Charge, Current and Voltage

This is the first lesson of the course, and it assumes you know nothing about electronics. That is not a problem. The ASOC paper does not want an engineer; it wants someone who understands what is happening inside their own transmitter well enough to operate it without causing harm. Everything in Section A is built out of three quantities — charge, current and voltage — and if you get those three straight now, the rest of the module is arithmetic. Get them muddled and every later lesson costs twice as much effort.

Charge: the thing that is actually there

Matter is made of atoms, and every atom has a nucleus of protons and neutrons with electrons moving around it. The proton carries a positive electric charge, the electron an equal negative one, and the neutron none at all. Charge is not something we invented for the exam — it is a physical property of those particles, in the same way that mass is.

The SI unit of charge is the coulomb, symbol C. One coulomb is a very large amount of charge: about 6.24 × 10¹⁸ electrons' worth. You will rarely count coulombs directly, but you must know the word, because the examiner asks for it flatly — "Unit of electric charge is…" — and because both of the next two definitions are built on it.

Current: charge on the move

In a metal, the outermost electrons of each atom are only loosely held, and they drift about randomly from atom to atom. Randomly means in no particular direction, and random movement carries nothing anywhere. Apply an electrical push and that aimless drift acquires a direction: now there is a net movement of charge past any given point in the wire. That net movement is current, symbol I.

Current is a rate. It is not an amount of charge, it is charge per second:

The unit is the ampere, symbol A, and its definition is the one sentence to memorise from this section: one ampere is one coulomb per second. Two ampere-sized traps sit next to it in the options list — one joule per second is a watt, and one coulomb per volt is a farad. Neither is a current.

Small currents get the usual prefixes: 1 milliampere (mA) is a thousandth of an ampere, 1 microampere (µA) is a millionth. A handheld on receive draws perhaps 200 mA; the same handheld transmitting draws two or three amperes.

Which way does it flow?

Here is a genuine oddity that the exam and every circuit diagram you will ever read depend on. Long before the electron was discovered, it was agreed that current flows from the positive terminal of a source, round the external circuit, to the negative terminal. That convention was written into every diagram, every arrowhead and every component symbol. Then, in 1897, the electron was found — and it turned out that what actually moves in a metal is negative, so the electrons drift the other way, from negative to positive.

Nobody redrew the world's circuit diagrams. So we live with two descriptions of the same thing:

• Conventional current flows from + to −, outside the source. Every arrow on a schematic means this — including the arrow on a diode symbol and the arrow on a transistor's emitter.
• Electron flow is the physical drift, from − to +.

Both give identical answers for everything the exam asks. Use conventional current, because that is what the symbols are drawn for; know that electron flow exists, because the paper occasionally asks which way the electrons really go.

Voltage: the push that causes the flow

Electrons do not organise themselves. Something has to do work on them, and that something is a source — a cell, a battery, a dynamo, a mains supply. The electrical pressure it produces goes by three names, and the exam uses all three interchangeably:

Electromotive force (EMF)

The pressure a source generates by its own action, symbol E. It is not a force in the mechanical sense; the name is historical.

Potential difference (PD)

The difference in electrical pressure between two points in a circuit — for example across a resistor. Always between two points; a single point does not have a voltage on its own, only relative to something else, usually the chassis.

Voltage

The everyday word for either of the above.

All three are measured in volts, symbol V. And the volt has a clean definition worth knowing, because it turns one exam question into arithmetic: one volt is one joule of energy per coulomb of charge.

Worked example

A circuit delivers 60 joules of energy for every 15 coulombs of charge that pass through it. What is the voltage?

V = W ÷ Q = 60 ÷ 15 = 4 V. Dividing the wrong way round gives 0.25, and 0.25 V is always one of the offered options.

The water analogy, and where it stops working

A tank of water plumbed to a tap is a fair picture of a simple circuit. The height of the tank — the pressure — is the voltage. The litres per second through the pipe is the current. A narrow pipe that restricts the flow is the resistance. Raise the tank and more flows; pinch the pipe and less does. That is Ohm's law in plumbing, and the next lesson, Ohm's Law, Power and Energy, turns it into an equation.

Use the analogy, but know its limits, because a student who leans on it too hard gets three things wrong:

• Water can spill out of a broken pipe; charge cannot. A circuit must be a complete loop. Break it anywhere and everything stops instantly — there is no puddle on the floor.
• The electrons do not race round. Their actual drift is millimetres per second. What travels near the speed of light is the electrical effect, like the pressure wave down an already-full pipe. The lamp lights the moment you close the switch, not when the first electron completes the lap.
• Nothing is consumed. Exactly as much charge returns to the source as leaves it. What the circuit uses is energy, not charge, and that distinction is what the joule-per-coulomb definition of the volt is telling you.

Conductors, insulators and semiconductors

Whether a material carries current comes down to how tightly it holds its outer electrons.

Two habits of this examiner are worth naming now. First, the conductor and insulator questions are asked as lists of four, and the wrong lists are spoiled by exactly one item — "glass, wood, copper, porcelain" is not four insulators, because copper is a conductor. Read every item. Second, carbon is a conductor, which is why it can be made into resistors at all; it appears in a list of insulators purely to catch you.

Semiconductors are the whole basis of Module 3. For now, note that they exist and that silicon and germanium are the two named in the syllabus.

The cell and the battery

A cell converts chemical energy into electrical energy. Two different electrodes sit in an electrolyte, a chemical reaction strips electrons from one and piles them on the other, and the resulting difference in potential is the cell's EMF. Strictly, a cell is one such unit and a battery is several of them connected together, though nobody outside an examination hall is fussy about it.

This next point is a favourite question, so learn it as a sentence: the EMF of a cell depends on the nature of its electrodes and its electrolyte — on the chemistry, and on nothing else. Not the size of the plates. Not the spacing between them. Not the size of the case.

An AAA alkaline cell and a D alkaline cell both give 1.5 V. The D cell is larger because it holds more chemical material, so it can supply more current for longer — more capacity, in ampere-hours, at the same voltage. Size buys stamina, not pressure.

That is also what decides how you wire cells together:

• In series — positive to negative, in a chain — the EMFs add. Six 2 V lead-acid cells in series make the 12 V battery in your car.
• In parallel — all positives together, all negatives together — the voltage stays the same and the capacities add. Two 12 V, 7 Ah batteries in parallel give 12 V at 14 Ah, not 24 V. Series for volts, parallel for ampere-hours.

Internal resistance

No source is perfect. The electrolyte and the plates themselves oppose the current they are producing, and that opposition is the cell's internal resistance, written r. It sits, in effect, in series inside the cell, so the voltage you can actually measure at the terminals sags as soon as current is drawn:

A tired 12 V battery with 0.2 Ω of internal resistance, asked for 5 A on transmit, delivers 12 − (5 × 0.2) = 11 V at its terminals. This is why a handheld's display dims on transmit and why a car's headlamps dip while the starter turns. A fresh lead-acid battery has an internal resistance of a few hundredths of an ohm; as it ages that figure rises, which is exactly what a battery tester measures.

DC and AC, in one paragraph

The EMF from a cell is steady: one terminal stays positive, current flows one way, and that is direct current (DC). The EMF from a generator or from the mains reverses over and over, so the current flows first one way and then the other, and that is alternating current (AC). There is a third case worth naming because the examiner does: a fluctuating DC, which varies in size but never goes negative — a rectified and imperfectly smoothed supply looks like this. Everything in this module is DC. AC gets a module of its own, starting with Sine Waves: Peak, RMS and Average, because a radio signal is an alternating current and nothing about transmitting makes sense until you have it.

Measuring: which meter, and where

Three instruments, three quantities, and the connection matters as much as the instrument.

The reasoning is worth having, because it explains both halves at once. An ammeter has to have the current pass through it, so it goes in the path of that current — and it must add as little opposition as possible, or the act of measuring would reduce the very current being measured. Hence a very low resistance. The consequence is a real hazard: connect a low-resistance ammeter straight across a supply and you have made a short circuit, which is how meters get destroyed.

A voltmeter is the mirror image. It goes across the two points whose difference you want, in parallel with whatever is already there. If it drew appreciable current it would be an extra branch of the circuit and would change the voltage it was supposed to report. So it is made with a very high resistance — a modern digital meter is around 10 MΩ — and draws so little that the circuit does not notice.