prep 3 second term unit 2 lesson 1 Electric resistance .ohm's law .part 3
Date: NaT

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شرح دروس science للمراحل الابتدائيه والاعداديه
Unit 2: Electric energy and radioactivity lesson 1: properties of the electric currentThe electric resistance (resistor)_car face bumps when they pass on the way similarly The electric current face an obstruction (resistance )when it passes through conductor which affect its intensity
The electric resistance :it is the opposition that the electric current faces during its passing through conductor
 The apparatus used in measuring the electric resistance is the “ohmmeter ” 
The measuring unit of the electric resistance is the “ohm”
The electric resistance :it is the opposition that the electric current faces during its passing through conductor
 The apparatus used in measuring the electric resistance is the “ohmmeter ” 
The measuring unit of the electric resistance is the “ohm”
Type of electric resistance 
1-The constant (fixed)resistance Its symbol in the electric circuit is 
2-The variable resistance (sliding rheostat)Its symbol in the electric circuit is  
The variable resistance (sliding rheostat ) It is a resistance that you can change it´s value in order to adjust the value of the current and potential difference in the different parts of the circuits. 
Components of the variable resistance:
1-Metal wire of high resistance,     wrapped around a cylinder     made of an insulating    substance as porcelain
2-Thin copper sheet is touching     the wire and  can slide on it for 
    the whole length of the cylinder    and known as the slider
The  importance 
It is used to control the electric current intensity flowing through the circuit and potential difference in the parts of the circuit 
Its scientific base 
The resistance of the conductor (metallic wire coil is directly to its length
The resistance increases by increasing the length of conductor (metallic )wire coil)
The resistance decrease  by decreasing the length of conductor (metallic )wire coil)
Theory of operation of the variable resistance: Through the sliding of the flexible sheet on the wire, you can control the resistance the current will face while passing in the wire, and that is by controlling the length of the wire that enters the circuit and has the current passing through.
 Therefore, the current intensity subsequently changes, hence, if we increase the length of the wire the resistance increases and the current intensity decreases. 
By adjusting the sliding sheet, you can control the length of the wire that enters the circuit where the current passes and thus control the current in the circuit. Hence, if the length the wire increases, the resistance increases and the current intensity decreases. 
Enriching information: 
Inside the fuel tank of a car, there is a pointer connected to a changeable resistance that controls the flow of the electric current in the car’s fuel scale. When the level of the fuel is low, an electric current flows in a circuit causing the deviation of fuel pointer. 
Ohm’s Law: 
The electric current intensity passing through a conductor is directly proportional with the potential difference between its ends when the temperature is constant.
      v          I
       V= constant value  ×  I 
        V=R ×I