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Ch 20ELECTRICITY
Static electricity • the accumulation of
electric charges (e-) on an object
• opposite charges attract, like charges repel
• There are 3 ways to transfer e- from one object to another:
#1Charging by Friction
• Dragging feet on carpet
• Rubbing balloon on hair
• Running comb through your hair
• All have e- transferring causing negative charge
#2Charging by Induction
• charged objects cause e- to rearrange their position on a neutral object, w/o contact
• Ex: Electroscope or a door knob
• An electroscope detects the presence of electric charges, NOT the type of charge!
• If a negatively charged item touches the knob, the leaves repel
• If a positively charged item touches the knob, the leaves repel
• An uncharged object has no effect on the leaves
Electroscope
#3 Charging by Contact
• Touching hand to Van De Graff generator
Conductors and Insulators
• Conductor-A material that allows e- to move easily
Ex: metals, earth
• Insulators-A material that does NOT allow e- to move easily
Ex: plastic, wood, glass
Star Questions
1.1. What is static electricity?What is static electricity?– The accumulation of charges on an objectThe accumulation of charges on an object
2.2. How do like charges behave?How do like charges behave?– They repelThey repel
3.3. What is charging by friction?What is charging by friction?– Electrons move from one object to another causing a Electrons move from one object to another causing a
chargecharge
4.4. What is a conductor? Give an example.What is a conductor? Give an example.– A material that allows electrons to move easily; copper A material that allows electrons to move easily; copper
wirewire
5.5. What is an insulator? Give an example.What is an insulator? Give an example.– A material that does not allow electrons to move easily; A material that does not allow electrons to move easily;
plasticplastic
Electric Current • Flowing Electrons• a negatively charged object has more
potential energy than uncharged objects • Electrons flow from places of higher PE
to places of lower PE
• There are 2 types of Electric Current:
–Direct Current: Charge flows in one direction
–Alternating Current: Charge can reverse it’s direction
Conductor(wire)SwitchBatteryBulb, light, lampMotorVoltmeterAmmeterResistor/resistance
Circuit Symbols
Circuit
• a closed path through which e- can flow
Batteries1. Dry Cell• E- flow from + to -
Contains an electrolyte paste
2. Wet Cell• Two metallic
(different metals) plates in electrolyte solution
Potential Difference • difference in potential
between 2 different places
• Measured in Volts (V)
• Also called “Voltage”
• Depends on a comparison of the energy carried by e- at different points
• Measured w/ a voltmeter
Current
• the flow of e- through a wire or any conductor
• measured in Amperes (A)
• Depends on the # of e- passing a point in a given time
• Measured w/ an Ammeter
Voltmeter
Ammeter
resistor
Resistance
• The tendency for a material to resist the flow of e-
• Measured in Ohms
( : omega)
• Two things affect the amount of resistance in a conductor...
Resistance
1. Type of material
– materials such as tungsten (filament in light bulbs) are very resistant to e- flow
– higher resistance higher resistance changes electrical energy changes electrical energy into thermal energy & into thermal energy & lightlight
Resistance
2. Size of conductor– Thin wires have higher
resistance
– Thick wires have low
– Short wires have less resistance
– Long wires have more
Ohm’s Law
• Potential difference = current x resistance
• V = I x R
• V=Voltage [V]
• I = Current [A]
• R=Resistance []V
I R
Ohm’s Law• R=160
• I=15A• V=?
• V=120V• R=3• I=?
Star Questions• Name the symbol• Motor• Name the symbol• Battery• Unit for current• Amps• Very large discharge of static
electricity• Lighting• Device with measures
potential difference• voltmeter
Electrical Circuits• Current has only
one path it can travel along
• No current flows if path is open
• Current stays the same in series circuit for the circuit & across each resistor
Electrical Circuits-SERIES• If looking for Resistance for circuit
use Req=R1+R2+R3+…
(eq = Equivalent Resistance)
• If looking for voltage (p.d.) of circuit use V=IReq
• If looking for voltage across each resistor use V1=IR1 V2=IR2 V3=IR3 V4=IR4, etc. for however many resistors you have
Electrical Circuits-SERIES
• If looking for current of circuit use I=V/Req
• If looking for current across each resistor use I=V/Req
Current stays the same in series circuit for the circuit & across each resistor
Electrical Circuits-series
A 9V battery is connected to four resistors: 2 , 4 , 5 , and 7 . Find the
a) Equivalent Resistance
b) Current in the circuit
Electrical Circuits-series
A 9V battery is connected to four resistors: 2 , 4 , 5 , and 7 . Find the
c) Voltage in each resistor
Electrical Circuits-series
A 9V battery is connected to four resistors: 2 , 4 , 5 , and 7 . Find the
d) Current in each resistor
A. Req
B. V circuit
C. Veach resistor
D. I circuit
E. I each resistor
12V
=6.75
=21.6
15.3=
Star Questions
• Tungsten, the filament in light bulbs, is (very resistant/not resistant) to e- flow?
• Very resistant• Higher resistance changes electrical energy into
(thermal energy/chemical energy) and light• Thermal energy• What property is the same no matter where in the
circuit it is measured?• Current• What property changes depending on where in the
circuit it is measured?• voltage
Electrical Circuits-parallel
• Contains separate branches for current to move through
• More current flows through the paths of lowest resistance
Electrical CircuitsPotential difference (Voltage) stays the same in parallel circuits across each resistor & for the circuit•Current continues to flow through other branches if one branch is open
Electrical Circuits-PARALLEL
• If looking for Resistance for circuit, use Inverse, add, (equal) inverse
1 = 1 + 1 + 1Req R1 R2 R3 …
• If looking for current of circuit use I=V/Req• If looking for the current in each
resistorI1=V I2=V
R1 R2…
Electrical Circuits-PARALLEL
• If looking for voltage of circuit use, look it is usually given to you or use V=IR
• If looking for voltage across each resistor
Potential difference (Voltage) stays the same in parallel circuits across each resistor & for the circuit
Electrical Circuits-parallel
• A 9V battery is connected to four resistors: 2 , 4 , 5 , and 7 . Find the
a) Equivalent Resistance
Electrical Circuits-parallel
• A 9V battery is connected to four resistors: 2 , 4 , 5 , and 7 . Find the
b) Current in the circuit
Electrical Circuits-parallel
• A 9V battery is connected to four resistors: 2 , 4 , 5 , and 7 . Find the
c) Current in each resistor
Electrical Circuits-parallel
• A 9V battery is connected to four resistors: 2 , 4 , 5 , and 7 . Find the
d) Voltage in each resistor
A. Req
B. I circuit
C. I each resistor
D. V circuit
E. V each resistor
24V
=41
120
=8
Star Questions
• Thin wires have a (higher/lower) resistance thank thick wires
• higher• Find the Req of a series circuit with 2 , 4 , 6 ?• 12 • Find the Req of a Parallel circuit with 2 , 4 , 6 ?• 1.09 • What property is the same no matter where in the
Parallel circuit it is measured?• voltage• What property changes depending on where in the
Parallel circuit it is measured?• current
Electrical Power
• the rate at which electrical energy is converted to another form of energy
• Measured in Watts (W) (or kW)
• Power=current x voltage
• P = I x V
• [W] = [A] x [V]
Electrical Power• An air conditioner used 1800W of
power. The voltage source is 120V. How much current is flowing through the air conditioner?
• A radio requires 0.25A of current when operated on a 120V source. How much power is used by the radio?
Electrical Energy • The amount you use depends
on the power required by appliances & how long they are used.
• Energy=Power x time• E = P x t• [kWh] = [kW] x [h]• Most common unit is the
kilowatt-hour, but other acceptable combinations are okay: Ws (Watts-second)
Electrical Energy• The reading on an electric
meter was 2345 kWh for Jan and one month later it was 3456 kWh. How much electrical energy was used for the month of Feb?
• How much would the electric bill for the month of Feb be if the electricity cost $0.10 per kilowatt-hour?
Electrical EnergyElectrical Energy• A Blow Dryer is rated at 2,600 watts
by the manufacturer. This is how much electric power it uses when it operates.
• If the Blow Dryer is operated for a total of 5 hours each month (10-15 min/day M-F), the Blow Dryer consumes 2,600 watts x 7 hours = 18,200 watt-hours.
• Since "Utility Rates" are based on kilowatt-hours, divide by 1,000 to get 18.2 kilowatt-hours.
• This shows how power consumption and operating time are important in determining energy use.
• If Progress Energy charges 20 cents/kWh. How much are spending on your blow dryer each month?
• $3.64
Electrical EnergyElectrical Energy
Star Questions
• Rate at which electrical energy is converted to another form
• Electrical power• Depends on the power required by
appliances and how long they are used• Electrical energy• Unit for electrical energy• Kilowatt-hour• Device used to detect electric charges• electroscope