Unit E: Electrical Applications
Chapter 12: Static and Current Electricity
12.3: Charging by Contact
pg. 468
Key Concepts:
1. Static electricity is the result of an imbalance of charges
in the surface of an object.
2. Objects can be charged by contact or by induction
Charging by Friction
Charging by Friction: is the charging of two neutral
objects, made of different materials; by rubbing them
together.
When neutral objects are brought together, they may
transfer electrons. Both objects become charged. One
object received electrons while the other lost electrons.
The transfer of electrons occurs when to objects are rubbed
together, creating friction, which speeds up the transfer
process.
Example; Combing your hair, the comb attracts electrons
and becomes negatively charged. Your hair loses electrons
and become positively charged. Your hair becomes
attracted to the comb.
Figure 1: a) The hair and the comb start out electrically neutral. b) After combing, the
comb is negatively charged and the hair is positively charged.
Effects of Humidity
Humid air, water molecules have a greater opportunity to
bump together, transferring electrons to nearby objects. As
a result charged particles lose their charge quickly in humid
weather.
In dry weather there are less water particles in the air,
therefore there is less of a chance for objects to lose their
charge, and stay charged.
Charging by Conduction
Charging by Conduction: is the charging an object by
contact with a charged object.
Neutral objects can become charged by direct contact with
already charged objects. Electrons moved from the charged
object to the neutral object, causing it to become charged.
Conduction between Charged and Neutral Objects
When an object is negatively charted, they have more
electrons then protons. The electrons try to get further away
from each other (like charges repel). When a negatively
charged object touches a neutral charged object, the
electrons jump over to the neutral object. The two objects
now have the same charge and repel each other.
Figure 3: The negatively charged rod transfers electrons to the sphere by direct contact.
The sphere gains a negative charge by conduction.
The opposite occurs when you touch a positively charged
object to a neutral object. The positively charged object has
more protons then electrons. When the positively charge
object touches the neutral object, electrons are transferred
from the neutral object to the positive object. Both objects
have an evenly distribution of electrons, and are now
positively charged and repel each other.
Conduction between Two Charged Objects
Charging by direct contact does not have to involve a
charged object and a neutral object.
Two charged objects can pass electrons onto one another.
Objects with similar charges will pass electrons from an
object with greater negative charge to an object with a
lower negative charge.
Figure 4: Electrons will move from the rod with the greater negative charge to the one
with the lesser charge until the charges are evenly distributed. Both rods will have a
negative charge, but now they both have the same amount of charge.
Check Your Learning:
Questions 1 – 5, pg. 471
Wrap Up:
- When two different neutral materials rub against each
other, electrons are transferred between them. This is
called charging by friction.
- Charging by friction results in charged objects that
have opposite charges.
- When a neutral object is charged by contact with a
charged object, the neutral object becomes charged
with the same type of charge as the object that touched
it.
- When there is a difference in charge between two
objects that are the same size, contact causes electrons
to be transferred between the objects until the charge
is balanced.
Try This: Charging by Contact
pg. 469