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Electricity

Electricity. Introduction. Electricity is a very useful thing in our daily life. Whenever we think of Electricity,we think of electrical appliances that we use at home. Eg : when we switch on,the bulb glows.

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Electricity

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  1. Electricity

  2. Introduction • Electricity is a very useful thing in our daily life. • Whenever we think of Electricity,we think of electrical appliances that we use at home. • Eg: when we switch on,the bulb glows. • When we put our fingers in the socket, we get Electric shock. • Batteries which are used for the working of Electrical Appliances such as Camera,cell phone, Computer etc. • Electric Fans,Transformers…..

  3. Electric current • The Flow of Electric charges constitute Electric Current. • It is measured as Rate of flow of charge. • That means,the rate at which charges are flowing. • I = Q/t. Where Q-charge, t-time. • S.I unit of charge-coulomb, It is the charge contained in 6x10 18 electrons. • Denoted by I. • S.I unit : Ampere.(A). • A device called Ammeter is connected in series to measure the current flowing in the circuit.

  4. Direction of current Two types of current: Electronic current. Conventional current. The direction of Electronic current or the flow of Electrons is from the negative Electrode to the positive Electrode. The direction of conventional current is from positive to Negative Electrode. Whenever we say that a current is flowing in a circuit, we mean the conventional flow of current.

  5. Electric current When the switch or key is closed,the bulb glows. This means that current flows in a closed circuit.

  6. Definition of Ampere • 1 Ampere is defined as the current ,when a number of electrons,having one coulomb of charge,move past a given point in one second. • I=Q/t • 1 Amp= 1 coulomb/sec.

  7. What causes the charge to flow? • There is something that initiates the charge flow.

  8. Application 1: Suppose the ball is at rest in the beginning. On applying force, what will happen? Is there any force while moving through the slope? The Force that is acted on the ball in the beginning is the trigger for the movement of the ball.

  9. Application 2 Suppose a hot cup of coffee is placed on the table.On keeping some time, what happens? The coffee becomes cold.As the temp of the coffee is very high with respect to the surroundings,heat flows from the higher temperature to a lower temperature.What is the trigger in this case? The change in Temperature.

  10. Application 3 Why does a copper wire conducts Electric current? Why does the Electric charges flow through a copper wire? Charges flow due to the difference in potential between the two ends. Or difference in Electric pressure.

  11. How does the charges flow? • Charges flow from the region of Higher potential to the region of Lower potential. • The positive end has more potential than the negative end. • So the charges flow from the positive end to the negative end.

  12. How a potential difference is created in the circuit? • By connecting a battery or a combination of cells. • In a cell, chemical Energy is converted into Electrical Energy. • A Battery has a positive end and a Negative end. • Due to which there is a difference in potential. • This initiates the flow of Electric current in the circuit.

  13. Electric Potential and Potential Difference. • Electric potential Difference • Work required to be done by an external force in moving a charge from one point to another. • Electric Potential. • Work done per unit charge in bringing the charge from infinity to that point against Electrostatic Force.

  14. How do we measure potential difference? • Measured by voltmeter which is connected in parallel across the points between which the potential difference is to be measured. • S. I unit : Volt. • 1 Volt is the potential difference between two points in a current carrying conductor when 1 J of work is done to move a charge of 1 coulomb from one point to another. • V = W/ Q. • 1 V= 1 J/ Coulomb.

  15. Is current a scalar or a vector quantity? • S calar-A Quantity having only magnitude and no direction. • But current flow has a direction,still it is not a vector.why? • Comparison of current to the distance travelled.The net current will be the sum of the currents in the different directions. So it is a scalar.

  16. Types of current • Direct current • Alternating current

  17. Electric circuit An Electric circuit is represented by a circuit diagram. It is drawn using the conventional symbols of different circuit elements.

  18. Ohm’s Law • Ohm was a scientist who conducted an experiment and found that as the potential difference is increased, the current also increased. • According to Ohm’s Law, • Current flowing through a conductor is directly proportional to the potential difference applied between the ends of the conductor, provided the temperature remains the constant.

  19. Mathematical Representation of Ohm’s Law • I∝V. • I = (1/R) V. • V= IR, where R is the resistance of the conductor.

  20. Resistance • As the name suggests, resistance resists the flow of current or it opposes the flow of current. • It is a property that resists the flow of current. • For Eg, Metals are good conductors of Electricity. • Which means that the resistance is very low. • For Insulators,there is no flow of current because the resistance is high. • S. I unit: ohm. (Ω).

  21. Who carries current in solids? • Free electrons. • Eg : A copper wire contains number of copper atoms. • Valence electrons:1 • This electron will jump from one atom to the other atom and reach the other side of the conductor.

  22. Low Resistance and High Resistance • Low Resistance. • Number of atoms will be less. • Free electrons will collide with less number of atoms. • High Resistance • Number of atoms will be more. • Free Electrons will collide with more number of atoms.

  23. Low Resistance and High resistance Low Resistance High Resistance

  24. Factors on which the resistance depends • The Length of the conductor. • More the length of the conductor,more will be the collision of the free electron with the atoms • So H igh resistance. • Cross sectional area of the conductor. • If the cross sectional area is increased, the number of collisions will decrease. So Low Resistance.Number of atoms will not increase. • Nature of material of the conductor. • Temperature of the conductor. • As the temperature increases, atoms will move randomly,collide frequently , so the resistance also increases.

  25. Mathematical Representation • R∝l/A • R=ρl/A. • Where ρ is the resistivity of the conductor. • It is also known as the specific resistance of the conductor. • It is dependent on the nature of the material of the conductor. • For a metal,it is low. • For an insulator, it is high.

  26. Resistivity • Denoted by ρ. • Ρ =RxA/l • When A =1 m2 and l=1m, • Ρ = R. • Resistivity of a substance is numerically equal to the Resistance of a rod of that substance which is 1 m long and 1 square metre in cross-section • S. I unit: Ohm-metre. ( Ωm). • Resistivity of a substance does not depend on its length or thickness.It depends on the nature of the substance and Temperature. • A Good conductor should have a low resistivity. • Resistivity of alloys are much higher than those of the pure metals.so alloys like constantan, manganin ,nichrome, are used to make resistance wires in the circuits. • An alloy does not undergo oxidation even at a very high temperature, when it is red hot.

  27. Limitations of Ohm’s Law • Ohm’s Law is not universal in nature. • It is not true for all the applications. • Behaviour of Diodes: • Diodes are electronic devices which allow the flow of current in one direction. • If we plot the V-I graph for diodes, we get a curved line. • Behaviour of Ga As: • V-I graph is not a straight line

  28. Resistors • The materials that resist the flow of current. • Types of Resistors: Carbon resistors, Wirebound Resistors. • Carbon resistor: Resistance can be calculated by using the colour code. • Wirebound resistor- It is a metal in which some turns of wire are wounded.

  29. Combination of Resistors • When we design Electric circuits, we often use resistors. Why? • To prevent huge flow of Electric current.When high current flows , the electrical elements will be spoiled. • Eg : For Electrical appliances used in our homes. • There are two ways by which resistors are combined. • When we design a circuit,we want a particular current flow, for that we want a particular resistance. • All values of resistances are not available in the market. So we are combining resistances to get the appropriate resistance.

  30. Resistors in series The same amount of current flows through the resistances.I remains the same. But V across each resistor will be different.V1,V2,V3. V1=IR1, V2= IR2,V3=IR3

  31. continuation • Potential difference between the two points, • V=V1+V2+V3 • V= IR, V1= IR1,V2=IR2,V3=IR3. • Therefore, • IR= IR1 + IR2 + IR3 • Cancelling I, • R= R1 + R2 + R3

  32. Resistors in parallel V will be the same throughout the circuit.. I will be different. V/R= V/R1 + V/R2 +….. V/ Rn 1/R=1/R1 + 1/R2 +… 1/ Rn

  33. Heating effect of Electric current. • Dissipation of source energy in the form of heat in an electrical circuit. • Battery-a source of potential difference,thereby energy. • Some amount of this energy is dissipated in the form of heat.This is known as the heating effect of Electric current.

  34. Heating effect of Electric current • I through R. • Work done in moving a charge Q through P.d V= QV. • Power Input to circuit=W/t = QV /t • P=VI. • Energy supplied to the circuit E = Pxt = Vit. • From ohm’s Law, V=IR. • Therefore, E= I2 R t. • Heat Energy H = I2 Rt. • This is known as Joule’s Law of Heating.

  35. Joule’s Law of Heating • According to Joule’s Law of Heating, • Heat Energy produced across the resistor is d irectly proportional to the Square of the Current for a given resistance, • Resistance for a given current and the time for which the current flows through the circuit. • So, If current flows through the circuit, then resistor gets heated up gradually.

  36. Applications Of Heating Effect : Electric Bulb There is a filament inside the Electric Bulb. It retains the heat that is generated across the resistor.Filament stores the heat. It is made up of Tungsten which has very high melting point.when it is sufficiently hot, it emits light.Most portion is consumed as heat; only small amount as Light.

  37. Electric Fuse Prevents excessive flow of current.in the circuit. Connected in series with the device. Rated as 1A, 2A,3A,5A ,10 A etc.

  38. Electric Fuse • It consists of a specific metal wire with a low melting point, so that if excessive current is flowing, the wire will break and the circuit will not be damaged.

  39. Electric Power • Rate at which Electric Energy is dissipated or consumed in a circuit. Denoted by P. • S. I unit : Watt. • 1 Watt is the power consumed by a device carrying 1 A of current when operated at a potential difference of 1 V. • P= V I • P= I2R • P= V2/R

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