SNC1DB
Charging by Contact & Induction
Content
Electricity is an important part of our everyday lives. It powers our televisions,
microwaves, computers, iPods, and virtually all other electronic devices. Current
electricity may come from electrical plants that transport the electricity to your homes
and the outlets in which your appliances are plugged into, or batteries may provide
electrical current for smaller devices.
We use so many electrical devices it is hard to find where to plug them all in.
Static electricity is also important in our lives, and most of you have experienced it. If
you brush your hair on a cold dry day, it can make your hair stand on end, or when you
walk across some carpet and touch a doorknob you may have received a shock. Static
electricity is the build up of a stationary electric charge on an object.
Did you know?
Greek philosophers first documented static electricity when they
rubbed a piece of amber with a cloth. They noticed that amber
attracted small pieces of straw. The phenomenon was thought by
many to be magic because the observations could not be
explained as the molecular model of the atom was not yet known.
In 1600 Dr. William Gilbert coined the term “electrica”, a Latin
word describing the electrical charge that builds up when some
substances are rubbed with amber.
Electric Charges
Mini-Lab
Try blowing up a balloon and sticking it to a wall. What happens to the balloon?
Now rub the balloon on your hair and then try sticking the balloon to a wall. What
happens?
In the mini-lab above the balloon initially did not stick to the wall, however after rubbing
it on your hair it did stick to the wall. Why did this happen?
Atomic
structure of a
helium atom.
Recall from the chemistry unit that atoms consist of a nucleus with
protons and neutrons surrounded by electrons. The protons and
neutrons are relatively large and heavy compared to electrons and
they are tightly bound in the nucleus. The nucleus has an overall
positive charge because of the positive charge of the protons.
Electrons are small and negatively charged particles in constant
motion as they orbit around the nucleus. Electrons are able to move
to other atoms because they are not held tightly inside the nucleus.
This implies the first rule of electricity; it has to do with the motion of
electrons.
Initially the balloon and your hair are both electrically neutral with the number of protons
and electrons in each atom of the balloon and in your hair being equal. After rubbing the
balloon on your hair, your hair transfers some of the electrons to the balloon resulting in
a negative charge on the balloon (more electrons now than protons), and leaving your
hair positively charged (less electrons then protons).
Laws of Electric Charges
There are three laws that describe what happens when charged objects are brought
together.
1. Like charges will repel.
2. Unlike charges will attract.
3. Charged objects will attract neutral objects.
Although electrons are much smaller than protons their electrical charges are
approximately equal but opposite in charge. The interaction between charged objects is
also dependant on the distance, and the farther apart they are the less force they will
have on each other.
When two oppositely charged objects are brought close
together they will be attracted to each other – this also includes a charged object, either positive or
negative, being brought near a neutral object. When two objects with the same charge are brought close
together they will repel.
Creating Static Charges
Charging By Friction:
Static charges can be produced by rubbing two different objects together resulting in the
movement of electrons from one object to another. For example when a glass rod is
rubbed with silk some of the electrons from the glass are transferred to the silk, resulting
in a negatively charged silk cloth and a positively charged glass rod. The net negative
charge produced on the silk cloth is equal to the net positive charge produced on the
glass rod.
The charges on objects due to friction can be predicted with an electrostatic series. An
electrostatic series is a list that shows how strongly a substance holds onto its
electrons. The following electrostatic series is in increasing order of strength in ability to
hold onto the electrons. If two objects are rubbed electrons will be ripped off the one
that holds them more loosely and onto the one that holds them more strongly.
Electrostatic Series
Weak hold on electrons
Acetate
Glass
Nylon
Wool
Human hair
Silk
Aluminium
Cotton
Ebonite
Plastic
Carbon
Rubber
Gold
Strong hold on electrons
Example
Locate silk and plastic in the electrostatic series. Silk is first and therefore does not hold
onto its electrons as strongly as plastic. When silk is rubbed over a plastic rod, the silk
will transfer electrons to the plastic rod and the net charge on the plastic will be negative
leaving the silk with a positive charge.
Look at the electrostatic series above. Do you see why kids sometimes get a charge from going
down a slide?
Charging By Contact:
A neutral object can be charged by touching it to a charged object.
When a negatively charged
rod touches a neutral sphere the electrons will move into the neutral object. The rod will remain negative
and the sphere will have a negative charge after contact.
A neutral object could also be given a positive charge by touching it with a positively charged rod.
The electrons move from the sphere to the rod leaving the sphere with a positive charge as well as the
rod.
Charging By Induction:
How can an object be charged without touching it? No, it is not by magic. Recall two of
the facts about static charges:


like charges repel each other
electrons move from one object to another
Induction
Charges on two neutral spheres in contact could be induced by bringing a charged
object close to but not touching the two spheres. In the example a negatively charged
balloon repels the electrons in the spheres resulting in the electrons being transferred to
the sphere on the left. If the spheres are moved apart with the balloon still close by then
the electrons are not able to redistribute back to the two neutral spheres and as a result
the sphere on the left remains negatively charged leaving the sphere on the right with a
positive charge equal to the negative charge.
Applications of Static Electricity
The build up of static charges may be potentially dangerous and harmful to electrical
equipment. Computer data may be lost due to static shock, and people who work with
constructing electrical circuits wear specialized suits that prevent damaging the
equipment and the devices being constructed.
Computer technicians wear wrist straps to make sure they are grounded.
Grounding
Static charges can easily be removed by discharging the charge into the Earth.
Grounding objects to the Earth works in both ways as electrons may be removed from a
negatively charged object until it becomes neutral, or electrons may be provided from
the Earth to positively charged objects that are grounded. The Earth is large so it
remains relatively neutral when electrons are added or removed.
A lightning rod on top of a tall building acts as a direct path to the ground. A metal
conductor (usually copper) is connected from the pointed rod to a metal plate buried in
the ground. The metal conductor will carry the electrons safely to the plate buried in the
ground. This prevents the electrons from going through the building and causing a fire.
Did you know?
The fumes in grain elevators are explosive and a single shock
from a person touching a door could ignite the explosive gases.
As a result there are special regulations and equipment to
prevent the unfortunate shock that might turn into an explosive
situation.
Applications of static electricity have also been beneficial in technology and
society. Photocopiers used to make copies, electrostatic precipitators used in
reducing pollution, and electrostatic spray painters used to paint objects more
efficiently are all devices that operate based on electrostatic theory.