Electric Circuits: Series and Parallel Concepts

Current

Same through all components in a series circuit.

Potential Difference

Shared across components; sum of individual voltages equals source voltage.

Resistance

Total resistance in a series circuit is calculated as R=R1+R2+....

Total Voltage in Series Cells

Total voltage is the sum of individual cell voltages (e.g., three 1.5V cells in series = 4.5V).

Total Resistance of Resistors in Series

Example: R1=3Ω, R2=5Ω → Rtotal=3+5=8Ω.

Adding Resistors in Series

More resistors create a longer path for electrons, resulting in greater opposition.

Current in Parallel Circuit

Current splits across branches.

Potential Difference in Parallel Circuit

Same across each branch.

Current through Resistor in Parallel Circuit

Example: V=12V, R=6Ω → I=12/6=2A.

Total Resistance of Resistors in Parallel

Total resistance is less than the resistance of the smaller individual resistor due to more pathways reducing total opposition: 1/Rtotal=1/R1+1/R2.

Adding Resistors in Parallel

Additional branches provide alternative current paths, lowering overall resistance.

Reversing Potential Difference Across Resistor

Current flows in the opposite direction; resistance remains constant for ohmic resistors.

Resistance of Filament Lamp with Temperature Increase

Resistance increases due to increased electron-ion collisions.

Current through a Diode

Conducts only in forward direction above threshold voltage; negligible current in reverse.

Resistance of Thermistor with Temperature Increase

Resistance decreases for NTC thermistors.

Resistance of LDR with Light Level Increase

Resistance decreases.

Direct Current (DC)

Flows in one direction (e.g., batteries).

Alternating Current (AC)

Reverses direction periodically (e.g., mains: 230V, 50Hz).

Live Wire in Mains Circuit

Carries alternating voltage (230V in Europe).

Neutral Wire in Mains Circuit

Completes the circuit at approximately 0V.

National Grid

A system for distributing electrical power across a region.

Network of power stations

A network of power stations, transformers, and cables distributing electricity nationwide.

Casing of a mains plug

Material: Insulating plastic/rubber. Purpose: Prevents electric shocks.

Mains cable contents

Live, neutral, earth wires + insulation.

Wire colors in mains cable

Live: Brown, Neutral: Blue, Earth: Green/Yellow.

Earth pin in three-pin plug

To safely divert fault current to the ground, preventing electric shock.

Electric circuit diagrams

Using standardized symbols (e.g., ⏚ for cell, ⏛ for resistor).

Difference between battery and cell

Cell: Single unit (e.g., AA). Battery: Multiple cells connected in series.

Determining electric current size

Charge flow (QQ) and time (tt): I=Qt I=tQ.

Calculate current from charge flow and time

Example: Q=10C, t=2s → I=5A.

Ohm's Law

V=I×R.

Useful energy

Energy transferred to perform desired work (e.g., light from a bulb).

Wasted energy

Energy not used for the intended purpose (e.g., heat from a bulb).

Fate of wasted energy

Dissipated as heat into the surroundings.

Usefulness of energy after use

No—it becomes less concentrated (e.g., heat energy in a room).

Efficiency

Efficiency=(Useful Output Energy / Total Input Energy)×100%.

Maximum efficiency of energy transfer

Always <100% due to wasted energy.

How machines waste energy

Through heat, sound, or friction.

Energy supply to homes

Via the National Grid: power stations → transformers → homes.

Usefulness of electrical appliances

Convert electrical energy into useful forms (e.g., light, motion).

Everyday electrical appliances

Lighting, heating, cooling, communication, etc.

Choosing an appliance for a job

Consider power rating, efficiency, and safety.

Power

Rate of energy transfer: P=Et. Unit: Watts (W).

Calculate power of an appliance

Use P=IV, P=I²R, or P=V²/R.

Calculate efficiency in terms of power

Efficiency=(Useful Power Output / Total Power Input)×100%.

Calculate power wasted by an appliance

Pwasted=Ptotal−Puseful.