Electric Circuits

Eleventh Edition, Global Edition

Chapter 3
Simple
Resistive
Circuits

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Electronic Components

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Measuring Instruments: ammeter, voltmeter, ohmmeter
You must be able to calculate currents and voltages in circuits that contain “real” measuring
instruments.
 Measuring Voltage

• multimeter can measure voltage or current, and resistance.

• To measure voltage, the meter’s probes are touched to two places in a
circuit or across a battery.

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 Measuring Current

• To measure current you must force the current to pass

through the multimeter.

• Multimeters can measure two types of currents:

Alternating current (AC)

Direct current (DC).

• Circuit breakers and fuses are two kinds of devices that
protect circuits from too much current by making a break
that stops the current.

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Ammeter:
• measures current (A)
• connected in series
(current must go through instrument)

Voltmeter:
• measures potential difference (V)
• connected in parallel

Ohmmeter:
• measures resistance of an isolated
resistor (not in a working circuit)
 Measuring Dc Power
• Both current and voltage measurements are necessary to measure
power.

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Calculating Power in Series Dc Circuit

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Measuring Power in Series Dc Circuit

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Measuring Power in Parallel Dc Circuits

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 Electrical Short Circuit

• Consider a resistor whose value is zero ohms. An equivalent representation

of such a resistance, called a short-circuit

• A short circuit is a direct connection between two points in a circuit that

aren't supposed to be directly connected, such as the two terminals of a
power supply. Electric current takes the path of least resistance, so in a short
circuit, the current will bypass other parallel paths and travel through the
direct connection. (Think of the current as being lazy and taking the path
through which it doesn't have to do much work.)

By Ohm’s Law: ( For short Circuit, R = 0 )

v = Ri = 0i = 0 V

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 Electrical Open Circuit

• Consider a resistor having infinite resistance. An equivalent representation of

such a resistance, called an open-circuit

• You need a closed path, or closed circuit, to get electric current to flow. If there's
a break anywhere in the path, you have an open circuit, and the current stops
flowing — and the metal atoms in the wire quickly settle down to a peaceful,
electrically neutral existence.

By Ohm’s Law: ( For open Circuit, R = ∞ )

i = v/R = v/∞ = 0 A

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 passive elements:

• Three important passive circuit elements:

1) Resistor
2) Capacitor
3) Inductor

• Resistor dissipates energy

• Capacitor and inductor can store energy and they can neither
generate nor dissipate energy.

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 Resistors

• Therefore, changing the resistance by adding resistors allows

you to manipulate the voltage and current in a circuit
 Different Types Of Resistors

Wire wound resistor A 1/4-watt

metal film resistor

Small thick-film resistor chips

Resistor Color Code Chart
The chart below shows how to determine the resistance and tolerance for resistors.

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Series Resistors
• When components are connected in series,
the same electric current flows through them.

Rtotal = R+R+R+R
• Charge conservation : current cannot disappear!
 Parallel Resistors
• When components are connected in parallel, the same
potential difference drops across them

• Points connected by a wire are at the same voltage!

𝟏 𝟏 𝟏
= + + 𝟑
𝑹𝟏 𝑹𝟐 𝑹
Example: Current in series
resistive circuit

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Example: Voltage in series
resistive circuit

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Example: Current in parallel
resistive circuit

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The circuit for Example 1.

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Simplifying the circuit in Example 1. (1 of 2)

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Simplifying the circuit in Example 1.(2 of 2)

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REQ?

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 V2
P  SMALLER RESISTANCE FOR HIGHER POWER
R

A : R   (OFF )
B : R  R2 LOW
C : R  R1 MEDIUM
D : R  R1 || R2 HIGH
230[V ]2
PMEDIUM  1200W   R1  44.08
R1
230[V ]2
PHIGH  2000W   R1 || R2  26.45
230[V ]2
R1 || R2 OR PR  1200W  PR  800W 
R1 R2
1 2
R2
 26.45
R1  R2
 Voltage-Divider Circuit
(a) A voltage-divider circuit and (b) the voltage-
divider circuit with current i indicated.

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 v0=?

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The current-divider circuit.

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Examples
Figure 3.23: An ammeter connected to measure
the current in R1, and a voltmeter connected to measure
the voltage across R2.

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A short-circuit model for the ideal ammeter, and an open-
circuit model for the ideal voltmeter.

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Measuring Resistance: The Wheatstone
bridge circuit.

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A balanced Wheatstone bridge (ig = 0).

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A ∆ configuration viewed as a 
configuration.

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A Y structure viewed as a T structure.

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The -to-Y transformation.

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Example

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The equivalent Y resistors.

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A transformed version of the circuit
( -to-Y ).

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The final step in the simplification of the
circuit

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