Category Physics

A hovercraft is an air-cushion vehicle without wheels. It rides on a cushion of air which could be as much as 1.5 metres thick. This cushion of air reduces friction. A hovercraft moves on this cushion which is created within a rubber skirt at the bottom of the hovercraft. This rubber skirt is about two and half metres deep. The air cushion is created by powerful fans blowing air downward along the periphery inside the skirt. Propellers on the deck push the hovercraft forward. A gas turbine engine provides necessary power to both the fans and the propellers.

Hovercrafts make an ideal ferry as they can travel from water straight on to land. The speed of a hovercraft can be as high as 120 km/hr which is faster than any ship. However, a hovercraft becomes unstable in rough weather.

Fire extinguishing agents work basically in two ways — either by cooling the burning materials or by blanketing them with an inert coating that cuts off the supply of oxygen. There are different types of fire extinguishers. One of the most common ones uses water which has a high heat capacity. Water is released out of such extinguishers at high pressure so it acts like a powerful cooling agent. However, water type fire extinguishers cannot be used on electrical fires or where the burning substance involves oil. Foam type fire extinguishers use foaming agents that have a smothering and cooling effect on the fire. A dry chemical extinguisher sprays a very fine power of sodium bicarbonate or potassium bicarbonate or monoammonium phosphate. These solids coat the fuel and smother the fire. A safe and effective extinguisher for all confined fires uses carbon dioxide ( ) which acts as an inert blanket.

Based on the nature of the burning substance, different kinds of fire extinguishers are used, each having a specific design and mode of functioning. 

Fire alarm is a safety device which gives warning of a fire before it spreads. Most fire alarms work on the principle of detecting heat, smoke or flame. Heat detectors sense the rise in temperature due to fire and trigger an alarm by either melting a low fusing alloy or bending a bimetallic strip. In smoke detectors, a beam of light falling on a photocell is interrupted by smoke particles triggering the alarm. Flame detectors usually detect infrared radiation given off by flames to set off an alarm.

The picture on a television screen is nothing but a pattern of glowing dots, or pixels. The pixels are made up of fluorescent chemicals called phosphors that are coated on the back of the screen. These glow on being hit by a beam of electrons. In a black and white television, the pixels are made of a single phosphor and are lit up by a single electron beam that rapidly sweeps across the screen. In a colour television, however, each pixel contains three phosphors each producing a different colour – green, red and yellow. Three electron beams produced by three electron guns are used to light up the different phosphors. The instructions specifying which of the phosphors in a pixel are to be lit are contained in the TV signal transmitted by the telecasting station. A perforated screen also called the shadow mask, placed behind the screen ensures that an electron beam reaches only the dot producing a specific colour. Different combinations of these three basic colours give illusion of different hues of colours in the final picture.

Cordless telephone is a wireless version of a conventional telephone. It is made up of two units – a mains-powered base unit which is connected to the telephone line, and a battery-powered wireless hand set which is used for talking. Both the base unit and the hand set have built in low-power radio transmitters and receivers for two-way communication which operate at a pre-set frequency. When an incoming call arrives it is received by the base unit which passes it on to the handset via coded radio signals. The handset antenna picks up the signals and gets linked to the calling line. Similarly when the person called talks into the handset his or her voice is transmitted to the base unit via radio waves and on to the telephone line. The two parties can now talk as on a conventional phone. Most cordless phones operate only within a range of a few hundred metres.

The word ‘radar’ stands for Radio detecting and ranging. It makes use of very short radio waves called microwaves to find out how far away an object is and in case of a moving object in what direction it is moving and at what speed. These waves are transmitted from a special type of antenna which can direct a narrow beam of the radio-waves at a distant object. The antenna not only sends out the radio beam but also receives back the reflected signals from the distant object. The reflected beam is processed to produce an image on a screen similar to a TV screen, indicating the location of the distant object. Usually the antenna is made to rotate or oscillate horizontally to scan the horizon. By observing the position of the image produced over a period of time on the screen, which is marked with a scale of distances, the direction and speed of the object can be determined.