Chapter 8 - Electricity
Electricity can be studied under 2 branches
Static Electricity AND Current Electricity
Static Electricity
Structure of an atom
Net charge of an atom
+6 (from proton)
-6 (from electron)
0 (from neutron)
Total charge = 0
Hence, any atom will be neutral in nature
Charging by friction
What will happen if we rub 2 insulators?
● T
he friction causes electrons to jump from one insulator to another. In this case,
from Perspex to wool. Let’s see what happens in the atom then…
Balloon has gained 2 electrons, leaving it with a negative charge.
here are more protons in the atom of woolen cloth than electrons –
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leaving it with a +ve charge.
The material that gains electrons gets a –ve charge.
●
● The material that loses electrons is left with atoms with
fewer electrons, hence gaining a positive charge
● This charge collected is referred to asSTATIC charge,
as it stays in the materials.
● This process is called –charging by friction
EMEMBER
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ProtonsDO NOTmove in static charge formation.
PROPERTIES OF STATIC CHARGES:
● In charging by friction,both the objects get staticcharges
– onenegativeand onepositive.
● The amount of negative charge is equal to the amount of
positive charge (as the no: electrons gained by one object
is equal to the no: of electrons lost by positive object)
● Opposite(unlike) chargesattract,likechargesrepel.
● S
tatic charges have a tendency toneutralizethemselves,when
there is a path to a conducting object. This is called
DISCHARGING (OR) EARTHING.
● S
tatic charges tend to getaccumulatednear sharppoints / edges /
corners.
2 nd method of charging – CHARGING BY INDUCTION
● Let us assume we have apositivelycharged glass rod(which is
charged by friction).
● Now we bring it close to anuncharged (neutral object),in this
case a sphere (kept on an insulated base)
● When we bring the rod close, theelectronsin thesphere will be
attractedtowards the positively charged rod, as opposite
charges attract.
● So, theelectrons move to the left, leaving atomswith excess
protons on the right.
● This causes the sphere to have negative charge in the left, and
positive charge in the right.
● This is calledCharge separation, and the processis called
Charging by Induction.
emember–
R
Protons DO NOT move,
The sphere is still neutral, as its charges are just separated.
The glass rod and left side of sphere will attract each other.
HAT HAPPENS IF WE BRING A NEGATIVELY CHARGED ROD
W
NEAR A NEUTRAL SPHERE?
he opposite happens - Electrons in the sphere are repelled, so they move to the
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right, resulting in charge separation again!
A practical example of Electrostatic Induction
hen a charged plastic comb (charged by friction by
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r ubbing on hair) is brought near paper bits, the paper bits
are attracted to the comb.
Video link:
ttps://www.youtube.com/watch?v=74RvmPFkwQo
h
https://www.youtube.com/watch?v=VhWQ-r1LYXY
Sparks and Flashes caused by static charge
● L
et’s assume that a person keeps walking with his shoes on a woolen
carpet.
● S ince shoes and carpet are insulators, and since they rub, the person’s
shoes will gain electrons from carpet. (carpet can also gain e-) .
● This leaves the person to be left with a negative charge.
● Generally, these charges in the person will stay till there is a
conducting path.
● Now, let’s say the person tries to open a door with this metal knob.
● In many countries, it’s a common phenomenon for the person to see a
small spark from his fingers to the metal knob. The person may also
feel a slight shock in her fingers. All these, even before she touches the
knob.
BUT, WHY??
● W e all know that air is anINSULATOR(it doesn’t allowelectrons to
f low).
● But when a huge amount of static charge is collected, and there is a
thin layer of air separating the source and a conductor,AIR IS
IONIZED.
● Meaning, air is broken intofree electrons and positiveions. This air is
conductingin nature, and is calledPLASMA. The processis called
AIR BREAKDOWN.
● S o, in the above example, the thin layer of air between the
person’s fingers and knob becomesPlasma.
● So electrons jump from the person’s fingers, through
conducting air, to the knob.
● When this happens, we see af lashand hear aspark.The
person may also feelshock.
xamples of static shocks:When a person walks withhis shoes on a
E
carpet, when a person travels in a car rubs his clothes with the car seat,
when a person walks with his shoes on a treadmill etc.
Lightning
● W hen clouds move, they rub with dust particles, small hail stones etc.
This results in clouds getting static charges.
● Generally, the bottom of the cloud has electrons (-ve charge), while the
top has positive charge.
● Since the charge is very high, air getsbroken downintoPlasma. Now
the electrons in the cloud flow to the ground, through plasma. When this
happens, we see a flash, which is what we callLightning.
Lightning arrestors:
⮚ Tall buildings have lightning arrestors to prevent any harm to the
buildings. Lightning arrestors are long metal rods that run across the
height of the building to the ground.
⮚ Since charges take the easiest path, the charges take the metal
rod/lightning arrestor.
