Unit 3: Commonly used appliances and devices

Home Appliances

This section provides a brief description of the working principles of common household appliances.

Pressure Cooker

Principle: The boiling point of water depends on the surrounding pressure. At higher pressure, water boils at a higher temperature.

• Working: A pressure cooker is a sealed pot. As the water inside boils, the steam (water vapor) cannot escape.
• This trapped steam increases the pressure inside the pot to about 2 times normal atmospheric pressure.
• At this high pressure, the boiling point of water is raised from 100°C (212°F) to about 121°C (250°F).
• The food inside is therefore cooked at a much higher temperature, making the cooking process significantly faster.
• A safety valve (or "whistle") releases excess steam to prevent the pressure from becoming dangerously high.

Water Purifiers (UV & RO)

These are two different technologies often used together to purify water.

• UV (Ultraviolet) Purifier:
  • Principle: Uses ultraviolet (UV-C) light to kill microorganisms.
  • Working: Water is passed through a chamber containing a UV lamp. The high-energy UV light penetrates the cells of bacteria, viruses, and other pathogens, damaging their DNA. This makes them unable to reproduce and renders them harmless.
  • Limitation: UV *only* kills germs. It does *not* remove dissolved salts (like arsenic, fluoride, heavy metals) or suspended particles (like mud).
• RO (Reverse Osmosis) Purifier:
  • Principle: Uses high pressure to force water through a semi-permeable membrane, leaving contaminants behind.
  • Working: "Osmosis" is the natural tendency of water to move from a low-salt-concentration area to a high-concentration area. "Reverse Osmosis" does the opposite. An electric pump creates high pressure on the "tap water" (high-concentration) side. This pressure forces *only* the water molecules through the tiny pores of the RO membrane, leaving behind dissolved salts, heavy metals, and even microorganisms.
  • Limitation: RO is very effective at removing dissolved solids, but it also wastes a significant amount of water (the "reject" water) and can strip the water of beneficial minerals.

Electric Motors

Principle: The motor effect. A current-carrying wire placed in a magnetic field experiences a force.

F = ILB sin(θ)

• Working:
  1. A coil of wire (the "armature") is placed inside a magnetic field (created by permanent magnets or electromagnets).
  2. An electric current (from a battery or AC source) is passed through the coil.
  3. One side of the coil experiences a force pushing it *up*, and the other side experiences a force pushing it *down*.
  4. This pair of forces (a "couple") creates a torque, causing the coil to spin.
  5. A "commutator" (in DC motors) is a clever switch that reverses the direction of the current in the coil every half-turn, ensuring that the coil keeps spinning in the same direction.
• Application: Converts electrical energy into mechanical energy. Found in fans, blenders, washing machines, etc.

Fans

A fan is a simple application of an electric motor. The motor's axle (the spinning part) is connected to blades. The blades are angled so that as they spin, they push air in a specific direction (downwards for a ceiling fan), creating a "wind" or air current.

Sound Systems and Microphones

These two devices work on opposite principles.

• Microphone (Converts Sound to Electricity):
  • Principle: Electromagnetic induction. A changing magnetic field through a coil induces a voltage (current).
  • Working: You speak into the microphone. Your sound waves (which are pressure waves) hit a thin diaphragm, causing it to vibrate.
  • In a dynamic microphone, this diaphragm is attached to a small coil of wire. The coil vibrates back and forth *inside* a permanent magnet.
  • This movement of the coil in the magnetic field induces a small, changing electrical voltage in the wire. This electrical signal is an "analog" (a copy) of the original sound wave.
• Sound System (Loudspeaker):
  • Principle: The motor effect (opposite of a microphone). A current in a magnetic field feels a force.
  • Working: The electrical signal from an amplifier (the "sound") is fed into a coil of wire attached to a large cone (the speaker).
  • This coil (the "voice coil") sits inside a powerful permanent magnet.
  • As the electrical signal (current) rapidly changes, the coil experiences a rapidly changing magnetic force, pushing it back and forth.
  • The coil pushes the large cone, which in turn pushes the air, creating pressure waves... which are the sound you hear.

Micro-ovens (Microwave Ovens)

Principle: Dielectric heating. Certain molecules (especially water) are "polar," meaning they have a positive and a negative end. These molecules will try to align themselves with an electric field.

• Working:
  1. A device called a magnetron generates high-frequency electromagnetic waves called microwaves (frequency ~2.45 GHz).
  2. These waves fill the metal box of the oven.
  3. The electric field of the microwaves changes direction billions of times per second.
  4. Water molecules in the food (which are polar) try to "follow" this rapidly flipping field. They twist and turn, bumping into each other.
  5. This intense microscopic friction creates heat, which cooks the food from the inside-out.
Why doesn't a microwave oven cook your plate?
Ceramic and glass are typically non-polar. Their molecules don't have a + and - end, so the microwaves pass right through them without causing them to vibrate. The plate only gets hot from the *food* touching it.
Why no metal? Metals *reflect* microwaves, which can bounce back to the magnetron and damage it. It also causes sparks (arcing).

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Modern Electronic Devices

Thermistors

A thermistor is a "thermal resistor" — a resistor whose electrical resistance changes significantly with temperature. They are made from semiconductor materials.

• NTC (Negative Temperature Coefficient): This is the most common type.
  • As temperature increases, resistance decreases.
  • Use: As highly sensitive temperature sensors (e.g., in digital thermometers, car engine temperature gauges, thermostats).
• PTC (Positive Temperature Coefficient):
  • As temperature increases, resistance increases.
  • Use: As resettable "fuses." If the current gets too high, the PTC heats up, its resistance shoots up, and it chokes off the current, protecting the circuit.

Displays (LCD and LED)

These are the two main types of flat-screen display technologies.

• LCD (Liquid Crystal Display):
  • Principle: Liquid crystals are materials that can "twist" or "untwist" to block or pass light when a voltage is applied.
  • Working: An LCD screen has two main parts: 1. A constant backlight (usually made of LEDs) that is always on. 2. A "shutter" system made of pixels. Each pixel is a tiny liquid crystal cell sandwiched between two polarizing filters.
  • To make a pixel dark, a voltage is applied. The crystal untwists and blocks the backlight.
  • To make a pixel bright, no voltage is applied. The crystal twists the light so it can pass through the second filter.
  • Summary: LCDs *block* light to create an image. They are not light emitters.
• LED (Light Emitting Diode):
  • Principle: A diode is a semiconductor device that lets current flow in one direction. An *LED* is a special diode that emits light when current flows through it.
  • Working (as a display): 1. True LED Displays (like in giant stadium screens or OLED TVs): Each *pixel* is a tiny, individual LED (or a group of red, green, and blue LEDs). 2. To make a pixel bright, current is sent to it, and it lights up. 3. To make a pixel dark, the current is turned off.
  • Summary: LEDs *create* their own light. This is why "OLED" (Organic LED) displays have perfect "blacks" (the pixel is just turned off) and better contrast than LCDs.

Digital Watches

A digital watch is a mini-computer dedicated to keeping time.

• Time Base: The "heart" of the watch is a tiny quartz crystal. This crystal is piezoelectric, meaning it vibrates at a *precisely* constant frequency (exactly 32,768 times per second) when a voltage from the battery is applied.
• Counter Circuit: An integrated circuit (IC) "counts" these vibrations. It knows that after 32,768 vibrations, exactly one second has passed.
• Logic: The circuit then updates the "seconds" count. When "seconds" reaches 60, it resets to 0 and adds 1 to the "minutes" count, and so on.
• Display: The IC sends signals to the LCD display (see above) to show the correct numbers.

Mobile Phones

A mobile phone is a highly complex, portable two-way radio and computer.

• As a Radio:
  • When you speak, a microphone converts your voice to an electrical signal.
  • A processor digitizes this signal (converts it to 1s and 0s).
  • A transmitter encodes this digital signal onto a radio wave and sends it via the antenna.
  • The wave is received by the nearest cell tower, which routes your call through the network.
  • The reverse happens for incoming calls: the tower sends a radio signal, the phone's receiver decodes it, and a speaker turns the digital signal back into sound.
• "Cell" Network: A region is divided into "cells," each with its own tower. This allows many people to use the same frequencies without interference. As you move from one cell to another, your call is "handed off" to the next tower.
• As a Computer: It has a CPU (Processor), RAM (Memory), Storage (Flash memory), a battery, and an Operating System (like Android or iOS), allowing it to run apps, browse the internet (via Wi-Fi or the cell data network), and act as a camera, GPS, etc.

Computers

A computer is a programmable device that processes data. Its basic working is based on the von Neumann architecture.

1. Input: You provide data and instructions (e.g., from a keyboard, mouse, or touch screen).
2. Memory (RAM): This is the fast, temporary "workspace." The data and the program instructions are loaded into RAM.
3. CPU (Central Processing Unit): This is the "brain." It follows the program instructions, fetching them one by one from RAM.
   • It performs calculations (using its Arithmetic Logic Unit - ALU).
   • It makes decisions (control flow).
   • It reads and writes data to and from RAM.
4. Storage (Hard Drive/SSD): This is the slow, permanent "filing cabinet" where the operating system, your apps, and your files are kept when the power is off.
5. Output: The CPU sends the results of its processing to an output device (e.g., the display to show an image, or a speaker to play sound).