Download Study Notes Lesson 18 Electric Current

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Lesson 18 Electric Current
Introduction
a.
Electric potential (voltage)— the electrical potential energy per coulomb at a location in an electric field;
an electric pressure that can produce an electric current within a conductor, measured in volts (V).
b.
Electric current— the flow of electric charge, measured in amperes (A); the flow is restrained by the
resistance it encounters.
c.
Electric resistance— the resistance of a material to the flow of electric current through it, measured in
ohms (Ω).
d.
Electric power— the rate at which energy is transferred by electric current and converted into another
form, such as light, heat, or mechanical energy.
e.
Direct current (DC)— an electric current takes place in one direction.
f.
Alternating current (AC)— an electric current flows to and fro.
Flow of Charge
a.
When the ends of an electric conductor are at different electric potentials (potential difference), charges
flow from one end to the other. The flow of charge will continue until both ends reach a common potential.
b.
To attain a sustained flow of charge in a conductor, some potential difference must be maintained. Like the
water will flow from the high-pressure end of the pipe to the low-pressure end. The flow will cease when
the difference in pressure ceases.
Electric Current
a.
Electric current is the flow of charge. In solid conductors the electrons (called conducting electrons) carry
the charge through the circuit because they are free to move through the atomic network.
b.
In fluids, positive and negative ions as well as electrons may compose the flow of electric charge.
c.
Electric current is measured in amperes (A). An ampere is the flow of 1
coulomb of charge per second. (1 coulomb is 6.24 x 1018 electrons.)
d.
When electrons flow in a wire, the number entering one end is the same as the
number leaving the other. The net charge of the wire is normally zero at every moment.
Voltage Source
a.
Something, such as dry cell, wet cell or generator, that provides a sustained potential difference is called a
voltage source.
b.
In dry cells and wet cells, chemical energy is converted to electric energy. In generators, mechanical energy
is converted to electric energy.
c.
The potential energy per coulomb of charge available to electrons moving between terminals is the voltage
(sometimes called the electromotive force, or emf).
Electric Resistance
a.
The amount of charge that flows in a circuit depends on the voltage provided by the voltage source and also
depends on the electric resistance that the conductor offers to the flow of charge.
b.
The resistance of the wire depends on the conductivity of the material used in the wire and also the
thickness and length of the wire.
ρL
where
ρ
resistivity (Ω•m)
R=
A
L
length (m)
A
cross-sectional area (m2)
c. €For a given pressure, more water pass through a large pipe than a small one. Similarly, for a given voltage,
more electric current passes through a large-diameter wire than a small-diameter one.
d.
The thick wire has less resistance than the thin one. The longer wire has more resistance than the short one.
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e.
For most conductors, increased temperature means increased resistance.
f.
Electric resistance is measured in ohms (Ω).
Ohm’s Law
a.
Ohm discovered that the current in a circuit is directly proportional to the voltage impressed across the
circuit, and is inversely proportional to the resistance of the circuit.
b.
Ohm’s law:
current(I) =
c.
Units:
V,
or
R
1 ampere = 1 volt/ohm,
a.
The damaging effects of electric shock are the result of current passing through the body. The current
depends on the voltage applied and also on the resistance of the human body.
b.
The resistance of the body range from about 100 Ω if you are soaked with salt water to about 500 KΩ if
your skin is very dry.
c.
Effect of various electric currents on the body
I=
6
voltage(V )
resistan ce(R)
R=
V,
I
or
V = IR
or
1 A = 1 V/Ω
€
d. In electric circuit, resistors are used to provide resistance.
€
€
Ohm’s Law€and Electric Shock
Current (A)
0.001
0.005
0.010
0.015
0.070
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Lesson 18 Electric Current
Effect
can be felt
painful
involuntary muscle contractions (spasms)
loss of muscle control
if through the heart, serious disruption; probably fatal if current lasts for more than 1 second
d.
Handling a wet hair dryer can be very dangerous if you are standing barefoot in a wet bathtub. Because the
resistance between you and the ground is very small, the 120 V potential difference can produce harmful
current through the body.
e.
The birds perched on the high-voltage wires are safe because every part of their bodies is at the same high
potential as the wire.
f.
The third prong of the plug connects the body of the appliance directly to ground.
Direct Current (DC) and Alternating Current (AC)
a.
Direct current refers to a flow of charge that flows in on direction,
even if the current moves in unsteady pulses.
b.
A battery produces direct current. Electrons always move through
the circuit in the same direction from the repelling negative
terminal and toward the attracting positive terminal.
c.
Alternating current means that the flow of electrons is alternating
its directions. This is accomplished by alternating the polarity of
voltage at voltage source.
Current
DC
Time
Current
AC
Time
d.
Nearly all of the commercial AC circuits in North America involves 120 V and 60 Hz. Europe adopted 220
V as their standard.
e.
The 120 V refers to the “root-mean-square” (RMS) average of the voltage. The actual voltage in a 120 V
AC circuit varies between +170 V and – 170 V peaks. It delivers the same power as a 120 V DC circuit.
f.
Because most electric service in the United States is three-wire: one wire at +120 V, one wire at 0 V
(neutral), and the other wire at -120 V. Most of the appliance use +120V/-120 V and the neutral wires,
producing 120 V. When use both +120V and -120 V wires, a 240 V jolt is produced.
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Lesson 18 Electric Current
Converting AC to DC
a.
Instead of using batteries, the AC-DC converter can convert the AC in your home to DC.
b.
A typical diagram of DC power supply:
High Voltage AC
Rectifing
Smoothing
Regulating
AC to DC
Great Ripple DC
to
Small Ripple DC
Eliminate Ripple
Bridge Rectifier
Diodes
Capacitor
Zener Diode
Regulator
Transforming
High Voltage AC
to
Low Voltage AC
Transformer
c.
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Low Voltage DC
Single diode vs. bridge rectifier
The Speed of Electrons in a Circuit
a.
The electrons’ thermal motion (random motion) speed inside a metal wire is about 1/200 the speed of
light.
b.
Under electric field, the electrons’ actual drift speed (net speed) is only about 0.01 cm/s. The electrons will
“bump into” the anchored metallic ions in their path and transfer some kinetic energy to them. That’s why
the wire will be heated up.
c.
So the extremely high speed of electricity is not due to the electrons but due to the signal. The signal is
traveling at near high speed.
d.
The electrons will follow the electric field lines inside the conductor and shift forward (DC) or forward and
backward (AC).
10 The Source of Electrons in a Circuit
a.
The source of the electrons is the conducting circuit material itself. The AC outlets in your home do not
supply you electrons but supply you energy. The electrons vibrate to and fro about relative fixed positions.
Energy flow into your home AC appliance instead of electrons.
11 Electric Power
a.
A charge moving in a circuit expends energy. This may result in heating the circuit or turning a motor. The
rate at which electrical energy is converted into another form such as mechanical energy, heat, or light is
called electric power.
b.
Electric Power = Current x Voltage = (Charge/Time) x (Energy/Charge) = Energy/Time
c.
Unit: 1 watt = (1 ampere) x (1 volt)
d.
P = IV = I2R = V2/R
e.
Since Energy/Time = Power, so Energy = Power x Time; hence, energy can be represented in units of
kilowatt-hours (kW·h), where 1 kW·h = 3.6 x 106 J. A kilowatt is 1000 watt, and a kilowatt-hour is the
energy consumed in 1 hour at the rate of 1 kilowatt.
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