11.2: Charging by Contact
pg. 472
Key Concepts:
1. Static electricity results from an imbalance of electric charge on the surface of an
object.
2. Objects can be charged by contact or by induction.
- Greek Philosopher, Thales of Miletus, rubbed amber over a piece of fur, after the fur
and amber were able to attract other objects, such as; feathers and straw.
- It was determined that neutral objects can be charged through direct contact.
- Two common methods of charging by contact are created by friction or conduction.
Charging Objects by Friction
Charging by Friction: is the transfer of electrons between two neutral objects, (made
from different materials), that occurs when they are rubbed together or come in contact
(touch).
- If two neutral objects are rubbed together, then each will become charge (friction).
- Electric charges are transferred from one object to the other.
- One object becomes negatively charged as it accumulates electrons, the other one
becomes positively charged as it loses electrons.
- Combing hair is an excellent example of charging by contact, or by friction.
- Objects are easily charge during the winter because the air is dry.
- During the summer the humidity is high (water vapour in the air), it is more difficult to
charge objects.
- Water vapour makes contact with charged objects and electrons are passed form the
object to the water vapour molecules.
- Some objects can be charge just by touching (contact).
Figure 2: a) The comb and the hair are both neutral. b) After rubbed together, the comb
is negatively charged and the hair is positively charged.
The Electrostatic Series
Electrostatic Series: a list of materials arranged in order of their tendency to gain
electrons.
- A list of materials in order of increasing tendency to gain electrons is known as an
Electrostatic Series.
- Moving down the list there is a greater affinity to gain electrons.
- Using the Electrostatic Series, you can predict the charge of objects that are rubbed
together.
- Important to know the objects higher up on the list will lose electrons (positive) will the
ones below will gain electrons (negative).
Sample Problem: Using Electrostatic Series
Ex. Rub a piece of wool against you skin. What charge does each material now have?
Step 1: Compare the positions of the materials on the electroscope series.
Step 2: Compare the attraction for electrons of the two materials.
Charging Objects by Conduction
Charging by Conduction: is the charging an object, by contact, with another charged
object.
- Charging by conduction occurs when electrons move from a negatively charged object
to a neutral object, changing the neutral object to negatively charged object.
- Two charged objects can also exchange charges. An object with more negative charge
then another negative charge object, will create two objects with the same amount and
type of charge.
Figure 4: A neutral metal sphere a) becomes charged by conduction when brought in
contact with a negatively charged bar. Some of the electrons are transferred from the bar
to the sphere, resulting in an overall negative charge on the sphere b).
Figure 5: Two metal rods being charged by conduction
Grounding
Grounding: connecting an object to a large body, like Earth, that is capable of
effectively removing an electric charge that the object might have.
- Objects that are either positively or negatively charged can have the excess charge
remove, turning them back to neutral objects by the process of grounding.
- Grounding occurs when a charge object loses their charge to large neutral object such as
the Earth.
- When a positively charged object is grounded electrons from the ground are passed into
the positively charged object creating a neutral object.
- When a negatively charged object is grounded, the excess electrons in the negative
object are passed into the ground. The negative charged object becomes neutrally charged.
Figure 6: The symbol for grounding.
Figure 7: a) Before the discharge, the hand is negatively charged and the faucet is neutral.
b) During the discharge, excess electrons are transferred from the hand into the ground. c)
After the discharge, the negative charge on the hand has been removed.
Putting Charged Objects to Work.
- Many products and technologies have been created by the advancements in
Electrostatics.
Electrostatic Dusters
- Friction causes dust to be attracted to the electrostatic duster.
Electrostatic Precipitators
- Particles of soot, dust and other pollutants are released into the air, when coal, oil and
natural gas are burnt to produce electrical energy.
- An electrostatic precipitator attracts these particles as they pass up a factories chimney
or smoke stack.
- This prevents pollution from entering the atmosphere.
- The particles pass over a negatively charged plate, taking on a negative charge. They
then pass over a positive plate and are attracted to the plate and removed form the air.
- Approximately 99 % of the particles are removed.
Figure 9: Electrostatic precipitators use charged plates to filter the particles out of
flowing gases such as exhaust and household air.
Evidence of Learning …. Students can
- correctly using the electrostatic series to predict static electric charges.
- predict and explain how static charges will redistribute themselves when two objects
with different amounts of charge are brought into contact.
- explain how grounding can remove excess charge from an object.
- describe and explain how electrostatic dusters and precipitators work.
Check Your Learning
Questions 1 – 8, page 477
Summary:
- Charging by friction occurs when two neutral objects made of different materials rub
against or touch each other and electrons are transferred between them.
- When objects are charged by friction, one material is more likely to accept electrons,
while the other is more likely to give up electrons. This is because some kinds of atoms
are more strongly attracted to electrons than others.
- The electrostatic series is a list that ranks the tendency of different materials to gain
electrons. It can be used to predict the charge that will be gained by two objects (made
from different materials) when they come in contact.
- Charging by conduction occurs when two objects with different amounts of electric
charge come in contact and electrons are transferred from one object to the other.
- A neutral object is charged by conduction when a charged object touches it. The neutral
object becomes charged with the same charge as the object that touched it.
- When two charged objects with different amounts of electric charge come in contact,
electrons are transferred between them.
- When we ground an object, we transfer electrons between the object and a large neutral
object such as Earth (the ground).
- Charging objects by friction or by conduction has practical applications.