Electrical Charges and How They Transfer

Lesson 1 of 5: Static Electricity

In this lesson:

• What charge is and where it lives in the atom
• Why charge is always conserved
• How charge moves from object to object

What You Will Learn Today

By the end of this lesson, you will be able to:

1. Describe the two types of charge and how they interact
2. State conservation of charge and apply it
3. Distinguish conductors from insulators
4. Explain the three methods of charging
5. Connect atomic structure to electrical neutrality

What Happens When You Rub a Balloon?

Something invisible transfers during rubbing — and it has real, measurable effects

Two Types of Electric Charge

• There are exactly two types of charge: positive and negative
• Like charges repel — positive-positive or negative-negative pairs push apart
• Unlike charges attract — positive and negative pull together
• SI unit of charge: the coulomb (C)

Charge Lives in the Atom

• Protons (positive, ) — locked in the nucleus; do not move
• Neutrons (no charge) — locked in the nucleus
• Electrons (negative, ) — in the outer cloud; can transfer
• Elementary charge: C

Only electrons move. "Positive charge appearing" = electrons left.

Rubbing Moves Electrons Between Objects

Rubbing moves electrons from hair → balloon. Hair becomes positive; balloon becomes negative.

Which Object Becomes Negatively Charged?

When you rub a balloon on your hair, which becomes negative?

• A) Your hair
• B) The balloon
• C) Both become negative
• D) Neither — they stay neutral

Think: which particle moved, and which direction?

Rubbing Check — Balloon Gains Electrons

B) The balloon

• Electrons move from hair → balloon during rubbing
• Balloon gains electrons → becomes negatively charged
• Hair loses electrons → becomes positively charged
• The moving particle: electrons (protons never leave the nucleus)

Charge Cannot Be Created or Destroyed

• Charge cannot be created or destroyed — only transferred
• Total charge of an isolated system stays constant
• This is a fundamental conservation law — like conservation of energy

Worked Example: Two Conducting Spheres Touch

• Sphere A: nC · Sphere B: nC → they touch, then separate

Step 1: Total charge before contact:

Step 2: Charge distributes equally (same size spheres):

Conservation Check: What Is the Other Charge?

Two neutral objects are rubbed together. One ends up with nC.

What is the charge on the other object?

Use conservation of charge — total must stay the same.

Conductors Have Mobile, Free Electrons

• Conductors (metals, salt water): free electrons move through the material

  • Charge distributes across the whole surface
  • Examples: copper, aluminum, iron

• Insulators (rubber, glass, plastic): electrons are tightly bound — they don't flow

  • Charge stays where it's placed; examples: rubber, glass, dry wood

Conductors and Insulators in Real Life

• Electricians wear rubber gloves — insulator; current can't reach their body
• Copper wire (conductor) inside plastic sheath (insulator)
• Charged rod on metal sphere → charge spreads everywhere
• Charged rod on rubber ball → charge stays at contact point

Semiconductors (silicon): controllable conductivity — basis of transistors

Classify These Five Common Materials

Match each to: Conductor or Insulator

1. Copper wire
2. Rubber glove
3. Salt water
4. Dry wood
5. Silicon chip

Can electrons move freely through each?

Method 1: Charging by Friction

• Two materials rubbed together — electrons transfer between them
• Direction depends on the materials (triboelectric series)
• Both objects end up charged with opposite signs; total = zero

Example: Balloon on wool — balloon gains electrons (negative); wool loses electrons (positive)

Method 2: Charging by Conduction

Charged object touches neutral conductor → electrons flow → sphere gets same sign as rod

Method 3: Charging by Induction

No contact needed → grounding step → sphere gets opposite sign to rod

Why Induction Produces Opposite Sign

Walk through with a positive rod:

1. Rod near sphere → electrons cluster near rod end
2. Ground connected → electrons flow in from Earth
3. Ground removed → extra electrons trapped
4. Rod removed → sphere is negatively charged

The key: grounding lets electrons enter permanently

Conduction vs. Induction at a Glance

Worked Example: Induction Step by Step

Negative rod near neutral sphere → grounded → ground removed → rod removed.

What sign does the sphere acquire?

• Rod repels electrons → flee to far side
• Ground → electrons leave into Earth
• Ground removed → fewer electrons → positive
• Rod removed → sphere stays positive

Induction Quick Check: Positive Rod

A positive glass rod is held near a neutral metal sphere. Sphere is grounded, ground removed, rod removed.

What is the final charge on the sphere?

A) Positive — same as the rod
B) Negative — opposite to the rod
C) Neutral — charges cancelled

Practice: Identify the Charging Method

1. Rubber rod rubbed with fur becomes negative. Which method? What charge does fur acquire?

2. Charged rod touches neutral sphere; both end up nC. What was the rod's charge?

3. Negative rod charges sphere by induction. What sign does the sphere acquire?

Try all three before continuing.

Practice Answers: All Three Methods

1. Friction. Fur → positive charge; total stays zero.

2. Conduction. Total after = nC; sphere was nC → rod started at nC.

3. Induction. Negative rod → electrons leave → sphere is positive (opposite sign)

Key Takeaways: Charge and Atomic Structure

✓ Two types: positive and negative. Like repels, unlike attracts.

✓ Only electrons move — "positive charge" = electron deficit

✓ Charge is conserved — transferred, never created

Positive charges do not move — electrons do

Rubbing redistributes charge — it doesn't create it

Key Takeaways: Methods of Charging

✓ Conductors: free electrons, charge spreads. Insulators: charge stays local.

✓ Friction: contact, opposite signs on both objects

✓ Conduction: contact, same sign result

✓ Induction: no contact, opposite sign result

Watch out: Induction → opposite sign. Conduction → same sign. Don't mix these up.

Coming Up: Coulomb's Law and Force

Lesson 2 of 5: Coulomb's Law

• Force depends on charge amount and distance
• Electric force vs. gravity — same formula, very different scale

Prepare: review the sign rule and conservation of charge.