Name: ________________

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CURRENT ELECTRICITY
Name ________________ Class _________________
Index ________________
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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
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-- 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 Ohms 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
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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
Electric charges flow from the negative to the
positive ends
Conventional current flows from the positive to
the negative ends
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Measuring current
Conventional Current and Electron Flow

• An ammeter is an instrument used for measuring
  electric current.
• Ammeters must be connected in series in a circuit

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
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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

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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

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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
nonelectrical 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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Measuring p.d./e.m.f.
Potential Difference

• 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)

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

?1 ?2 V1 V2 V3
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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
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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
• SI Unit of Resistance ohms (?)

Where R resistance in ohms (?) V p.d.
across the component in volts (V) I current
in ampere (A)
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Ohms 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.
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Resistance
If a cell is connected to a resistance, the
current gets smaller as the resistance increases.
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Resistance
Uses of high and low resistances materials.
All metals have finite resistance.
Materials Uses
Low resistance copper, gold, silver, aluminium connecting wires, conductors or connectors
High resistance tungsten used in light bulbs
High resistance nichrome (an alloy of nickel and chromium) heaters, such as coils of electric kettles
High resistance carbon resistors for radio and television sets
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Resistors
Resistance

• 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

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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

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Measuring Resistance
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 flows in the circuit initially. 3.
Record the ammeter reading (I) voltmeter
reading (V). 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.
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
Gradient V / I resistance
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Factors Affecting Resistance
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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Factors Affecting Resistance
Resistance

• 2. Length of wire

• longer wire
• larger resistance
• (R ? l)

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Factors Affecting Resistance
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 Om)
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Resistance

• These factors can be placed together to find
  resistance

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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Example
Resistance

• 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
Rt
R1
R2
R3
is equivalent to
I
I
V
V1
V2
V3
Rseries R1 R2 R3
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Resistance
resistors in parallel

• since resistors are in parallel, potential
  difference across each resistor is the same

R1
I1
effective resistance
R2
I2
R
is equivalent to
I
I
R3
I3
V
V
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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
Roa(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 a is the percentage change
in resistivity per unit temperature. The constant
a depends only on the material being considered.
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The uniform gradient shows uniform resistance
V
Ohmic Conductors
I
O
(a) Pure metal
Pure metal, carbon and copper sulphate
V
I
O
(b) Copper sulphate solution
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Non-Ohmic Conductors
V
Higher resistance due to higher temperature
At low temperature, the tungsten wire obey Ohms
Law but at higher temperature it is not obeyed
the Law.
Constant resistance
I
O
filament bulb
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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.
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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.
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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.
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