Energy, power and the units that measure them: Electricity and the nucleus

5.1 Electricity is a carrier, and high voltage keeps losses down

Electricity is a flow of electric charge, usually electrons moving through a metal wire. It is not a primary source of energy like coal or sunlight. It is a carrier, made from those sources and delivered to wherever it is needed, almost instantly and with few moving parts.

Two quantities describe the flow. Voltage, measured in volts, is the push that drives charge. Current, measured in amperes, is the rate at which charge flows. Power is the product of the two: power = voltage x current. A 230 volt Indian socket supplying 10 amperes delivers 2,300 W.

Wires resist the flow, and that resistance turns some electrical energy into heat. The loss grows with the square of the current. Double the current and the loss quadruples. This simple fact shapes every grid in the world. To send a large amount of power a long way, engineers raise the voltage so that the same power travels with a small current. India's main transmission lines run at 400,000 and 765,000 volts, and its long-distance direct current links at up to 800,000 volts. Raising voltage tenfold cuts the current tenfold and the resistive losses a hundredfold for the same wire.

This is why electricity is generated at moderate voltage, stepped up for transmission, and stepped down in stages before reaching homes. Even so, losses are real. Well-run grids lose about 5 to 8% of electricity in transmission and distribution. Indian distribution companies have lost far more: the Ministry of Power puts their aggregate technical and commercial (AT&C) losses at about 22.6% in 2013-14 and about 15% in 2024-25. A large share of that is commercial loss, meaning theft, unbilled supply and unpaid bills, rather than physics.

Electricity's flexibility is its great virtue. It can be turned into motion at over 90% efficiency, into light, into heat at nearly 100%, and it powers every digital device. That is why the share of energy reaching users as electricity keeps rising.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Electricity and the nucleus'
• Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M1 'Electricity and the nucleus'
• Outlook Business (PTI), 'Discoms post Rs 2,701 cr profit in FY25 after years of losses', citing Ministry of Power, January 2026 · AT&C losses 22.62% in FY 2013-14 and 15.04% in FY 2024-25. https://www.outlookbusiness.com/corporate/discoms-post-2701-cr-profit-in-fy25-after-years-of-losses
• GE Vernova, 'Champa 800 kV HVDC: delivering efficient energy highways in India' (case study) · Champa-Kurukshetra +/-800 kV, 3,000 MW, 1,365 km; India's 765 kV substations. https://www.gevernova.com/electrification/sites/default/files/case_studies_pdfs/case-study-champa-800-kv-hvdc-delivering-efficient-energy-highways-in-india.pdf

5.2 Induction, transformers and alternating current built the grid

Nearly all the world's electricity comes from one discovery. In 1831 Michael Faraday showed that moving a conductor through a magnetic field induces a current in it. Spin a coil of wire inside a magnetic field, or spin a magnet inside a coil, and you have a generator. Coal, gas, nuclear, hydro and wind plants differ only in what turns the shaft. Solar photovoltaics are the main exception, turning light directly into current without moving parts.

Run induction backwards and you get a motor: feed current into a coil in a magnetic field and it turns. Motors are the great consumers of electricity. Smil points out that electric motors, from tiny phone vibrators to factory compressors and locomotive drives, use roughly half of all the electricity produced in the world. Pumps, fans and compressors are among the largest users in Indian industry and agriculture alike.

The transformer is the third pillar. It is a pair of coils wound on an iron core, with no moving parts, that steps voltage up or down with efficiencies above 99% in large units. Smil calls transformers unsung, silent devices without which large grids could not exist.

Transformers only work with alternating current, which reverses direction many times a second, 50 times in India and Europe and 60 times in the US. That is why alternating current beat Thomas Edison's direct current in the 1890s: AC voltage could be changed cheaply, so power could be sent far at high voltage and used safely at low voltage. Direct current has returned for very long and undersea links, because modern power electronics can now convert it efficiently and DC lines lose less over great distances.

• Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M1 'Electricity and the nucleus'
• Smil, Creating the Twentieth Century: Technical Innovations of 1867-1914 and Their Lasting Impact (2005) · via Smil Mastery Course, M1 'Electricity and the nucleus'

5.3 Nuclear energy is a million times more concentrated than chemical fuel

Chemical reactions rearrange the outer electrons of atoms. Nuclear reactions rearrange the protons and neutrons in their cores, which are bound together far more tightly. The energy involved is roughly a million times larger per reaction, and that ratio is the whole story of nuclear power's concentration.

Fission splits heavy nuclei such as uranium-235 or plutonium-239. When a neutron strikes a uranium-235 nucleus, it splits into two lighter fragments, releases a large burst of energy and frees two or three more neutrons, which can split further nuclei. A reactor controls this chain reaction so that, on average, exactly one neutron from each split causes another. Fissioning one kilogram of uranium-235 completely releases about 80 TJ, roughly what burning 3,000 tonnes of good coal provides. Reactors use fuel enriched to only 3 to 5% uranium-235 and burn only part of it, but a kilogram of natural uranium used once through a typical reactor still yields something like 15,000 to 20,000 times more heat than a kilogram of coal (calculated).

The energy comes from mass. The fission fragments weigh very slightly less than the original nucleus and neutron, and the missing mass appears as energy according to Einstein's E = mc2. Because c, the speed of light, is so large, a tiny mass yields a vast energy: converting one gram of mass entirely into energy would release 9 x 10^13 J, about the energy in 2,100 tonnes of crude oil.

Fusion joins light nuclei, such as the hydrogen isotopes deuterium and tritium, into helium. It powers the Sun and releases even more energy per kilogram than fission, but it needs temperatures of over 100 million degrees to start on Earth. Smil counts controlled fusion for electricity among the inventions we keep waiting for: decades of steady scientific progress, but still no plant selling power to a grid.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Electricity and the nucleus'
• Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M1 'Electricity and the nucleus'

See it in the table

Check your understanding

If a line's voltage is doubled while carrying the same power, what happens to its resistive losses?

Why did alternating current win over direct current for early grids?

Why is nuclear fuel so much more concentrated than coal?