Ch 20 ELECTRICITY

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.What is static electricity? –The accumulation of charges on an object 2.How do like charges behave? –They repel 3.What is charging by friction? –Electrons move from one object to another causing a charge 4.What is a conductor? Give an example. –A material that allows electrons to move easily; copper wire 5.What is an insulator? Give an example. –A material that does not allow electrons to move easily; plastic

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) Switch Battery Bulb, light, lamp Motor Voltmeter Ammeter Resistor/resistance Circuit Symbols

Circuit a closed path through which e- can flow

Batteries 1. 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 changes electrical energy into thermal energy & light

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 IR

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 V 1 =IR 1 V 2 =IR 2 V 3 =IR 3 V 4 =IR 4, 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. R eq B. V circuit C. V each 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 Circuits Potential 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 = Req R 1 R 2 R 3 … If looking for current of circuit use I=V/Req If looking for the current in each resistor I 1 =VI 2 =V R 1 R 2 …

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. R eq 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 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 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