Notes
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Topics to be learn :
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Introduction :
Electrical conductor : A material which has very low electrical resistance is called a good conductor of electricity.
- Examples: silver, copper, aluminium.
Electricity Insulator : A material which has extremely high electrical resistance is called an insulator of electricity.
- Examples : rubber, wood, glass.
Energy transfer in electric circuit :
Electric power : Electric power is the electric work done per unit time or electric energy used per unit time. Its SI unit is the watt (W).
Electric power (P) = \(\frac{\text{electrical work (W) or electrical energy used}}{time(t)}\)
P =VI = I2R = V2/R.
Mechanical power : Similar to the electric power we can write,
Mechanical power (P) = \(\frac{\text{Mechanical work (W)}}{time(t)}\)
P = W/t = Fscos/t
The watt : If one joule of electric work is done per second, the electric power is 1watt.
1 watt (W) = 1joule (J) / 1 second (s)
Commercial unit of electric energy : The commercial unit of electric energy is the
kilowatt-hour (kW.h),
1 kW.h -3.6 x 106J
It is commonly known as the unit.
The unit of electric power 1W is a very small unit, hence 1000W or 1 kW is used as a unit to measure electric power, in practice. If 1kW power is used for 1 hour, it will mean 1 kWx 1 h of electric energy is used
1KW.h= 1 kilowatt-hour = 1000W x 3600s
= 3.6 x 106 w.s = 3.6 x 106J
Electricity bill : Electricity bill shows the consumption of units (i.e., kWh) and the cost of using electric energy. When 1 kWh electric energy is used, it is termed as 1 unit of energy.
Heating effect of electric current :
The production of heat in a resistance due to the electric current flowing through it when it is connected in an electrical circuit, is called the heating effect of electric current.
Origin of the heating effect of electric current :
- When a potential difference is applied across a metallic conductor, free electrons in the conductor move from the end at the lower potential to the end at the higher potential giving rise to electric current.
- These electrons collide with the atoms and positive ions and transfer some kinetic energy to them. This energy is converted into heat.
- Hence, the temperature of the conductor begins to rise i.e., the conductor becomes hot. This is the origin of the heating effect of electric current.
Joule's law about heating effect of electric current: The quantity of heat produced (H) in a conductor of resistance R, when a current I flows through it for a time t is directly proportional to
- The square of the current
- The resistance of the conductor
- The time for which the current lows.
Mathematical expression for the heat generated in a metallic conductor by electric current (Joule’s law) : If Vis the potential difference applied across a metallic conductor of resistance R, the current through the conductor, given by Ohm’s law, is I = V/R. …..(1) The charge passing through the conductor in time t when the current I flows in the conductor is Q = It ….(2) The work done in this process is W = VQ …..(3) From Eqs. (1), (2) and (3), we have, W = (IR) (It) = I2Rt = VIt = \(V\frac{V}{R}t\) = \(\frac{V^2}{R}t\) This work is converted into heat. When I is expressed in ampere, R in ohm, z in second and V in volt, W is expressed in joule. In that case, _ W = I2Rt = VIt = \(\frac{V^2}{R}t\) (in joule) Usually heat energy (H) is expressed in calorie Using the relation 4.18 J = 1 cal, we have H = W = \(\frac{I^2RT}{4,18}\) cal = \(\frac{VIT}{4,18}\) cal = \(\frac{V^2T}{4,18R}\) cal This is the required expression.
Application : The working of an electric bulb, electric iron, fuse wire, etc., is based on the heating effect of electric current.
Electrical conductors having higher resistivity are used here.
Examples :
- A coil made up of an alloy Nichrome is used in the electric heater-cooker as a resistor.
- Tungsten wire is used in an electric bulb. Because of the current, this wire gets heated (to nearly 3400 0C) and emits light. The hot wire also radiates heat to a certain extent.
Fuse : A fuse protects electrical circuits and appliances by stopping the flow of electric current when it exceeds a specified value. For this, it is connected in series with the appliance (or circuit) to be protected.
- A fuse is a piece of wire made of an alloy of low melting point (e.g. an alloy of lead and tin). If a current larger than the specified value flows through the fuse, its temperature increases enough to melt it. Hence, the circuit breaks and the appliance is protected from damage.
Know This :
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Short circuiting :
- When a bare live wire (phase wire) and a bare neutral wire come into direct contact or are very close to each other, the resistance of the circuit becomes very small and hence a large (very high) electric current flows through it. This is referred to as a short circuit or short circuiting
- In this case, a large amount of heat is produced and the temperature of the components involved becomes very high. Hence, the circuit catches fire.
Overloading : A flow of large amount of current in a circuit, beyond the permissible value of current, is called overloading.
- It occurs when many electrical appliances of high power rating, such as a geyser, a heater, an oven, a motor, etc., are switched on simultaneously. This causes fire.
- Overloading can be avoided by not connecting many electrical appliances of high power rating in the same circuit.
Magnetic effect of electric current :
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