Name: ____________________
SNC1
Static Electricity
1) a) In static electricity, the charges do not move.
b) Static electricity makes us feels strange, but it does not produce a painful shocks.
c) Current electricity (like the electricity from a wall socket) involves moving charges.
d) Current electricity causes painful shocks.
2) Like charges repel, unlike charges attract, charged objects attract uncharged ones.
Show where charges are in an atom
dark circles
small dots
light circles
3) In an atom: protons have a “+” charge
electrons have a “-” charge
neutrons have no charge.
protons can not easily be transferred from one object to another,
electrons can easily be transferred from one object to another.
4) In a solid:
5) Objects become charged when electrons are moved from one object to another.
6) a) All objects contain both positive and negative charges.
b) Positively charged objects have more positive than negative charges.
c) Negatively charged objects have fewer positive than negative charges.
7) For the following objects which are i) positively charged,
iii) neutral,
a
b
positive neutral
ii) negatively charged,
iv) not likely to occur naturally
c
d
e
f
g
negative unnatural positive unnatural positive
Charging by Friction
8) Why can we not charge to objects by touching them together, why must we rub them together?
It takes energy to remove electrons from atoms.
9) Show what will happen to the charges when the following objects are rubbed together.
a) balloon and hair
b) cotton shirt and
c) cotton towel and
wool socks
copper pipe
+
e- from hair to balloon
+
+ e- from sock to shirt
e- from towel to pipe
copper should have been on the list near gold
10) How do cars and aircraft get static charges? Rubbing against air and dust.
Charging by Contact
11) Static charges can be transferred from one object to another when they touch.
12) a) Show what happens to the charges when each of the following pairs of objects come in contact.
b) What charge does each object have i) before and ii) after they come in contact?
<—
o
-
<—
-
+
o
<—
o
+
+
o
+
Conductors and Insulators
13) a) Substances in which electrons can not move easily are called insulators. (plastic, wood, rubber, etc.)
b) Substances in which electrons can move easily are called conductors.(anything metallic, salt water)
c) Substances in which electrons can move somewhat easily are called semiconductors. (silicon, carbon)
List four household examples of each of the above.
14) a) Which type of substance do we use to protect us from electric shocks? insulators
b) Which type of substance do we use to transport electric charges?
conductors
c) Which type of substance can build up large static charges?
insulators
15) Insulators, like air, can still conduct electricity if the charges are large enough. They are ionized.
An example of this around the house is a spark. An weather example is lightning.
When filling a gas tank, why must the gas nozzle be kept in contact with the gas tank?
To keep the tank grounded so static charges cannot build up and cause a spark.
17) Why do ships not build up static electric charges the way that cars and aircraft do?
Water usually has enough dissolved ions to conduct the static charges away.
Charging by Induction
When a charged object is brought near a neutral conductor, the charged object will push electrons in the neutral
conductor around. This will make one part of the neutral object positive and the opposite end negative. The
two objects do not touch in this type of charging.
16) What would happen to the metal bar if the positive rod was brought close while the metal bar is grounded
(electrically connected to the earth)?
e- would be drawn through the ground wire from the earth into the metal bar.
If the connection to the earth was broken before the rod was taken away the electrons would be trapped in the
bar giving it a negative charge. This would it be a permanent charge achieved by induction.
17) Describe what happens when a neutral pith ball and a charged pith are brought close to each other.
Explain each step in the process, the type of charging, and the type of interaction between the pith balls.
a) Why will the two balls be attracted?
The negative pith ball will push electrons towards the far side of the neutral one making the near side of the
neutral ball positive (it is still neutral overall, though). This near side is attracted to the negative ball more
strongly than the more distant negative side is repelled, so the neutral ball and negative ball move towards
each other.
b) What will happen when they touch?
Electrons are transferred from the negative ball to the neutral one.
c) What will happen after they touch and why?
They both will be negatively charged at this point, so they will be repelled and move apart.
Discharging
18) a) What does it mean to be discharged (use the term excess charge)? The excess charge is balanced by
gaining or removing electrons to achieve a charge ratio of 1:1
b) How is discharging in a positive object different from discharging in a negative object?
e- donated to another object (if negative) versus electrons taken from another object (if positive)
19) a) Is the earth a good conductor, a good insulator, or somewhere in between? in between
b) Which contains more atoms, the earth, or an object like a car, person, or house? the earth
c) Why does contact with the earth cause objects to discharge?
e- can be added to or taken from the earth without causing a significant change in its charge ratio.
20) Grounding: attach a charged object to the earth, or some other large object, by a conductor. Electrons
will move through the conductor as in questions 16 and 18 to balance out the electrons and protons.
Humidity: water molecules are especially good at transporting electrons from one object to another.
Thus, air molecules in the air collide with a charged object and either take away electrons or add them. The
now charged water molecules eventually will collide with the earth and be grounded and neutralized.
Sharp Points: especially in conductors, excess charges accumulate on the surface of an object where
they try to push away from each other. If the object has a smooth edge, the only excess charges push
parallel to the surface and can’t push each other off the object. If the object has sharp points anywhere, the
excess charges at the point can be pushed off by the ones behind them. This eventually discharges the
object.
21) a)Why does static electricity build up more in the winter than the summer?
The humidity is lower in the winter so fewer water molecules are available to discharge objects.
b) Why do aircraft need discharge tips?
They rub against dust and other things in the air to become charged and they cannot be grounded.
If they were to land while charged, a dangerous spark would be generated between the aircraft and
ground just before landing.
c) How do cars get rid of their static build-up?
The tires are conductive enough to keep the car grounded.
22) a) How do we think that the top and bottom of a thunder cloud become charged?
Vertical air movement (warm, dusty air moving up, cold air with ice crystals moving down) within the cloud
cause charging by friction.
b) How does the ground under a thunder cloud become charged?
The negative base of the cloud is close to the ground and positively charges the ground by induction.
c) What happens to the air between a thunder cloud and the ground?
They can be ionized by the strong electric field between the cloud and ground which allows electric current
to flow between cloud and ground.
23) List two things that you should not do during a thunder storm.
You should NOT
talk on a land line phone
take a bath or shower
wash dishes (or come into contact with any metal plumbing or wiring)
stand in open spaces
stand under a tree
You SHOULD
get into a building or a car
unplug any electronics that are not on surge protected electrical circuits
crouch down on the balls of your feet if you start feeling statically charged
24) Describe how to detect charged objects with an electroscope.
Bring the object near the top of the electroscope.
If the object is negative, electrons in the top of the electroscope will be pushed down into the foil leaves.
They will both become negative and move apart.
If the object is positive, electrons in the foil leaves will be pulled up to the top of the electroscope. The foil
leaves will both become positive and move apart.