Transcript I. Electric Charge

Unit Electricity
I. Electric Charge
 Static Electricity
 Conductors
 Insulators
 Electroscope
What do these have in
common?
A. Static Electricity
 Static Electricity
 the net accumulation of electric
charges on an object
 Electric Field
 force exerted by an e- on anything
that has an electric charge
 opposite charges attract
 like charges repel
A. Static Electricity
 Static Discharge
 the movement of
electrons to relieve a
separation in charge
B. Conductors
 Conductor
 material that allows electrons to
move through it easily
 e- are loosely held
 ex: metals like copper and silver
C. Insulators
 Insulator
 material that doesn’t allow electrons
to move through it easily
 e- are tightly held
 ex: plastic, wood, rubber, glass
D. Electroscope
 Electroscope
 instrument that
detects the presence
of electrical charges
 leaves separate
when they gain either
a + or - charge
II. Electric Current
 Circuit
 Potential Difference
 Current
 Resistance
 Ohm’s Law
Potential Difference
B. Potential Difference
 Potential Difference (voltage)
 difference in electrical potential
between two places
 large separation of charge creates
high voltage
 the “push” that causes e- to move
from - to +
 measured in volts (V)
C. Current
 Current
 flow of electrons through a conductor
 depends on # of e- passing a point in
a given time
 measured in amperes (A)
D. Resistance
 Resistance
 opposition to the flow of electrons
 electrical energy is converted to
thermal energy & light
 measured in ohms ()
Copper - low resistance
Tungsten - high resistance
Resistance
 Caused by internal friction in conductor
and slows the movement of charges.
 A 40 W light bulb is dim and has a
higher resistance than a bright 100W
light bulb.
 https://www.youtube.com/watch?v=KvV
Th3ak5dQ
 Resistance video
D. Resistance
 Resistance depends on…
 the conductor
 wire thickness
• less resistance
in thicker wires
 wire length
• less resistance in shorter wires
 temp - less resistance at low temps
E. Ohm’s Law
 Ohm’s Law
V=I×R
V: potential
difference (V)
I: current (A)
R: resistance ()
• Voltage increases when current increases.
• Voltage decreases when resistance increases.
E. Ohm’s Law
 A lightbulb with a resistance of 160  is
plugged into a 120-V outlet. What is the
current flowing through the bulb?
KNOWN/UNKNOWN:
R = 160 
V = 120 V
I=?
I = 0.75 A
V
I
EQUATION: WORK:
I = (120 V) ÷ (160 )
I=V÷R
R
Superconductors
Some metals have zero resistance
when their temperature falls below a
certain temperature (critical
temperature).
The range is from -458º F to -189ºF.
III. Electrical Circuits
 Circuit components
 Series circuits
 Parallel circuits
 Household circuits
 https://www.youtube.com/watch?v=l8JS
8BbrVOg
 Video
 https://www.youtube.com/watch?v=lYZ
UXV-v71Y
 https://www.youtube.com/watch?v=D2
monVkCkX4
Circuits
A. Circuit
 Circuit
 closed path through
which electrons can flow
A. Circuit Components
A - battery
B - switch
C - light bulb
D - resistor
B. Series Circuits
 Series Circuit
 current travels in a single path
• one break stops the flow of current
 current is the same throughout circuit
• lights are equal brightness
 each device receives a fraction of the
total voltage
• get dimmer as lights are added
C. Parallel Circuits
 Parallel Circuits
 current travels in multiple paths
• one break doesn’t stop flow
 current varies in different branches
• takes path of least resistance
• “bigger” light would be dimmer
 each device receives the total voltage
• no change when lights are added
D. Household Circuits
 Combination of parallel circuits
 too many devices can cause wires to
overheat
 Safety Features:
 fuse - metal melts, breaking circuit
 circuit breaker - bimetallic strip bends
when hot, breaking circuit
IV. Measuring Electricity
 Electrical Power
 Electrical Energy
A. Electrical Power
 Electrical Power
 rate at which electrical energy is
converted to another form of energy
P: power (W)
P=I×V
I: current (A)
V: potential
difference (V)
A. Electrical Power
 A calculator has a 0.01-A current flowing through it.
It operates with a potential difference of 9 V. How
much power does it use?
KNOWN/UNKNOWN:
EQUATION: EQUATION:
I = 0.01 A
V=9V
P=?
P=I·V
P
I
V
P = (0.01 A) (9 V)
P = 0.09 W
B. Electrical Energy
 Electrical Energy
 energy use of an appliance depends
on power required and time used
E: energy (kWh)
E=P×t
P: power (kW)
t: time (h)
B. Electrical Energy
 A refrigerator is a major user of electrical power. If
it uses 700 W and runs 10 hours each day, how
much energy (in kWh) is used in one day?
UNKNOWN/KNOWN EQUATION: WORK:
:
E
=
P
·
t
E=P·t
P = 700 W = 0.7 kW
E = (0.7 kW)
t = 10 h
E=?
E = 7 kWh
E
P t
(10 h)