2.1 Charge, Voltage & Current
Charge
Electric charge is represented by Q and measured in coulombs (C).
One coulomb corresponds to approximately 6.25 × 10¹⁸ elementary charges. [1, pp. 30-32]
We normally do not count individual electrons. The coulomb gives us a practical unit for talking about an amount of charge.
Voltage: potential difference
Voltage is a potential difference between two points. It is measured in volts (V). [1, pp. 33-35]
A useful relationship is:
1 V = 1 J/C
That means voltage can be understood as energy available per unit of charge.
Raised-glass analogy
Imagine choosing the floor as a reference height.
- a glass on the floor is at the reference level
- lifting the glass gives it more gravitational potential energy
- a point above the reference can be treated as positive
- a point below a chosen reference can be treated as negative
Voltage works similarly as a difference relative to a reference point. In an electronic system, that reference is often called ground.
The analogy is about potential, not about electricity literally behaving like water.
A battery can have a voltage across its terminals even when nothing is connected and no current is flowing. [1, p. 49]
Current: charge in motion
Current is represented by I and measured in amperes (A).
I = Q / t
Therefore:
1 A = 1 C/s
Current is a rate. A useful water analogy is the rate at which water is being poured, not the total amount of water in the glass. [1, pp. 35-38]
Why coulombs matter later
Capacitors give charge a practical meaning.
A capacitor stores separated charge and electrical energy. Its capacitance tells us how much charge can be stored at a given voltage:
C = Q / V
We will use that relationship later. For now, the important connection is:
charge + voltage -> capacitor behaviour
Modern multilayer ceramic capacitors are extremely common. Open Circuits notes that a smartphone may contain hundreds of MLCCs, many used to help stabilize power supplies. [2, p. 36]
Quick check
- What does a coulomb measure?
- What is the difference between voltage and current?
- Can voltage exist when current is zero?
- If 3.0 C of charge passes a point in 2.0 s, what is the current?
- Why might capacitors make the idea of charge useful when looking at a motherboard?
Add one better analogy, diagram, or real component example for charge, voltage, or current.