Circuit Components
Your TV remote runs on two AA cells (a battery). The small LED inside flashes infrared light when you press a button — that's why it only works when pointed at the TV. The LED current flows in one direction only, just like we learnt!
The word "battery" technically means multiple cells connected together — but we use it for a single cell too (like in your phone). Benjamin Franklin first used the word "battery" for a group of capacitors he was experimenting with!
When a filament "fuses" in a bulb, the thin tungsten wire breaks inside. This creates a gap in the circuit (open circuit), stopping current flow. That's why the bulb goes dark even though the battery and switch are fine.
Types of Circuits
Open Circuit
A circuit with a break in the path. Current cannot flow. Switch in OFF position = open circuit. A broken filament also creates an open circuit.
Closed Circuit
A complete, unbroken path for current to flow. Switch in ON position = closed circuit. The lamp glows only in a closed circuit.
Series Circuit
All components in a single loop. Current is the same at every point. Voltage divides across components. If one component fails, the entire circuit breaks.
Parallel Circuit
Components connected on multiple branches. Voltage is the same across each branch. If one branch fails, others keep working. Your home is wired this way!
Short Circuit
A path of zero or very low resistance created accidentally. Draws massive current (could be 1500 A!). Can melt wires or start fires. Circuit breakers protect against this.
Parallel: multiple branches
Parallel: divides at branches
Parallel: same across all branches
Parallel: other branches still work
Cheap fairy/festival lights are in series — if one bulb fuses, all go out. Premium ones are parallel — if one fails, the rest glow on. That's physics saving your Diwali decoration!
Your body's nervous system is a network of electric circuits. Motor nerves carry electrical signals from the brain to muscles. A spinal cord injury creates an "open circuit" — the signal can't reach the muscle, causing paralysis. This is exactly why doctors call it nerve damage!
Key Formulas
V = Voltage (Volts, V) ·
I = Current (Amperes, A) ·
R = Resistance (Ohms, Ω)
If voltage increases, current increases (resistance constant). If resistance increases, current decreases (voltage constant).
A hair dryer connected to 120V draws 10 A of current. Its resistance = 120 ÷ 10 = 12 Ω. Double the voltage, double the current — that's why using a 220V appliance on a 110V socket makes it run slower!
P = Power (Watts, W) ·
V = Voltage (V) ·
I = Current (A)
Power tells you how fast energy is being used. 1 Watt = 1 Joule per second. A 100W bulb uses 100 joules of energy every second.
An LED bulb uses only 7–10 watts to produce the same brightness as a 60-watt incandescent bulb. That's 6× less power for the same light! Over a year, switching to LEDs can save hundreds of rupees on your electricity bill.
E = Energy (Joules, J) ·
P = Power (W) ·
t = Time (seconds)
Electricity companies charge in kilowatt-hours (kWh). 1 kWh = 1000W × 3600s = 3.6 million joules!
In series, resistances add up — total resistance increases. In parallel, total resistance is always less than the smallest individual resistance — more paths = easier flow.
When you plug multiple appliances into the same socket (parallel), the total resistance decreases and total current increases. If you plug in too many, the current exceeds 15–20 A and the circuit breaker trips — a safety mechanism preventing fires!
Energy supplied by battery equals energy consumed by all resistors. In a 3V circuit with 3 equal bulbs, each gets 1V — the energy is shared.
In parallel circuits, the total current from the battery equals the sum of all branch currents. Current is conserved — it never disappears or is created.
Laws & Principles
Ohm's Law
This is the single most important relationship in circuit analysis. It works for most metallic conductors. Devices that obey Ohm's Law show a straight line on a V–I graph. LEDs and diodes do NOT obey Ohm's law.
Kirchhoff's Voltage Law (KVL)
This is simply the law of conservation of energy applied to circuits. Energy given by the battery = energy taken by all resistors. Battery raises voltage, resistors lower it. Total change = zero.
Kirchhoff's Current Law (KCL)
This is conservation of charge. Electric charge cannot be created or destroyed. At every branch point, what flows in must flow out. Like water in a network of pipes — it can split but never vanish.
Electromagnetic Induction
Moving a magnet through a coil of wire generates current. This is how all generators and power plants produce electricity. The faster the magnet moves, the greater the induced current.
The Bhakra Nangal Dam (Punjab/Himachal Pradesh) uses electromagnetic induction at a massive scale. Falling water spins giant turbines, rotating magnets inside wire coils — generating gigawatts of electricity that powers millions of homes across North India.
For Ohm's Law to apply: Temperature must remain constant. Works best for metallic conductors at fixed conditions.
For Kirchhoff's Laws: Applies to all circuits — series, parallel, or complex. Essential for analysing multi-loop circuits.
For all circuit calculations: We usually ignore the resistance of connecting wires and batteries unless stated otherwise. This is a standard simplifying assumption.
The resistance of metals increases with temperature. That's why a tungsten filament has much higher resistance when hot (glowing) than when cold. Some materials become superconductors at extremely low temperatures (near −269°C) — their resistance drops to exactly zero! Hospitals use superconducting magnets in MRI machines.
Units & Constants
| Quantity | Unit | Symbol | Named After | Definition |
|---|---|---|---|---|
| Electric ChargeProperty of matter responsible for electricity | Coulomb | C | Charles-Augustin de Coulomb (1783) | Charge carried by ~6.24 × 10¹⁸ electrons |
| Electric CurrentFlow of electric charge | Ampere (Amp) | A | André-Marie Ampère (1775–1836) | 1 A = 1 coulomb per second |
| Voltage (Potential Difference)Energy difference per unit charge | Volt | V | Alessandro Volta (1745–1827) | 1 V = 1 joule per coulomb |
| ResistanceOpposition to current flow | Ohm | Ω | Georg Simon Ohm (1787–1854) | 1 Ω = 1 V per 1 A of current |
| PowerRate of energy transfer | Watt | W | James Watt (1736–1819) | 1 W = 1 joule per second |
| Energy (Electrical)What utility companies sell | Kilowatt-hour | kWh | — | 1 kWh = 1000 W × 3600 s = 3.6 MJ |
| Energy (Physics)SI unit of energy | Joule | J | James Prescott Joule (1818–1889) | 1 J = 1 W·s = energy to lift 102g by 1m |
| Electrical ConductivityAbility to conduct current | Siemens per metre | S/m | Ernst Werner von Siemens | Inverse of resistivity |
| Frequency (AC current)How often current reverses direction | Hertz | Hz | Heinrich Hertz (1857–1894) | India AC supply: 50 Hz (50 reversals/sec) |
The force between 1 coulomb of positive and 1 coulomb of negative charge placed 1 metre apart is 9 billion newtons — roughly 10 times the weight of the entire Earth! This is why atoms stay together so strongly, and why separating charges in a storm cloud creates lightning.
When MSEB/BEST charges you per unit, they mean per kWh. If you run a 1000W AC for 10 hours = 10 kWh = 10 units. At ₹7 per unit, that's ₹70. Knowing P = VI helps you estimate and save on your electricity bill!
Conductors & Insulators
Metals are good conductors because their atoms release "free" electrons that can move through the material. Insulators keep electrons tightly bound to their atoms — no free electrons means no current flow.
✅ Good Conductors
- Silver (best, most expensive)
- Copper (used in wiring)
- Gold (used in electronics)
- Aluminium
- Iron / Steel
- Tungsten (filament)
- Carbon / Graphite (pencil lead!)
- Human body (⚠️ danger!)
- Water (with impurities)
❌ Insulators (Poor Conductors)
- Rubber (used to cover wires)
- Plastic / PVC
- Glass
- Wood
- Paper
- Wax / Candle
- Air (breaks down at very high voltage)
- Ceramics / Porcelain
- Pure distilled water
Semiconductors
Not full conductors, not full insulators. Silicon and germanium are the most common. Computer chips, LEDs, solar cells, and transistors are all made from semiconductors. They can be "switched" between conducting and not conducting — the foundation of all digital electronics.
Superconductors
At extremely low temperatures (near absolute zero), some materials lose ALL resistance. Current flows forever with zero energy loss. Used in MRI machines and particle accelerators. Scientists are working on room-temperature superconductors — it would revolutionise energy transmission.
Silver conducts better than copper but is too expensive. Aluminium is cheaper but less flexible and has higher resistance. Copper strikes the perfect balance — excellent conductivity, abundant supply, affordable cost, and easy to draw into thin wires. That's why 90% of electrical wiring worldwide uses copper.
Your pencil lead is actually graphite (a form of carbon), which is a semiconductor. You can use it as a simple resistor in a circuit! The longer and thinner the pencil stroke, the higher the resistance. Ancient electrical experiments used carbon rods for exactly this purpose.
Ohm's Law Calculator
Solve for V, I, or R
Fill in any two fields and click the button to find the third.
Power Calculator
Calculate electrical power (P = V × I)
Electricity Bill Estimator
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Use the Ohm's Law calculator to verify: A hairdryer uses 10 A on a 120 V circuit. What's its resistance? (Answer: R = 120 ÷ 10 = 12 Ω). Now try the Bill Estimator with a 2000W geyser running 1 hour a day. You might be surprised!