Charge

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Static Electricity
CHARGE
Principles of Physics
CHARGE
Charge is the ability to attract or repel
Q (large charges)
q (small charges)
Units: coulombs (C)
Types of charge: positive or negative
neutral → no charge
CHARGE
Charged particles:
Electron – around nucleus
Charge = -1 elementary charge (-1e)
Proton – in nucleus
Charge = +1 elementary charge (+1e)
MAGNITUDE OF CHARGE
Amount of charge a particle has is proportional to
the number of excess electrons or protons
Q = Ne
Q = charge (C) = q
N = number of excess charges
e = elementary charge
= charge of a proton when +
= charge of an electron when -
MAGNITUDE OF CHARGE
Example:
A charged sphere has an excess of 15 protons.
What is the charge of the sphere?
Q = Ne = +15e
Like charges REPEL
Unlike
charges
ATTRACT
ELECTROSTATIC FORCE
A force exists between all charged bodies

When the charges are like the force causes them to move
away

When the charges are unlike the force causes them to
toward each other

According to Newton’s 3rd law, if one charged body acts on
another, the other acts back on it with the same force
F1
+Q
F2
F2
+q
F1
-q
ELECTROSTATIC FORCE
A force exists between all charged bodies

Inverse relationship between force and distance



Force increases when distance decreases
Force decreases when distance increases
Direct relationship between force and charge


Force increases when charge increases
Force decreases when charge decreases
MATERIALS
Conductors: metals
 lots of loosely connected, easily moved electrons
Insulators: plastic, ceramics
 Tightly bound electrons
Semiconductors: Silicon, Germanium
 Semi-tight electrons – more movement at high temps
 Conductivity affected by doping (adding materials that
help control conduction)
GROUND
Source (donator) or sink (acceptor) of an infinite
amount of charge without significantly changing
its own charge
Examples:
1. Earth
2. You
TRANSFER OF CHARGE
Methods
1. Inducing a charge (Induction)
 Neutral objects can act charged when
placed near a charged object
2. Charge by Conduction
 Charging by touch
3. Permanently charge by induction
 Charging by touching to ground
INDUCING A CHARGE
+ + + + + + + + +
-- -- -- -- -- -- -- -- --


When a negatively charged
object is brought near a neutral
object, electrons move away
from it
The object attract because the
unlike charges are near
+ +
-- ---+ -- + +
--+ -- + +
-- + +
+ --+ -+ + -- + -+ + -- + -+ + --
 When
a positively charge object is
brought near a neutral object,
electrons in the neutral object
move toward it
 The objects attract because
unlike charges are near
WHY DOES THE BALLOON
STICK TO THE WALL?
Balloon (rubbed
over hair)
Wall (Neutral)
The charges in the wall separate
CHARGE BY CONDUCTION
Charge transfers directly from one object to
another
 Results in equal charge on all objects
 Charges distribute evenly between objects

CHARGE BY CONDUCTION
Example: A sphere with a charge of -4e touches a
sphere with a charge of -6e. What is their charge
after they separate again?
Together they have:
-6e + -4e = -10e
After they separate they
each have:
-10e/2 = -5e
CHARGE BY CONDUCTION
Example: A sphere with a charge of -4e touches a
sphere with a charge of +8e. What is their charge
after they separate again?
Together they have:
+8e + -4e = +4e
After they separate they
each have:
+4e/2 = +2e
PERMANENTLY CHARGING BY INDUCTION

Charging by touching to ground
PERMANENTLY CHARGING BY INDUCTION

Transfer of opposite charge, but same amount
Example: If charged object has a charge of +24e,
then object charged by induction will have -24e.

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