Current Electricity:
The amount of charge that flows in unit time through any cross-section of a conductor is called an electric current. Electric current is the flow of electric charge across a surface, point, or region.
The amount of energy required by an electric source to transfer unit positive charge from one point of the circuit and bring it there again by traveling the total circuit is called the electromotive force of that source.
The work done by an energy source to drive a unit positive charge around a circuit from one point back to the same point is called the electromotive force of that source.
The work done to bring a unit positive charge from an infinitive distance to any point of the electric field is called the electrical potential of that point.
The materials through which electric current can flow very easily are called conductors. Electrons can flow freely within these materials. In metal wires, the charges are carried by electrons. So, metallic materials are good conductors of electricity.
The substance which has no free electron for conducting electricity is called an insulator. Plastic, rubber, wood, and glasses are examples of insulators.
There are some substances whose conductivity lies between insulators and conductors at normal temperature, but their conductivity increases with the rise of temperature. These types of substances are called semiconductors. Silicon or germanium examples of semiconductors.
Ohm's Law:
The current passing through a wire at constant temperature is proportional to the potential difference between its ends.
A conductor for which this relationship is true is said to obey Ohm's law. This law may also be expressed as
potential difference / current = constant
The ohm (Ω) is the resistance of a conductor such that, when a potential difference of 1 volt is applied to its ends a current of 1 ampire flows through it.
The current passing through a wire at constant temperature is proportional to the potential difference between its two ends.
A conductor for which this relationship is true is said to obey Ohm's law. This law may also be expressed as
potential difference / current = constant
It follows that, volt/ampire = ohm
or, in symbols V/I = R
So, V= I/R
The equipment used for measuring voltage is called a voltmeter, and the equipment used for measuring current is known as ammeter.So, the higher the resistance, the lower is the electric current. And the lower the resistance, the higher is the current.The unit of resistance is Ohm and it is expressed by the Greek letter Ω. If 1 V potential difference is applied in an electric circuit and if it is seen that 1 A current flows as a result, it can be said that the resistance of the circuit is 1 Ω.Resistance:Resistance is the obstacle to the flow of current in a conductor. So the more the length of a material L, the higher the hindrance and resistance.R ∝ LAgain current can flow more easily through a wide path than through a narrow path. So, the higher the cross-sectional area A of a conductor, the lower is the resistance.
If we want to write these two things as an equation rather than in proportional form, a constant ρ has to be used. So, resistance R is,
Where the constant ρ is,For a certain material, ρ is specific resistance and its unit is Ω m.To measure how conducting a substance is, a term called conductivity σ has been introduced. The more a substance is electrically conducting, the more is the value of the electrical conductivity. It is reciprocal to the specific resistance σ.
σ = 1/ρ
The unit of conductivity σ is : (Ω m ) ¯¹
Q. The specific resistances ρ of silver, copper, tungsten and nichrome wire are 1.6 x 10 ¯⁸, 1.7 x 10 ¯⁸, 5.5 x 10 ¯⁸, 100 x 10 ¯⁸ Ω m respectively. Using these make a 1 Ω resistance.
You can see, to create a 1 Ω resistance, the length has to be very very long. Practically, very thin wires are used, so A = 1 m² is not usually feasible. If we fix the radius as 0.1 mm, then how long will a wire need to be to make a 1 Ω resistance?
We know,
L = RA/ ρ
A = лr² = π(10ー⁴)²m² = 3.14 x 10ー⁸m²
For silver, L = 1.96m
For copper L = 1.84 m
For tungsten L = 0.57 m
For Nichrome, L = 0.03 m
Circuit:
The path through which electric flow occurs is called the electric circuit.
An electric circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors, and diodes, connected by conductive wires or traces through which electric current can flow
The power of the lens having one-meter focus length is called one diopter.
Electric Power:
The rate at which the work is being done in an electrical circuit is called electric power. If a charge is moved by applying a potential then some work is done or some energy is lost. So, if V potential is applied to move a charge Q in a circuit then the amount of work is done.W = VQ JoulePower P is the capacity of work done per second, so if Q charge is moved in t time thenP = W/t= VQ/t= VI WattIf we use this on a single resistance R, then applying Ohm's law we can write,since V = RIP = I²RI = V/RTherefore, P = (V/R)² R= V²/RQ. What is the resistance of the filament of a 100W bulb?Ans: 100W is written on a 220V bulb, asP = V²/RTherefore, R = v² / P= 220²/100 Ω= 484 ΩCurrent is flowing hereI = V / R= 220/484=0.45 AIt can be done in other ways too: P = VII = P/V= 100/220= 0.45 A
Q. If a bulb of 60 Watts is lit everyday for 5 hours for 30 days then what would be the electric energy usage? If each unit has a price of 10 Tk, then what will be the total cost for this electricity?
Ans: We know,
spent energy = (P x t) / 1000 kW-h
P = 60 W and t = 5 x 30 hour
Spent energy = 60 x (5 x 30 ) / 1000 unit = 9 unit
Total cost of electricity with 10 Tk per unit = 9 x 10 Tk = 90 Tk
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