AP Physics B Ch 18 Review PPT

8/9/2019 AP Physics B Ch 18 Review PPT

1/18


2/18

Electric Current & Circuits

3 things necessary for electric current to flowat a useable rate through a circuit

1. Source of potential difference (increase)- battery

2. Closed loop conducting pathway- connecting wires

3. Potential downhill (decrease)- resistor

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You need to be able toread circuit diagrams

and identify direction of

current flow

battery

resistor

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4/18

Voltage (potential increase) source

battery or power supply charge pump does work on charges supplying

them with electric potential energy

creates a potential difference ((V)

higher potential (V) at +, lower potential at terminal

current flow through circuit results

does NOT supply charge carriers

where are they?

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5/18

Electric Current

number of + charge carriers passingthrough a cross-sectional area per second

not velocity of charge carriers water flow

through pipe or hose is good equivalent

qI

t

(!

(

11 ( )

1 second

coulomb Ampere A!Units:

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Electric Current

conventional current = movement of +charge carriers

electrons actually move through wires

+ + +

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Conventional current

Electron flow

Conventional current

What could be done to increase the flow of chargesthrough the conductor?

+

+

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Electrical Resistance

impedance or opposition to the flow ofelectric charge by the conductor current is

moving through

electrical friction all conductors have resistance

electrons contacting atoms in metal

8drift velocity 1resistance


9/18

Ohms Law definition of resistance

what does resistance depend on? this ratio is constant for resistors that obey Ohms Law,;

resistance is independent of voltage and current

VR

I!

Units: 1 1 ( )1

Voltoh

pere! ;

voltage

current

graph of voltage vs current is linear

greater potential difference causes

increased current

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10/18

Resistance depends on

material property called resistivity V length of conductor L

cross sectional area A

LRA

V!

L

A

short and wide =

low resistance

long and narrow =

high resistance

capacitor C=Q/V

was definition; capacitance depended on A, d 10


11/18

Current Control

2 ways to increase current

increase voltage (greater potentialdifference supplying more energy tocharge carriers

decrease resistance by changing length,area

VI

R!

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Resistors

Electrical circuit element with purpose to regulate current flow

create a potential downhill

decrease in potential

take energy (potential) out of the charge carriers thatflow through resistor

Resistors do NOT take charge carriers out of the

circuit

Baseboard hot water heating system in house

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3 ways to use Ohms Law expression

V

1) I = V/R to calculate current

2) R = V/I to calculate resistance of resistor

3) V = IR to calculate either

voltage drop when current flows through resistor

voltage required to cause current flow throughresistor


15/18

Power

in energy = q VWork change! y (

Work energy transfer q VPower

time time t

y! ! !

Units WattsPower I V! y

alternative expressions based on substituting Ohms Law V=IR or I=V/R

2P I IR I R! y !

2V V

P VR R

! y !15


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Energy Transfer Rate

Watt = Joules of energy used per second

Energy used (J) = Powery time

secondssecond

JoulesJoules x!

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Electrical Power

Heating of resistor or brightness of bulb isresult of taking EPE from charge carriers

in current

Resistors use (dissipate) electrical energy

at a certain rate = power

Rate of energy transfer

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18

Efficiency of a motor or heat engine po er output work output

efficiency epowerinput energy input

! !

all motors or heat engines are less than

100% efficient since some of the inputenergy is converted to heat or

overcomes friction and cannot be used

to do work

AP Physics B Ch 18 Review PPT

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