Page 120 - Science Class 08

Actually, a magnet is surrounded by an invisible
field of a force. Any piece of iron, if enters this
magne c field, gets a racted by the magnet.
A magnet also a racts another magnet. A
magnet has two poles — posi ve pole (South
pole) and nega ve pole (North pole). When the
posi ve pole of a magnet is brought near the
nega ve pole (North pole) of another magnet
they a ract each other. But, similar poles of
two magnets repel each other.
Posi ve Pole
(South Pole)
Nega ve Pole
Magnet (North Pole)
Electrosta c Force : When two objects of
different materials are rubbed together, they
acquire electric charges. They may acquire
nega ve or posi ve charge, depending on the
nature of the objects rubbed together. The
object which acquires posi ve charge a racts
the nega vely charged object. This force of
a rac on is called electrosta c force.
When you rub a glass rod with a piece of silk,
the rod acquires posi ve charge on the silk, an
equal amount of nega ve charge. On the other
hand, if you rub a rubber rod with a piece of
flannel, the rubber rod acquires nega ve
charge. If we hold a charged glass rod near a
charged rubber rod, the two a ract each other
and move towards each other.
1. What is gravita onal force?
2. What is lodestone?
3. What is the use of magne c compass?
4. What is electrosta c force?
5. How can we generate electrosta c force?
120
Electrosta cs is the study of electric charge
at rest, the forces between them and the
electric field associated with them.
PRESSURE
You have studied about what a force is. You
know that when we pull a spring or push a
vehicle, then we are applying a force. A push or
pull implies a force.
Let us think about another aspect of force:
We will study about the factors that may affect
the magnitude of force. There are two factors
that can affect the effect of force.
They are:
v The more we apply a force, the more is its
effect. It means the amount of force
applied is directly propor onal to its effect.
v The area over which the force is applied. It
means the area of contact between the
two objects. If the area of contact is less,
the effect of force is more.
It is therefore, the nails have pointed p. The
edge of the knife is sharp so as to make the
force applied more effec ve. We find it easy to
cut vegetables with a sharp knife. But, when
the knife in blunt it becomes tough to cut
vegetables. In this case, we apply more force.
Unit of Pressure
Now, we can define pressure as a force ac ng
perpendicularly on a unit area of the object.
Therefore,
Force(F)
Pressure (P) =
Area (A)
The unit of force is Newton (N) and the unit of
2
2
area is metre (m ).
Therefore, the unit of pressure, is N/m 2
Science-8