Effects of Electric Current Science Notes

Important Points :

• A material which has very low electrical resistance is called a good conductor of electricity. Examples: silver, copper, aluminium.
• A material which has extremely high electrical resistance is called an insulator of electricity.
• Examples : rubber, wood, glass.
• When we pick up a piece of iron resting on the ground, we don’t get electric shock because that piece does not carry any electric current at that time.

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)
=
P = VI = I2R = V2/R.

→ Mechanical power (P)

→ The watt: If one joule of electric work is done per second, the electric power is 1 watt.
1 watt (W) = $$\frac{1 \text { joule }(\mathrm{J})}{1 \text { second }(\mathrm{s})}$$

→  Commercial unit of electric energy : The commercial unit of electric energy is the kilowatt-hour (kWh).
1 kW.h -3.6 × 106 J
It is commonly known as the unit.

→  The unit of electric power 1 W is a very small unit, hence 1000 W or 1 kW is used as a unit to measure electric power, in practice. If 1 kW power is used for 1 hour, it will mean 1 kW × 1h of electric energy is used 1 kW.h -1 kilowatt-hour = 1000 W × 3600 S
= 3.6 × 106 W.s = 3.6 × 106 J

→  Electricity bill : Electricity bill shows the consumption of units (i.e., kW.h) and the cost of using electric energy Check monthly electricity bill received from the Electricity Distribution Co. Ltd. Observe various details and get information about them. The electricity bill specifies the usage in ‘Units’. What is this unit? When 1 kW.h electric energy is used, it is termed as 1 unit of energy.

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 flows.

→  H = I2 Rt = VIt = $$\frac{V^{2}}{R}$$t, where V is the potential difference (RI) across the conductor. Here, H is expressed in joule, V in volt, I in ampere, R in ohm and t in second. 1 calorie (cal) = 4.18 joules (J). With V in volt, I in ampere, R in ohm and t in second, we have:

→  The working of an electric bulb, electric iron, fuse wire, etc., is based on the heating effect of electric current

Magnetic effect of electric current :

→  When an electric current is passed through an electric resistor (electric conductor), heat is produced in it. Passage of electric current through a conductor also produces a magnetic field around it. This effect, called magnetic effect of electric current, was discovered by Hans Christian Oersted. The unit of intensity of magnetic field, the oersted, is named after him.

→  Oersted’s discovery : Hans Christian Oersted (1777 – 1851), Danish physicist, discovered the magnetic effect of electric current in 1820. He observed the deflection of a compass needle when placed near a wire carrying an electric current. The experiment described on page 51 of the textbook refers to his discovery.

→  Right hand thumb rule : Imagine that you have held a current-carrying straight conductor in your right hand in such a way that your thumb points in the direction of the current. Then turn your fingers around the conductor. The direction of the fingers is the direction of the magnetic lines of force produced by the current.

Magnetic field due to a current-carrying conductor :

• The intensity of the magnetic field produced at a given point is directly proportional to the current passing through the conductor.
• The intensity of the magnetic field produced by a given current in the conductor decreases as the distance from the conductor increases.

→ A magnetic field is produced around a straight current-carrying conductor. If the current is unchanged, this magnetic field reduces as the distance from the wire increases. Therefore, the concentric circles representing the magnetic lines of force are shown bigger and rarefied as conditioners, use of electricity in shops, etc.

→ As a result, excessive current is drawn from the transformer supplying the electricity, and if the capacity of the transformer is insufficient, its fuse wire melts and the supply gets shut down. Such events occur due to overloading.

→ These days miniature circuit breaker (MCB) switches are used in homes. When the current in the circuit suddenly increases this switch opens and current stops. Different types of MCBs are in use. For the entire house, however the usual fuse wire is used.

• Lamps, TV, computer, electric fan, electric bell.
• Heating effect, magnetic effect, production of light, conversion of electric energy into mechanical energy and the conversion of that mechanical energy into sound.