One Way Current Flow
Topic
Simple electronic circuits
Introduction
Electronics can be defined as the study and design of systems using the flow of
electrons through components such as semiconductors, resistors, and capacitors.
It uses many of the concepts underlying electric circuits, but many of the
components used in electronic circuits have specialized characteristics. For
instance, a diode (whose name refers to the fact that it has two electrodes, an
anode and a cathode) is a component that allows electrons to flow around a
circuit in one direction only (see diagram 1 below). In this experiment, you will
use light emitting diodes (LEDs) to demonstrate that diodes only allow current to
flow in one direction. An LED (see diagram 2 below) is a special type of diode
that lights up when current flows through it; the color of the LED depends on
the material used to make it. In this experiment, you will also construct a simple
electronic circuit to find out more about the properties of LEDs.
1
anode
cathode
–
+
current allowed this way
Current flow in a diode
2
Selection of light emitting diodes (LEDs)
Time required
15 minutes for Part A
10 minutes for Part B
Materials
9 volt battery and snap connector
5 mm (0.2 in.) green LED
560 ohm resistor (0.25 watts)
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clip leads or other suitable connection
system (e.g. breadboard*)
clock or watch
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*A breadboard is a device that allows temporary connections to be made
between electronic components. It consists of a plastic block with holes through
which the wires from the components are inserted. Beneath each hole is a clip,
which holds the wire in place and makes electrical contact with other
components. The breadboard shown in the experiments in this section has two
central blocks where connections are made along each horizontal row and two
lines down each side in which connections are made down the vertical rows.
If you are using a breadboard, you will also need a short length of connecting
wire (e.g., approximately no. 20 gauge, insulated, tinned copper wire with about
0.5 cm of insulation stripped away at each end).
Electronic components are available from suppliers such as RadioShack
(http://www.radioshack.com). The appearance of the components may vary
among suppliers. Simple wiring diagrams and circuit diagrams are given in this
experiment to show the arrangement of the components. Diagram 3 below
shows the symbols used in the circuit diagrams in this experiment.
3
resistor
LED
supply voltage
(longer line identifies positive pole)
Symbols used in circuit diagrams
Safety note
Do not use an electrical outlet.
Procedure
Part A: Current flow and LEDs
1. Look at the LED. It has a plastic body with two wires coming out of it; these
are the electrodes. The wires may be different lengths; if so, the shorter wire is
the cathode. If the LED has a flat side, this should be on the cathode side.
Identify the anode and the cathode of your LED.
2. Set up the equipment as shown in diagram 4 on the next page (i.e., anode of
the LED connected to the positive supply voltage). Observe the LED and
record your observations in data table A on the next page.
3. Disconnect the LED and reverse the connections (i.e., cathode connected to
the positive supply voltage). Observe the LED and record your observations.
Disconnect the battery from the circuit.
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4A
4B
battery
9 volt
connecting wire
breadboard
LED
560 ohm
resistor
Circuit diagram (A) and wiring diagram (B) for Part A
DATA
TABLE
A
Observations
Appearance of LED
Appearance of LED with
connections reversed
Part B: Sensitivity of components
Warning: make sure your teacher knows you are doing this experiment, as it will
make the LED unusable.
1. Set up the equipment as shown in diagram 5 below (anode connected to the
positive supply voltage).
2. Observe the LED for about 5 minutes and then disconnect the circuit. Gently
touch the LED. Record your observations in the first row of data table B on
the next page.
3. Reconnect the same LED in the circuit. Observe and touch the LED. Record
your observations in the second row of data table B on the next page.
Disconnect the battery from the circuit.
5A
5B
battery
connecting wire
9 volt
breadboard
LED
Circuit diagram (A) and wiring diagram (B) for Part B
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or transmittal is copyright protected by the publisher.
DATA
Glowing?
TABLE
B
Color (if glowing)
Temperature
Color and condition
of LED (step 2)
Condition of LED
(step 3)
Analysis
Part A: Current flow and LEDs
1. What was the appearance of the LED when the circuit was connected as
shown in diagram 4?
2. What was the appearance of the LED when the connections were reversed?
Part B: Sensitivity of components
1. What was the appearance of the LED when the circuit was connected as
shown in diagram 5?
2 How did the LED feel when you touched it after it had been disconnected
from the circuit in step 2?
3. What was the appearance of the LED after the circuit had been reconnected in
step 3?
Want to know more?
Click here to view our findings.
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10.41 • OUR FINDINGS
PHYSICS EXPERIMENTS ON FILETM
5. When the magnet is stationary, no current is recorded.
6. When the magnet is moving quickly in and out of the coil, there is a larger
reading on the meter than when it is moving slowly. These results follow part
of Faraday’s law of induction, which relates the speed with which a magnet
moves relative to a conductor to the size of the induced current.
8.07 Electric Motor Construction
1. The motor spins in the opposite direction when the poles of the magnets are
reversed.
2. The motor spins in the opposite direction when the connections to the cells are
reversed.
Electricity flows through the coil when the wire from one end of the coil is in
contact with one of the brushes, and the wire from other end of the coil is in
contact with the other. As the coil is in a magnetic field, a force acts on it
(force on an electrical conductor in a magnetic field) and it moves round on
the axle, breaking the electrical connection. The wire from the other side of
the commutator now touches the brush and current flows through the coil
again; the force acts on it again and the coil continues to rotate. The effect of
these events is to cause the coil to continue turning as the circuit for the
current to flow through the coil is continually made and broken.
The direction of rotation is reversed if the magnetic field is reversed (it acts
north to south) following Fleming’s Left Hand Rule (see Experiment 8.05:
Invisible Force). If the direction of the electric current flow is reversed, the
direction of rotation is also reversed.
Electronics
9.01 One Way Current Flow
Part A: Current flow and LEDs
1. The LED glowed green when the circuit was connected as shown in
diagram 4.
2. The LED did not glow when the circuit was connected with the LED the other
way around.
A semiconductor diode is a silicon or germanium crystal of which part
(p-type) has been treated so that it has excess positive charges; the other part
(n-type) has been treated to have an excess of electrons. If a cell is connected
across such a diode, electrons will flow as indicated in the diagram on the next
page. Virtually no current flows if the semiconductor diode is connected the
other way around (the diode is said to be “reverse biased” in this situation),
unless a very large voltage is used and the diode breaks down.
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PHYSICS EXPERIMENTS ON FILETM
OUR FINDINGS • 10.42
p-type
n-type
many excess
positive
charges
many
electrons
junction
appreciable
current
Electron flow through a diode
Part B: Sensitivity of components
1. The LED glowed yellow, then orange. It then began to glow less brightly. This
shows that the current through the LED was greater than it was designed to
take (this is usually 10 mA).
2. The LED felt hot. This shows that the LED was overheating.
3. Our LED did not glow, showing that it had been destroyed and was no longer
capable of conducting. It is important to protect electronic components such
as LEDs by connecting them in series with a resistor.
Many examples of output devices (LED, buzzer, motor) can be incorporated in
electronic circuits. As in electrical circuits, they can be connected in series or in
parallel.
When studying electronic circuits, it is important to bear in mind the idea of the
positive and negative supply rails – the points in the circuit that notionally
supply and return the electric current (see the diagram below left).
Circuit diagrams in electronics are often complicated so, to simplify them, the
supply is often not shown. The positive supply rail is then drawn as a line, which
represents the positive side of the circuit. Another line is drawn representing the
negative side and is shown at 0 volts (see the diagram below right).
positive supply rail
+9V
0V
+9V
0V
negative supply rail
Positive and negative supply rails
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Example circuit diagram for electronics circuit
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