Transcript Current Electricity - HSphysics

CURRENT ELECTRICITY
Name: ________________
Class: _________________
Index: ________________
Objectives
--state that a current is a rate of flow of charge measured in amperes
--distinguish between conventional current and electron flow
--recall and apply the relationship charge = current x time to new
situations or to solve related problems
-- define electromotive force (e.m.f.) as the work done by a source in
driving a unit charge around a complete circuit
-- calculate the total e.m.f. where several sources are arranged in series
--state that the e.m.f. of a source and the potential difference across a
circuit component is measured in volts
--define the p.d. across a component in a circuit as the work done to
drive a unit charge through the component
--state the definition that resistance = p.d./ current
-- apply the relationship R= V/I to new situations or to solve related
problems
--describe an experiment to determine resistance using a voltmeter and
an ammeter and make the necessary calculations
-- recall and apply the formulae for the effective resistance of a number
of resistors in series and in parallel to new situations or to solve related
problems
--recall and apply the relationship of the proportionality between
resistance and length and the cross-sectional area of a wire to new
situations or to solve related problems
--state Ohm’s law
--describe the effect of temperature increase on the resistance of a
metallic conductor
--sketch and interpret the V-I characteristic graph for metallic conductor
at constant temperature, a filament lamp and for a semiconductor diode
-- show an understanding of the use of a diode as a rectifier
Electric Current
• An electric current I is a measure of the rate of flow of
electric charge Q through a given cross section of a
conductor.
• Symbol of Electric Current = I
• SI Unit of Electric Current = ampere (A)
I = Q/t
where
I = current in ampere (A)
Q = amount of charges in coulombs (C)
t = time in seconds (s)
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Conventional Current and Electron Flow
Conventional current
flows from the positive to
the negative ends
Electric charges flow
from the negative to the
positive ends
5
Conventional Current and Electron Flow
Measuring current
• An ammeter is an instrument used for
measuring electric current.
• Ammeters must be connected in series
in a circuit
A
ammeter symbol
Positive (negative) side of
ammeter is connected to
the positive (negative)
terminal of the cell /
battery.
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Conventional Current and Electron Flow
Measuring current
Since the circuit consists of only one loop, the same
current flows through the circuit; does not matter
where the ammeter is placed on the circuit
A1
-
+
A6
cell
A2
A5
resistor
A3
A4
Conventional Current and Electron Flow
Measuring current
The digital multimeter (DMM) is starting to replace the
ammeter.
 has a wide range of
between a few hundred
A to several A
 can be used for direct
current (D.C.) and
alternating current
(A.C.)
 able to read voltage and
resistance too
Electromotive Force (e.m.f)
 electric current is produced when there is a flow of
charges
 a source of energy (provided by a cell, group of cells or
generator) is needed to enable charges to be pumped or
forced around a circuit
 electromotive force is the electric force that provides
the pumping action for electric current to flow from
the positive terminal to the negative terminal of the
+
battery
cell
I
lamp
Electromotive Force (e.m.f)
Electromotive Force (e.m.f)
Definition
• The electromotive force (e.m.f.) of an electrical source
is the work done by the source in driving a unit charge
round a complete circuit.
– is the potential difference between the two
terminals of the cell or battery. (From higher p.d.
to lower p.d)
– A point of high potential is a region where there is a
large number of positive charges whereas a point of
low potential has lesser positive charges
(more negative charges)
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Electromotive Force (e.m.f)
Electromotive Force (e.m.f)
• Symbol of Electromotive Force = 
• SI Unit of Electromotive Force = volts (V) or joules per
coulomb (JC-1)
 = W/Q
where
 = e.m.f. (V)
W = Energy converted from non–electrical forms to
electrical form (J) [work done]
Q = amount of charge in coulombs (C)
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Potential Difference
Potential Difference (p.d.)
• The Potential Difference (p.d.) between two points in an electric
circuit is defined as the amount of electrical energy converted to
other forms of energy when one coulomb of positive charge passes
between the two points
• Symbol of Potential Difference (p.d.) = V
• SI Unit of Potential Difference (p.d.) = volts (V)
V = W/Q
where
V = Potential difference (V)
W = Energy converted from electrical
form to other forms (J)
Q = amount of charge in coulombs (C)
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Potential Difference
Measuring p.d./e.m.f.
• An voltmeter is an instrument used for measuring
potential difference or electromotive force.
• As charges flow round a circuit, they lose their P.E.,
transforming P.E. into other forms of energy.
• It is connected in parallel to the circuit.
• The SI unit for p.d. / e.m.f. is volt (V)
V
voltmeter symbol
Voltmeters will measure the potential difference
across 2 points of the circuit, so we connect it in
parallel with respect to those 2 points
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Potential Difference
Potential difference around a simple circuit
 sum of all the e.m.f.’s of the cells must be equal to
the sum of potential differences across all the
components in the circuit
V
+
-
1
2
V
V
V1
V2
 1 +  2 = V 1 + V2 + V 3
V3
Resistance
In a circuit, the size of the
current depends on the
resistance in the circuit.
Any component of a circuit
resisting the flow of electricity is
called a resistor
The greater the resistance in a
circuit, the lower the current.
different types of resistors
Resistance
Definition:
• Resistance R of a component is the ratio of the
potential difference V across it to the current I
flowing through it.
• Symbol of Resistance = R
R
I
• SI Unit of Resistance = ohms ()
V
Where
R = resistance in ohms ()
V = p.d. across the component in volts (V)
I = current in ampere (A)
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Ohm’s Law
Ohm's law states that the current through a conductor between two
points is directly proportional to the potential difference or voltage across
the two points, and inversely proportional to the resistance between
them.
where I is the current through the resistance in units of amperes, V is the
potential difference measured across the resistance in units of volts, and
R is the resistance of the conductor in units of ohms. More specifically,
Ohm's law states that the R in this relation is constant, independent of the
current.
Resistance
If a cell is connected to a resistance, the current gets
smaller as the resistance increases.
Resistance
Uses of high and low resistances materials.
All metals have finite resistance.
Materials
Low
resistance
High
resistance
Uses
copper, gold, silver, aluminium
connecting wires,
conductors or connectors
tungsten
used in light bulbs
nichrome
(an alloy of nickel and chromium)
heaters, such as coils of
electric kettles
carbon
resistors for radio and
television sets
Resistance
Resistors
• Is a conductor that has a known value of resistance
• Primary purpose is to control the size of the current
flowing in the circuit.
• Two types: fixed resistors & variable resistors (or
rheostats)
• Variable resistor (or rheostat) allows resistances to be
changed easily
fixed resistor symbol
variable resistor symbol
20
Resistance
Rheostats
 are variable
resistors used for
controlling the size
of the current in a
circuit
 are used as
brightness controls
for lights, volume
controls on radio and
television sets
Resistance
Measuring Resistance
• To determine the resistance of a
metallic conductor, we use the
following circuit:
• We can find the current
flowing through R from the
ammeter reading.
• We can find the potential
difference across R from the
voltmeter reading
• R can be calculated from the
equation:
R=V/I
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Resistance
Experiment to Determine Resistance of a resistor
1. Set-up the apparatus as shown in the diagram.
2. As a safety precaution, adjust the rheostat to the
maximum resistance so that a small current
battery
rheostat
flows in the circuit initially.
3. Record the ammeter reading (I) & voltmeter
R
reading (V).
ammeter
4. Adjust the rheostat to allow a larger current to flow
in the circuit. Again record the values of I and V.
5. Repeat Step 4 for at least 5 sets of I and V
readings.
6. Plot the graph of V(V) against I (A). Determine
the gradient of the graph.
voltmeter
Note that:
Always connect:
Voltmeter in Parallel
Ammeter in Series
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Resistance
Experiment to Determine Resistance of a resistor
Result:
The gradient of the graph gives the resistance of the
load, R
V/V
Gradient = V / I
= resistance
0
I /A
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Resistance
Factors Affecting Resistance
There are several factors that affect the resistance of
an object such as a wire:
1. Cross-sectional area of
wire / thickness of wire
thicker wire
smaller resistance
(R  1/A)
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Resistance
Factors Affecting Resistance
2. Length of wire
longer wire
larger resistance
(R  l)
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Resistance
Factors Affecting Resistance
3. Type of material
Wires of the same length and thickness but made of
different materials will have a different resistances.
This is because they have different resistivities.
(Units: Ωm)
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Resistance
• These factors can be placed together to find
resistance
R = l /A
Where R = resistance in ohms ()
 = resistivity in ohm meter (m)
l = length of wire (m)
A = cross-sectional area in meter square (m2)
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Resistance
Example
• The diameter of the copper wire used in a circuit is
2.0 mm. If the resistively for copper is 1.7 x 10-8
m, what is the resistance for 50 cm of the wire?
Solution
L = 50 cm = 0.5 m
diameter = 2.0 mm = 0.002 m
A =  (d/2)2 =  (0.002/2)2 = (0.001)2 m2
R = (1.7 x 10-8)(0.5) / (0.001)2
= 0.0027 
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Resistance
resistors in series
 since resistors are in series, current I passing
through each resistor is the same
effective
resistance
I
R1
R2
R3
V1
V2
V3
is
equivalent
to
I
Rseries = R1 + R2 + R3
Rt
V
Resistance
resistors in parallel
 since resistors are in parallel, potential difference
across each resistor is the same
I1
I2
R1
R2
I
I3
effective
resistance
R3
is
equivalent
to
I
R
V
V
Temperature Dependence
Near room temperature, the electric resistance of a typical metal
increases linearly with rising temperature, while the electrical resistance
of a typical semiconductor decreases with rising temperature. The
amount of that change in resistance can be calculated using the
temperature coefficient of resistivity of the material using the following
formula:
R = Ro[α(T-To)+1] -- Formula not in syllabus
where T is its temperature, To is a reference temperature (usually room
temperature), R0 is the resistance at T0, and α is the percentage change in
resistivity per unit temperature. The constant α depends only on the
material being considered.
V
Ohmic
Conductors
The uniform gradient
shows uniform
resistance
I
O
(a) Pure metal
Pure metal,
V
carbon and copper
sulphate
O
I
(b) Copper sulphate solution
Non-Ohmic Conductors
At low
temperature,
the tungsten
wire obey
Ohm’s Law
but at higher
temperature
it is not
obeyed the
Law.
V
Constant
resistance
Higher
resistance
due to higher
temperature
I
O
filament bulb
Non-Ohmic Conductors
Semiconductor diode
A diode allows an electric current to pass in one direction (called the
diode's forward direction) while blocking current in the opposite
direction (the reverse direction). Thus, the diode can be thought of as an
electronic version of a valve.
Forward Voltage Drop
Electricity uses up a little energy pushing its way through the diode, rather like a
person pushing through a door with a spring. This means that there is a small voltage
across a conducting diode, it is called the forward voltage drop and is about 0.7V for
all normal diodes which are made from silicon. The forward voltage drop of a diode is
almost constant whatever the current passing through the diode so they have a very
steep characteristic (current-voltage graph).
Reverse Voltage
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes
leak a very tiny current of a few µA or less. This can be ignored in most circuits
because it will be very much smaller than the current flowing in the forward direction.
However, all diodes have a maximum reverse voltage (usually 50V or more) and if
this is exceeded the diode will fail and pass a large current in the reverse direction, this
is called breakdown.
Bridge Rectifiers
Rectifier diodes are used in power supplies to convert alternating
current (AC) to direct current (DC), a process called rectification. There
are several ways of connecting diodes to make a rectifier to convert AC
to DC. The bridge rectifier is one of them and it is available in special
packages containing the four diodes required.
References
http://www.cartft.com/image_db/1n4001.jpg
http://image.wistatutor.com/content/current-electricity/vacuum-diodegraph.gif
http://cyberchalky.files.wordpress.com/2010/03/web_ohms_law_triangle.gif
http://www.kpsec.freeuk.com/components/diode.htm