 1 joule/coulomb. Electric Circuit  A continuous conducting path between the terminals of a source of electricity is called an electric circuit.  A drawing showing the way various electric devices are connected in a circuit is called a circuit diagram.  Some commonly used circuit elements are given below: Sr. No. Element Symbol 1 An electric cell 2 A battery 3 Plug key or switch (open) 4 Plug key or switch (closed) 5 A wire joint PHYSICS ELECTRICITY www.topperlearning.com 3 6 Wires crossing without joining 7 Bulb 8 Resistor 9 Variable resistor or Rheostat 10 Ammeter 11 Voltmeter Ohm’s law  According to Ohm’s law, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends, provided its physical conditions remain the same. V  I V/ I = Constant V/ I = R V = IR where R is a constant of proportionality called resistance of the conductor.  Resistance is the property of a conductor to resist the flow of charges through it.  The S.I. unit of resistance is ohm (Ω). From VRI  , 1 ohm = 1 volt/ampere Resistivity  The resistance of a conductor is directly proportional to its length (l) and inversely proportional to its area of cross section (A). R  l/ A R = ρ l/A where ρis a constant of proportionality called specific resistance or resistivity of the material of the conductor.  The S.I. unit of resistivity is ohm metre (Ωm). PHYSICS ELECTRICITY www.topperlearning.com 4 Combination of Resistances Resistances in Series  The current flowing through each resistance is the same.  The potential difference across the ends of the series combination is distributed across the resistances.  The equivalent resistance (Rs) of a series combination containing resistances R1, R2, R3... is s 1 2 3 R R R R ...      The equivalent resistance is greater than the greatest resistance in the combination. Resistances in Parallel  The potential difference across each resistance is the same and is equal to the potential difference across the combination.  The main current divides itself, and a different current flows through each resistance.  The equivalent resistance (Rp) of a parallel combination containing resistances R1, R2, R3... is given by p 1 2 31 1 1 1...R R R R      The equivalent resistance is lesser than the least of all the resistances in the combination Heating Effect of Electric Current  The effect of electric current due to which heat is produced in a conductor, when current passes through it, is called the heating effect of electric current.  The total work (W) done by the current in an electric circuit is called electric energy and is given as W = VIt = I²Rt W = V² t /R This energy is exhibited as heat. Thus, we have 2 H VIt I Rt.  This is called Joule’s Law of Heating, which states that the heat produced in a resistor is directly proportional to the o Square of the current in the resistor o Resistance of the resistor o Time for which the current flows through the resistance Practical Applications of the Heating Effects of Electric Current  Electrical appliances like laundry iron, toaster, oven, kettle and heater are some devices based on Joule’s Law of Heating.  The concept of electric heating is also used to produce light, as in an electric bulb.  Another application of Joule’s Law of Heating is the fuse used in electric circuits. PHYSICS ELECTRICITY www.topperlearning.com 5 Electric Power  Electric power is the rate at which electrical energy is produced or consumed in an electric circuit P = VI = I²R P = V²/R  The S.I. unit of power is watt (W).  One watt of power is consumed when 1 A of current flows at a potential difference of 1 V. The commercial unit of electric energy is kilowatt hour (kWh), commonly known as a unit. 1 kWh = 3.6 MJ