Introduction to Electricity
Electricity is a fundamental form of energy that powers our modern world. In this chapter, we'll explore:
- Basic concepts of electric charge and current
- Circuit components and Ohm's Law
- Resistance and its factors
- Practical applications of heating effect
- Electrical power and energy calculations
Electric Charge
Charge is the fundamental property of matter that causes it to experience a force in an electric field.
Key Concepts:
NCERT Example: How many electrons make 1 Coulomb?
n = Q/e = 1C/(1.6×10-19C) ≈ 6.25 × 1018 electrons
Electric Current
The flow of electric charge constitutes electric current.
I = Q/t
Where:
I = Current (Ampere, A)
Q = Charge (Coulomb, C)
t = Time (seconds, s)
Key Points:
- Direction: Conventional current flows from positive to negative terminal (opposite to electron flow)
- Ammeter: Always connected in series to measure current
- 1 Ampere: Flow of 1 Coulomb charge per second
- Types:
- Direct Current (DC): Flows in one direction (cells, batteries)
- Alternating Current (AC): Changes direction periodically (household supply)
[NCERT Diagram: Fig 12.1 - Conventional current vs electron flow]
NCERT Definition: The amount of charge flowing through a cross-section of a conductor in unit time is called current.
Electric Potential and Potential Difference
Electric Potential:
- Work done to bring unit positive charge from infinity to a point
- Unit: Volt (V)
Potential Difference:
The difference in electric potential between two points in a circuit.
V = W/Q
Where:
V = Potential difference (Volts, V)
W = Work done (Joules, J)
Q = Charge (Coulomb, C)
NCERT Activity 12.1: The potential difference between two points is 1V if 1J of work is done to move 1C of charge between them.
Key Points:
- Voltmeter measures potential difference (connected in parallel)
- Battery maintains potential difference in a circuit
- Positive terminal is at higher potential than negative terminal
[NCERT Diagram: Fig 12.2 - Measuring potential difference]
Electric Circuit Components
An electric circuit consists of various components connected with conducting wires:
| Component |
Symbol |
Function |
Connection |
| Cell |
| | (long line is +ve) |
Provides potential difference (1.5V) |
- |
| Battery |
| | | (multiple cells) |
Higher voltage source |
- |
| Switch |
⎔— (open/closed) |
Controls current flow |
Series |
| Resistor |
⎺⎺⎺⎺⎺ or ⏢⏢⏢ |
Limits current |
Series/Parallel |
| Ammeter |
A in circle |
Measures current |
Series |
| Voltmeter |
V in circle |
Measures voltage |
Parallel |
| Bulb |
⨀ with × inside |
Converts electrical energy to light |
Series/Parallel |
[NCERT Diagram: Fig 12.3 - Simple circuit diagram]
Circuit Conditions:
- Open circuit: No current flows (switch open)
- Closed circuit: Current flows (switch closed)
- Short circuit: Very high current flows (dangerous)
Ohm's Law
At constant temperature, the current (I) through a conductor is directly proportional to the potential difference (V) across its ends.
V ∝ I ⇒ V = IR
Where R is the constant of proportionality called resistance.
NCERT Verification Activity (12.2):
- Set up circuit with resistor, ammeter, voltmeter, battery and rheostat
- Record current (I) for different voltages (V) by adjusting rheostat
- Plot V-I graph → Straight line verifies Ohm's law
- Slope gives resistance (R)
[NCERT Diagram: Fig 12.6 - Ohm's law verification setup]
Resistance (R):
- Opposition to flow of current
- SI unit: ohm (Ω)
- Slope of V-I graph gives resistance
R = V/I
Limitations:
- Valid only when physical conditions (especially temperature) remain constant
- Doesn't apply to semiconductors, electrolytes, gases
Factors Affecting Resistance
As per NCERT experiments, resistance depends on:
| Factor |
Relation |
Explanation |
Formula |
| Length (L) |
R ∝ L |
Longer wire → more collisions → higher resistance |
R = ρ(L/A)
ρ = resistivity |
| Area (A) |
R ∝ 1/A |
Thicker wire → more space for electrons → lower resistance |
| Material (ρ) |
Depends on ρ |
Silver (best conductor) → Copper → Tungsten (high ρ) |
ρ = RA/L |
| Temperature |
Metals: R increases
Semiconductors: R decreases |
Metals: Increased vibrations obstruct flow
Semiconductors: More charge carriers |
- |
Resistivity (ρ):
- Characteristic property of material
- Independent of dimensions
- Unit: Ωm (ohm-meter)
- Increases with temperature for metals
[NCERT Diagram: Fig 12.5 - Resistance depends on length and area]
Resistor Combinations
Series Combination (NCERT Fig 12.7):
- Same current flows through all resistors
- Total voltage gets divided (V = V₁ + V₂ + V₃)
- Equivalent resistance is sum of all resistances:
Req = R₁ + R₂ + R₃ + ...
- Used when higher resistance needed
Parallel Combination (NCERT Fig 12.8):
Practical Applications:
- Household wiring: All appliances connected in parallel (each gets full voltage)
- Decorative lights: Sometimes in series (but if one fails, all stop working)
- Resistance boxes: Use combination to get desired resistance
[NCERT Diagram: Fig 12.7 & 12.8 - Series and parallel combinations]
Heating Effect of Electric Current (Joule's Law)
When current flows through a conductor, heat is produced due to resistance.
H = I2Rt
Where:
H = Heat energy (Joules, J)
I = Current (Amperes, A)
R = Resistance (Ohms, Ω)
t = Time (seconds, s)
Derivation:
- Work done to move charge Q: W = VQ
- Since I = Q/t ⇒ Q = It
- From Ohm's law: V = IR
- Therefore: W = VIt = I2Rt
- This work appears as heat: H = I2Rt
Practical Applications (NCERT Examples):
- Electric heater: Nichrome wire (high ρ) gets red hot
- Fuse: Thin wire melts when excess current flows (safety device)
- Light bulb: Tungsten filament heats up to glow (melting point 3380°C)
- Electric iron: Heating element converts electrical energy to heat
- Electric kettle: Heats water quickly
Why Tungsten in Bulbs?
- High melting point (3380°C)
- High resistivity
- Doesn't oxidize easily in vacuum/argon
Electric Power
Rate at which electric energy is consumed or dissipated.
P = W/t = VI = I2R = V2/R
Unit: Watt (W) = Joule/second
Commercial Unit of Energy:
Electricity bills use kilowatt-hour (kWh) instead of joule:
1 kWh = 1000 W × 1 hour = 1000 W × 3600 s = 3.6 × 106 J
1 kWh = 1 "unit" of electricity
Power Rating Examples:
- LED bulb: 5-15 W
- Fan: 50-80 W
- Geyser: 2000-3000 W
- AC: 1500-2500 W
NCERT Example 12.8:
An electric bulb is connected to a 220V generator. The current is 0.50A. What is the power of the bulb?
P = VI = 220 × 0.50 = 110 W
Energy Conservation Tip:
Using a 15W LED bulb (equivalent to 100W incandescent) for 5 hours daily:
Daily consumption = 15W × 5h = 75 Wh = 0.075 kWh
Monthly savings = (100W-15W) × 5h × 30 = 12.75 kWh
Important NCERT Questions
- Define 1 ampere current. (Q12.1)
- Why are coils of electric toasters made of alloy rather than pure metal? (Q12.4)
- Compute heat generated while transferring 96000C through 1Ω in 1 hour. (Q12.7)
- Compare power used in 2Ω resistor in both circuits (series vs parallel). (Q12.8)
- What determines the rate at which energy is delivered by a current? (Q12.5)
- An electric motor takes 5A from 220V line. Determine power and energy consumed in 2h. (Q12.11)
Numerical Problem Solving Approach:
- Identify given quantities and what to find
- Choose appropriate formula
- Convert units if necessary
- Substitute values and calculate
- Include proper units in final answer
Example Solution (Q12.7):
Calculate heat produced when 96000C charge is transferred in 1 hour through a potential difference of 50V.
Given: Q = 96000C, t = 1h = 3600s, V = 50V
I = Q/t = 96000/3600 = 26.67A
H = VIt = 50 × 26.67 × 3600 = 4.8 × 106 J
Alternative: H = VQ = 50 × 96000 = 4.8 × 106 J
Chapter Summary
| Concept |
Formula |
Unit |
| Current |
I = Q/t |
Ampere (A) |
| Potential Difference |
V = W/Q |
Volt (V) |
| Ohm's Law |
V = IR |
- |
| Resistance |
R = ρL/A |
Ohm (Ω) |
| Resistivity |
ρ = RA/L |
Ωm |
| Series Combination |
Req = R₁ + R₂ + ... |
Ω |
| Parallel Combination |
1/Req = 1/R₁ + 1/R₂ + ... |
Ω |
| Heating Effect |
H = I2Rt |
Joule (J) |
| Electric Power |
P = VI = I2R = V2/R |
Watt (W) |
| Electrical Energy |
E = Pt = VIt |
kWh |