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