# Short Notes & Video Lectures : Electricity ## What is Electric Current ?

• Flow of charges constitutes electric current.
• It is measured as rate of flow of charge.
• Denoted by “I”.
• S.I unit = Ampere.

## Direction of Current

Electronic currents flow from negative(-) terminal to positive(+) terminal of the battery , whereas conventional current flows from (+) to (-).

## Circuit Symbols # Ohm’s Law

Current flowing through a conductor is directly proportional to the potential difference applied between the ends of the conductor , provided temperature remains constant. V=IR

V = Potential Difference

I = Current

R = Resistance

## Resistance

• It resists the flow of charges.
• It is denoted by “R”.

## Factors on which Resistance depend

• Length of the conductor
• Cross Sectional area of the conductor
• Material of the conductor
• Temperature of the conductor

# R = ρ l/a

L = length

A = Area of cross section

ρ= Resistivity

### Combination of Resistors

• Series – In series circuit electrons travel only in one path. Here the current will be the same which passes through each resistor. The voltage across resistor in a series connection will be different. In series connection if one resistor is broken or any fault occurs, the entire circuit is turned off. Series circuits do not overheat easily. The design of series circuit is simple compared to parallel circuits.
• Parallel – In parallel circuit electrons travel through many branches in it. In this case the voltage remains the same across each resistors in the circuit. Here the current in the circuit is divided among each branches and finally recombines when the branches meet at a common point. A parallel circuit can be formed in many ways, which means resistors can be arranged in different forms. It can be used as a current divider.

### Series Combination Current is same in all resistors , potential difference is different across each resistor.

## Parallel Combination Potential difference across each resistor is same but current is different.

## Heating effect of Current

If an electric circuit is purely resistive (only resistors are connected to a battery), the energy from the source continually gets dissipated totally in the form of heat. This effect is called as heating effect of electric current.

#### H=VIT

where,

V= Potential Difference

I = Current

T = Time

## Joule’s Law of Heating

• Directly proportional to
1. square of the current
2. to the resistance
3. Directly proportional to the time

## Practical Applications

Heat produced by the electric circuits is used in many appliances , sometimes heat is used to produce light . Light bulb is an example of light produced by the electric heat . Below are some examples :

Electric Oven Electric Heater ## Power

Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.

#### P=VI

Where ,

P = Power input

V = Potential Difference

I = Current

### Some Important Questions :

1. Question: Two bulbs have ratings 100 W, 220 V and 60 W, 220 V respectively. Which one has a greater resistance?

Answer: P=VI=  V2/R For the same V, R is inversely proportional to P.

Therefore, the bulb 60 W, 220 V has a greater resistance.
2. Question: A torch bulb has a resistance of Ω when cold. It draws a current of 0.2 A from a source of 2 V and glows. Calculate
(i) the resistance of the bulb when glowing and
(ii) explain the reason for the difference in resistance.
(i) When the bulb glows:
V = I R —- Ohm’s law R = V/I = 2/.2 =10 Ω

(ii) Resistance of the filament of the bulb increases with increase in temperature. Hence when it glows its resistances is greater than when it is cold.

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