electric circuits chapter 20 - physicsskills /...

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Chapter 20Electric Circuits

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20.1 Electromotive Force and Current

In an electric circuit, an energy source and an energy consuming deviceare connected by conducting wires through which electric charges move.

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Within a battery, a chemical reaction occurs that transfers electrons fromone terminal to another terminal.

The maximum potential difference across the terminals is called the electromotive force (emf).

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The electric current is the amount of charge per unit time that passesthrough a surface that is perpendicular to the motion of the charges.

One coulomb per second equals one ampere (A).

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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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Example 1 A Pocket Calculator

The current in a 3.0 V battery of a pocket calculator is 0.17 mA. In one hourof operation, (a) how much charge flows in the circuit and (b) how much energydoes the battery deliver to the calculator circuit?

(a)

(b)

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Conventional current is the hypothetical flow of positive charges that wouldhave the same effect in the circuit as the movement of negative charges thatactually does occur.

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20.2 Ohm’s Law

The resistance (R) is defined as the ratio of the voltage V applied across a piece of material to the current I throughthe material.

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20.2 Ohm’s Law

OHM’S LAW

The ratio V/I is a constant, where V is thevoltage applied across a piece of mateiraland I is the current through the material:

SI Unit of Resistance: volt/ampere (V/A) = ohm (Ω)

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20.2 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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20.2 Ohm’s Law

Example 2 A Flashlight

The filament in a light bulb is a resistor in the formof a thin piece of wire. The wire becomes hot enoughto emit light because of the current in it. The flashlightuses two 1.5V batteries to provide a current of0.40 A in the filament. Determine the resistance ofthe glowing filament.

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20.3 Resistance and Resistivity

For a wide range of materials, the resistance of a piece of material of length L and crosssectional area A is

resistivity in units of ohm∙meter

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Example 3 Longer Extension Cords

The instructions for an electric lawn mower suggest that a 20gauge extensioncord can be used for distances up to 35 m, but a thicker 16gauge cord shouldbe used for longer distances. The cross sectional area of a 20gauge wire is5.2x107Ω∙m, while that of a 16gauge wire is 13x107Ω∙m. Determine the resistance of (a) 35 m of 20gauge copper wire and (b) 75 m of 16gauge copper wire.

(a)

(b)

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Impedance Plethysmography.

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temperature coefficient of resistivity

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20.4 Electric Power

Suppose some charge emerges from a battery and the potential differencebetween the battery terminals is V.

energypower

time

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20.4 Electric Power

ELECTRIC POWER

When there is current in a circuit as a result of a voltage, the electricpower delivered to the circuit is:

SI Unit of Power: watt (W)

Many electrical devices are essentially resistors:

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How much work does the electric field do in moving a proton from a point with a potential of +105 V to a point where it is 21 V? Express your answer both in joules and electron volts

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(a) Determine the electric field E at the origin O in Fig. 1642 due to the two charges at A and B. (Type your answer using k for proportionality constant, Q for charge and l for the length l.)(b) Repeat, but let the charge at B be reversed in sign.

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20.4 Electric Power

Example 5 The Power and Energy Used in aFlashlight

In the flashlight, the current is 0.40A and the voltageis 3.0 V. Find (a) the power delivered to the bulb and(b) the energy dissipated in the bulb in 5.5 minutesof operation.

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20.4 Electric Power

(a)

(b)

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20.5 Alternating Current

In an AC circuit, the charge flow reverses direction periodically.

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In circuits that contain only resistance, the current reverses direction each time the polarity of the generator reverses.

peak current

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Example 6 Electrical Power Sent to a Loudspeaker

A stereo receiver applies a peak voltage of34 V to a speaker. The speaker behavesapproximately as if it had a resistance of 8.0 Ω.Determine (a) the rms voltage, (b) the rmscurrent, and (c) the average power for this circuit.

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(a)

(b)

(c)

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Conceptual Example 7 Extension Cords and a Potential Fire Hazard

During the winter, many people use portable electric space heaters to keepwarm. Sometimes, however, the heater must be located far from a 120V wall receptacle, so an extension cord must be used. However, manufacturers oftenwarn against using an extension cord. If one must be used, they recommenda certain wire gauge, or smaller. Why the warning, and why are smallergauge wires better then largergauge wires?

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The drawing shows four point charges. The value of q is 2.2 µC, and the distance d is 0.63 m. Find the total potential at the location P. Assume that the potential of a point charge is zero at infinity.

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The electric field has a constant value of 3.5 103 V/m and is directed downward. The field is the same everywhere. The potential at a point P within this region is 135 V. Find the potential at the following points. (a) 9.5 103 m directly above P(b) 3.3 103 m directly below P(c) 5.0 103 m directly to the right of P

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A charge of +8q is fixed to one corner of a square, while a charge of 7q is fixed to the diagonally opposite corner. Expressed in terms of q, what charge should be fixed to the center of the square, so the potential is zero at each of the two empty corners?

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20.6 Series Wiring

There are many circuits in which more than one device is connected toa voltage source.

Series wiring means that the devices are connected in such a waythat there is the same electric current through each device.

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20.6 Series Wiring

Series resistors

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20.6 Series Wiring

Example 8 Resistors in a Series Circuit

A 6.00 Ω resistor and a 3.00 Ω resistor are connected in series witha 12.0 V battery. Assuming the battery contributes no resistance to the circuit, find (a) the current, (b) the power dissipated in each resistor,and (c) the total power delivered to the resistors by the battery.

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20.6 Series Wiring

(a)

(b)

(c)

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20.6 Series Wiring

Personal electronic assistants.

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20.7 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 eitherresistor alone.

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The two parallel pipe sections are equivalent to a single pipe of thesame length and same total cross sectional area.

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parallel resistors

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Example 10 Main and Remote Stereo Speakers

Most receivers allow the user to connect to “remote” speakers in additionto the main speakers. At the instant represented in the picture, the voltageacross the speakers is 6.00 V. Determine (a) the equivalent resistanceof the two speakers, (b) the total current supplied by the receiver, (c) thecurrent in each speaker, and (d) the power dissipated in each speaker.

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(a)

(b)

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(c)

(d)

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Conceptual Example 11 A ThreeWay Light Bulband Parallel Wiring

Within the bulb there are two separate filaments.When one burns out, the bulb can produce onlyone level of illumination, but not the highest.

Are the filaments connected in series orparallel?

How can two filaments be used to produce threedifferent illumination levels?

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20.8 Circuits Wired Partially in Series and Partially in Parallel

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20.9 Internal Resistance

Batteries and generators add some resistance to a circuit. This resistanceis called internal resistance.

The actual voltage between the terminals of a batter is known as theterminal voltage.

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20.9 Internal Resistance

Example 12 The Terminal Voltage of a Battery

The car battery has an emf of 12.0 V and an internalresistance of 0.0100 Ω. What is the terminal voltagewhen the current drawn from the battery is (a) 10.0 Aand (b) 100.0 A?

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20.10 Kirchhoff’s Rules

The junction rule states that the total current directed into a junction mustequal the total current directed out of the junction.

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The loop rule expresses conservation of energy in terms of the electric potential and states that for a closed circuit loop, the total of all potentialrises is the same as the total of all potential drops.

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KIRCHHOFF’S RULES

Junction rule. The sum of the magnitudes of the currents directedinto a junction equals the sum of the magnitudes of the currents directedout of a junction.

Loop rule. Around any closed circuit loop, the sum of the potential dropsequals the sum of the potential rises.

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Example 14 Using Kirchhoff’s Loop Rule

Determine the current in the circuit.

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Reasoning Strategy

Applying Kirchhoff’s Rules

• Draw the current in each branch of the circuit. Choose any direction. If your choice is incorrect, the value obtained for the current will turn outto be a negative number.

• Mark each resistor with a + at one end and a – at the other end in a waythat is consistent with your choice for current direction in step 1. Outside abattery, conventional current is always directed from a higher potential (theend marked +) to a lower potential (the end marked ).

• Apply the junction rule and the loop rule to the circuit, obtaining in the processas many independent equations as there are unknown variables.

• Solve these equations simultaneously for the unknown variables.

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20.11 The Measurement of Current and Voltage

A dc galvanometer. The coil ofwire and pointer rotate when thereis a current in the wire.

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An ammeter must be inserted intoa circuit so that the current passesdirectly through it.

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If a galvanometer with a fullscalelimit of 0.100 mA is to be used to measure the current of 60.0 mA, a shunt resistance must be used so thatthe excess current of 59.9 mA can detour around the galvanometer coil.

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To measure the voltage between two pointsin a circuit, a voltmeter is connected betweenthe points.

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20.12 Capacitors in Series and Parallel

Parallel capacitors

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20.12 Capacitors in Series and Parallel

Series capacitors

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20.13 RC Circuits

Capacitor charging

time constant

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20.13 RC Circuits

Capacitor discharging

time constant

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20.13 RC Circuits

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20.14 Safety and the Physiological Effects of Current

To reduce the danger inherent in using circuits, proper electrical grounding is necessary.

electric circuits chapter 20 - physicsskills /...

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