Transcript Chapter 20 - Teacher Pages

Chapter 20
Electricity
Section 20-1
• Electric Charge and Static Electricity:
– An electric charge exerts a force through the
electric field that surrounds the charge. An
electric field extends outward from every
charged particle, if the charged particles are
placed in the electric field of another charged
particle.
20-1
–If they are the same, the charges
push away.
–(+)(+)
(-)(-)
If they are different, the charges
pull towards each other:
(+) (-) or (-) (+)
20-1
• Electrons can sometimes leave their
atoms. When an object has gained
electrons the object is negatively
charged. When an object has lost
electrons the object is positively
charged.
20-1
• Static electricity is the build-up of charges
on an object. Charges build up, but do not
flow. For charges to build-up, charges must
be transferred from one object to another by
one of three ways:
20-1
1. Friction: electrons are transferred from
one object to another by rubbing:
•
•
Ex: a balloon against your hair.
Ex: comb rubbed on the carpet.
20-1
2. Conduction: electrons are transferred
from one object to another by direct
contact.
3. Induction: Charges are moved to one part
of an object as a result of electric fields to
another object.
20-1
• Charges are not created or destroyed, only
transferred. If an object gives up electrons,
another object gains those electrons. This is the
law of: Conservation of charge.
• When a negatively charged object and a positively
charged object are brought together, electrons
move until both objects have the same charge.
• The loss of static electricity as charges move from
an object is called the static discharge “shock”
20-2 Circuit Measurements
• Current depends on the resistance of the
material through which it travels.
– The greater the resistance, the less current there
is for a given voltage.
• Thickness of wire
• Length of wire
• What the wire is made of: copper, etc.
20-2
• The potential energy per unit of electric
charge is called electrical potential.
• The difference in electrical potential
between two places is called the potential
difference.
• Voltage is the unit of measurement of
potential difference.
20-2
• Voltage is measured with a device called a
voltmeter. Current is measured with a
device called an ammeter.
• Ohm’s law states that the resistance is equal
to the voltage divided by the current:
– Resistance = V/C or Ohm’s = Volts/Amps
20-2
Insert Diagram
• When the battery is charged, and we
connect the wires, voltage causes current to
flow in the circuit.
• Adding a second batter would increase the
voltage, therefore increasing brightness.
• If you added a 2nd light-bulb, more
resistance would occur. Less current flows.
20-2
• R = Resistance
• I = Current
• V = Voltage
I = Voltage/Resistance
V = Current multiplied by Resistance
Electrical Safety
• If a person touches a downed power line,
electricity can flow through the person in a short
circuit. (a connection that allows current to take
an unintended path).
• Insulation on wires protect you from getting
shocked.
• Rubber, thick-soled shoes increases the resistance
between you and the wire – reduces the current
going through your body.
• A wire that carries more current than it is designed
to carry will become hot – leading to a fire.
– In order to prevent circuits from overheating, devices
called fuses and circuit breakers are added to circuits.
– A fuse contains a thin strip of metal that will melt if too
much current flows through it. When the strip of metal
melts, or blows, the circuit stops.
• Lightening rod is a metal rod mounted on
the roof of a building in order to protect the
building.
• A lightening rod is connected to a
grounding wire. When lightening strikes
the rod, charges flow through the rod, into
the wire, and then into Earth – this protects
the building.
• When a fuse burns out, it cannot be used
again. To avoid this problem, we use circuit
breakers, safety devices that use an
electromagnet to shut of the circuit when
the current gets too high.
The human body depends on electrical signals. For
example, tiny electrical pulses control the beating
of the heart. If your body receives an electric
current from a source outside the body, the current
interferes with the normal process with your body.
Current greater than 0.2 amp cause burns and stop
the heart.
Chapter 20 Test
1. How do electric charges interact?
2. How does static electricity differ from
electric current?
3. What causes electric current to flow?
4. How does increasing voltage affect
current?
5. How does increasing resistance affect
current?