1. What is heating effect of current? What is its cause?

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1 GRADE: X PHYSICS (ELECTRICITY) DOMESTIC ELECTRIC CIRCUITS: SERIES OR PARALLEL Disadvantages of series circuits for domestic wiring : In series circuit, if one electrical appliance stops working, due to some defect, then all other appliances also stop working. In series circuit, all the electrical appliances have only one switch due to which they cannot turn on or off separately. In series circuit, the appliances do not get the same voltage (220v) as that of the power supply line because the voltage is shared by all appliances. In series connection of electrical appliances, the overall resistances of the circuit increases too much due to which the current from the power supply is low. ADVANTAGES OF PARALLEL CIRCUITS IN DOMESTIC WIRING In parallel circuits if one electrical appliance stops working due to some defect, then all other appliances keep working normally. In parallel circuits, each electrical appliance has its own switch due to which it can be turned on or turned off independently, with out affecting other appliances. In parallel circuits, each electrical appliances, the overall resistance of the household circuit is reduced due to which the current from the power supply is high. In the parallel connection of electrical appliances, the overall resistance of the household circuit is reduced due to which the current from the power supply is high. HEATING EFFECT OF ELECTRIC CURRENT: 1. What is heating effect of current? What is its cause? When an electric current is pass through a conductor, heat is produced. This effect is called heating effect of current. When a potential difference is apply across the ends of a conductor, its electron gets accelerated from negative terminal to positive terminal. During the cause of their motion this electron collide frequently against one another and against the atoms of the conductor. At each collision a part of kinetic energy of the electron gets converted into heat and this increases the temperature of the conductor. 2. Derive an expression for the heat produce in a resistor R, when a voltage drop across it is V.

2 OR Prove that heat produced in a conductor of resistance R on passing current I through it for time t is H = I 2 Rt. Consider a resistor AB of resistance R connected across a cell of voltage V. suppose a current I is flowing through it. The amount of charge that flows from A to B in time t is given by Q = I x t {I = Q/t} By definition of potential difference, the work done in carrying unit charge from A to B = V Therefore, work done in flowing charge Q from A to B is W = V x Q = V x It This energy is spend in overcoming the resistance offered by the resistor. If whole of the energy appears as heat then the amount of heat produced is H = VIt = I 2 Rt The above equation represents Joules law of heating. This laws state that the amount of heat produced in a conductor is a. Directly proportional to the square of the current b. Directly proportional to the resistance of the conductor and c. Directly proportional to the time for which the current is pass. 3. Define the term electric energy. Write an expression for electric energy consumed in an electric circuit.? It is defined as total work done to maintain an electric current in an electric circuit for a given time. A conductor offers resistance to the flow of current. So work has to be done continuously to maintain it. The work done in carrying a charge through a potential difference V is W = V x Q = V x It = I 2 Rt This work done is the electric energy consumed in the circuit in time t Electric energy = VIt = I 2 Rt 4. Define the term electric power. Write an expression for it. Power = workdone Time taken

3 P = w/t (1) W = VxIxt sub in equ 1 P = VI watts Electric power = voltage x current It is define as the rate at which work is done in maintain an electric current in an electrical circuit. When a current I flows through a circuit for a time t at constant potential difference V, then the work done is W = VIt Electric power P = W/t = VIt/t P = VI = I 2 R = V 2 /R The power is inversely proportional to the resistance. The S.I unit of power is Watt. First Formula of power = P = VI Second formula for power : P = I²R P = V²/ R Define one watt? Ans : One watt is the power consumed by an electrical device which when operated at a potential difference of 1 volt carries a current of 1 ampere 1W= 1VA 5. Practical applications of heating effect of current: (a) In electrical heating appliances: Such as electric iron, water heaters and geysers, room heaters, toasters, hotplates are fitted with heating coils made of high resistance wires such as nichrome wire (b) Electric filament of bulb: This filament is made from a very thin high resistance tungsten wire. When current flows through this filament, it gets heated up and stats emitting lights. (c) Electric fuse: When large current pass through the circuit, this fuse wire get heated up and melts away. As a result the circuit is broken and further damage is prevented. Electrical Energy Electrical energy = Power x Time

4 E= P x t The electrical energy consumed by an electric appliance depends on two factors : (1) power rating of the appliance and (ii) time for which the appliance is used. The unit of an electrical energy is Watt-hour (Wh). One watt-hour is the amount of electrical energy consumed when an electrical appliance of 1 watt power is used for 1 hour. COMMERCIAL UNIT OF ELECTRICAL ENERGY : KILOWATT-HOUR The commercial unit of electrical energy is kilowatt-hour (1kWh) One kilowatt hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour. Relation between kilowatt-hour and joule 1 kilowatt-hour = 3.6 x 106

5 CLASS : X PHYSICS STUDY MATERIAL ELECTRICITY Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges: positive and negative. Like charges repel and unlike attract. SI unit of charge is Coulomb (C). A continuous and closed path of an electric current is called an electric circuit. Electric current is defined as the rate of flow of electric charges. 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. SI unit of electric current is Ampere (A). Define the unit of current? Ampere is the flow of electric charges through a surface at the rate of one coulomb per second, i.e if I coulomb of electric charge through a cross section for 1 second, it would be equal to I ampere. 1 ampere = 1C/ 1 S An instrument called ammeter measures electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured. Q: Calculate the number of electrons constituting one coulomb of charge? 1 electron = 1.6 x coulomb Therefore 1 C of charge = elctrons x = 6.25 x 10¹8 electrons

6 Electric Potential and Potential difference Electric field is the region or space surrounding the charge where another charge experiences a force of attraction or repulsion depending upon the nature of charge. Electric potential is defined as the work done in carrying a unit positive charge from infinity to a point in an electric field. Electric potential difference between two points in an electric circuit carrying current is defined as the work done to move a unit charge from one point to the other. Potential difference (V) between two points = Work done (W)/Charge (Q) V = W/Q. 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. 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. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured. OHM S LAW Ohm s Law states that the potential difference across the ends of a metallic wire is directly proportional to the electric current flowing through it, provided its temperature remains the same. Mathematical expression: V I or V/I = constant = R or V = IR Where R is a constant for the given metallic wire at a given temperature and is called its resistance. Resistance is the property of a conductor to resist the flow of charges through it. SI unit is ohm ( Ω ). Definition of 1 Ohm: 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 Ω.

7 1 ohm = 1 volt/1 ampere. FACTORS ON WHICH THE RESISTANCE OF A CONDUCTOR DEPENDS ARE: 1. Length of the conductor. 2. Area of cross-section of conductor. 3. Nature of material. 4. Temperature of the conductor R l (1) R 1/A (2) Combining (1) & (2) R l /A That is R = ρ l/ A (3) where ρ (rho) is a constant of proportionality called the electrical resistivity of the material of the conductor. The SI unit of resistivity is Ω m. Electrical resistivity is a characteristic property of the material. It does not depend on the length or thickness of wire. From (3) ρ =RA/l If l=1m, A=1m ², then ρ=r. The electrical resistivity / specific resistance of the material is defined as the resistance offered by a metallic wire having a unit length and a unit area of cross-section. Q: Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source. why? Ans : Resistance is indirectly proportional to the area of cross section, thus current flows easily through a thick wire compared to a thin wire of the same material. Q: Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal? Ans : * The melting point, resistivity of an alloy is much higher than that of a pure metal. *. An alloy does not undergo oxidation easily even at high temperature. (1) Prepared by ginilasajeev

8 CLASS 10 PHYSICS NOTES Combination of Resistors In electrical gadgets, we need to connect two or more resistors. Various resistors may be connected with one another to form a network. The resistors may be connected in the following two ways (a) In series (b) In parallel Series combination of resistors: Two or more resistors are said to be connected in series if they are connected one after the other such that the same current flows in one path through all the resistors when some potential difference is applied across the combination. Derive an equation for the equivalent resistance of three resistors connected in series. As shown in the figure, consider three resistors R1, R2and R3 connected in series. Suppose a current I flows through the circuit when a cell of V volt is connected across the combination. If V1,V2 and V3 be the potential differences across the resistors R1,R2 and R3 respectively, then V= V1+ V2+ V3 (1) By Ohm s law, the potential differences across the three resistors will be V1 = IR1, V2 = IR2, V3 = IR3 Since the current flowing through all the resistors is same

9 Eqn (1) becomes V= IR1+ IR2+ IR3

10 V = I (R1+ R2+ R3) (2) If Rs be the equivalent resistance of the series combination, then V =IRs From eqns (2) and (3), we get IRs= I (R1+ R2+ R3) Rs= R1+ R2+ R3 Hence, when several resistors are joined in series, the resistance of the combination Rs equals the sum of their individual resistances, R1, R2, R3, and is thus greater than any individual resistance. The following are the characteristics of the series circuit Current through each resistance is same. Total voltage across the combination = sum of the voltage drops across each resistor. Voltage drop across any resistor is proportional to its resistance. Equivalent resistance = sum of the individual resistances. Equivalent resistance is larger than the largest individual resistance. Q: Calculate the current flowing in the circuit, the three resistors of 5 Ω,8Ω 12 Ω are connected in series? Ans Total Resistance = = 25Ω p.d = 6 V Current I =? V I = IR = V/ R = 6/25 I = 0.24 A

11 Parallel combination of resistors: If a number of resistors are connected in between two common points so that each of them provides a separate path for current, then they are said to be connected in parallel. Derive an equation for the equivalent resistance of three resistors connected in parallel

12 As shown in the figure, consider three resistors R1, R2 and R3 connected in parallel. Suppose a current I flows through the circuit when a cell of voltage V is connected across the combination. The current I at point A is divided into three parts I1, I2 and I3 through the resistors R1, R2 and R3 respectively. These three parts recombine at point B to give same current I. I = I1+ I2+ I3 As all the three resistors have been connected between the same two points A and B, voltage V across each of them is same. By Ohm s law, If RP be equivalent resistance of parallel combination, then, 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. The following are the characteristics of the parallel circuit Voltage across each resistor is same and is equal to the applied voltage.

13 Total current = sum of the currents through the individual resistances. Currents through various resistance are inversely proportional to the individual resistances. Reciprocal of equivalent resistance = sum of reciprocals of individual resistances. Equivalent resistance is less than the smallest individual resistance.