Transcript Slide 1 - LPS.org

Unit 5 Electricity
Chapter 7
Electric Charge
• Protons have + charge
• Electrons have – charge
• Static electricity is the
accumulation of excess
electric charges on an
object
• Law of Conservation of
Charge – charge can be
transferred but not
created nor destroyed
Transferring charge
• Conductors – allow
electrons to transfer
easily
• Insulators – don’t allow
electrons to transfer
easily
• Induction –
rearrangement of
electrons on neutral
object caused by a
nearby charged object
• Common conductors:
• Metals
• Water
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Common insulators:
Air
Plastic
Rubber
Wood
Glass
Grounding
• Lightning consists of negative charges
• Buildings should provide a path for the
charges to reach earth
• This path is a lightning rod that extends to the
highest point on the building and extends to
the ground
Detecting electric charge
• An electroscope can detect the presence of
electric charges
• When the device is not charged, the metal
leaves hang straight down
• When charged, the leaves move away from
each other
Electric Current
• Diff in voltage allows charge to move from
high V to low V
• Electric current – flow of charges through a
wire or any conductor
• Electric current is measured in Amperes (A)
Batteries
• A common source of voltage difference is a
battery
• 2 types:
• 1. Dry cell- most common around the house,
has a positive and negative terminal on ends
• 2. Wet cell- car battery, has positive and
negative terminal as knobs on top
Sockets
• Another source of a voltage difference are the
sockets in your house.
• Most sockets in your walls provide a 120V
difference
• Electric ovens and dryers need a socket with a
240V difference
• Circuit- closed loop which allows charge to
flow
• Resistance - tendency for material to oppose
the flow of electrons, converting electrical E
into thermal E and light
• Resistance is measured in Ohms
• The thinner the wire, the more resistance
Equation time!
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Ohm’s Law
I = current (in Amps)
V = voltage (in Volts)
R = resistance (in Ohms)
I=V/R
So greater the voltage, the greater the current
Greater the resistance (distance), the smaller
the current
Ohm’s Law
• Calculate the voltage difference across a 25
Ohm resistor if a 0.3 A current is flowing
through it.
• What happens when the voltage is doubled?
Electric Circuits
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2 types:
1. Series- only one loop for the current to flow
If one light bulb goes out, they all go out
2. Parallel- two or more branches for the
current to flow through
• If one bulb goes out, the rest stay on
Household Circuits
• Houses are wired in parallel circuits
• The fuse box or circuit breaker is the electrical
headquarters of your house
• If the wires get too hot from too much current
flowing through them, the fuse will blow and the
circuit will be broken
• This protects your home from fires
• Similar process in circuit breakers, except those
can be reset and don’t need to be replaced like
fuses
Electrical Power
• Electrical energy is converted to mechanical,
light, heat, etc
• The rate that it is converted is electrical power
• Power = current x voltage difference
• P (watts) = I (amps) x V (volts)
• The unit for power is W (watts)
Electrical Energy
• The amount of electrical energy used depends
on the amount of power required to run the
appliance and how long it is used
• Energy = power x time
• E (kWh) = P(kW) x t(h)