SPEED OF SOUND IN WINTER NIGHT

Speed of sound decreases with the fall in temperature as density of medium increases due to cooling.

FOG REDUCES VISIBILITY, WHY?

Why does fog reduce visibility range?

The fog particles scatter  light of all colour to the same extent as the size of particles  of fog is same or greater than that of  wavelength of light. As colours are scattered, the fog appears white and on the other hand the light from the other side does not reach the observer, so it reduces visibility.

USES OF ULTRASOUND

ULTRASONIC WAVES

What are the properties of ultrasonic waves?

1 They carry large amount of energy with them as they have high
  frequency.
2. They travel in straight path.

COMMERCIAL UNIT OF ENERGY

The commercial unit of electric energy is kilowatt hour. We have 

Energy =power ×time. Accordingly, Killowatt hour is the commercial unit as this unit is used to  measure electric energy and to prepare electricity bill.

What is Electroscope?

It is a device to detect and to identify electric charge. Examples of electroscope: Pith Ball and Gold leaf electroscope.

Why is loud sound heard during the process of Resonance?

Loudness depends on the amplitude of sound wave . In resonance, amplitude increases when natural frequency matches with the frequency of external force. As amplitude increases, loudness of the sound increases.

FOCUS OF A CONCAVE MIRROR

Focus is a point on the principal axis where the incident rays parallel to the principal axis meet after reflection. Highly diminished image is formed at focus if the object is
placed at infinity.

When does a nucleus become unstable?

A nucleus becomes unstable when number of neutrons in the nucleus is  more than the number of protons

Why is handle fitted near the edge of a door?

The moment of force directly proportional to the force and perpendicular distance of force from the axis of rotation. For a given moment of force, smaller force is required if it is applied at a longer distace from the axis of rotation. To open the door with least force ,handle is fitted near its edge.

Principle of moments

Principle of MOMENTS states that in EQUILIBRIUM the sum of clockwise MOMENT is equal to the sum of anti clockwise MOMENT.

Why do we need moisturizers in winters?

Winters in most places have very low humidity. For that,  the moisture of our body is lost easily and skin becomes dry quickly.

Temperature of surrounding falls during or after the heavy shower. Why?

Temperature of any medium or object falls when it loses the heat. After the heavy shower, water absorbs heat from the surrounding air. Water has high specific heat capacity (i.e. the quantity of heat required by unit mass for the unit rise of temperature)  so water absorbs large amount heat from the surrounding without much rise in it's own temperature. Hence, temperature falls.

Why is food cooked faster in pressure cooker?

Water boils at 100°C at normal pressure of 1 arm or 100000 Pascal. Boiling temperature increases  with the increase in pressure above the liquid surface. When we cook food in pressure cooker, the pressure increases due the retention of vapour and hence the boiling temperature increases in the pressure cooker upto 125°C As food is cooked at higher temperature than 100°C , it is cooked faster in the pressure cooker.