Voltage, current, resistance, Ohm's Law, AC vs DC, power, and circuits — explained clearly for electricians and apprentices. This is the foundation of everything you do on site.
Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians
Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
1,000+
UK electricians
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer — DP Electrical
Electricity is the flow of tiny charged particles called electrons through a conductor — typically a copper or aluminium wire. Every atom has electrons orbiting its nucleus. In conductive materials like copper, some electrons are loosely bound and can move freely from atom to atom. When a voltage (electrical pressure) is applied across a conductor, these free electrons drift in one direction, creating an electric current.
That is the simplest explanation. In practice, electricity is the energy source that powers everything from the lights in your home to the motors in an industrial plant. As an electrician, you work with electricity every day — installing the cables that carry it, fitting the protective devices that control it, and testing the installations that use it safely.
To work safely and competently, you need to understand three fundamental quantities: voltage, current, and resistance. These three are related by Ohm's Law, and every electrical calculation on site comes back to them.
Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
These are the three pillars of electrical theory. Get these right and everything else follows.
Voltage is the electrical pressure — the force that pushes electrons through a conductor. It is measured in volts (V). The UK mains supply is 230V. A car battery is 12V. A lightning bolt can be 300 million volts. Voltage is sometimes called electromotive force (EMF) when referring to the source, or potential difference (PD) when measured across a component.
Current is the rate of flow of electrons, measured in amperes (A). One ampere means approximately 6.24 x 10^18 electrons passing a point every second. A typical UK socket circuit is protected at 32A. A lighting circuit is usually protected at 6A. The current drawn depends on the load — plug in a 3kW kettle and the circuit draws about 13A.
Resistance opposes the flow of current, measured in ohms (Ω). Every material has some resistance. Copper has very low resistance (good conductor). Rubber has extremely high resistance (good insulator). The resistance of a cable depends on its material, length, and cross-sectional area. Longer cables and thinner cables have more resistance — which is why cable sizing matters.
The water pipe analogy is useful: voltage is the water pressure, current is the flow rate, and resistance is the narrowness of the pipe. Increase the pressure (voltage) and more water (current) flows. Narrow the pipe (increase resistance) and less water flows for the same pressure.
Ohm's Law is the relationship between voltage, current, and resistance. It states:
V = I x R
Voltage (volts) = Current (amps) x Resistance (ohms)
You can rearrange it to find any value:
On site, you use Ohm's Law constantly — even if you do not realise it. When you measure earth fault loop impedance (Zs) and calculate prospective fault current (PFC), you are using I = V / Zs. When you check voltage drop across a cable run, you are using V = I x R. When you size a cable for a given load, the resistance per metre of the conductor matters because of Ohm's Law.
Elec-Mate includes built-in calculators for cable sizing, voltage drop, maximum demand, prospective fault current, and more.
Try it free for 7 days
There are two types of electrical current: alternating current (AC) and direct current (DC). The UK mains supply is AC. Batteries, solar panels, and most electronic devices run on DC.
In AC, the direction of electron flow reverses at regular intervals. The UK supply alternates at 50 hertz (Hz), meaning the current changes direction 50 times per second. AC is used for mains power because it can be easily stepped up and down using transformers. This makes it efficient for long-distance transmission — power stations generate at around 25kV, step up to 400kV for the National Grid, then step down through substations to the 230V that reaches homes and businesses.
In DC, electrons flow in one direction only. Batteries produce DC. Solar photovoltaic panels produce DC (which is then converted to AC by an inverter for connection to the mains). Electronic circuits inside computers, phones, and LED drivers all run on DC. Electric vehicles charge using DC (or AC converted to DC by the on-board charger). As renewable energy and battery storage grow, electricians are working with DC circuits more frequently.
As an electrician, you need to understand both. Most of your installation work involves AC circuits, but solar PV installations, EV charger installations, and battery storage systems all involve DC components. The testing and safety procedures differ between AC and DC, and BS 7671 has specific requirements for both.
16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.
Start free trialPower and energy are related but different. Power is the rate of doing work — how much electrical energy is being used per second. Energy is the total amount used over time.
When you calculate maximum demand for an installation, you are working out the total power (in kW or kVA) that all the connected loads could draw at the same time. This determines the size of the main incoming cable, the main switch, and the supply fuse. Get it wrong and the supply trips under load.
A circuit is a closed loop that allows current to flow from the supply, through the load, and back to the supply. If the loop is broken — by a switch, a fuse, or a fault — current stops flowing.
Components connected in a single path. The same current flows through each component. The total resistance is the sum of all resistances. If one component fails open, the whole circuit stops. Practical example: the trip coil inside an MCB is in series with the circuit it protects — when the coil trips, it breaks the series path and disconnects the supply.
Components connected across the supply so each has the full voltage. Current divides between branches. If one branch fails, the others continue working. Almost all domestic circuits connect loads in parallel — each socket on a ring circuit gets 230V regardless of how many other sockets are in use.
Unique to UK wiring, a ring final circuit connects sockets in a loop — the cable leaves the consumer unit, visits each socket in turn, and returns to the same MCB terminal. This means current can flow to any socket from both directions, effectively halving the maximum current in any section of cable. This allows 2.5mm² cable to serve a 32A circuit.
Understanding circuit types is essential for fault finding. A break in a ring circuit turns it into a radial — it still works, but the cable may be overloaded because current can only flow from one direction. The R1+R2 continuity test during an EICR checks for exactly this kind of fault.
Electrical theory is not just something you learn for exams and forget. Every task on site depends on it — whether you realise it or not.
Whether you are a first-year apprentice learning the basics or a qualified electrician preparing for C&G 2391, strong electrical theory makes everything else easier. It is the difference between following procedures by rote and genuinely understanding what you are doing and why.
Elec-Mate's Study Centre covers all the theory you need — from basic Ohm's Law through to three-phase calculations and BS 7671 regulations.
Try it free for 7 daysVerified reviews from the UK App Store.
Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.
I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.
I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.
Real feedback from real sparks
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer
Sole Trader · DP Electrical
“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”
Nathan Perry
Electrician · NP Electrical Services
“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”
Jake Pizey
3rd Year Apprentice · Apprentice
Structured study courses, AI tutor, practice questions, and built-in calculators. Everything an apprentice or qualified electrician needs to master electrical theory. 7-day free trial.
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer, DP Electrical
From £6.99/mo after trial — less than a coffee a week
We use cookies to improve the app and measure what works. Cookie Policy