Electric Circuits and Kirchhoff's Rules, Summaries of Physics

Download Electric Circuits and Kirchhoff's Rules and more Summaries Physics in PDF only on Docsity!

Chapter 20

Electric Circuits

RC Circuits

Alternating Current

Kirchhoff’s Rules

Capacitors in Series and Parallel

Physics2 (APHYT2A) Vaal University Of Technology

20.

Alternating Current

More electric circuit uses battery and involved

Direct Current

(dc).However, there

are more circuits that operate with

Alternating Current

(ac). In an

AC circuit

the charge flow reverses direction periodically. The

ac generators

serve the same

purpose as the battery serves in a dc circuit, that is they give energy to the movingcharges. Electric outlets in a house provide ac current, we all use ac circuit regularly.

Alternating Current^ In circuits that contain only resistance, the current reverses direction each timethe polarity of the generator reverses.

(^

)^

(^

) ft

I

ft

V R

V R I^

o

o

π

π

2

sin

2

sin

=

=

=

peak current

Alternating Current

(^

) ft

V I

IV

P

o

o

π 2

sin

=

= (^

) ft

I I^

o

π 2 sin =^

(^

) ft

V V^

o

π 2 sin =

Alternating Current

R I

V

rms

rms

=

rms rms

I V

P

=

R I

P

(^2) rms

V R

P

(^2) rms

When the instructions for an electrical appliance specifythe voltage, it is an rms voltage that is meant. Similarlywhen we specify an ac voltage or current in this textbook it is an rms value, unless indicated otherwise.Likewise, when we specify an ac power, it is an averagepower, unless stated otherwise.

Alternating Current 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 thiscircuit.

Alternating Current 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 120-V wallreceptacle, 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 smaller-gaugewires better then larger-gauge wires?

Kirchhoff’s Rules^ The junction rule states that the totalcurrent directed into a junction mustequal the total current directed out ofthe junction.

Kirchhoff’s Rules

KIRCHHOFF’S RULES Junction rule.

The sum of the magnitudes of the currents directed

into 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 drops

equals the sum of the potential rises.

Kirchhoff’s Rules

Example 14

Using Kirchhoff’s Loop Rule

Determine the current in the circuit.

Kirchhoff’s Rules

Kirchhoff’s Rules Reasoning Strategy Applying Kirchhoff’s Rules 1.^

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.2.

Mark each resistor with a + at one end and a – at the other end in a way that 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 -).3.

Apply the junction rule and the loop rule to the circuit, obtaining in the process as many independent equations as there are unknown variables.4. Solve these equations simultaneously for the unknown variables.

RC Circuits

Capacitor charging

[^

]

RCt

o^

e

q

q

−

RC

time constant^ τ

RC Circuits

Capacitor discharging

RCt eo q

q

−

=

RC

time constant^ τ