Unit-6 | CURRENT ELECTRICITY AND MAGNETISM | Class-09

Overview: 
Electricity is a form of energy which is either produced due to the flow of electrons through a conductor or change in number of electrons in a body. Magnetism is the phenomenon by which a magnet attracts other magnetic substances. Here we will study electricity, its sources, electric circuit, electric devices, magnetism, angle of dip and angle of declination.


Electricity: 

Electricity is a form of energy which is either produced due to the flow of electrons through a conductor (current electricity) or change in number of electrons in a body (static electricity). All the objects which can produce electricity are called sources of electricity. Some sources of electricity are: cell, battery, dynamo, etc.

Cell:

Cell is a source of electricity which produces direct current (DC). The combination of cells forms batteries. The advantages of cell over other sources of electricity are: they are cheap and portable.

Hydroelectricity:  

Hydroelectricity is a source of electricity which produces alternating current (AC). It can produce current electricity in a large amount. Since, it operates with water, it is a very reliable and renewable source of energy. 

Electric Circuit:

Electric circuit is a closed path made using a conducting wire, electric load, source of electricity and a switch in which the current can flow. There are two types of electric circuits:

1. Open Electric Circuit: This is the type of electric circuit in which the electric load doesn't glow because either the connecting wire is faulty or the switch is turned off. This is why the current cannot flow completely throughout the circuit.




2. Closed Electric Circuit: This is the type of electric circuit in which the electric load glows and current can completely flow throughout the circuit because the connecting wires are good and the switch is also turned on.


Direction of flow of current:

The electric current in an electric circuit always flows from the positive terminal to the negative terminal.


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Conductors and Insulators:

Those substances which can conduct electricity through them are called conductors. Example are: silver, gold, etc.

Those substances which can not conduct electricity through them are called insulators. Example are: rubber, plastic, etc.

Electric Current: 

The rate of flow of electric charge in a circuit is called electric current. The SI unit of electric current is Ampere (A). 

Mathematically,
Electric Current (I) = Charge (Q)/ Time (T) 
The SI unit of Charge is Coulomb (C).

One ampere Current:

Current is said to be one ampere if one coulomb charge is being flowed in an electric circuit in one second.

Ammeter:

Ammeter is a device used to measure the total current flowing through an electric circuit. It is connected in series in an electric circuit. It is because it has a low resistance which doesn't change the overall resistance of the electric circuit. 

Voltmeter:

Voltmeter is a device used to measure the potential difference between any two points of an electric circuit. It is connected in parallel in an electric circuit. It is because it has high resistance that affects the total resistance of the electric circuit as a whole.


Ohm's Law:

Ohms law states that, "The electric current (I) flowing through a conductor is directly proportional to the potential difference (V) across the ends, provided that the physical conditions like temperature, pressure, etc. do not change.

From Ohm's Law, we get,
I ∝ V
or, V ∝ I
or, V = IR [ Where 'R' is a constant called resistance of the conductor ]

The resistance is said to be one ohm (1Ω) if one ampere (1A) current flows through a circuit with the applied voltage of one volt (1V). 

Some Important Terms:

Resistance: The property of a conductor by the virtue of which it opposes the flow of current is called resistance.

Resistivity: Resistivity is defined as the resistance of a unit length (1m) and unit cross-sectional area (1m^2). 

Conductivity: The reciprocal of resistivity is called conductivity.



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Important terms related to Magnetism:


Magnet:
A magnet is a substance that can attract magnetic substances.

Magnetism:
Magnetism is the attractive or repulsive force exerted by a magnet.

Magnetic Compass:
A magnetic compass is a magnetic device consisting of a magnetic needle that can rotate freely on a pivot at the centre of a round box.

Magnetic Field:
The effective range of magnetic influence shown by a magnet is called magnetic field. 

Magnetic Lines of Force:
The path along which a unit north pole moves in a magnetic field is called magnetic line of force.

Neutral Points:
Those points where the resultant magnetic influence of a magnet is zero is called neutral points. 

Terrestrial Magnetism:
The magnetic property shown by the Earth is called Terrestrial Magnetism.

Angle of Dip:
The angle between the dip needle and the horizontal line at a certain place is called angle of dip.

Angle of Declination:
The angle between the geographical meridian and the magnetic meridian at a place is called angle of declination. 

Uses of Magnetic Compass:
  1. It is used to find the direction of a magnetic field. 
  2. It is used to test the polarity of a magnet.
  3. It is used to find the magnetic north-south direction.
The value of angle of dip at the equator is zero. The value of angle of dip at magnetic poles is 90 degrees.

Uses of angle of dip and angle of declination:
They are used by cave-searchers, navigators, pilots, travellers, etc. to find the position on the Earth's surface.

Numerical Problems:
1. The power of the bulb in a car is 60W. Calculate the current that flows when it is connected to a batter of 6V. Also calculate the resistance of the bulb. 
Solution:
We have, 
Power (P) = 60W
Potential difference (V) = 6V
Current (I) = P / V
= 60 / 6
= 10 A

And, 
Resistance (R) = ?
We know,
R = V/ I
=  6 / 10
= 0.6 Î©

2. A current of 0.33 A is flowing through a circuit. If the potential difference between the two points of a wire is 5V, calculate the resistance. 
Solution:
Current (I) = 0.33 A
p.d. = 5V
We have,
R = V / I
= 5/ 0.33
= 15.15 Î©


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3. The voltage of an electric line is 220 V. If 4.54 A current flows through the line when a heater is connected to the circuit, calculate the power of the heater. 
Solution:
p.d. = 220 V
Current (I) = 4.54 A
Power of the heater (P) = ?
We have,
P = V x I
= 220 x 4.54
= 998.8 W




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