Electric current and Circuit:
- The rate of flow of charges through a metallic wire is called as electric current.
- And switch is used to make the connection between cell and the bulb or any device.
- Electric circuit is the closed and continuous loop or path of an electric current. The electrons flow from negative terminal of battery to positive terminal of battery. And hence the direction of electric current would be from positive terminal of battery to the negative terminal.
- If Q is the total charge flowing through a cross section of the conductor, then an electric current through the conductor will be I = Q/t
- The SI unit of electric charge Q is coulomb.
- The SI unit of electric current is ampere A.
1 ampere:
- The 1 coulomb of charge flowing through unit cross section of the conductor per second is called as one ampere current.
- Thus, 1A = 1C/1S
Electric potential and potential difference:
- The amount work done to move or displace the unit positive charge from one point to another point is called as electric potential difference between two points of an electric circuit.
- Thus, we can write
- Potential difference between two points = work done*charge
V = W/Q
- The SI unit of electric potential difference is volt V.
1 Volt:
- The potential difference between any two points of the circuit is said to be 1 volt when 1 joule of work is required to move a charge of 1 coulomb from one point to the other point in an electric circuit.
- Thus,
- 1 volt = 1 joule/ 1 coulomb
1V = 1J/ 1C
- The instrument used to measure the potential difference between two points in an electric circuit is called as voltmeter.
- And voltmeter is connected in parallel across the points between which we have to measure the potential difference.
- The instrument used to measure an electric current flowing through the circuit is called as ammeter.
Ohm’s law:
- According to Ohm’s law, the potential difference between two end points of the conductor is directly proportional to the electric current flowing through it, provided that the temperature remains constant.
- Thus, V α I
- Hence, V/I = constant
V/I = R
Or V = IR
- Where R is the constant for the given conductor and which is called as resistance of the conductor.
- Resistance of the conductor is the opposition to the flow of electric charges.
- The SI unit of resistance is ohm.
- Thus, R = V/I
1 ohm:
- The resistance of the conductor is said to be one ohm when the potential difference between two ends of the conductor is 1 V and current flowing through it is 1 A.
- Thus, 1 ohm = 1V/ 1A
- And we observe that, I = V/R
- Thus, the electric current flowing through the conductor is inversely proportional to the resistance of the conductor.
- The instrument or component used to change an electric current without changing the voltage is called as variable resistance.
- And the device used to change the resistance in the circuit is called as rheostat.
- The materials having very low resistance are good conductor of electricity.
And materials having high resistance are called as insulators and they do not conduct electricity.
Factors on which the resistance of the conductor depends:
The resistance of the conductor mainly depends on the following factors:
- Length of the conductor
- Area of cross section of the conductor
- And nature of the material of the conductor
- Thus, the resistance of the conductor is directly proportional to length of the conductor and inversely proportional to the area of cross section of the conductor.
- Hence, R α l
And R α 1/A
Or R α l/A
Thus, R = ρ*l/A
- ρ rho is the constant of proportionality which is called as electrical resistivity of the conductor.
- The SI unit of resistivity is ohm meter.
- The metals and alloys which are good conductor of electricity has resistivity in the range 10-8 to 10-6 ohm m.
- Glass, rubber are insulators which are having resistivity in the range 1012 to 1017ohm m.
Resistance in series:
- When the resistors are connected end to end as shown in figure below then we can say that resistors are connected in series combination.
- When resistors are connected in series then current flowing through each resistor is same but the potential difference across each resistor is different in series combination of resistors.
- Let V be the total potential difference given and V1, V2 and V3 be the potential difference across resistors R1, R2 and R3 connected in series.
- Then we can write,
V = V1 + V2 + V3
- Let I be the current through each resistor flowing then the equivalent resistance for series combination of resistors is given by
R = R1 + R2 + R3
- Thus, in series combination of resistors the equivalent resistance is the sum of individual resistance and hence equivalent resistance must be greater than each individual resistance also.
Resistors in parallel:
- When the three resistors are connected in between two points XY as shown in figure then we can say that it is the parallel combination of resistors.
- In parallel combination of resistors, the current flowing through each resistor is different but the potential difference across each resistor is same.
- Let I be the total current flowing through the circuit.
- Let I1, I2 and I3 be the current flowing through resistors R1, R2 and R3 Then we can write
I = I1 + I2 + I3
- Thus, for parallel combination of resistors
1/R = 1/R1 + 1/R2 + 1/R3
- Thus, in parallel combination of resistors the reciprocal of equivalent resistance is the sum of reciprocals of resistance connected in parallel combination.
- And hence, the equivalent resistance is less than each individual resistance.
Heating effect of electric current and its applications:
Joule’s law of heating gives the heating effect of an electric current. According to Joule’s law of heating, the heat produced in a resistor R is directly proportional to
- Square of the current through the resistor
- Resistance for a given current
- Time for which the current flows through the resistor
Mathematically, H = I2Rt
- This heat energy produced depends on current, resistance and time for which the current is flowing. This proves the Joule’s law experimentally.
- In fuse wire circuits also heating effect occurs. The fuse wires are made from metal or an alloy of particular melting point like aluminium, copper, iron, lead etc. if large current flows through the circuit, the temperature of the fuse wire increases and due to which fuse wire get melted and circuit breaks. To protect the circuit from large current or heat produced due to large current fuses are used.
- The electric iron we use in daily life is also working on the Joule’s law of heating. In which electrical energy is converted into heat energy that can be used to press the cloths.
- Also, in electric heaters the Joule’s heating effect is observed, in which large current produces heat energy. This heat energy produced is used to heat the water.
Electric power:
- Electric power is defined as it is the rate at which electrical energy get dissipated or consumed in an electric circuit.
- The electric power is given by,
- P = VI
- P = I2R = V2/R
- The SI unit of electric power is watt W.
1 watt W:
- The power consumed by an electric circuit is said to 1 watt when it carries current of 1 ampere when it is operated at potential difference of 1 volt.
- Thus, 1W = 1V*1A
- In practical use we use unit of power as kilowatt.
- And 1 kilowatt = 1000 watt
- Also, the commercial unit of electric energy is kilowatt hour kWh.
- Thus, 1kWh = 1000watt*3600 second
- 1kWh = 3.6*106 joule