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Electric Circuit
Circuit diagrams and component
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Current in series circuit
The parts follow one after the other and
only one path for the current to flow
Reading of the ammeter will
be not changed if it is placed
in position of B, C or D.
The current is the same at any
point in series circuit.
Current in parallel circuit
Lamps are side by side
-There are alternative paths for the current
-the current splits, some goes through one
lamp
-the rest goes through the second lamp.
- The current recombines
The current from the source is larger
Example: if the ammeter reads
than the current in each branch.
0.4 A, and the lamps are
The splitting of the current depends on identical, the current at P and Q
the lamps will be 0.2 A.
If lamps identical, the current split If lamps are not identical, the
equally. current might divided so that 0.1
A at Q and 0.3 at P.
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Current at a junction
Electric current in a circuit cannot be stored
At any junction in the circuit, the total current
going into a junction must be equal to the total
current leaving the junction.
Potential difference in a series circuit
The total p.d. across the components
in a series circuit is equal to the sum
of individual p.d.s across each
component
V = V1 + V2 +V3
Example
If V1 =1.5V and V2 = 1.2 V then V3
=?
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Potential difference in a series circuit
The total p.d. across the components
in a series circuit is equal to the sum
of individual p.d.s across each
component
V = V1 + V2 +V3
Example
If V1 =1.5V and V2 = 1.2 V then V3
= 1.8 V
1.5V + 1.2 V +1.8 V =4.5 V
Potential difference in a parallel circuit
The p.d. across an arrangement of
parallel resistance is the same of p.d.
across one branch
V = V1 = V2
Example
If V1 =1.5V and V2 = 1.5 V then V
= 1.5 V
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Cells, batteries and e.m.f.
Cell is the source of electric current.
Battery consists of two or more
electric cells.
If cells are connected (joined) in
series; + of one to – of next, the
e.m.f. are added together.
If cells are connected (joined) in
series; + of one to – of next, the e.m.f. at A,B is 3.0 V
e.m.f. are added together. e.m.f. at X,Y is zero
If cells are in opposition; + of one to
+ of next, (or - to -) the e.m.f. are
subtracted.
Cells, batteries and e.m.f.
If two cells are connected in parallel,
e.m.f. across P,Q is still 1.5V
Two cells in parallel behaves like a
larger cell and will last longer
e.m.f. at P,Q is 1.5V
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Internal resistance of a battery
If the battery has e.m.f. of 1.5 V, the p.d. at the terminals
of the battery decreases slightly when current is drawn
from it and may become 1.4 V
e.m.f. of the battery doesn't equal p.d..
The reason is the battery has internal resistance which
transfers electrical energy to thermal energy when
current flows through the battery.
when no current in the circuit it is called open circuit
When the circuit is open, the p.d. across the battery will
have its maximume value and equal to the e.m.f.
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Work done
The work done by the battery on the charge Q is
W = e.m.f. × Q
e.m.f. =
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Resistors in series
If three resistors are in series, then the total voltage V is
V = V1 + V2 +V3 I =I1 = I2 = I3
From ohm’s law V= IR, so V1 = IR1 , V2 = IR2 , V3 = IR3 , I is the same
because the three resistors are in series.
V = V1 + V2 +V3 Combined resistance of
IR = IR1 + IR2 +IR3 resistors in series is more
IR = I (R1 + R2 +R3) divide both sides by I than that of any resistor
R = R1 + R2 +R3 by itself
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Resistors in series
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Resistors in parallel
If three resistors are in parallel,
then the total current I is
I = I1 + I2 + I3
From ohm’s law I =
so, law I1 = , I2 = , I3 =
V is the same because the three
resistors are in parallel.
I = I1 + I2 + I3
I= + + Combined resistance of
two resistors in parallel is
=V( + + ) Dividing both sides by V less than that of either
𝟏 𝟏 𝟏 𝟏 resistor by itself
= + + 16
𝑹 𝑹𝟏 𝑹𝟐 𝑹𝟑
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Advantages of connecting lamps in
parallel
How do the lamps in the house connect to each other?
1- The p.d. across each lamp is fixed as the supply p.d.
So, all lamps shines with the same brightness irrespective of
how many other lamps are switched on
2- each lamp can be switched on and off independently.
And if one lamp fails, the other can still operated.
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Examples
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Effect of temperature on resistance of
metals
- Metals have free electrons
- If temperature increases, the atoms vibrate faster.
- It becomes more difficult for the electrons to move
through the material
- It means that the resistance of metal increases as its
temperature increases
- In metals V = IR
- T increases, R increases, and if I is constant then it
causes the p.d. to increase
- As it discussed in a filament lamp
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Variable potential divider
In semiconductor thermistor
The resistance decreases as its temperature increases
Thermistor in a potential divider
-If temperature increases, thermistor resistance
decreases.
-then the combined resistance of the two resistors
decreases
I=
- If the supply voltage remains constant, the current
in the circuit increases as the resistance decreases.
- For the fixed resistor, increasing the current
with R is constant will cause the p.d. to increase.
In the thermistor potential divider, p.d. across the fixed resistor increases as
temperature increases.
This circuit (variable potential divider) can be used to monitor temperature
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Potential divider
Total resistance R= R1 + R2
supply voltage
I=
I=
R1 + R2
The supply voltage is divided into the
two resistors
V= V1 + V2 The ratio of the voltage across the two
the current is the same in the two resistors in series (potential divider)
resistors V1 I × R
= = 1
R1 V2 × R2
V1 = I× R1 = × R1 = V ×
R1 + R2 R1 + R2
V1 R1
=
V2 = I× R2 =
R1 + R2
× R2 = V ×
R2
R1 + R2
V2 R2
Remember:!
V1 + V2 = V 23
Example
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1-Light-dependent resistor (LDR)
LDR is formed by cadmium sulfide
Cadmium sulfide is a semiconductor
The resistance decreases as the intensity of the light falling on it
increases.
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Light-dependent resistor (LDR)
When light from a lamp falls on the
window of the LDR, its resistance
decreases. Then the current increases.
As a result the lamp lights.
Applications of LDR
1- LDR is used in photographic exposure
meters
2- LDR is used in series with a resistor to
provide an input signal in switching
circuits as in light-operated intruder alarm
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2-Relays
Relay is a switch that uses a small
current to control a large current
needed to operate a large devices
Applications of relays
1- temperature-operated switch
2- to switch on the main supply for
electrical appliances in the home
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Relays
In the relay circuit
- If the output of the
switching circuit is high
(5V), a small current
flows to the relay which
closes the mains switch.
- Note that the relay
isolates the low voltage
circuit from the high
voltage mains supply.
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Applying LDR in a circuit with a relay
LDR is used to switch a relay
-LDR, in the circuit, is part of a potential
divider across 6 V supply.
-as light falls on LDR, its resistance
decreases and so the p.d. does.
-Current in the circuit increases.
-As a result the p.d. across R increases as
The bell stops if the light is
well as across the relay. removed which cause the
-When relay reaches a high enough p.d., it potential across R to
acts as a switch and the circuit is closed decrease below the
operating p.d. of the relay.
allowing the current to flow to the bell And the relay contact open.
which rings.
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3-Thermistor
A negative temperature coefficient (NTC) thermistor contains
semiconducting metallic oxides
The resistance decreases markedly as the temperature increases.
The temperature of thermistor rises because
1- thermistor is directly heated
2- a current passes in it
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Thermistor
When thermistor is heated, its
resistance decreases. Then the
current increases. As a result the lamp
lights.
Applications of thermistor
1- in series with a meter marked in oC can
measure temperatures.
2- in series with a resistor it can provide
an input signal to switching circuits.
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Applying thermistor in a circuit with a
relay
Thermistor is used to switch a relay
-relay, in the circuit, is part of a potential
divider across 6 V supply.
-as the temperature of the thermistor
increases, its resistance decreases and so
the p.d. does.
-Current in the circuit increases.
-As a result the p.d. across R increases as The temperature at which
well as across the relay. the alarm sounds can be
controlled by adding a
-When relay reaches a high enough p.d., it variable resistor to the
acts as a switch and the circuit is closed circuit.
allowing the current to flow to the bell
which rings. 32
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4- Semiconductor diode
A diode is a device that lets current pass in one direction only.
A diode is a device that lets current
pass in one direction only.
The wire nearest the
band is the cathode and the one at
the other
end is the anode.
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Diode in forward-biased
Forward biased
If the anode is connected to the positive terminal of the voltage
supply and the cathode to the negative terminal, the diode
resistance is small and the conventional current passes in the
direction of the arrow on its symbol.
The lamp in the circuit shows when
the diode is conducting, as the lamp
lights up.
The lamp also acts as a resistor to limit the
current when the diode is forward-biased.
Otherwise the diode might overheat and
be damaged.
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Diode in reverse-biased
Reverse biased
If the anode is connected to the negative terminal of the voltage
supply and the cathode to the positive terminal, the diode
resistance is large and it does not conduct.
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Diode as a rectifier
Diode can be used as a rectifier to change the a.c.
current to d.c. current.
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5-Light emitting diode (LED)
LED is a diode made from the semiconductor gallium arsenide
phosphide.
When LED is forward biased, the current makes it emit red,
yellow or green light.
When LED is reversed biased, no light is emitted.
If reverse biased voltage exceeds 5 V it may cause damage.
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LED
There should be a suitable resister R connected in series
with LED
R will limit the current The suitable resistance is 300
(10 mA) Ω on a 5 V supply.
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Applications of LED
LED is used as indicator lamps on computers, radios
and other electronic devices.
LED is used as seven-segment red or green numerical displays.
Clocks, calculators, video recorders and measuring instruments.
In seven-segment numerical displays, each segment is an LED.
The one has voltage across it will light up the numbers 0 to 9.
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LED
LED is
-Small
-Reliable
-Has long life
-operating speed is high
- The current requirements are very low
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diode lasers
Diode lasers operate in a similar way to LEDs
But
Diode lasers emit coherent laser light, and
are used in optical fibre communications as
transmitters
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