Grade 9||Electricity|| Notes

1. Electricity

Electricity is a major source of energy.
It is transmitted through conducting wires.
Electrical appliances convert electrical energy into other forms of energy like heat, light, or motion.

Examples:

1. Heater → heat
2. Bulb → light and heat
3. Fan → motion

The rate of energy conversion differs for different appliances.
A meter is used to measure electrical energy consumption to calculate the electricity bill.

2. Electric Current

Electric current is the flow of electric charges through a conductor.

Cause of Current

1. Metals have free electrons
2. These electrons move easily due to weak nuclear attraction
3. In solutions, positive and negative ions move freely

When a battery is connected, charges move in a particular direction, producing electric current.

3. Definition of Electric Current

Electric current is defined as the rate of flow of electric charge per unit time.

Where:
I = current
Q = charge
t = time

4. Units

1. SI unit of charge = Coulomb (C)
2. SI unit of current = Ampere (A)

1 ampere = flow of 1 coulomb per second

Smaller units:

1. 1 mA = 10⁻³ A
2. 1 μA = 10⁻⁶ A

Current is measured by an ammeter, connected in series.

5. Direction of Current

1. Conventional direction: from positive terminal to negative terminal
2. Electron flow: from negative to positive

Exams prefer the conventional direction.

6. Electromotive Force (EMF)

Electromotive force is the energy supplied by a source per unit charge.

1. Unit: Volt (V)
2. Provided by cells, batteries, generators
3. It is the cause of potential difference
4. Independent of external resistance

7. Potential Difference (PD)

Potential difference is the energy transformed when a unit charge flows through a conductor.

1. Unit: Volt (V)
2. It is the effect of EMF
3. Depends on resistance of the circuit

Measured by a voltmeter, connected in parallel.

8. Difference Between EMF and Potential Difference

EMFPotential DifferenceEnergy supplied by sourceEnergy used in circuitCauseEffectIndependent of resistanceDepends on resistanceMeasured without currentMeasured with current

9. Meaning of 1.5 V on a Cell

It means the cell provides 1.5 joule of energy per coulomb of charge.

10. Ohm’s Law

According to Ohm’s law, current through a conductor is directly proportional to the potential difference, provided temperature and physical conditions remain constant.

Where:

R = resistance

11. Resistance

Resistance is the opposition offered by a conductor to the flow of current.

​

SI unit: Ohm (Ω)

1 Ω = resistance when 1 A flows under 1 V

12. Factors Affecting Resistance

Length – longer wire → more resistance

Thickness – thinner wire → more resistance

Material – nichrome > copper

Temperature – higher temperature → higher resistance

13. Combination of Resistors

Series Combination

Same current flows through all

Potential difference is divided

Total resistance:

Used in decorative lights

If one fails, all stop working

Parallel Combination

Same potential difference across all

Current is divided

Total resistance:

Used in domestic wiring

If one fails, others still work

If resistors of 10 2, 20 2 and 30resistance are connected with a 12 V battery at first in series combination and then in parallel combination. Calculate the current in both cases and compare these values. Science and Technology, Grade 9 183 According to the given question, Resistance of the first resistor (R1) =10 Resistance of the second resistor (R2) = 20 Ω Resistance of the third resistor (R3) = 30 Potential difference (V) = 12 V

Series CombinationParallel CombinationResistors are connected end to endResistors are connected side by sideSame current flows through all resistorsSame voltage across all resistorsTotal resistance is highTotal resistance is lowIf one resistor fails, the circuit breaksIf one resistor fails, others still workUsed in decorative lightsUsed in domestic wiring

Formula for Total Resistance

SeriesParallel

14. Heating Effect of Electric Current

Conversion of electrical energy into heat due to resistance is called the heating effect of electric current.

Examples:

1. Heater
2. Iron
3. Rice cooker
4. Kettle

Nichrome is used because:

1. High resistance
2. High melting point
3. Does not oxidize easily

15. Electric Power

Electric power is the rate of energy conversion per second.

Unit: Watt (W)

1 W = 1 J/s

Meaning of 5 W:

Converts 5 joules per second

16. Electric Energy Consumption

Energy consumed depends on:

Power

Time of use

Commercial unit:

kilowatt-hour (kWh)

1 unit = 1 kWh = 3.6 × 10⁶ J

17. Electricity Bill

Electric bill is calculated by:

Where:

P = power (kW)

n = number of appliances

t = time (hours)

Monthly bill = total units × cost per unit