Download Simple Circuits

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Why use circuit symbols?
When designing or recording results, scientists and engineers
use diagrams to record their work in a simple, clear manner.
Electrical circuits also require
diagrams to record the
relative position of different
components.
Circuit symbols are used,
as they allow complex circuits
to be drawn in a clear and
precise manner, which is
easily understood by anyone
studying the image.
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Basic circuit symbols
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Specialized circuit components
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Current
Current is a measure of the amount of
charge moving per second.
I (A) =
ΔQ (C)
Δt (s)
Electrons usually drift in a
random direction, but there is
no net movement of charge.
When a potential difference is applied across a conductor a
current is produced. This causes a net movement of charge
in one direction, rather than a rapid flow.
Electrons flow from a circuit’s negative terminal to positive
terminal. However, traditionally current is drawn flowing in the
opposite direction, and this is called conventional current.
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Voltage
Voltage, also known as potential
difference, is a measure of the energy
provided to the charge carriers. It can
be defined as the amount of work
done per unit charge.
W (J)
V (V) =
Q (C)
Voltage is measured as a difference in potential between two
points. Thus a voltmeter must be connected in parallel and
used to measure the difference in potential across a device.
If a cell supplies 23 coulombs of charge with 776 J of
energy, what is its voltage?
776
V=
23
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= 33.7 V
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Resistance
Resistance is a measure of the opposition a material exerts
against the flow of electrons.
The resistance of a material can be
calculated from the current and voltage
passing through it.
V (V)
R (Ω) =
I (A )
Calculate the resistance in
this simple circuit.
9.7 V
3.2 A
R=
9.7
3.2
= 3.0 Ω
Resistance can also be calculated from a V/I graph.
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Investigating resistance
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V
I
5.0
4.5
0.50
0.45
4.0
0.40
3.5
0.35
3.0
0.30
2.5
0.25
2.0
0.20
1.5
1.0
0.15
0.10
voltage (V)
Plotting the V/I graph for a resistor
current (A)
voltage
5
resistance =
= gradient =
= 10 Ω
current
0.5
As the graph is linear, R is constant.
Therefore a resistor is an ohmic device.
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V
I
4.5
4.0
0.415
0.415
3.5
0.410
3.0
0.400
2.5
0.370
2.0
0.330
1.5
0.270
1.0
0.5
0.190
0.090
voltage (V)
Plotting the V/I graph for a bulb
current (A)
The graph is curved, therefore resistance is not constant.
A bulb is a non-ohmic device.
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The resistance of a non-ohmic device
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Current, voltage and resistance summary
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Kirchhoff’s first law
The German physicist Gustav Kirchhoff established two laws
which help us to understand the function of electric circuits.
Kirchhoff’s first law states that:
The sum of the currents leaving any junction is always
equal to the sum of the currents that entered it.
This law is based upon the idea of the conservation of charge:
no charge can be lost or made in a circuit.
I
IN
Thus the sum of the currents at a junction
should be zero.
IIN = I1 + I2 … + In
ΣI = 0
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I1
I3
I2
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Using Kirchhoff’s first law
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The energy in a circuit
What happens to the energy supplied to a circuit?
Batteries and power supplies supply electrical energy to a
circuit. Devices within the circuit transduce this energy: bulbs
produce heat and light, resistors produce heat.
What is the conservation of energy?
Energy cannot be created or destroyed. All of the energy
provided by a power supply must be used by the circuit.
How does the voltage of a battery relate to the voltage
measured across the devices in a circuit?
Voltage is the energy transferred to the charge in a circuit.
The battery’s voltage is shared between the components,
which transduce this energy into different forms.
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Kirchhoff’s second law
Kirchhoff’s second law is based upon the law of the
conservation of energy. It states that:
The total voltage across a circuit loop is equal to the
sum of the voltage drops across the devices in that loop.
Essentially, the energy you put
into the circuit equals the energy
you get out of each circuit loop.
An equation can be produced
for each loop in a circuit. For
example:
VIN = V1 + V2
VIN
I
V1
V2
R1
R2
VIN = IR1 + IR2
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Simple uses of Kirchhoff’s second law
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Further uses for Kirchhoff’s law
Use Kirchhoff’s laws to find the values for each current.
E = 12 V
V1
10 Ω
V2
100 Ω
V3
40 Ω
As I =
I1
I
I2
I3
V
R
I = 1.6 A
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 There are 3 loops in the circuit. Each
has a voltage drop equal to the input
voltage according to the 2nd law.
Therefore: E = V1 = V2 = V3
 The 1st law means that current
entering each junction equals the
current leaving.
Therefore: I = I1 + I2 + I3
I = 12 + 12 + 12
10
100
40
I1 = 1.2 A
I = 1.2 + 0.12 + 0.3
I2 = 0.1 A
I3 = 0.3 A
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Using Kirchhoff’s laws
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Summary: Kirchhoff’s laws
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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