# Electricity | Revision Notes

# Revision Notes

## Electric Current

- Electric current is expressed as the amount of charge flowing through a particular area in unit time.
- Quantitatively,
**electric current**is defined as the rate of flow of electric charge.

- The S.I. unit of current is ampere (A), where 1 ampere = 1 coulomb/second.
- 1 mA = 10
^{−3}A, 1 μA = 10^{−6}A - The conventional direction of electric current is the one in which positive charges move orderly.

## Electric Potential Different

- A battery provides the driving force required to move the charges along the wire from one terminal to another.
- The chemical reaction within a cell generates the potential difference across the terminals of the cell.

This constant potential difference between the two terminals of the cell maintains a constant electric current through the circuit. Thus, in order to maintain a constant electric current, the cell has to spend the chemical energy stored in it. - Electric
**potential difference**between two points in an electric circuit, carrying some current, is the amount of work done to move a unit charge from one point to another.

where V is potential difference, W = workdone and Q = amount of charge moved.

- The S.I. unit of potential difference is volt (V), where 1 volt = 1 joule/coulomb.
**One volt**is the potential difference between two points in a current carrying conductor when**one joule of work**is done to move**a charge of one coulomb**from one point to another.- Potential difference is measured in a circuit by an instrument called the
**Voltmeter**. It is always connected**in parallel**across the points components where it is to be measured.

## Electric Circuit

- A continuous conducting path between the terminals of a source of electricity is called an electric circuit.
- A drawing showing the way various electric devices are connected in a circuit is called a circuit diagram.
- Some commonly used circuit elements are given below:

## Ohm’s law

- According to Ohm’s law, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends, provided its physical conditions remain the same.
*V ∝ I**V / I = Constant**V / I = R**V = IR***R**is a constant of proportionality called**resistance**of the conductor. **Resistance**is the property of a conductor to resist the flow of charges through it.- The S.I. unit of resistance is
**ohm**(**Ω**). - If we plot the V-I graph for a conductor, then it shows a linear nature.

- The slope of the graph gives the resistance of the conductor.
- If the potential difference across two ends of conductor is 1 V and the current through it is 1 A, then the resistance of the conductor is 1 Ω.
- From R = V / I , 1 ohm = 1 volt/ampere
- In practice, the current needs to be increased or decreased. This is achieved by a variable resistor called a
**rheostat**. - A conductor offering less resistance is called a
**good conductor**, and a conductor offering high resistance is called a**poor conductor**. An insulator offers a very**high resistance.**

### Factors on which the Resistance of a Conductor depends

The resistance of a conductor is

**Directly proportional to the length**(l) of the conductor.**Inversely proportional to the area of cross-section**(A) of the conductor.- Depends
**on the nature of the material**of the conductor. - Depends on the temperature.

## Resistivity

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

R ∝ l / A |

R = ρ l / A |

where **ρ** is a constant of proportionality called **specific resistance** or **resistivity** of the material of the conductor.

- The S.I. unit of resistivity is
**ohm metre**(**Ω m**).

## Combination of Resistances

### Resistances in Series

- When two or more resistors are joined from end to end, the resistances are said to be connected in
**series**.

- The current flowing through each resistance is the same.
- The potential difference is the sum of potential differences across all the individual resistors.

i.e. V = V_{1} + V_{2} + V_{3} ...(1)

were V_{1}, V_{2}, V_{2} are the potential difference across R_{1}, R_{2} and R_{3} respectively.

- The equivalent resistance (R
_{s}) of a series combination containing resistances R_{1}, R_{2}, R_{3}... is

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

- The equivalent resistance is greater than the
**greatest resistance in the combination**.

### Resistances in Parallel

- The potential difference across each resistance is the same and is equal to the potential difference across the combination.
- The main current divides itself, and a different current flows through each resistance.
- The equivalent resistance (R
_{p}) of a parallel combination containing resistances R_{1}, R_{2}, R_{3}... is given by

1 / R_{p} = 1 / R_{1} + 1 / R_{2} + 1 / R_{3} + ... |

- The equivalent resistance is lesser than the
**least of all the resistances**in the combination

## Heating Effect of Electric Current

- The effect of electric current due to which heat is produced in a conductor, when current passes through it, is called the heating effect of electric current.
- The total work (W) done by the current in an electric circuit is called electric energy and is given as

W = VIt = i^{2}Rt |

This energy is exhibited as heat. Thus, we have * H = VIt = i^{2}Rt*.

This is called

**Joule’s Law of Heating**, which states that

the heat produced in a resistor is directly proportional to the

- Square of the current in the resistor
- Resistance of the resistor
- Time for which the current flows through the resistance

### Practical Applications of the Heating Effects of Electric Current

- Electrical appliances like laundry iron, toaster, oven, kettle and heater are some devices based on Joule’s Law of Heating.
- The concept of electric heating is also used to produce light, as in an electric bulb.
- Another application of Joule’s Law of Heating is the fuse used in electric circuits.

## Electric Power

- Electric power is the rate at which electrical energy is produced or consumed in an electric circuit

P = Vi = i^{2}R |

P = V^{2}/ R |

The S.I. unit of power is watt (W).

**One watt**of power is consumed when 1 A of current flows at a potential difference of 1 V.- The commercial unit of electric energy is
**kilowatt hour**(**kWh**), commonly known as a unit.

1 kWh = 3.6 MJ |