AC AND DC

Physics Notes: Direction of Current Flow, AC & DC, Electricity Production, and Electric Motors

1. Electric Current

Definition

Electric current is the flow of electrons through a conductor caused by a difference in electric potential (voltage).

• Symbol: I

• Unit: Ampere (A)

2. Battery

A battery converts:

Chemical Potential Energy → Electrical Potential Energy

A battery has:

• Positive (+) terminal

• Negative (−) terminal

Electron Flow

Electrons always move:

[
\boxed{\text{Negative Terminal} \rightarrow \text{Positive Terminal}}
]

3. Electron Kinetic Energy

Since electrons have mass and velocity, they possess kinetic energy.

Formula:

[
\boxed{KE=\frac12 mv^2}
]

Where

• KE = kinetic energy (J)

• m = mass (kg)

• v = velocity (m/s)

4. Why a Light Bulb Lights Up

As electrons move through the bulb:

Electrical Energy

↓

Kinetic Energy of Electrons

↓

Heat Energy

↓

Light Energy

The hot filament glows, producing light.

5. Types of Electric Current

There are two kinds of electric current:

A. Direct Current (DC)

Definition

Electrons move in one direction only.

Direction:

[
\boxed{- \rightarrow +}
]

Examples

• Batteries

• Flashlights

• TV remotes

• Phones

• Laptops

Characteristics

• Constant direction

• Usually one voltage level

• Best for low-voltage systems

• Common voltage: 12 V

B. Alternating Current (AC)

Definition

Electrons move back and forth, continually reversing direction.

Direction

→ ← → ← → ←

Characteristics

• Direction changes continuously

• Used for homes and businesses

• Voltage can easily be increased or decreased

• Standard frequency in North America:

[
\boxed{60\ \text{Hz}}
]

Meaning:

Current changes direction 60 times every second.

6. AC vs DC

7. Why AC Is Used Instead of DC

Electric companies prefer AC because:

• Voltage can be increased.

• Higher voltage reduces energy loss.

• Electricity can travel long distances efficiently.

• Voltage can later be reduced for homes.

8. Frequency

Frequency tells how many complete cycles occur each second.

Symbol:

[
f
]

Unit:

Hertz (Hz)

[
\boxed{1\ Hz=1\ cycle/second}
]

North American electricity:

[
\boxed{60\ Hz}
]

9. Electromagnetic Induction

Definition

Electromagnetic induction is the process of producing electricity by moving a conductor through a magnetic field.

Mechanical Energy

↓

Magnetic Field Changes

↓

Voltage Produced

↓

Electric Current

10. How Electricity Is Generated

Power plants convert:

Mechanical Energy

↓

Electrical Energy

Sources of Mechanical Energy

Renewable

• Wind

• Water (Hydroelectric)

Nonrenewable

• Coal

• Oil

• Natural Gas

11. Power Plant Process

1. Fuel heats water.

2. Water becomes high-pressure steam.

3. Steam spins a turbine.

4. Turbine spins a generator.

5. Generator rotates magnets.

6. Rotating magnets create AC electricity.

7. Electricity enters transmission lines.

8. Transformers reduce voltage.

9. Electricity enters homes.

Flow Diagram:

Fuel

↓

Steam

↓

Turbine

↓

Generator

↓

AC Electricity

↓

Transmission Lines

↓

Transformer

↓

House

12. Transmission Lines

Power plants send electricity through:

High-voltage transmission lines

Typical transmission voltage:

[
\boxed{35,000\ V}
]

Before entering homes:

Step-down transformers reduce voltage to

• 120 V

• 240 V

13. Transformers

Definition

A transformer changes AC voltage.

Two types:

Step-Up Transformer

Increases voltage.

Used before electricity enters transmission lines.

Step-Down Transformer

Decreases voltage.

Used before electricity enters homes.

14. AC Generator

An AC generator contains:

• Rotating coil

• Magnet

As the coil rotates:

• Magnetic field changes.

• Voltage changes.

• Current reverses direction.

Produces:

[
\boxed{\text{Alternating Current (AC)}}
]

Advantages

• Simple design

• No commutator

• Efficient

• Easy voltage transformation

15. DC Generator

A DC generator also uses electromagnetic induction.

Additional components:

• Commutator

• Brushes

Purpose of commutator:

Converts alternating current into direct current.

Produces:

[
\boxed{\text{Direct Current (DC)}}
]

16. Commutator

The commutator:

• Reverses electrical contacts every half turn.

• Keeps current flowing in one direction.

Without a commutator:

Generator would produce AC.

17. Electric Motors

Definition

Electric motors convert:

[
\boxed{\text{Electrical Energy} \rightarrow \text{Mechanical Energy}}
]

18. DC Motor

Uses

• Battery

• Brushes

• Commutator

Advantages

• Works directly with batteries

• Good speed control

Examples

• Laptop

• Electric wheelchair

• CD player

• Car starter

Disadvantages

• More complex

• More moving parts

19. AC Motor

Uses

• Coil

• Magnet

No commutator needed.

Alternating current changes magnetic poles:

North

↓

South

↓

North

↓

South

This continuous switching causes the motor to rotate.

Advantages

• Simple

• Reliable

• Low maintenance

Examples

• Refrigerator

• Fan

• Washing machine

• Television

• Air conditioner

20. Historical Scientists

Thomas Edison

• Developed the first commercial DC power system.

Michael Faraday

• Invented the first electric motor.

• Discovered electromagnetic induction.

William Sturgeon

• Built the first practical motor capable of powering machinery.

Emily Davenport & Thomas Davenport

• Patented the commutator-type DC motor.

Nikola Tesla

• Invented the first practical AC motor (1888).

• Revolutionized electrical power systems.

Important Energy Conversions

Battery

Chemical Energy

↓

Electrical Energy

Power Plant

Mechanical Energy

↓

Electrical Energy

Motor

Electrical Energy

↓

Mechanical Energy

Light Bulb

Electrical Energy

↓

Heat Energy

↓

Light Energy

Key Vocabulary

Essential Formulas

Current:

[
\boxed{I=\frac{Q}{t}}
]

Power:

[
\boxed{P=VI}
]

Voltage:

[
\boxed{V=\frac{P}{I}}
]

Current:

[
\boxed{I=\frac{P}{V}}
]

Kinetic Energy:

[
\boxed{KE=\frac12 mv^2}
]

Frequency:

[
\boxed{f=\frac{\text{cycles}}{\text{second}}}
]

Summary

• Electrons move from the negative terminal to the positive terminal.

• DC flows in one direction and is commonly supplied by batteries.

• AC reverses direction about 60 times per second (60 Hz) and powers homes.

• AC is preferred for long-distance transmission because its voltage can be changed efficiently with transformers.

• Power plants convert mechanical energy into electrical energy using electromagnetic induction.

• Generators produce electricity; motors convert electricity back into mechanical motion.

• DC motors use brushes and a commutator, while AC motors have a simpler design and operate directly from alternating current.