std10 electricity
TRANSCRIPT
Standard/ Grade/ Class 10 Electricity
Gurudatta K Wagh
Electric current and circuit
Electric potential and potential difference
Ohm's Law
Factors responsible for the resistance of a conductor
Resistance of a system of resistors
• Resistors in series
• Resistors in parallel
Heating effect of electric current
• Practical applications of Joule's heating
Electric power
Electric current and circuitIf the electric charge flows through a conductor (a
metallic wire) we say that there is an electric current in the conductor.
A switch makes a conducting link between the cell and the bulb. A continuous and closed path of an electric current is called an electric circuit
If the circuit is broken anywhere (or the switch of the torch is turned off), the current stops flowing and the bulb does not glow
Electric current The amount of charge flowing through a particular area in unit time, i.e. the rate of flow of electric charges
In metallic wires, electrons constitute the flow of charges
Electrons were not known at the time when the phenomenon of electricity was first observed. So, electric current was considered to be the flow of positive charges and the direction of flow of positive charges was taken to be the direction of electric current
Conventionally, in an electric circuit the direction of electric current (positive to negative) is taken as opposite to the direction of the flow of electrons
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 circuit
Current formulaIf a net charge Q flows across any cross-section of a conductor in time t, then the current I through the cross-section is
current I = net charge Q/ time t SI unit electric charge = coulomb 'C' equivalent to the charge contained in nearly 6 x 1018 electrons
An electron possesses a negative charge of 1.6 X 10-19 C
Electron/s Charge (C)1 1.6 x 10-19
6 x 1018 6 x 1018 x 1.6 X 10-19 = 9.6 x 10-1
Charles-Augustin de Coulomb
The chemical action within a cell generates the potential difference across the terminals of the cell, even when no current is drawn from it
In order to maintain the current in a given electric circuit, the cell has to expend its chemical energy stored in it
SI unit electric current = ampere 'A'
One A is constituted by the flow of one C of charge per second
1A = 1C/ 1s
André-Marie Ampère
Small quantities of current
milliampere 1mA = 10-3A
microampere 1µA = 10-6A
Ammeter measures electric current in a circuit; always connected in series in a circuit•YouTube video explain working of ammeter. Add
Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters
Flow of charges inside a wireElectrons are able to travel through a perfect solid crystal smoothly and easily, as if they were in a vacuum. The motion of electrons in a conductor is different from that of charges in empty space.
When a steady current flows through a conductor, the electrons in it move with a certain average drift speed. The drift speed of electrons for a copper wire carrying a small current is small, 1mm s-1
Electric potential and potential difference
What makes the electric charge to flow?The electrons move only if there is a difference
of electric pressure – potential difference – along the conductor
The potential difference may be produced by a battery consisting of one or more electric cells
Definition Electric potential difference between two points in an electric circuit carrying some current = work done to move a unit charge from one point to the other
Potential difference between two points (V) = Work done (W)/ Charge (Q)SI unit 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 across one point to other
1 volt = 1 joule/ 1 coulomb
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
Alessandro Giuseppe Antonio Anastasio Volta
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit.
One joule is the equivalent of one watt of power radiated or dissipated for one second
James Prescott Joule
Ohm's LawDefinition The potential difference across the
ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it, provided its temperature remains same
V I, V/ I = constant (R), ∝ V = IR
Relationship between the potential difference across a conductor and the current through itV-I graph is a straight line passing through the origin. V/I is a constant ratio
Georg Simon Ohm
R is a constant for the given metallic wire at a given temperature and is called resistance.
Resistance The property of a conductor to resist the flow of charges through it
SI unit = ohm ΩOhm'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 Ω•1 ohm = 1 volt/ 1 ampere
The current through a resistor is inversely proportional to its resistance. If the resistance is doubled the current gets halved
A component used to regulate current without changing the voltage source is called variable resistance. A rheostat is often used to change the resistance in the circuit
The word rheostat was coined about 1845 by Sir Charles Wheatstone. It is a two-terminal variable resistor. The term "rheostat" is becoming obsolete, with the general term "potentiometer" replacing it
The most common way to vary the resistance in a circuit is to use a rheostat
Factors responsible for the resistance of a conductor
The current is different for different components. Certain components offer an easy path for the flow of electric current while the others resist the flow.
The ammeter reading decreases to one-half (half) when the length of the wire is doubled.
The ammeter reading increases when a thicker wire of the same material and of the same length is used
The ammeter reading changes when a wire of different material of the same length and the same cross-section is used
On applying Ohm's law, it is observed that,The resistance of the conductor depends 1)on its length, 2)on its area of cross-section, and 3)on the nature of its material
Resistance (R) of a uniform metallic conductor is directly proportional to its length (l) and inversely proportional to the area of cross-section (A)
R l and R 1/ A∝ ∝
Hence R l/ A = p l/ A∝
ρ (rho) = constant of proportionality = electrical resistivity of the material of the conductor
SI unit = Ω m
It is a characteristic property of the material
Metals and alloys have very low resistivity, range 10-8 – 10-6 Ω m = good conductors of electricity
Conductor tungsten (ρ = 5.20 x 10-8) is used almost exclusively for filaments of electric bulbs, whereas copper (ρ = 1.62 x 10-8) and aluminium (ρ = 2.63 x 10-8) are generally used for electrical transmission lines
Resistivity (ρ) of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at high temperatures. For this reason alloys are commonly used in electrical heating devices like electric iron, toasters, etc.
Nichrome (ρ = 100 x 10-6) = Alloy of nickel, chromium, manganese, and iron
The above values are at temperature 20 °C
Insulators
rubber, glass – high resistivity
1012 – 1017 Ω m
Resistance (R) and resistivity of a material (ρ) vary with temperature
Resistance of a system of resistorsCurrent through a conductor depends upon its
resistance and the potential difference across its ends.
Application of Ohm's law to combination of resistors
Two methods (series and parallel)
Series
Parallel
• Resistors in seriesHaving resistances R1, R2 and R3 connected end to
end in series
Value of the current in the ammeter is the same, independent of its position in the electric circuit
In a series combination of resistors the current is the same in every part of the circuit or the same current through each resistor
Potential difference is equal to the sum of potential differences V1, V2 and V3
The potential difference across a combination of resistors in series is equal to the sum of potential difference across the individual resistors,V = V1 + V2 + V3
It is possible to replace the three resistors joined in series by an equivalent single resistor of resistance R, such that the potential difference across it, and the current I through the circuit remains the same,V = IR
Applying Ohm's law to the three resistors separately,V1 = IR1 V2 = IR2 V3 = IR3
Hence IR = IR1 + IR2 + IR3
OR
Rs = R1 + R2 + R3
When several resistors are joined in series, the resistance of the combination Rs equals the sum of their individual resistances R1, R2,
and R3 and is greater than any individual resistance
• Resistors in parallelTotal current I = sum of the separate currents
through each branch of the combinationI = I1 + I2 + I3
Rp is the equivalent resistance of the parallel combination of resistors
I = V/ Rp
Applying Ohm's law to each resistorI1 = V/ R1, I2 = V/ R2, I3 = V/ R3
V/ Rp = V/ R1 + V/ R2 + V/ R3
Resistors in parallel
The reciprocal of the equivalent resistance of a group of resistances joined in parallel = sum of the reciprocals of the individual resistances
Disadvantages of series circuitIn a series circuit the current is constant throughout the electric circuit. It is obviously impracticable to connect an electric bulb and an electric heater in series because they need currents of widely different values to operate properly
When one component fails the circuit is broken and none of the components works, e.g. fairy lights
Advantages of parallel circuit•Divides the current through the electrical gadgets•The total resistance is decreased •It is helpful particularly when each gadget has different resistance and requires different current to operate properly
Heating effect of electric currentA part of the source energy in maintaining the
current may be consumed into useful work (like in rotating the blades of an electric fan)
Rest of the source energy may be expended in heat to raise the temperature of the gadget
An electric fan becomes warm if used continuously for longer time
Heating effect of electric current If the electric circuit is purely resistive, i.e. a configuration of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat, e.g. electric heater, electric iron, boiler, geyser
Current I, Resistor of resistance R, Potential difference V, time t, charge QWork done P = VQPower input in time t, P = V x Q/t = VI
Energy supplied to the circuit by the source in time t = P x t = VI x t
Energy expended by the source gets dissipated in the resistor as heat
For a steady current I, the amount of heat H produced in time t, H = VIt
Applying Ohm's law,
H = I2Rt Joule's lawHeat produced in a resistor is 1) directly proportional to the square of current for a given resistance, 2) directly proportional to resistance for a given current, and 3) directly proportional to the time for which the current flows through the resistor
H = I2Rt is used after calculating the current through it, using the relation I = V/ R
Heat energy is expressed in calories, 4.18 J = 1 calorieH = I2Rt/4.18 cal = V2t/4.18R cal = VIt/4.18 cal
• Practical applications of Joule's heatingThe generation of heat in a conductor is an
inevitable consequence of electric current. In many cases it is undesirable as it converts useful electrical energy into heat. The unavoidable heating can increase the temperature of the components and alter their properties
Heating effect of electric current has many useful applications. E.g. iron, toaster, oven, kettle, heater
Electrical heating is also used to produce light, as in a bulb. The filament must retain as much of the heat generated as is possible, so that it gets very hot and emits light. It must not melt
A strong metal with high melting point such as tungsten (melting point 3380 °C) is used for making bulb filaments. The filament should be thermally isolated as much as possible, using insulating support
The bulbs are usually filled with chemically inactive nitrogen and argon gases to prolong the life of the filament
Most of the power consumed by the filament appears as heat, but a small part of it is in the form of light radiated
Fuse used in electric circuits is a common application of Joule's heating. The fuse protects circuits and appliances by stopping the flow of any unduly high electric current. The fuse is placed in series with the device.
If a current larger than the specified value flows through the circuit, the fuse wire melts due to heating
Fuses for domestic purposes are rated 1 A, 2 A, 3 A, 5 A, 10 A etc. For an electric iron which consumes 1 kW electric power when operated at 220 V, a current 4.54 A flows in the circuit. Here a 5 A fuse must be used.
Electric powerRate of doing work is power = rate of
consumption of energy
H = I2Rt gives the rate at which electric energy is dissipated or consumed in an electric circuit = electric power
Power P is given by P = VIORP = I2R = V2/ RSI unit watt (W)
Watt Power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V1 W = 1 VA
James Watt
Unit watt is very small
In actual practice a larger unit called kilowatt (kW = 1000 W) is used
Unit of electric energy watt hour (Wh)One watt hour is the energy consumed when 1 watt of power is used for 1 hour
The commercial unit of electric energy is kilowatt hour (kWh)
1 kWh = 1000 watt x 3600 second= 3.6 x 106 watt second= 3.6 x 106 joule (J)
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