Electricity runs our homes, offices and hydropower projects, so Lok Sewa science questions often ask about current, Ohm’s law, circuits, electric power, fuses, magnets and the units used to measure them. This note explains each idea in simple words with formulas, tables and examples, and lists the scientists and instruments that are asked again and again.
1. Electric charge and current
All matter is made of atoms. An atom has positively charged protons, negatively charged electrons and neutral neutrons. Like charges repel and unlike charges attract. The SI unit of charge is the coulomb (C). The charge of one electron is about 1.6 × 10⁻¹⁹ C.
Electric current is the rate of flow of electric charge. Current I = Q ÷ t, where Q is charge and t is time. Its SI unit is the ampere (A); 1 ampere = 1 coulomb per second. Current is measured with an ammeter, which is always connected in series. In metal wires, current is carried by moving electrons. By custom, conventional current flows from the positive terminal to the negative terminal, which is opposite to the flow of electrons.
2. Potential difference, resistance and Ohm’s law
Potential difference (voltage) is the work done to move a unit charge between two points. It pushes current through a circuit. Its unit is the volt (V) and it is measured with a voltmeter, which is connected in parallel.
Resistance is the opposition a conductor gives to the flow of current. Its unit is the ohm (Ω). The resistance of a wire increases with its length, decreases when it is thicker (larger cross-section), depends on the material, and for metals increases with temperature.
Ohm’s law (Georg Simon Ohm): at constant temperature, the current through a conductor is directly proportional to the potential difference across it. So V = I × R. Example: if a 12 V battery is connected to a 4 Ω resistor, the current is 12 ÷ 4 = 3 A.
3. Conductors, insulators and semiconductors
| Type | Meaning | Examples |
|---|---|---|
| Conductor | Allows current to flow easily | Silver (best), copper, aluminium, iron, salt water, the human body |
| Insulator | Does not allow current to flow | Rubber, plastic, glass, dry wood, mica, porcelain |
| Semiconductor | Conducts more than an insulator but less than a conductor; used in chips, diodes, transistors and solar cells | Silicon, germanium |
| Superconductor | Has zero resistance at very low temperature | Some metals and alloys cooled to very low temperatures |
Copper is used for household wires because it conducts well and is cheaper than silver. Tungsten is used for the filament of a bulb because of its very high melting point. Nichrome is used in heaters and electric irons because of its high resistance.
4. Electric circuits: series and parallel
A circuit is a complete path for current. It has a source (cell or battery), wires, a switch and a load (bulb, fan). A closed circuit lets current flow; an open circuit is broken and no current flows. A short circuit happens when current takes a path of almost no resistance, causing a large current, heat and danger of fire.
| Point | Series connection | Parallel connection |
|---|---|---|
| Arrangement | Components joined end to end in one path | Components joined across the same two points, in separate paths |
| Current | Same through every component | Divides among the branches |
| Voltage | Divides among the components | Same across every branch |
| Total resistance | R = R1 + R2 + R3 (increases) | 1/R = 1/R1 + 1/R2 + 1/R3 (decreases, less than the smallest) |
| If one bulb fuses | All bulbs go off | Others keep working |
| Example | Old decorative light strings | Household wiring |
Example: two 6 Ω resistors give 12 Ω in series but 3 Ω in parallel.
5. Electric power, energy and safety
Electric power is the rate at which electrical energy is used. P = V × I = I²R = V²/R. Its unit is the watt (W); 1 kilowatt (kW) = 1,000 W. Electrical energy = power × time. The SI unit of energy is the joule (J), but electricity bills use the kilowatt-hour (kWh), also called one unit. 1 kWh = 3.6 million joules (3.6 × 10⁶ J).
Example: a 100 W bulb used for 10 hours uses 100 × 10 = 1,000 Wh = 1 unit (1 kWh). Five such bulbs used for 4 hours a day for 30 days use 0.5 kW × 4 × 30 = 60 units.
- Heating effect of current (Joule’s law): heat produced depends on I²Rt. It is used in heaters, irons, kettles and bulbs.
- Fuse: a thin wire of low melting point (often a tin and lead alloy) connected in series with the live wire. It melts and breaks the circuit when too much current flows.
- MCB (Miniature Circuit Breaker): a switch that trips automatically on overload or short circuit and can be reset; it is replacing fuses.
- Earthing: the metal body of an appliance is connected to the ground so that leaking current goes to the earth and does not give a shock.
- Household wiring has three wires: live (phase), neutral and earth. The switch and fuse are always placed in the live wire.
6. AC and DC
| Point | Direct current (DC) | Alternating current (AC) |
|---|---|---|
| Direction | Flows in one direction only | Changes direction again and again |
| Sources | Cells, batteries, solar cells | Generators in power houses (for example hydropower stations) |
| Frequency | Zero | In Nepal, household supply is about 230 V at 50 hertz (Hz) |
| Uses | Mobile phones, torches, laptops (after conversion), electroplating | Homes, offices, factories; easy to send over long distances |
A transformer raises (step-up) or lowers (step-down) AC voltage. It works only on AC, not on DC. Power is sent over long distances at high voltage to reduce loss. An inverter changes DC into AC, and a rectifier (adapter or charger) changes AC into DC.
7. Magnets and magnetism
- A magnet attracts iron, nickel and cobalt (magnetic materials). Wood, plastic, copper and aluminium are non-magnetic.
- Every magnet has two poles, north and south. Like poles repel; unlike poles attract. Attraction is strongest at the poles.
- Poles always exist in pairs: breaking a magnet gives two smaller magnets, each with both poles.
- A freely hanging magnet always comes to rest in the north-south direction. A magnetic compass uses this to show direction.
- The Earth behaves like a huge magnet. Magnetic field lines go from the north pole to the south pole outside a magnet.
- Natural magnet: lodestone (magnetite). Permanent magnets are made of steel or special alloys; temporary magnets such as soft iron lose magnetism quickly.
- Heating, hammering or dropping a magnet weakens its magnetism.
- SI unit of magnetic flux is the weber (Wb); of magnetic flux density (field strength) it is the tesla (T).
8. Electromagnetism: motors and generators
In 1820, Hans Christian Oersted found that an electric current produces a magnetic field around a wire. A coil of wire around a soft iron core becomes an electromagnet when current flows; its magnetism can be switched on and off. Electromagnets are used in electric bells, cranes that lift scrap iron, loudspeakers, relays and MRI machines.
In 1831, Michael Faraday discovered electromagnetic induction: a changing magnetic field produces a current in a coil. This is the principle of the generator (dynamo) and the transformer.
| Device | Energy change | Rule |
|---|---|---|
| Electric motor (fan, pump, mixer) | Electrical energy into mechanical energy | Fleming’s left-hand rule |
| Generator or dynamo (hydropower plant, bicycle dynamo) | Mechanical energy into electrical energy | Fleming’s right-hand rule |
9. Units and measuring instruments
| Quantity | SI unit (symbol) | Instrument |
|---|---|---|
| Electric charge | Coulomb (C) | Electroscope detects charge |
| Electric current | Ampere (A) | Ammeter (in series) |
| Small current | Ampere (A) | Galvanometer detects small current |
| Potential difference | Volt (V) | Voltmeter (in parallel) |
| Resistance | Ohm (Ω) | Ohmmeter |
| Power | Watt (W) | Wattmeter |
| Electrical energy | Joule (J); commercial unit kWh | Energy meter |
| Capacitance | Farad (F) | Capacitance meter |
| Frequency | Hertz (Hz) | Frequency meter |
| Magnetic flux density | Tesla (T) | Gaussmeter or teslameter |
A multimeter can measure current, voltage and resistance with one instrument.
📌 Key points to remember
- Current I = Q ÷ t, unit ampere, measured by an ammeter in series; voltage in volts by a voltmeter in parallel.
- Ohm’s law: V = I × R at constant temperature; resistance is measured in ohms.
- Silver is the best conductor; copper is used for wires, tungsten for bulb filaments, nichrome for heaters.
- Series: same current, resistances add. Parallel: same voltage, total resistance falls. Houses are wired in parallel.
- P = V × I; 1 unit of electricity = 1 kWh = 3.6 million joules.
- A fuse melts and an MCB trips on overload; switches and fuses go in the live wire; earthing prevents shock.
- Nepal’s household supply is AC at about 230 V and 50 Hz; a transformer works only on AC.
- Like poles repel; a freely hanging magnet points north-south; iron, nickel and cobalt are magnetic.
- Oersted (1820): current makes a magnetic field. Faraday (1831): electromagnetic induction. A motor turns electrical energy into motion; a generator does the reverse.
Test yourself: Electricity and Magnetism in Daily Life
20 questions check what you read about Electric charge and current, Electric power, energy and safety and Units and measuring instruments.