CHAPTER AT A GLANCE
1. Electricity: Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge.
There are two types of electricity –
(a) Static electricity: Static electricity causes physical effects due to the charge at rest.
(b) Current or dynamic electricity: Current electricity causes physical effects due to the moving charge.
2. Conductors: It is a material which allows the current (charge) to pass through it, e.g., aluminium, copper, silver.
3. Insuators: It is a material which does not allow the current (charge) to pass through it, e.g., rubber, wood, plastic.
4. Electric current: Electric current is the rate of flow of charge.
If a net charge Q flows across any cross-section of a conductor in time t, then the current/, through the cross-section is
The SI unit of electric charge is coulomb (C).
5. Electric potential and potential difference: When a unit test charge is placed outside an electric field which is produced by another charge it experience no force on it. When this test charge is brought inside the field, some work is done because the unit positive charge experiences a force which becomes more and more when the test charge moves near to the given charge. This amount of work done is known as electric potential.
Potential difference is the difference between electric potential of two distinct points inside an electric field. Suppose A and B are two points inside an electric field. When a unit positive test charge moves from A to B, some work is done. This amount of work done is known as potential difference.
6. Circuit diagram: It is a closed conducting path containing a source of potential difference or electric energy and a device utilising the electric energy.
7. Ohm’s law: According to Ohm’s Law, “Electric current is directly proportional to the potential difference between the two ends of a conductor at constant temperature” i.e.,
I oc V or V oc IorV = R I
8. Resistance: A conductor has large number of free electrons. When a potential
difference is maintained across the ends of a conductor, the free electrons drift from one end to the other end of the conductor. During their movement, they collide with each other. These collisions oppose the movement of free electrons from one end to the other end of the conductor. This opposition to the flow of free electrons due to the collision is called resistance. More is the collision suffered more is the resistance.
9. Factors affecting resistance of a conductor: There are four factors on which resistance ofa conductor i.e., wire depends. These are-
- Length of wire or conductor.
- Area of cross-section.
- Nature of material of wire.
- Temperature of a conductor.
10. Resistivity or specific resistance : Resistance of a conductor depends on length of wire and area of cross-section i.e.
P is a constant. This constant of proportionality is known as resistivity,
which is represented by P (rho). Resistivity, which is also known as specific resistance is also defined as the resistance offered by a conductor of length 1 m having area of cross-section lm2.
11. Resistance of alloy: It is found practically that the resistance of alloy is more than the resistance of its constituent metals. It means alloys have higher resistivity than their constituent metals, e.g., nichrome which is an alloy of nickel and chromium has very high resistivity than its constituent metals i.e., nickel and chromium.
12. Resistance of a system of resistors: Resistances can be connected in two ways:
(i) Combination of resistances in series: When two or more resistances are connected end to end, they are said to be in series. When two or more resistances are connected in series, then current flowing in the circuit remains same.
R= R1 + R2 + R3 + ……………….
(ii) Combination of resistances in parallel: If two or more resistances are connected to the same end, they are said to be in parallel.
13. Heating effect of electric current: When the electric current is passed through a conductor then conductor gets heated, this effect is known as the heating effect of electric current.
Joule’s law of heating: It states that the amount of heat produced in a resistor is
Practical applications of heating effect of electric current
- Electric heater, electric iron and water heater, etc. works on the principle of the heating effect of current.
- Electric bulb glows when electric current flows through the filament of the bulb.
- Electric fuse in the electric circuit melts when large current flows in the circuit.
14. Electric power: The rate at which electric energy is consumed is called electric power.
where P is the power, I is current flowing, V is the potential difference and R is the resistance.