Unit E: Electrical Applications
Chapter 12: Static and Current Electricity
12.5: Charging by Induction
pg. 474
Key Concepts:
1. Static electricity is the result of an imbalance of charges
on the surface of an object.
2. Objects can be charged by contact or by induction.
Induced Charge Separation
Induced Charge Separation: is a shift in the position of
electrons in a neutral object that occurs when a charged
object is brought near it.
Charging by Induction: is the charging of a neutral object
by bringing another charged object close to; but not
touching, the neutral object.
Rubbing a balloon against your sweater or hair, the balloon
will gain electrons, becoming negatively charged. The wall
is neutral having an equal number of positive and negative
charges.
As you bring the balloon closer to the wall, the negative
charge of the balloon repels some negative electrons of the
wall. Some of the wall’s electrons move away from the
balloon. The balloon becomes attracted to the wall; this is
caused by induction, or charging by induction.
Figure 1: a) When a negatively charged balloon is brought near a neutral wall, b) the part
of the wall near the balloon is temporarily charged by induction. The positive charges on
the surface of the wall attract the balloon.
The induce separation causes the wall to become positively
charged on the surface.
The negatively charged balloon is attracted to the positively
charge wall. This is only temporary as the negative
electrons evidently move back to their position once the
balloon is removed.
Charging More Than One Object by Induction
A positively charged object can be used to induce a charge
in a neutral object.
You can also use two objects at the same time to
permanently charge the objects.
In sphere A, the electrons are attracted by the positive
charge on the balloon. The electrons the balloon draw
electrons in sphere A to the left side. The right side of the
sphere becomes negatively charged.
Electrons in sphere B are attracted to sphere A and are
transferred.
Keep the balloon in place and remove sphere B from sphere
A, makes sphere B permanently charged positively.
Electrons are unable to move back.
Remove the balloon form sphere A allows the electrons to
spread throughout the sphere, yet it is still negatively
charged.
Figure 2: a) Two identical neutral spheres are in contact. b) A charged balloon induces
the movement of electrons. c) Separating the spheres prevents further electron transfer. d)
Both spheres have been charged by induction.
Grounding
Grounding: is connecting an object to a large body, like
Earth, that can remove an electric charge from the object.
By adding or removing electrons from a charged object you
can return it to neutral. Large objects, like Earth; can
receive or lose electrons from charged objects, returning
them to neutral, without becoming charged itself.
The process of removing or adding electrons to make an
object neutral is called grounding.
When a negative charged object is grounded, electrons
move from the object to the ground.
When a positive charged object is grounded, electrons
move from Earth to the object.
Figure 3: This symbol indicates an object is grounded.
Discharge and Grounding
When a person caries a negative charge and then comes in
contact with neutrally charged object, grounding can take
place. An electric discharge will take place, electrons
rapidly transfer, and a shock is received and a spark may be
created. If the object is grounded the electrons will not stay
in the object, but will enter the ground.
Both the hand and the object become neutral.
Figure 4: a) The person’s hand is negatively charged. the faucet is neutral. b) The excess
electrons move from the hand through the faucet to the ground. c) Both the hand and the
faucet are neutral.
Technicians working on electronic devices must always be
sure to be grounded. An electrostatic discharge from the
technician to the electronic equipment can damage the
device.
Permanent Charges and the Law of Electric Charges
As a negatively charged rod, gets closer to a neutrally
charged pith ball, the force of repulsion increases. This
repulsion will create a temporary positive charge in the side
of the pith ball closest to the rod. Electrons move to the
other side of the pith ball causing this side to be negatively
charged while the near side is positively charged. The rod
is attracted to the oppositely charged side of the pith ball.
(negative to positive)
If you touch the negative side of the pith ball, you can
ground it, the electrons leave, causing the pith ball to be
permanently charged positive.
This process, charging by induction, causes a neutral object
to gain a charge opposite to that of the object that induced it.
Figure 5: a) A negatively charged rod induces a positive charge on one side of the pith
ball. b) Electrons that moved to the other side of the ball can travel to the ground. c)
Removing the ground before the charged rod is moved away leaves the ball permanently
charged.
Check Your Learning:
Questions 1 – 6, pg. 479
Wrap Up:
- Charging by induction refers to charging a neutral
object y bringing it close to; but not touching, a
charged object.
- Charging by induction can be either temporary or
permanent.
- Charging by induction helps explain why neutral
objects are always attracted to charged objects.
- Charging permanently by induction always produces
an object with the opposite charge to the original
charged object
Try This: Storing Static Electricity
pg, 478