Combustion And Flame
The fire produced by oil cannot be controlled by _____________.
Water
What is pressure?
The force acting on a unit area of a surface is called pressure.
Friction produces _______.
Heat
The passage of an electric current through a solution causes _______ effects.
_________ is also an electric spark, but on a huge scale.
Lightning
Name the types of combustion.
i) Rapid Combustion
ii) Spontaneous Combustion
iii) Explosion
What are the Si units for force and pressure?
Force = Newton
Pressure = Pascal
What are lubricants?
The substances which reduce friction are called lubricants.
Define electroplating.
The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating.
Define the term "earthing".
The process of transferring charge from a charged object to the earth is called earthing.
Define combustion.
A chemical process in which a substance reacts with oxygen to give off heat is called Combustion.
What are the types of contact forces and non-contact forces?
Contact forces = Muscular force, Friction.
Non-Contact forces = Electrostatic force, gravitational force, magnetic force.
What is rolling friction?
When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.
Does pure water conduct electricity? If not, what can we do to make it conducting?
No, pure water doesn’t conduct electricity. But when salt is dissolved in pure water, it conducts electricity.
True or False:
It is a good idea to carry an umbrella during thunderstorms. Justify
False, thunderstorms are accompanied by lightning, electric charges from clouds that travel to the metallic rod of the umbrella. Therefore metal is a good conductor of electricity so we may get an electric shock if carrying an umbrella.
Explain Spontaneous combustion with an example.
The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called Spontaneous combustion.
Ex: Forest Fire
In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
(a) Agents are fingers, object is lemon, effect of force changes the shape of lemon.
(b) Agents are fingers of the person squeezing the tube, object is toothpaste tube and effect of the force can be observed as the paste coming out of the tube (change in shape).
(c) Agent is the load suspended, object is the spring and effort can be seen in the form of elongation of spring on suspension of load (change in shape).
(d) Agent is muscles of athlete, object is athlete himself and effect of the force changes the state of motion of the athlete.
Explain why sliding friction is less than static friction.
The sliding friction is less than static friction because the sliding object get less time to interlock into the contact points on the floor. So it is somewhat easier to move an object already in motion than to get it started.
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.
The water used in the water hoses is not pure water and is a good conductor of electricity. So, the fire¬men shut off the electric supply before spraying water to save themselves and other people from electrocution.
Explain Lightning conductors.
A lightning conductor is a device used to protect buildings from the effect of lightning. A metallic rod, taller than the building, is installed in the walls of the building during its construction. One end of the rod is kept out in the air and the other is buried deep in the ground. The rod provides easy route for the transfer of electric charge to the ground.
It is difficult to burn a heap of green leaves but dry dry leaves catch fire easily. Why
The green leaves hold some amount of water, so its ignition temperature gets increased and it does not burn easily. On the other hand, dry leaves are waterless, so they catch fire easily (having low ignition temperature).
List the properties of force.
i) It may make an object move from rest.
ii) It may change the speed of an object if it is moving.
iii) It may change the direction of motion of an object.
iv) It may bring about a change in the shape of an object.
v) It may cause some or all of these effects
Explain why objects moving in fluids must have special shapes.
The objects moving in fluids must have a special shape to overcome the fluid friction acting on them. Efforts are therefore made to minimise the friction, so objects are given special shape having pointed fronts with little broader middle portion which gets tapered at the back called streamlined.
Santhosh had heard that rainwater is as good as distilled water. So he collected some rainwater in a clean glass tumbler and tested it using a tester. To his surprise, he found that the compass needle showed deflection. What could be the reasons?
Rainwater is pure water which is an insulator but it gets mixed with air pollutants like sulphur dioxide and nitrogen oxides and form acidic solution, which is a good conductor of electricity. So, the compass needle showed deflection.
What are the safety measures should be followed when your outdoor and indoor.
Outdoor: Don't travel in open vehicles and dont't stay in open places. Don't stand near tall buildings. Don't carry an umbrella. If no safe place is available, you are in an open place, stay far away from trees, poles and metal objects. Squat low in the ground. Place your hands on your knees with your head betweeen the hands. This will make you the smallest damage.
Indoor: Stay away from electrical wires etc. Don't call a person who is recieveing the call through a wired phone. Bathing should be avoided during thunderstorms. Electrical appliances like computers, TVs should be unplugged.