# CBSE – 10th Standard Science

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### Class 10 Chapter 12 – Electricity

ELECTRIC CURRENT AND CIRCUIT

• The electric charge flows through a conductor like a metallic wire, A continuous and closed path of an electric current is called an electric circuit.
• If a net charge Q, flows across any cross-section of a conductor in time t, then the current I, through the cross-section is I = Q/T.
• The SI unit of electric charge is coulomb (C), which is equivalent to the charge contained in 6 × 1018 electrons.
• The electric current is expressed by a unit called ampere (A), One ampere is constituted by the flow of one coulomb of charge per second, that is, 1 A = 1 C/1 s
• An ammeter measures electric current in a circuit.
•  Electric current flows in the circuit from the positive terminal of the cell to the negative terminal of the cell through the bulb and ammeter.

ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE

• The electrons move only if there is a difference of electric pressure called the potential difference along the conductor.
• We define the electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other –
• The SI unit of electric potential difference is volt (V).
• One volt is the potential difference between two points in a current carrying conductor when
• 1 joule of work is done to move a charge of 1 coulomb from one point to the other .therefore, 1 volt = 1 joule /1 coulomb , 1 V = 1 J C–1
• The potential difference is measured by means of an instrument called the voltmeter. Voltmeter is always connected in parallel across the points between which   potential difference is to be measured.

• V µI or V/I = constant= R   or V = IR
• R is a constant for the given metallic wire at a given temperature and is called its resistance. It is the property of a conductor to resist the flow of charges through it. The
• SI unit is ohm, represented by the Greek letter W. According to Ohm’s law, R = V/I
• If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 W. That is, 1 ohm = 1 volt /1 ampere I = V/R
• If the resistance is doubled the current gets halved. A device called rheostat is often used to change the resistance in the circuit.
• The resistance of the conductor depends (i) on its length, (ii) on its area of cross-section, and (iii) on the nature of its material.
• The SI unit of resistivity is Wm. It is a characteristic property of the material.
• There are two methods of joining the resistors to electric circuit in which three resistors are joined end to end. Here the resistors are said to be connected in series.ii) combination of resistors in which three resistors are connected together between 2 points. the resistors are said to be connected in parallel..

• Resistors in Parallel total current I, is equal to the sum of the separate currents through each branch of the combination. I = I 1 + I 2 + I 3 By applying Ohm’s law to the parallel combination of resistors, I = V/Rp
• The reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances.
• To maintain the current, the source has to keep expending its energy. A part of the source energy in maintaining the current may be consumed into useful work.
• The power input to the circuit by the source is P = Q/ t = V. P= VI steady current I, the amount of heat H produced in time t is H = VIt
• Applying Ohm’s law H = I2 Rt
• Heating effect of electric current has many useful applications. The electric laundry iron, electric toaster, electric oven, electric kettle and electric heater are some of the familiar devices based on Joule’s heating.
• The electric heating is also used to produce light, as in an electric bulb .tungsten (melting point 3380°C) is used for making bulb filaments.
• Another common application of Joule’s heating is the fuse used in electric circuits. It protects circuits and appliances by stopping the flow of any unduly high electric current.
• The fuse is placed in series with the device. It consists of a piece of wire made of a metal or an alloy of appropriate melting point, copper, iron etc.
• The unit ‘watt’ is very small. ‘kilowatt’ is equal to 1000 watts. Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (W h).
• One watt hour is the energy consumed when 1 watt of power is used for 1 hour.

RESISTANCE OF A SYSTEM OF RESISTORS

Series combination

Parallel combination

ELECTRIC POWER

Transmission of Power from generating station