CURRENT ELECTRICITY Q.1

Transcription

1 CUENT EECTCTY Q. Define Electric current an its unit.. Electric Current t can be efine as the time rate of flow of charge in a conuctor is calle Electric Current. The amount of flow of charge Q per unit time is calle Electric Current. t is a scalar quantity. t is enote by. Mathematically Ch arg e Current Electric time Q t The S unit of Electric Current is mpere (). mpere t can be efine as if one coulomb of charge flows in a conuctor in one secon time, then the current is sai to be one mpere. Q. Write a note on conventional current?. Conventional Current t can be efine as the current ue to the flow of positive charge is calle conventional current. Before the iscovery of electron the scientists thought that current is causes ue to the flow of positive charge proton. But later on the scientists state that current is causes ue to the flow of positive as well as negative charge. Q. State an explain Ohm's aw?. Statement German scientist George Simon Ohm in 86 state that the current flows in a conuctor is irectly proportional to the voltage supplie across the ens of the conuctor provie the temperature an esistance of the conuctor is kept constant. Mathematically Cons tan t( ) Where is constant of proportionality an are known as resistance of the wire. Experimental verification Take a thin wire of about one meter length. Connect both ens of the

2 wire to the positive an negative plate of the battery. The voltage across the wire is measure by voltmeter an the current in the wire is measure by ammeter. By rawing a graph between current an voltage it will give a straight line. aliity of Ohm's aw Ohm's aw is not vali for those conuctors whose temperature oes not remain constant. For examples Electric tubes, Transistors an onic plasma. Q.4 Define Electrical esistance an its unit?. Electrical resistance t is the property of a conuctor by which it opposes the flow of current in the conuctor. t is enote by "". Mathematically The S unit of electrical resistance is ohm. t is enote by omega " ". Definition of ohm The electrical esistance of a conuctor is sai to be one Ohm, if one ampere of current passes through it by having a potential ifference of one volt across its ens. Mathematically Q. What is meant by specific resistance?. Specific esistance The esistance of a conuctor is sai to be Specific esistance, if the conuctor have a unit cross section area an unit length. The esistance of a conucting wire is irectly proportional to the length an inversely proportional to the cross section area. Mathematically () () Combining uation () an (), we get Constant

3 Where is the constant of proportionality an it is calle specific esistance. The S unit of specific esistance is Ohm-meter ( m) Q.6 What is the effect of temperature on electrical resistance?. Effect of temperature on esistance The esistance of a conuctor irectly epens on the temperature an initial esistance of the conuctor. Explaination Consier a conuctor has an initial esistance " " an temperature" T ". fter a temperature change " T "the esistance of conuctor becomes" ". Mathematically T Combining both uations, we get T Constant T T T T T t Q.7 Define Electric circuit an its types?. Electric circuit The path of the flow of Electric current is known as Electric circuit. There are two types of an Electric circuit. i-e Close circuit complete circuit for the flow of Electric current is calle close circuit. current can easily flow in this circuit. Open circuit n incomplete circuit for the flow of Electric current is calle open circuit. current can not flow in this circuit ue to a break in the circuit. Q.8 Write a note on series combination of esistors?. Series combination of esistors The en to en connection of two or more than two esistors with each other is calle Series combination of the esistors. s shown in Fig

4 (i) (ii) Series combination has the following characteristics. There is a single path for the flow of current it means that current is constant in the circuit. Mathematically oltage across each esistor is rop. t means that the sum of the iniviual voltage is ual to the total voltage. Mathematically () But accoring to Ohm s law;,,, Put these values in uation () For "n" numbers of esistors, we have n Q.9 Write a note on parallel combination of esistors?. Parallel combination of esistors The connection of two or more than two esistors in which each an every esistor is parallel to each other is calle parallel combination of esistors. s shown in Fig below. (i) Parallel combination has the following characteristics. oltage across each esistor is constant. Mathematically

5 (ii) The current flows in each esistor is ifferent. The sum of the iniviual current across the esistors is ual to the uivalent current. Mathematically () But accoring to Ohm s law;,,,,,, Put these values in uation (), we get ( ) For "n" numbers of esistors n For only two esistors " " an " " Q.0 What is meant by Electric Power an Electric Energy? Explain. Electric Power The time rate of oing work is calle Power. Mathematically W P () t The work one in electric fiel on a unit charge is represente by "". So the work one on charge "Q", will be W Q Put the value of "W" in uation () Q P t ;

6 Q t So P But we also know that from Ohm s law; P ( ) Unit of Power The S unit of Power is watt. The other units of Power are Kilowatt, Horse Power. Mathematically KW 000W Horse Power 746watt Electric Energy The energy prouce by generators an consume by ifferent appliances such as Heater, fans, bulb, etc is calle Electric Energy. t is enote by "H". Mathematically Energy Power time H Pt () s we know that P Put this value of "P" in uation () H t t Unit of Energy The ifferent units of Energy are joule, kilowatt hour, electron volt, etc. KWh e Q. Differentiate between DC an C?. Direct Current D.C stans for irect current. This type of current always flow in one irection. This current flows from positive to negative terminal of the battery. lternating current.c stans for alternating current. This type of current changes its irection many times in a secon. lternating current changes its irection alternatively from the positive half to the negative half. 9 6 J J Q. What are the main components of a circuit?. The basic components of a circuit are as uner; Switches t is an electric circuit control element with two iscrete states ON an OFF. Current flows in a circuit if switch is ON, otherwise current can not flow. The symbol for a switch is.

7 esistors The evices connecte in a circuit to reuce current is calle resistor. They are mae from wire coil of special alloys or from carbon. The symbol for a resistor is. Batteries The combination of many cells is calle a battery. Battery is often mae from chemical substances. The symbol for a battery is. Q. Write note on the following instruments. i. Galvanometer ii. mmeter iii. oltmeter. Galvanometer t is a sensitive evice use to etect small amount of current in a circuit. t consist of a ial on which a neele can rotate. n orer to etect current in the circuit galvanometer is connecte in series with the circuit. The eflection of the neele inicates the presence of current in the circuit. mmeter The evice which measure current in a circuit is calle mmeter. t is a moifie galvanometer for measuring large currents. This moification is accomplishe by connecting a low resistance in parallel with galvanometer. This resistance is known as shunt resistance which provies an alternate path for the flow of current. To measure current mmeter is connecte in series with the circuit.

8 oltmeter The evice which measure oltage in a circuit is calle oltmeter. t is also a moifie galvanometer in which a large resistance is connecting in series. To measure voltage across the terminals oltmeter is connecte in parallel with the circuit. Q.4 Differentiate between Series an Parallel circuits.. Series circuit The circuit in which the current in ifferent paths flow one after the other is calle Series circuit. n case of Series circuit the voltage rops across each resistor. The total voltage is ual to the sum of iniviual voltage across each resistor. Mathematically = + Parallel circuit The circuit in which current flows on ifferent path is calle Parallel circuit.

9 n case of Parallel circuit the current rop across each resistor. The total current is ual to the sum of iniviual current across each resistor. Mathematically = + Q. What is electricity Hazars?. Hazars The electricity comes to our houses by means of cable. These cables consist of two wire live wire an neutral wire. The live wire an any terminal connecte to it are very angerous. nyone who touches a live wire woul suffer a fatal shock, because human boy conucts electricity. The electric shock also occurs ue to amage insulation, overheating of cables an ampe conition. We shoul not walk upon wires bare foot. Q.6 How electricity is use safely in homes?. The electricity supplie to our homes is alternating current (-C). t changes its irection 0 times in a secon. So its fruency is 0Hz an voltage is 0. For the safe use of electricity the following evices shoul be use in the house circuit. Fuse t consists of a thin piece of metal wire having low melting point. When too much current flows through the fuse wire. t melts before the copper wire in the circuit get hot. s a result the circuit breaks an no amage occur. Circuit breaker t consists of an electromagnet, when current excees the rate value of the circuit breaker. t automatically separates the contact of the circuit wires an breaks the circuit. Now a ay it is use instea of fuse, because it works quickly than fuse. Earth wire wire which goes to the top connection on all power sockets an is connecte with the earth. This wire is a safety precaution to prevent electric shock when an appliance evelops a fault.

10 SHOT QUESTONS. Electric current is a flow of charge, why two wires rather than a single wire is use to carry current.. The electric current flow just like water in a pipe. The water flows from a higher concentration en of the pipe to the lower concentration en. Similarly electric current also flow from a higher concentration en to the lower concentration en. Therefore two wires are use rather than a single wire for the flow of current.. Which solis are goo electrical conuctors an which are goo insulators? How well o these substance conuct heat.. Metallic solis are goo conuctor of electricity. For example copper, silver, iron etc. Non metallic solis are goo insulator of electricity. For example woo, plastic, rubber etc. Basically goo conuctors of electricity are goo conuctor of heat. Therefore metallic solis conuct heat. While non metallic solis can not conuct heat.. State how the resistance of a wire woul change (i) if its length is increase (ii) if its iameter is ecrease.. The resistance of a wire epens upon the length an area of the wire. Mathematically r ( ) 4 This uation shows that if we increase the length of the wire the resistance increases. lso if we ecrease the iameter of the wire the resistance will also be increases. 4. To reuce the brightness of a light bulb an auxiliary resistance be connecte in series with it or in parallel? Why. To reuce the brightness of the bulb auxiliary resistance must be connecte in series. Because in series combination potential ifference across the resistors ecreases. s a result the brightness of bulb will be reuce.. number of light bulbs are to be connecte to a single power outlet. Will they provie more illumination if connecte in series or in parallel? Why. To get more illumination the bulb must be connecte in parallel. Because in parallel combination of the resistors voltage remain constant. s voltage

11 reache ually to each an every bulb. Therefore the illumination will be greater. 6. What is meant by the statement that an electric kettle has power of KW?. Electric kettle has a power of KW means that its power is 000W. so if an operating voltage of (0 to 40volts) is applie across its heating element, then it will convert electrical energy into heat energy at the rate of 000 j/sec. mathematically E P t E Pt E 000W Sec 000j 7. When incanescent lamps burn out, they usually o so just after they are switche on. Why?. ncanescent lamps are generally use to get heat energy using electrical energy. That is why they may burn out when switche on. Most of the electrical energy is converte into thermal energy an some prouce light. 8. Why is it possible for a bir to perch on a high voltage wire without being electrocute?. it is possible for a bir to perch on a high voltage wire without being electrocute. Because the potential ifference between the wire an the feet of the bir is zero. 9. Why is it angerous to turn on a light when you are in the bath tub?. The human boy is a goo conuctor of electricity. Therefore when a person in a bath tub tries to turn on a bulb, he may get on an electric shock.

12 NUMEC POBEMS. steay current of 0. flows through wire. How much charge passes through the wire in one minute? Given ata 0 t min 60sec Q? Q t Q t Q C. wire carries a current of. How many electrons pass at any point in the wire each secon? Given ata t sec e 6 0 n? 9 C Q ne () Q t () Comparing uation () an () we get ne t t n e n 60 n electrons

13 . 0 toaster has a resistance of ῼ. What must be the minimum rating of the fuse in electrical circuit to which toaster is connecte? Given ata 0? n electric heater uses.0 when connecte across 0. Determine its resistance. Given ata 0 0? 0 0. metal ro is m long an 8mm iameter. Compute its resistance if the resistivity of the metal is.76 x 0-7 ῼm. Given ata m 8mm r m? m m

14 r r (.4)(40 6 ) 6. platinum wire 80cm long is to have a resistance of 0.ῼ. What shoul its iameter be? The resistivity of platinum is. x 0-7 ῼm. Given ata 80cm 0 8m 0 0? 7 m

15 mm m ) 4( ) ( copper wire has a resistance of 0ῼ at 0 C. What will be the resistance at 0 C? The temperature coefficient of resistance of copper is 0.004C -. Given ata? C C T T T C T C T 9 9) 0(0 08) 0 0( 0) ( ) ( T

16 8. What is the uivalent resistance of three ῼ resistors connecte in series? f a potential ifference of 60 is applie across the combination, what is the current in each resistor? Given ata,? 60,? We also know that 60 4 n series combination of resistors current remain constant. So 4 9. What is the uivalent resistance of three ῼ resistors connecte in parallel? f a potential ifference of 60 is applie across the combination, what is the current in each resistor? Given ata,? 60,?

17 67 We also know that n electric iron of resistance 0ῼ takes a current of.0. Calculate the thermal energy, in joules, evelope in 0sec. given ata? H Sec t

18 H H H H H t ()(0)(0) J KJ