Transcript Static and Current Electricity

Static and Current Electricity
Advanced Physical Science
Static Electricity
• Static Electricity = electric charge at REST on
an object
– remember static means not moving
• If something has static electricity, it means
there an EXCESS or LACK of electrons (e-)
• Examples:
– Clothes clinging when removed from dryer
– Lightning (before the strike)
– Your charged finger after walking on carpet
The Atom
• Normally, atoms are electrically neutral
• Each atom has the same number of electrons
(negative) as protons (positive)
• Electrons are whirling about, far away from
the nucleus and can be removed from the
atom by FRICTION!
Charge by FRICTION
• When certain materials are rubbed together,
electrons are transferred from 1 surface to
another.
• The blue balloon rubbed against sweater is
charged
• Excess e- = Negative Charge
• Lack of e- = Positive Charge
• Like charges repel
• Unlike charges attract
Charge by Conduction
• Conduction =e- move from 1 object to another by
DIRECT CONTACT
• Charging an electroscope shows conduction
• Electroscope
• After charging, the metal leaves have same
Charge by induction
• Charges in an uncharged object are rearranged
without direct contact with the object
• The negative charges on the yellow balloon
make a section of the wall have a positive
charge.
• Electrons in the wall are
repelled by and move away
from the balloon
Induction Example 2
• Lightning
• The negative charge at the bottom of the cloud
INDUCES a positive charge on the ground
Static Discharge
• When a surface has acquired a strong negative
charge, the extra e- may jump to a neutral or
positive object
• The “jump” of e- gives a spark
• Spark= a rapid movement of a # of e- through
the air
• Ex: lightning
Current Electricity
Current electricity is the flow of electric charges
through a wire or other conductor.
Current flows from high voltage to low voltage.
Current only occurs when there is a difference
in voltage.
Current flows from positive to negative.
More about Current
Current: the flow of electrons through a
conductor.
Current is measured in amperes (amps or A)
Symbol used for current in equations is I.
Why “I”?
Originally current was referred to as
electrical Intensity by French scientist,
André-Marie Ampère.
Measured using an ammeter
Notes - continued
Two types of electric current: AC & DC – No, this
is not the rock band!!
AC – alternating current – the electrons are
changing direction rapidly. (household current
– in U.S. current alternates 120 times per
second)
DC – direct current – the electrons flow in only
one direction (ex. = battery)
Important Terms
Current: the flow of electrons through a
conductor.
Conductor: a material through which electric
charges can flow.
Resistance: opposition to the flow of
electrons
Notes about Resistance
Resistance (R) is measured in ohms.
Named for Georg Ohm – German scientist who
developed mathematical descriptions of
electrical circuits.
The symbol for ohms = Ω (Greek letter omega)
A light bulb offers resistance to flow of eYour skin also offers resistance to flow of eWet skin offers less resistance than dry
OR
More on Resistance
Resistance depends on:
• Thickness and/or length of the wire
– Thicker wire = less resistance
• How well the material conducts electric
current
• Temperature
– In general for metals, as temperature increases,
resistance increases (direct relationship)
Voltage
Voltage: Push that causes electric charges to
flow
Also known as potential difference
Electrons flow from high potential to low
potential
The difference in voltage is what drives the
current through a resistor
More about Voltage
Without voltage, there will be no electric
current.
Voltage is measured in volts (V)
Measured using a voltmeter
Ohm’s Law
V
I
R
I = Current (amps or A)
V = Voltage (volts of V)
R = Resistance (ohms or Ω)
Instead of learning different formulas for current,
voltage and electricity, you can use this single
formula to find ANY of them
Use Ohm’s Law Triangle to solve
Reminders about Circuits
There are 2 Types of Circuits:
Series – one path for the electrons to travel
If one light goes out, they all go out. 
Circuits (Cont)
Parallel – more than one path for the electrons
to travel
If one light goes out, the remainder of the
lights remain lit.
Don’t you hope your house is wired this way?
Play with Circuits
• http://www.andythelwell.com/blobz/
• http://phet.colorado.edu/en/simulation/circui
t-construction-kit-dc
Use Ohm’s Law in a Circuit
• Given that you have a 12 volt battery and a
lightbulb that offers 3 ohms (Ω) of resistance,
what is the amount of current (I) going
through the wire?
• Solve…
• 12V = I*3 Ω
• I = 12V/3 Ω
• I = 4 amps
Electrical Power
Rate at which electrical energy is converted to
other forms of energy.
Measured in watts.
Formula:
P = Power P  V  I
V = Voltage
I = Current
Electric Energy
Electric energy = P  t
P = power (in kilowatts)
T = time in hours
Electric energy is measured in kilowatt hours.
1 kWh = 1000 watts of power for 1 hour of time
Convert watts to kilowatts by dividing by 1000
(dimensional analysis)
Ex: 220 watts * 1 kilowatt/1000 watts = 0.22 kW
Calculating Electrical Energy Cost
• From the HW the other night…
• A room was lighted with three 100 watt bulbs
for 5 hrs per day. If the cost of electricity was
$0.09 per kWh, how much would be saved by
switching to 60 watt bulbs?
• Step 1: You are using 3 bulbs, so figure out
how many watts in 3 bulbs
• 100 watts*3 = 300 watts
• 60 watts*3= 180 watts
Calculating Energy Cost (cont)
• Comparing 300 watts vs 180 watts
• Step 2: Convert 300 watt and 180 watt to
kilowatt (divide by 1000)
• 300 watt * 1 kilowatt/1000 watt = 0.3 kW
• 180 w * 1 kw/1000 w = 0.18 kW
• Step 3: Find the difference in kW used
• 0.3 kW -0.18 kW = 0.12 kW
Calculating Energy Cost (cont)
• Step 4: determine energy by multiplying by
the hours used
• 0.12 kW * 5 h= 0.6 kWh
• Step 5: What is the cost? $0.09 per kWh
• 0.6 kWh * $0.09/kWh= $0.054 or 5.4 (5) cents