Transcript Ch 1 Electricity Slides 042909

Chapter 1
Properties
of
Electricity
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Overview
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What is Electricity
Ohm’s Law
Boat Circuit Problems
Multimeters
Magnetic Lines of Force
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What is Electricity
• Electricity consists of electrons
 Negatively charged
 Electrons actually flow from negative to positive
• Current “flow”,
by convention,
from positive to negative
• Conductors have electrons that readily move
 e.g. silver, copper, metal and liquids (salt water)
• Insulation has tightly bonded electrons
 Charges do NOT move
 e.g. plastic, rubber, glass and ceramics
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Water Analogy
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Height of water = Voltage
Valve = Switch
Water wheel = Electric motor
Small pipe = Resistance
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Electrical Circuits
• Simple Series Circuit
 Same current flow through all devices
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Complex Series Circuit
• If one light bulb burns out…
 No lights will be on
 Therefore, not used for light circuits
Off
Off
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Complex Parallel Circuit
• If one light bulb burns out…
 Other will remain on
 Therefore, used for light circuits
On
On
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Ohm’s Law
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E=
I =
R=
P=
Voltage (pressure) in volts
Current (rate of flow) in amperes
Resistance (friction) in ohms
E
Power (work) in watts
I R
E=I*R
R=E/I
I=E/R
P=I*E
P = I2 * R
P = E2 / R
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Practice
(fig 1-5)
• Given 12 volts and 4 amps
• What is resistance?
E=I*R
R=E/I
R = 12 / 4
R = 3 ohms (or 3 Ω)
12 VDC
4.0 Amps
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Practice
(fig 1-6)
• Given 12 volts and 8 ohms
• What is current?
E=I*R
I=E/R
RT = R1 + R2
I = 12 / 8
I = 1.5 amps
4 ohms (each)
12 VDC
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Practice
(fig 1-8)
• What is current in each device?
6 amps in 2 ohms
2 amps in 6 ohms (each)
• What is total current?
IT = I 1 + I 2 + I 2
IT = 10 amps
2 Ohms
12 VDC
6 Ohms
6 Ohms
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Electrical Power
• Watt is unit of electrical power
• Given 12 VDC and total resistance of 6 Ω
• What is total power?
I=E/R
I = 12 / 6
P = E * I P = 12 * 2
I = 2 amps
P = 24 watts
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What We Have Learned So Far
• Electricity is the negatively charged electrons that have
been freed from the structure of an atom
• For electricity to perform some useful function it must
flow in a circuit
• There are two basic types of electrical circuits
 The Series Circuit
 The Parallel Circuit
• Using Ohm's Law we can calculate the three electrical
values in a circuit, i.e. Voltage, Amperage, and
Resistance
• The fourth electrical value is Power
 Power is measured in Watts
 Using the PIE equation we can calculate Power
 Using Ohm's Law and PIE equation if we know any two values
we can find the other two
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Boat Circuit Problems
• Open Circuit is “break” in electrical wiring
 Current flow (path) is interrupted
• Sometimes deliberately by switch
• Short circuit is electrical wiring shortened
from original length
 Shorts to ground
 Interwiring short
 Internal short
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Multimeters
• Used to measure
 Voltage
 Current
 Resistance
• Types of Multimeters
 Analog
 Digital
• Preferred
• Easier to read
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Measuring Voltage
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Voltage to DC
Range to 20 volts
Red lead to positive
Black lead to negative
Meter across load
• Parallel with load
• Battery voltage is 12.6
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Measuring Voltage at Light
 with good fuse
 and switch “On”
• Should measure approx.
12 VDC at each light
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Measuring Voltage Drop
• Across a closed switch
 Should measure zero volts
• Across a high resistance “connection”
 Should measure a voltage drop
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Measuring Current
 Current to DC
 Range to 10 amps
 Red lead to current jack on
multimeter
 Insert meter in series with circuit
When measuring voltage
- meter is across the load
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Measuring Resistance
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Function to resistance
Touch test leads together
Adjust resistance to zero (0.0) ohms
Place test leads across device to be measured
• Continuity measurement
 When test leads are across low resistance
(Good circuit or device)
 Will hear a buzz indicating continuity
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Magnetic Lines of Force
• Results of Electrical Current
 Heat (due to resistance in wire)
 Chemical reaction (in current carrying solution)
 Magnetic Field (illustrated)
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Remote Control Circuit
AWG#2 (Large Wire)
AWG#16 (Small Wire)
Small current through solenoid coil
- Controls larger current, through contacts,
to Starter Motor
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Summary
• Electricity is negatively charged electrons
• Current flows from positive to negative
• Types of circuits
 Series (one path for current flow)
 Parallel (multiple paths for current flow)
• Ohm’s Law
 E=I*R
 P=E*I
• Boat Circuit Problems
 Open Circuits
 Short Circuits
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Summary - 2
• Multimeter
 Digital preferred
 Measure voltage across (parallel to) load
• Can also measure voltage drop
 Measure current in series with load
• Magnetic Lines of Force
 Concentrated in coil
 Solenoids use in remote control circuits
• Small current controls large current
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