【Physics Basics】Welcome to the World of Electricity!

Hello! Let’s start our journey into the study of "Electricity." When you hear the word "electricity," you might think, "It’s invisible and there are too many formulas—it seems difficult..." But don't worry! The true nature of electricity is actually very easy to visualize if you compare it to the flow of "water" around us.
In these notes, I’ve distilled the key points you need to secure your score on the Common Test, while avoiding overly complicated jargon. It’s perfectly fine to start small. Let’s unravel the mechanics of electricity one step at a time!

1. Static Electricity and Electric Charge

Have you ever felt a "snap" when touching a doorknob in winter? That is static electricity. All matter contains tiny particles of electricity called electric charges.

Basic Rules of Electric Charge

  • There are two types: positive charge (+) and negative charge (-).
  • Like charges (+ and +, or - and -) repel each other.
  • Opposite charges (+ and -) attract each other.
  • The amount of charge is called electric quantity, and its unit is the [C] (Coulomb).

【Key Point】Conservation of Charge
Electricity doesn't just disappear, nor does it appear from nowhere. When looking at the entire circuit, the total amount of charge is always constant. This is known as the Law of Conservation of Charge.

★Fun Fact★

When you rub your hair with a plastic ruler and your hair stands on end, it's because the negative charges from your hair move to the ruler, causing the individual hairs to become positively charged and repel each other!

2. Current, Voltage, and Resistance (The Electric Trio)

The most important part of studying electricity is understanding the relationship between these three. Let’s imagine them as "water flow."

① Current \(I\) [A] (Ampere)

Image: The "amount" of flowing water
Represents how many electric particles pass through a point per second. Formula: \(I = \frac{Q}{t}\) (\(Q\) is electric quantity, \(t\) is time)

② Voltage \(V\) [V] (Volt)

Image: The "pump force" pushing the water
This is the pressure that forces electricity to flow. Without this, electricity wouldn't move.

③ Resistance \(R\) [Ω] (Ohm)

Image: The "narrowness of the path" (difficulty of flow)
The degree to which the flow of electricity is obstructed.

Ohm's Law

The formula that expresses the relationship between these three is the super-important "Ohm's Law." \(V = RI\)
(Voltage = Resistance × Current)

【Common Mistake】
You might be tempted to think, "Since \(R = V / I\), increasing the voltage also increases the resistance," but resistance \(R\) is an intrinsic property of the material (the narrowness of the path). Changing the voltage does not change the value of the resistance itself!

【Section Summary】
・Current \(I\) is the amount of water, Voltage \(V\) is the water pressure, and Resistance \(R\) is the narrowness of the pipe!
・Definitely memorize \(V = RI\)!

3. How to Connect Circuits (Series and Parallel)

There are two patterns for connecting resistors. Let’s grasp the characteristics of each.

Series Circuit (Single Path)

Characteristic: The current \(I\) is the same everywhere!
Since it's a single path, the amount of flowing water doesn't change anywhere.
・Total resistance \(R = R_1 + R_2\)
・Total voltage \(V = V_1 + V_2\)

Parallel Circuit (Branching Path)

Characteristic: The voltage \(V\) is the same everywhere!
No matter which path the water takes, the "drop in height" (voltage) is the same.
・Total current \(I = I_1 + I_2\)
・Reciprocal of total resistance \(\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2}\)

【Memorization Tip】
Keep in mind that the combined resistance of a parallel circuit is "always smaller than any of the original individual resistors." This is because adding more paths makes it easier for the water to flow (reducing the overall resistance).

4. Electric Power and Electrical Energy

This refers to the ability to use electricity to generate heat, light, or other energy.

Electric Power \(P\) [W] (Watt)

"The power at this very moment."
Formula: \(P = VI\) (Power = Volts × Amperes)

Electrical Energy \(W\) [J] (Joule)

"The total amount of electricity used."
Formula: \(W = Pt = VIt\) (Energy = Power × Time used)

Joule Heat \(Q\) [J]

When current flows through a resistance, heat is generated. This is called Joule heat.
Formula: \(Q = RI^2 t\) (Derived by substituting Ohm's law into \(W = VIt\))

★Step-by-Step: Choosing the Right Formula★

1. First, check if the question provides the "time duration."
2. If asked for "per second" or "power," use \(P = VI\).
3. If asked for "heat quantity" or "energy," use \(W = VIt\).

【Section Summary】
・Electric power \(P\) is the "intensity," while electrical energy \(W\) is the "total."
・You don't need to rote-memorize the heat formula \(Q = RI^2 t\) if you remember how to derive it using \(V = RI\).

Conclusion

In the "Electricity" section of Physics Basics, the shortcut to passing is to master Ohm's Law (\(V=RI\)) and the Power formula (\(P=VI\)) perfectly.
You might stumble over calculations at first, but if you get into the habit of drawing diagrams and noting down "how much current is flowing where," you will definitely be able to solve these problems.
Let’s move forward one step at a time and enjoy the process. I’m rooting for you!