Haryana State Board HBSE 10th Class Science Notes Chapter 12 Electricity Notes.

## Haryana Board 10th Class Science Notes Chapter 12 Electricity

**Electric Current and Circuit**

- Electric charge and conduction of electricity:
- ‘Charge’ or ‘electric charge’ is the fundamental quantity of electricity.
- Electric charge or simply charge is of two types, (1) Positive (+) charge and (2) negative (-) charge.
- If electric charge flows through a conductor such as metal wire, we say that there is electric current in the conductor. The S.l. unit of electric charge is columb ‘C’.
- An electron possesses a ‘negative charge of 6 x 10
^{-19}C whereas a proton possesses a ‘positive charge of 6 x 10^{-19}C’. - Free (or conducting) electrons are responsible for conducting electricity or say conduction.
- Electric current can flow very easily through material which contain a large number of free electrons. Hence such a material is called a conductor. For example, metals such as copper, silver and aluminium are conductors.

Electric current:

The rate of flow of electric charge is known as electric current. In other words, the net quantity of electric charge that flows through any crosssection of a conductor is known as the electric current.

Thus, electric current \(=\frac{\text { Quantity of electric charge }}{\text { Time }}\)

If Q is the amount of electric charge passing through any cross-section of a conductor in time then, electric current \((I)=\frac{Q}{t}\)

SI unit of electric current is Coulomb/second (C/s). Electric current is also measured in Ampere (A).

Electric circuit:

A continuous and closed path along which electric current flows is called an electric circuit.

**Electric Potential and Potential Difference**

Electric potential difference:

The amount of work done to take a unit positive charge from a given point say A to point B in a circuit carrying some current is known as the electrical potential difference between the two points.

Thus, electric potential difference

\((V)=\frac{\text { Work done }(W)}{\text { Electric charge }(Q)}\) \(V=\frac{W}{Q}\)

In practice, electric potential difference is known as voltage. Its S.I unit is joule/coulomb or volt (V).

If the work done to bring 1 coulomb electric charge from one point to another is 1 joule, then the potential difference between these two points is called 1 volt.

\(\text { Thus, } 1 \text { volt }=\frac{1 \text { joule }}{1 \text { Coulomb }} \quad \mathrm{~V}=1 \mathrm{JC}^{-1}\)

Ohm’s law:

- In a definite physical situation, the electric current (I) flowing through the conductor is directly proportional to the potential difference (V) applied across it, provided its temperature and other physical conditions remain same.
- Thus, I α V or say V α I.

∴ V = I R (where, R is the proportionality by constant and it represents resistance R of the circuit) - The SI unit of resistance (R) is volt/ampere which is called ohm and it is denoted by Ω. (Omega).

∴ \(1 \mathrm{Ohm}(\Omega)=\frac{1 \text { volt }(\mathrm{V})}{1 \text { ampere }(\mathrm{l})}\)

**Factors on which the Resistance of a Conductor Depends**

The resistance (R) of a conductor is directly proportional to the length of the conductor and inversely proportional to the area of its cross-section.

Resistance R α length l and also R α \(\frac{1}{A}\)

∴ \(\mathrm{R} \alpha \frac{1}{\mathrm{~A}}=\rho \frac{1}{\mathrm{~A}}\)

∴ \(R \alpha \frac{1}{A}=\rho \frac{1}{A}\) (where ρ (rho) is constant and called the resistivity of conducting material.)

We can also say, \(\rho=\mathrm{R} \frac{\mathrm{A}}{l}\)

The S.l unit of resistivity ρ =Ωm (Ohm meter)

At constant temperature, the resistance of a conductor depends on:

- Length (L) i.e. R α L,
- Area of cross-section (A) i.e. \(R \propto \frac{1}{A}\) and
- Nature of material.

Combining all these factors we get:

\(R \propto \frac{L}{A} \text { or say } R=\rho \frac{L}{A}\)

**Resistance of a Sprem of Resistors**

Joining resistors in a series connection:

- When two or more resistors are connected end- to-end consecutively, such a connection is called a series connection.
- In a series connection, equivalent resistance

R_{s}= R_{1}+ R_{2}+ R_{3}

Joining resistors in a parallel connection:

When two or more resistors are connected between the same two points of a circuit, they are said to be connected in a parallel connection (because they are connected in parallel and not end-to-end).

In a parallel connection, equivalent resistance is denoted as \(\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}\)

Heating effect of electric current:

- When electric current is passed through a high resistance wire, the resistance wire becomes hot and produces heat. Here, electrical energy is converted into heat energy which is known as the heating effect of electric current.
- The energy spent by the source of the work done by the source can be calculated as

W=l.R x I.t =l^{2}Rt.

Thus, heat energy (H) = I^{2}Rt

**Practical Applications of Heating Effect of Electric Current**

Practical (daily life) applications of heating effect of electric current:

- Household heating appliances: Electric iron, toaster, sandwich maker, room heater, electric kettle, etc. all such appliances make use of heating effect of electric current.
- Electric bulbs: When electricity is passed through the filament of the electric bulbs, the bulbs light up. Here, heating effect is used foremitting light.
- Electric fuse: Electric tuse is a safety device which works on the heating effect of electric current.

Electric power:

- The electrical energy consumed (or heat energy generated) in unit time is called electric power.
- In other words, electric power is the rate of electric energy. It is denoted by R.

∴ Power P \(\mathrm{P}=\frac{\text { Electric energy consumed }}{\text { Time }}\)

\(=\frac{W}{t}=\frac{l^2 R t}{t}\)

(∴ W = electrical energy = I^{2}Rt)

∴ P=I^{2}R

The SI unit of power is joule/second or watt (W).

Practical unit of electrical energy:

Power \(\mathrm{P}=\frac{\text { electric energy }(\mathrm{W})}{\text { time }(\mathrm{t})}\)

∴ Electrical energy (W) = Power (P) x time (t)

= watt x second

= joule

Thus, the unit of electrical energy is watt. second.

The larger unit of electrical energy is kWh.

1 kWh = 1000 watt x 3600 seconds

= 3.6 x 10^{6} joules (J)