EE101 – Ohm’s Law & DC Circuits

At the heart of basic electrical engineering is a simple relationship between three quantities: voltage, current, and resistance. This is written as: Where:

• V = voltage (in volts, V) – how hard the circuit is being “pushed.”
• I = current (in amperes, A) – how much charge flows per second.
• R = resistance (in ohms, Ω) – how much the material opposes the flow.

Rearranging the formula gives you the three useful forms: That’s the basic rule for any simple DC resistor circuit.

For beginners, the notation is often the first stumbling block:

• Voltage → symbol V, unit volt (V). That’s intuitive.
• Resistance → symbol R, unit ohm (Ω). Also intuitive.
• Current → symbol I, but its unit is the ampere (A).

So why I for current instead of C or A?

Historically, current was described as the intensity of current, and the letter I came from that term. The unit “ampere” comes from André-Marie Ampère, but the symbol I stuck for the variable itself. It’s a quirk of history that every EE student just has to get used to.

The key mapping to remember:
• Variable I → current
• Unit A → amperes (“amps”)

A basic DC (direct current) circuit looks like:

• A voltage source (like a battery).
• A resistor (or several resistors).
• Wires connecting them in a loop.

In steady-state DC:
• The voltage is constant over time.
• The current flows in one direction and is steady.
• Ohm’s Law applies directly at every resistor.

Example: If you increase the resistance, the current goes down. If you increase the voltage, the current goes up. That’s the basic “push vs. opposition” picture that Ohm’s Law captures.

Besides resistors, two other important components show up everywhere:

• Inductor – measured in henries (H). It resists changes in current.
• Capacitor – measured in farads (F). It stores charge and resists changes in voltage.

For DC steady state (after everything has settled):

• A capacitor behaves like an open circuit – it eventually stops DC current once fully charged.
• An inductor behaves like a short circuit – once the current is stable, it looks like a piece of wire.

These behaviors are the DC limit. As soon as voltages and currents start changing with time (AC or transients), inductors and capacitors stop acting like simple opens and shorts and begin introducing frequency-dependent effects. Those effects are handled using a more general idea: impedance.

Ohm’s Law is perfectly fine for pure resistors in DC. But in real life:

• Voltages are often time-varying (AC power, audio, radio signals, digital pulses).
• Inductors and capacitors don’t behave like fixed resistors in those situations.

To handle that, electrical engineers generalize the idea of resistance to impedance, written as Z. Impedance can:

• Include resistance (R).
• Include the “reactive” behavior of inductors (L) and capacitors (C).
• Change with frequency.

The basic Ohm’s Law shape still survives in AC form: but now Z can be more complex and depends on frequency.

We’ll explore that next in:
AC & Impedance – how L and C depend on frequency