dc electricity 2
TRANSCRIPT
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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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