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Electromotive Force and Current
In an electric circuit, an energy source and an energy consuming device
are connected by conducting wires through which electric charges move.
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Electromotive Force and Current
Within a battery, a chemical reaction occurs that transfers electrons from
one terminal to another terminal.
The maximum potential difference across the terminals is called the
electromotive force (emf).
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Electromotive Force and Current
The electric current is the amount of charge per unit time that passes
through a surface that is perpendicular to the motion of the charges.
t q I
One coulomb per second equals one ampere (A).
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Electromotive Force and Current
If the charges move around the circuit in the same direction at all times,
the current is said to be direct current (dc).
If the charges move first one way and then the opposite way, the current is
said to be alternating current (ac).
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Electromotive Force and Current
Conventional current is the hypothetical flow of positive charges that would
have the same effect in the circuit as the movement of negative charges that
actually does occur.
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Ohm’s Law
OHM’S LAW
The ratio V/I is a constant, where V is thevoltage applied across a piece of mateiral
and I is the current through the material:
SI Unit of Resistance: volt/ampere (V/A) = ohm (Ω)
IRV R I
V orconstant
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Ohm’s Law
To the extent that a wire or an electrical
device offers resistance to electrical flow,
it is called a resistor.
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The Measurement of Current and Voltage
An ammeter must be inserted into
a circuit so that the current passes
directly through it.
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Safety and the Physiological Effects of Current
To reduce the danger inherent in using circuits, proper electrical grounding
is necessary.
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Series Wiring
There are many circuits in which more than one device is connected to
a voltage source.
Series wiring means that the devices are connected in such a waythat there is the same electric current through each device.
i i i
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Series Wiring
S i Wi i
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Series Wiring
Personal electronic assistants.
P ll l Wi i
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Parallel Wiring
Parallel wiring means that the devices areconnected in such a way that the same
voltage is applied across each device.
When two resistors are connected in
parallel, each receives current from the
battery as if the other was not present.
Therefore the two resistors connected inparallel draw more current than does either
resistor alone.
P ll l Wi i
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Parallel Wiring
Parallel Wiring
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Parallel Wiring
The two parallel pipe sections are equivalent to a single pipe of the
same length and same total cross sectional area.
Parallel Wiring
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Parallel Wiring
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RESISTORS IN
SERIES
RESISTORS IN
PARALLEL
RT = R1 + R2 + … 1/RT = 1/R1 + 1/R2 + …
IT = I1 = I 2 IT = I1 + I 2
VT = V1 + V2 VT = V1 = V2
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Determination of Resistance
Color DigitBlack 0
Brown 1
Red 2
Orange 3
Yellow 4Green 5
Blue 6
Violet 7
Gray 8
White 9Gold +5%
Silver +10%
Colorless +20%
D AB C
10
___ __________
___ __________
___ __________
3
2
1
R
R
R
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Activity 1: Series Resistors
Voltage (V) Current (I)
R1
R2
Theoretical RT
Experimental RT
% Error
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...
...
...
21
21
21
V V V
I I I
R R R
ConnectionSeries
T
T
T
Activity 1: Series Resistors
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Activity 2: Parallel Resistors
Voltage (V) Current (I)
R1
R2
Theoretical RT
Experimental RT
% Error
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......
...111
21
21
21
V V V I I I
R R R
Connection Parallel
T
T
T
Activity 2: Parallel Resistors
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...
...
...
21
21
21
V V V
I I I
R R R
ConnectionSeries
T
T
T
......
...111
21
21
21
V V V I I I
R R R
Connection Parallel
T
T
T
Series and Parallel Resistors
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Activity 4: Combination of Cells
In Series In Parallel
Electromotive force of Cell (E)
Known Resistance (R)
Current (I)
Internal Resistance of Cell (r)
Activity 5: Parts of a Cells
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Activity 5: Parts of a Cells