Chapter 3
Ohm’s Law
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C.Y. Lee
Objectives
Explain Ohm’s law, use Ohm’s law to determine
voltage, current, or resistance
Define energy and power, calculate power in a
circuit
Properly select resistors based on power
considerations
Explain energy conversion
Discuss power supplies and their characteristics
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Ohm’s Law
The effect of changing voltage on current, if
resistance is held constant
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Ohm’s Law
Ohm’s law describes mathematically how voltage,
current, and resistance in a circuit are related
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Formula for Current
If the values of Voltage and Resistance are know,
Current can be calculated as:
V
I=
R
Example:
=?
I = 50/(1.0×103) A
= 50 mA
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Formula for Voltage
If the values of Current and Resistance are know,
Voltage can be calculated as:
V = IR
Example:
Vs = 10 mA×3.3 kΩ
= 33 V
=?
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Formula for Resistance
If the values of Voltage and Current are know,
Resistance can be calculated as:
V
R=
I
Example:
R = 150 V/5 mA
= 30 kΩ
=?
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Ohm’s Law
Example: Determine the
current in below resistor
I = 25 V/10 kΩ
= 2.5 mA
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Ohm’s Law
Example: Compare R1, R2, and R3
R3 > R2 > R1
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Energy and Power
Energy (U) is the ability to do work
– Energy is measured in joules (J)
Power (P) is the rate at which energy is used
U
P=
t
(W)
– One watt is the amount of power when one joule
of energy is used in one second
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Energy and Power
Typical power rating for several household appliances
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Power in an Electric Circuit
Power in an electric circuit may be expressed as:
P = IV (W)
Using Ohm’s law, and substituting, we can also
2
obtain:
V
2
P = I R and P =
R
−
V
+
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The Power Rating of Resistors
The power rating is the
maximum amount of power
that a resistor can dissipate
without being damaged by
excessive heat buildup
– When a resistor is used in a
circuit, its power rating
should be greater than the
maximum power that it will
have to handle
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1/8 W
1/4 W
1/2 W
1W
(metal-film resistors)
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The Power Rating of Resistors
Example: Choose an adequate power rating (1/8, 1/4,
1/2, or 1 W) for each of the metal-film resistors
P = V2/R = (10)2/120
= 0.833 W
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P = I2R = (0.01)2(1000)
= 0.1 W
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Power Supplies
A power supply produces voltage across its output
terminals and provides current through the load
– The product IVOUT is the amount of power produced
by the supply and consumed by the load
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Power Supplies
Electronic power supplies normally convert 110 V
ac from a wall outlet into a regulated dc voltage
POUT
Efficiency =
PIN
POUT = PIN - PLOSS
PIN
POUT
(PLOSS)
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Power Supplies
Batteries have a certain capacity that limits the
amount of time over which they can produce a
given power level; this capacity is measured in
ampere-hours (Ah)
某牌 4號
1175 mAh
某牌 1號
18000 mAh
某牌 3號
2700 mAh
某牌 9V
550 mAh
某牌 2號
7750 mAh
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Summary
Ohm’s law (V = IR) gives the relationship of
voltage, current, and resistance
Current is directly proportional to voltage
Current is inversely proportional to resistance
Use: V = IR, when calculating voltage
Use: I = V/R, when calculating current
Use: R = V/I, when calculating resistance
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Summary
Power rating is not related to resistance value
Energy is equal to power multiplied by time
Electronic power supplies convert commercial
energy (ac) to a regulated dc voltage
A battery converts chemical energy into
electrical energy
Capacity of a battery is measured in amperehours (Ah)
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