Course: From Basic Science to Web Hosting

Module 01 — Basic Science

What Is an Electrical Circuit?

Difficulty: Beginner
Prerequisites: Lesson 004 — What Is Electric Charge?
Estimated time: 20 minutes

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1. Learning Objectives

After this lesson, you should understand:

• What an electrical circuit is
• The basic parts of a circuit
• Source, conductor, load, and control
• Open and closed circuits
• Voltage and current in a circuit
• Resistance
• Ohm’s Law
• How a simple circuit leads toward computer electronics

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2. What Is a Circuit?

An electrical circuit is a connected physical system that provides a path for electrical interaction and, when appropriately powered, allows current to flow through its components.

A very simple circuit:

       ┌───────────────┐
       │               │
     Battery         Lamp
       │               │
       └───────────────┘

The battery provides energy through electrochemical processes, and the circuit provides a path through which charge carriers can respond to the resulting electric fields.

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3. The Four Basic Parts

A simple circuit can contain:

Source
  ↓
Conductor
  ↓
Load
  ↓
Control

For example:

Battery → Wire → Switch → Lamp

Source

Provides the electrical potential difference.

Example:

Battery
Power supply

Conductor

Provides a conducting path.

Example:

Copper wire

Load

Uses/transfers electrical energy.

Examples:

Lamp
Motor
Resistor
Computer chip

Control

Controls the circuit condition.

Examples:

Switch
Transistor
Relay

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4. Source

A battery is one example of an electrical source.

It uses chemical processes to maintain a potential difference between its terminals.

For example:

     +              -
     │              │
   ┌──────────────────┐
   │     BATTERY      │
   └──────────────────┘

The battery establishes a potential difference.

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5. Conductor

A conductor provides a path through which charge carriers can move.

Copper is commonly used:

Battery
   │
Copper wire
   │
Load

Copper has many mobile charge carriers that can respond to an electric field.

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6. Load

A load is a component in which electrical energy is transferred to other forms.

For example, a lamp can convert electrical energy largely into:

Electrical energy
       ↓
Lamp
       ↓
Light + heat

A motor converts electrical energy primarily into mechanical energy, with losses such as heat.

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7. Open Circuit

An open circuit has a break in the conducting path.

Battery
   │
   │
  /      ← open switch
   │
 Lamp
   │
   └────────

The path is incomplete.

Therefore, in the idealized simple circuit:

Current ≈ 0

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8. Closed Circuit

A closed circuit provides a continuous conducting path.

     ┌──────── Lamp ────────┐
     │                      │
     │                      │
     └──── Battery ─────────┘

The circuit is complete.

With a suitable source and components:

Potential difference
       ↓
Electric field
       ↓
Charge-carrier response
       ↓
Current

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9. Switch

A switch changes the electrical connection.

OPEN:

──── / ────

CLOSED:

───────

Conceptually:

Switch
  ↓
Controls circuit state
  ↓
Current can flow / not flow

This simple idea becomes extremely important in digital electronics.

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10. The Circuit Must Be a Complete System

A common beginner misconception is:

“Electricity comes out of one terminal and disappears into the load.”

A circuit is better understood as a complete electrical system.

Source
  ↓
Circuit
  ↓
Load
  ↓
Return path
  ↓
Source

The complete circuit matters.

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11. Voltage in a Circuit

Voltage is a difference in electric potential between two points.

Suppose:

Point A = 5 V
Point B = 0 V

Then:

Voltage difference = 5 V

A circuit component can have a voltage across it.

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12. Current in a Circuit

Current measures the rate at which charge passes a point.

I = ΔQ / Δt

For example:

1 A = 1 C/s

Current is measured in amperes.

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13. Resistance

Real components oppose current to different degrees.

This electrical property is called resistance.

Resistance is measured in:

Ohms (Ω)

A resistor is a component designed to provide a controlled amount of resistance.

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14. What Does Resistance Do?

A resistor can limit current.

Conceptually:

Voltage
   ↓
Resistor
   ↓
Limited current

For an ideal resistor:

More resistance
       ↓
Less current

for the same applied voltage.

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15. Ohm’s Law

For an ideal resistor, Ohm’s law is:

V = I × R

Therefore:

I = V/R

and:

R = V/I

Where:

V = voltage
I = current
R = resistance

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16. Simple Example

Suppose:

Voltage = 10 V
Resistance = 5 Ω

Then:

I = V/R

I = 10/5

I = 2 A

So the idealized current is:

2 A

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17. Power

A circuit also involves energy transfer.

Electrical power can be calculated as:

P = V × I

For our example:

V = 10 V
I = 2 A

Therefore:

P = 10 × 2
P = 20 W

The resistor would dissipate 20 W in this idealized example.

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18. Energy vs Power

These are different.

Power

How quickly energy is transferred.

Watt (W)

Energy

The total amount transferred.

Joule (J)

A device consuming 20 W for 10 seconds transfers:

Energy = Power × Time
       = 20 × 10
       = 200 J

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19. Circuit Example

Consider:

          5 V
       ┌────────┐
       │        │
     Battery    R
       │        │
       └────────┘

Suppose:

R = 100 Ω

Then:

I = V/R
  = 5/100
  = 0.05 A

or:

50 mA

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20. What Happens Physically?

At a simplified physical level:

Battery
   ↓
Maintains potential difference
   ↓
Electric field established in circuit
   ↓
Charge carriers respond
   ↓
Current develops
   ↓
Energy is transferred in components

The detailed electromagnetic behavior is more complex than this simple picture, but this is a useful engineering model.

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21. Circuit Laws

As circuits become more complicated, we need mathematical rules.

Two important ones are Kirchhoff’s laws.

Kirchhoff’s Current Law

At a circuit node:

Current entering
     =
Current leaving

This reflects charge conservation.

Kirchhoff’s Voltage Law

Around an ideal closed loop:

Sum of voltage changes = 0

This reflects energy conservation within the circuit model.

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22. Series Circuit

Components connected one after another form a series path.

Battery
  │
  R1
  │
  R2
  │
  R3
  │
  └───

For ideal series components:

Same current
through each component

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23. Parallel Circuit

Components can also be connected in parallel.

       ┌── R1 ──┐
───────┤        ├──────
       └── R2 ──┘

For ideal parallel branches:

Same voltage
across each branch

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24. Why Circuits Matter to Computers

A computer is an enormous collection of electrical circuits.

The chain is:

Electrical circuit
       ↓
Electronic circuit
       ↓
Semiconductor circuit
       ↓
Transistor
       ↓
Logic gate
       ↓
Digital circuit
       ↓
CPU / memory

So when you eventually use:

systemctl status nginx

on your server, that command ultimately executes through electronic circuits built from transistors.

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25. The Important Transition

We have now moved from:

Atom
 ↓
Electron
 ↓
Charge
 ↓
Electric field
 ↓
Voltage
 ↓
Current

to:

Voltage + Current
       ↓
Electrical circuit
       ↓
Resistance
       ↓
Ohm's Law
       ↓
Circuit analysis

The next major step is:

Electrical circuit
       ↓
Electronic circuit
       ↓
Semiconductor

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26. Quick Check

What are the basic parts of a simple circuit?

Source
Conductor
Load
Control

What is voltage?

Electric potential difference.

What is current?

Rate of electric charge flow.

What is resistance?

A measure of opposition to current in a component/material under specified conditions.

What is Ohm’s Law?

V = IR

What is electrical power?

P = VI

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Next Lesson

Lesson 006 — What Is Resistance?

We will go deeper into:

Resistance
 ↓
Conductivity
 ↓
Resistivity
 ↓
Material properties
 ↓
Temperature
 ↓
Why copper conducts
 ↓
Why silicon behaves differently
 ↓
Ohm's Law
 ↓
Power and heat

Then we will move into capacitors, inductors, AC/DC, semiconductor physics, diodes, and finally transistors.