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Electric circuits are at the heart of modern technology. Every phone, computer, car, and medical device relies on the controlled flow of electric charge through circuits. Before you can analyse any circuit, you need to understand three foundational quantities: charge, current, and potential difference.
All matter is made of atoms, and atoms contain charged particles. Protons carry a positive charge and electrons carry a negative charge. The fundamental unit of charge is the coulomb (C).
The charge on a single electron is extremely small:
e = 1.60 × 10⁻¹⁹ C
This means that one coulomb of charge is equivalent to roughly 6.25 × 10¹⁸ electrons. In everyday circuits, we routinely deal with charges of several coulombs flowing every second.
Charge is quantised — it always comes in whole-number multiples of the elementary charge e. You cannot have half an electron's worth of charge. This quantisation was demonstrated experimentally by Robert Millikan in his famous oil drop experiment.
Charge is a conserved quantity. It cannot be created or destroyed — it can only be transferred from one place to another. This principle underpins Kirchhoff's first law, which you will meet in a later lesson.
Current is the rate of flow of electric charge past a point in a circuit. It is defined mathematically as:
I = Q / t
Or equivalently:
Q = It
where:
One ampere means one coulomb of charge flows past a point every second.
A current of 3.0 A flows through a lamp for 2.0 minutes. Calculate the total charge that flows.
Solution:
Historically, current was defined as the flow of positive charge from the positive terminal of a battery to the negative terminal. This convention was established before anyone knew that in metallic conductors it is actually the negatively charged electrons that move — and they move in the opposite direction, from negative to positive.
We still use the conventional current direction in circuit analysis. This is important to remember: conventional current flows from positive to negative, but electrons flow from negative to positive.
In electrolytes (solutions containing ions), both positive and negative charge carriers can move. Positive ions move in the direction of conventional current, and negative ions move in the opposite direction.
| Material | Charge Carriers |
|---|---|
| Metals | Free (delocalised) electrons |
| Electrolytes | Positive and negative ions |
| Semiconductors | Electrons and holes |
| Gases (ionised) | Electrons and positive ions |
Potential difference (p.d.) — often called voltage — is defined as the energy transferred per unit charge as charge moves between two points:
V = W / Q
where:
One volt means one joule of energy is transferred for every coulomb of charge that passes. The volt is therefore equivalent to J C⁻¹.
A 12 V battery drives 5.0 C of charge around a circuit. How much energy is transferred by the battery?
Solution:
The battery (or power supply) is an energy source. It does work on the charge carriers, giving them electrical potential energy. As charge carriers move through components in the circuit, they transfer this energy to other forms:
The key principle is that energy is conserved around any closed loop. The total energy given to the charges by the battery equals the total energy transferred by the charges to the components. This is the basis of Kirchhoff's second law.
Ammeters measure current and must be connected in series — the current you want to measure must flow through the ammeter. An ideal ammeter has zero resistance so it does not affect the circuit.
Voltmeters measure potential difference and must be connected in parallel across the component — they measure the difference in energy per unit charge between two points. An ideal voltmeter has infinite resistance so it draws no current from the circuit.
These three quantities are deeply interconnected. Current tells you how fast charge is flowing. Potential difference tells you how much energy each unit of charge transfers. Together, they determine the power delivered to or by a component (which you will explore in a later lesson on electrical energy and power).
Understanding these definitions precisely is essential because every circuit calculation you perform at A-Level builds on them. When you see V = IR or P = IV, remember that these are consequences of the fundamental definitions of charge, current, and potential difference.