Electricity and Magnetism: The Basics

In 1820 the Danish physicist Hans Christian Oersted noticed that a compass needle moved when it was placed near a wire carrying an electric current. He published his findings on 21 July 1820. Until then, electricity and magnetism had been studied as two separate subjects. Within about a decade, Michael Faraday showed the link works in both directions: electric currents make magnetic fields, and changing magnetic fields can make electric currents. Almost every motor, generator and transformer you will ever meet is built on those two ideas.

1.Electric Charge: Where It All Starts

Everything around you is made of atoms. An atom has a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge). The size of the charge on one electron is called the elementary charge, written e. Its value is exactly 1.602 176 634 × 10⁻¹⁹ coulombs (C) in the modern SI system. The coulomb is the unit of electric charge.

Two simple rules describe how charges behave: like charges repel, and unlike charges attract. A neutral object has equal amounts of positive and negative charge, so the effects cancel.

When you rub a plastic ruler with a cloth, electrons move from one material to the other. The material that gains electrons becomes negatively charged and the one that loses them becomes positively charged. Only electrons move; the protons stay fixed inside nuclei. This is static electricity. It explains why a rubbed balloon can pick up small pieces of paper, and it is also the basis of lightning, which is a very large electrical discharge between clouds and the ground.

Materials fall broadly into two groups. In a conductor such as copper or aluminium, some electrons are free to move through the material. In an insulator such as plastic, rubber or dry wood, almost all electrons stay bound to their atoms. Metals conduct well; salt solutions also conduct because they contain moving ions.

2.Current, Voltage and Resistance

An electric current is a flow of charge. It is defined as the charge passing a point each second:

In metals the moving charges are electrons. By an old convention, started before electrons were discovered, the arrow of conventional current is drawn from the positive terminal of a cell, round the circuit, to the negative terminal. Electrons actually move the opposite way. Circuit diagrams and exam answers use conventional current unless told otherwise.

Charges do not move unless something pushes them. The "push" supplied by a cell or battery is measured as voltage, also called potential difference. One volt (1 V) means one joule of energy is transferred for each coulomb of charge: V = W ÷ Q. A 1.5 V cell gives 1.5 J to every coulomb that passes through it, and the lamp or resistor later transfers that energy as heat and light.

Resistance measures how strongly a component opposes the current. It is defined by R = V ÷ I and measured in ohms (Ω).

A useful picture is water in a closed pipe loop with a pump. The pump plays the role of the cell, the flow of water is the current, and a narrow section of pipe resembles resistance. Like every analogy, it has limits, but it helps you see why the current is the same all the way round a single loop.

A common question is why a lamp lights almost at once when you press a switch, even though the electrons themselves drift very slowly (often well under a millimetre per second in ordinary wires). The reason is that the whole circuit is already full of free electrons. When the switch closes, an electric field spreads through the wire very quickly and all the electrons start moving together.

3.Ohm's Law and What Changes Resistance

For many conductors at constant temperature, the current is directly proportional to the voltage across them. This is Ohm's law, and it is usually written V = I × R. A graph of current against voltage for such a component is a straight line through the origin; these are called ohmic conductors. A filament lamp is not ohmic: as it heats up, its resistance rises, so its graph curves. A diode lets current pass in one direction only.

The resistance of a wire depends on four things: its length (longer means more resistance), its cross-sectional area (thicker means less resistance), its material, and its temperature (for most metals, hotter means more resistance). For a uniform wire these first three factors are combined in one equation:

4.Series and Parallel Circuits

Components can be joined in two basic ways. In a series circuit they are in one loop, one after another. In a parallel circuit the current has more than one path to follow, and each path is called a branch.

Houses are wired in parallel so that each appliance receives the full supply voltage and can be switched on or off independently.

5.Electrical Power and Energy

Power is the rate of transferring energy. Its unit is the watt (W), which equals one joule per second. For an electrical appliance:

Electricity bills use a bigger unit, the kilowatt-hour (kWh), often called a "unit". One kilowatt-hour is the energy used by a 1 kW appliance running for one hour: 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.

6.Mains Electricity and Safety

Mains electricity is alternating current (a.c.): the current reverses direction many times a second. Different countries use different standards. Many countries, including Pakistan and the United Kingdom, supply about 230 V at 50 Hz, while the United States uses about 120 V at 60 Hz. Always check your local rules; this article uses 230 V in its examples.

A standard three-pin plug has a live wire (carries the high voltage), a neutral wire (completes the circuit) and an earth wire (a safety connection to the ground). In the colour code used in the UK and many other places these are brown, blue and green-and-yellow respectively, but colour codes vary, so never rely on colour alone.

• Fuse: a thin wire placed in the live side that melts if the current is too large, cutting off the supply. Choose a fuse rated just above the normal working current of the appliance.
• Earth wire: connected to the metal casing of an appliance. If the live wire touches the casing, a large current flows to earth, the fuse melts, and the casing never stays live.
• Double insulation: appliances with plastic casings and extra insulation do not need an earth wire. They carry a square-in-a-square symbol.
• Circuit breakers and RCDs: switches that trip automatically when the current is too high or when current leaks to earth. They can be reset and are faster than fuses.

7.Magnets and Magnetic Fields

A magnet has two ends called poles: a north-seeking pole (N) and a south-seeking pole (S). Like poles repel and unlike poles attract. The only common materials strongly attracted to magnets are iron, steel, nickel and cobalt. These are called magnetic materials. Aluminium, copper, plastic and wood are not attracted. Steel keeps its magnetism (it is "hard") and is used for permanent magnets, while soft iron is easily magnetised and demagnetised, which makes it ideal for electromagnets.

The region around a magnet where a magnetic force acts is its magnetic field. We draw it with field lines. By agreement, the lines point away from the north pole and towards the south pole, and the closer together the lines are, the stronger the field. A small compass placed in the field lines up along the line, with its needle pointing in the field direction.

The Earth itself behaves like a huge bar magnet. A compass needle points roughly north because the north-seeking end of the needle is attracted to the Earth's magnetic pole in the northern hemisphere. In magnet terms that pole is actually a magnetic south pole, which is why it attracts the needle's north-seeking end. The magnetic poles are not fixed at the geographic poles and they slowly move over time.

8.Currents Make Magnetic Fields

This is Oersted's discovery. Any wire carrying a current is surrounded by a magnetic field. Around a straight wire, the field lines are circles centred on the wire. A convenient memory aid is the right-hand grip rule: point your right thumb along the conventional current and your curled fingers show the direction of the field.

Winding the wire into a coil called a solenoid adds the fields of every turn together. The field outside a solenoid looks just like that of a bar magnet, while inside it is strong and almost uniform. To find the north end, curl the fingers of your right hand in the direction of the conventional current round the coil; your thumb points towards the north end.

The strength of an electromagnet increases with more current, more turns of wire and by adding a soft-iron core. Unlike a permanent magnet, an electromagnet can be switched on and off, and its poles can be reversed by reversing the current. This is why electromagnets are used in scrapyard cranes, electric bells, relays (switches operated by a small current to control a large one), loudspeakers and MRI scanners.

9.The Force on a Current: The Motor Effect

If a wire carrying current is placed in a magnetic field, the two magnetic fields interact and the wire feels a force. This is the motor effect. When the wire is at right angles to the field, the size of the force is:

The direction of the force is found with Fleming's left-hand rule: hold your left thumb, first finger and second finger at right angles. The First finger points along the Field (north to south), the seCond finger along the Current, and the Thumb shows the direction of Thrust (motion). The force is zero if the wire is parallel to the field.

In a simple d.c. motor, a rectangular coil sits between the poles of a magnet. The two long sides carry current in opposite directions, so one side is pushed up and the other down, which turns the coil. A split-ring commutator reverses the current every half turn so the coil keeps turning the same way. The motor turns faster or with greater turning effect if you increase the current, use a stronger magnet or add more turns.

10.Electromagnetic Induction

In 1831 Michael Faraday showed that a voltage is produced in a conductor when the magnetic field through it changes. This is electromagnetic induction, and the voltage is called an induced e.m.f. (electromotive force). If the conductor is part of a complete circuit, a current flows. You can do it with a coil connected to a sensitive meter: pushing a magnet into the coil makes the needle flick; holding the magnet still gives nothing; pulling it out flicks the needle the other way.

The induced voltage is larger when you move the magnet faster, use a stronger magnet, or use more turns. Lenz's law tells us the direction: the induced current always flows in the direction that opposes the change that caused it. This is consistent with conservation of energy: you must do work to push the magnet in.

A generator uses this effect. A coil spins in a magnetic field (turned by steam, water, wind or an engine) and an alternating voltage appears at the slip rings. The output rises and falls smoothly each rotation, giving the sine-shaped graph below. Faster rotation raises both the voltage and the frequency.

11.Transformers and Power Lines

A transformer changes the size of an alternating voltage. It has a primary coil and a secondary coil wound on a shared soft-iron core. The alternating current in the primary makes a changing magnetic field in the core; this field induces an alternating voltage in the secondary. A transformer works only with a.c., because a steady d.c. gives no changing field. The voltages are related to the number of turns:

Transformers explain how electricity travels long distances. Power losses in cables are given by I²R, so reducing the current greatly reduces the loss. Because P = VI, raising the voltage lets the same power travel with a smaller current.

12.Common Mistakes to Avoid

13.A Short Timeline

14.Check Your Understanding

Try each question before opening the answer.

Summary

• Current is the flow of charge (I = Q/t); voltage is energy per coulomb (V = W/Q); resistance is R = V/I.
• Series: same current, resistances add. Parallel: same voltage, currents add, total resistance falls.
• Power P = VI; energy E = Pt; one kWh is 3.6 × 10⁶ J.
• A current makes a magnetic field; a field acting on a current gives a force (motor); a changing field induces a voltage (generator, transformer).

Keep Learning on EDUSAMBAM

Sources and Further Reading

This article is original EDUSAMBAM writing. Definitions, constants and historical dates were checked against the sources below. Diagrams are original and were not copied from any source.

• BIPM / NIST, SI units: definition of the ampere, coulomb and the elementary charge e = 1.602 176 634 × 10⁻¹⁹ C. BIPM — The International System of Units; NIST — SI Units.
• OpenStax, University Physics Volume 2: free university-level text covering current, resistance, magnetic fields and induction. OpenStax — University Physics Volume 2.
• OpenStax, College Physics 2e: circuits, magnetism and electromagnetic induction. OpenStax — College Physics 2e.
• Encyclopaedia Britannica: Electricity; Electromagnetism (history of Oersted, Ampère, Faraday and Maxwell).
• Further reading kept from the original article: Physics — CERN.

Note on the numbers: numerical examples use rounded values and a typical resistivity for copper. Please check them against your own syllabus or exam board specification before relying on them.