Dc Electricity 2

8/10/2019 Dc Electricity 2

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RESISTANCE

OHMS LAW

CURRENT- POTENTIAL DIFFERENCE

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DC ELECTRICITY


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Component Circuit Symbol Function

WireMade from a metallic conductor so current can easily pass from one

part of the circuit to another

Switch (open)On/Off switch in open position the circuit is broken so no current

flows

Switch (closed)On/Off switch in closed position the circuit is complete and current

flows

CellSupplies the electrical energy to the circuitthe larger terminal on the

left is the positive (+) terminal

Battery A battery is more than one cell.

Lamp Coverts electrical energy to light.

FuseA safety device which melts to break the circuit if the electrical current

flowing through it exceeds a specified value.

Voltmeter Instrument used to measure potential difference.

Ammeter Instrument used to measure electrical current

Resistor Restricts the flow of electrical current can be used to limit the flow ofcurrent to a particular component

Diode A device which only allows current to flow in one direction

Thermistor Converts heat to electrical resistance

Variable resistor Used to control current.

Light dependantresistor Converts light to electrical resistance


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CIRCUITS

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Studies on structure of metallic conductors suggeststhat:

Free electron in metals make up the current in an

electrical circuit

Electric current in a metal = flow of electrons carrying

negative charge

In electrolyte, positive and negative ions act as charge

carriers

Electric current flows in the direction of the flow of

positive charge


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Model of electricity

Water flow through pipe

represents the electric

current

Water circuit

Water causes turbine to

rotate


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Electric motor driven by battery

Proves that something travelsround a complete circuit

Energy is transferred to thesystem at one point andtransferred from it at another, andappears to be carried from one

place in the circuit to another bythe something that moves roundthe circuit


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The Something

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From the studies of metallic conductor structure:

Free electrons in metals make up the current in electrical

circuits.

Electric current in the metal

Flow of electrons carrying negative charge

In electrolyte

Positive and negative ions act as charge carriers

By convention

Flow of electric current = flow of positive charge

Flow of electron opposes the conventional electric

current flow


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Conventional current and the flow of electrons in acircuit


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CONVENTIONAL

current notation

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Electric charge moves from the positive side of thepower source to the negative.


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LDR

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The light dependent resistor (LDR) has a resistancethat decreases as the light level goes up.

The most obvious use for the LDR is as a light

detector. Light detectors can be found in:

counting systems;

burglar alarms;

automatic switches for street lights;

some kinds of smoke detectors;

flame-out detectors.


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THERMISTOR

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The thermistor is a component whose resistance goesdown as its temperature rises.

Thermistors have uses wherever temperature is

detected electronically.

Examples include:

electronic thermometers;

heat detectors.


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MEASURING

currents and potential difference

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Note that:

To measure current

ammeter connected in

series

To measure potential

difference voltmeter

connected in parallel


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CURRENT ELECTRICITY

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For current electricity to flow, we need:

a complete circuit;

conducting materials;

a source of voltage (e.g. a battery).

Different conduction in different materials

Conductor

Semiconductor

Insulator


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CURRENT ELECTRICITY

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METALS

All

metals areconducting

materials

SEMICONDUCTOR

Can conduct

electricity undercertain

circumstances

Has electrical

conductivitybetween those of a

conductor and an

insulator

INSULATOR

Keep parts

of the circuitseparate

For

example the

positive andnegative

terminals of

a battery


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RELATING VOLTAGE AND

CURRENT

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The voltmeter measures voltage and is in parallel across the

component; The ammeter measures current and is wired in series with

the component.

V

A


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Potential Difference (V) = Current (A) Resistance()

V = IR

V

I R


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RESISTANCE

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A tungsten filament lamp has a high resistance, butconnecting wires have a low resistance.

The greater the resistance of a component, the more

difficult it is for charge to flow through it.

The electrons make many collisions with the tungsten

ions as they move through the filament.

But the electrons move more easily through the copper

connecting wires because they make fewer collisionswith he copper ions.


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DEFINED RESISTANCE

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OHMS LAW

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ANSWER:

- R = V / I = 10 V 2.0 A = 5


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QUESTION:

What are the key points to Ohms Law?


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ANSWER:

Current and voltage are proportional

Provided the temperature is the same.


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QUESTION:

A 15 resistor is connected across a 3 V supply. What

is the current?


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ANSWER:

I = V / R = 3 V 15 = 0.2 A


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Current potential difference graph

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Resistance is defined by R = V/I

Ohms law: Provided the temperature and other physical factors

remain constant, the current through a wire is

proportional to the potential difference across its ends.

A conductor that obeys Ohms Law is called an ohmic

conductor.


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QUESTION:

Why does the line go through the origin?


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ANSWER:

If there is no current, there is no voltage.


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Non-ohmic

conductor

Two resistancesdepending on its

polarity

Which way it

connected

Forward directionlow resistance

Reverse directionhigh resistance

More currentflows the metalfilament gets hotter

Gain energyvibrate faster

Increase collisionwith the travelling

electros resiststheir motion

More current flow

temperature increasesTemperature increases thermistor makesavailable more freeelectrons to carry thecurrent


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QUESTION:

A light bulb is not an ohmic conductor. Its graph looks

like this:

Why is the graph of a light bulb shaped like this?


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ANSWER:

The filament gets hot.

The resistance goes up.

The gradient gets steeper with increased resistance


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DIODE

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Voltage-current graph The forward biased

diode starts to conduct

at +0.6 V.

The reverse biaseddiode breaks down at

about -30 V.

The breakdown voltage

varies according to thetype of diode.

+0.6

V

Voltage

(V)

Current

(mA)

-30 V

A

BC

D


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ANSWER: At A the diode is conducting as the forward biased

voltage is greater than 0.6 V.

At B the diode does not conduct as the forward biased

voltage is less than 0.6 V. At C the diode is reverse biased, so does not conduct.

At D, the reverse biased voltage is more negative than

the breakdown voltage, so the diode conducts (and may

be wrecked).


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QUESTION:

A lamp takes a current of 0.3 A at a voltage of 6 V. What

is its resistance? Some students measure the resistance

of the lamp with an ohmmeter and find that the resistanceis only 4 ohms. Explain why this happens.


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ANSWER:

R = V/I = 6 V 0.3 A = 20 ohms

When the students measure the filament's resistance with

an ohm meter, the filament is cold, so the resistance is

low. (In other words, when the bulb is on, the filament is

hot, so the resistance is higher)


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POP QUIZ

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Dc Electricity 2