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Introduction

Electronics and electrical devices surround us in daily life. From the street lights and water pumps to computers and digital phones, electronics have enabled the digital revolution to occur. All electronics are built on a backbone of simple circuits, and so an understanding of circuits is vital in understanding more complex devices.

This chapter will explain the basic physics principles of many of the components of electronic devices. We will begin with an explanation of capacitors and inductors. We see how these are used in tuning a radio. Next, we look at active components such as transistors and operational amplifiers. Lastly, the chapter will finish with an explanation of digital electronics, including logic gates and counting circuits.

Before studying this chapter, you will want to remind yourself of:

  1. The meaning of voltage ( V ), current ( I ) and resistance ( R ), as covered in Grade 10, and Grade 11.
  2. Capacitors in electric circuits, as covered in Grade 11 (see section 17.6).
  3. Semiconductors, as covered in Grade 11 (see chapter 20).
  4. The meaning of an alternating current (see section 28.3).
  5. Capacitance ( C ) and Inductance ( L ) (see section 28.4).

Capacitive and inductive circuits

Earlier in Grade 12, you were shown alternating currents (AC) and you saw that the voltage and the current varied with time. If the AC supply is connected to a resistor, then the current and voltage will be proportional to each other. This means that the current and voltage will `peak' at the same time. We say that the current and voltage are in phase . This is shown in [link] .

The voltage and current are in phase when a resistor is connected to an alternating voltage.

When a capacitor is connected to an alternating voltage, the maximum voltage is proportional to the maximum current, but the maximum voltage does not occur at the same time as the maximum current. The current has its maximum (it peaks) one quarter of a cycle before the voltage peaks. Engineers say that the `current leads the voltage by 90 '. This is shown in [link] .

The current peaks (has its maximum) one quarter of a wave before the voltage when a capacitor is connected to an alternating voltage.

For a circuit with a capacitor, the instantaneous value of V I is not constant. However, the value of V m a x I m a x is useful, and is called the capacitive reactance ( X C ) of the component. Because it is still a voltage divided by a current (like resistance), its unit is the ohm. The value of X C ( C standing for capacitor) depends on its capacitance ( C ) and the frequency ( f ) of the alternating current (in South Africa 50 Hz).

X C = V m a x I m a x = 1 2 π f C

Inductors are very similar, but the current peaks 90 after the voltage. This is shown in [link] . Engineers say that the `current lags the voltage'. Again, the ratio of maximum voltage to maximum current is called the reactance — this time inductive reactance ( X L ). The value of the reactance depends on its inductance ( L ).

X L = V m a x I m a x = 2 π f L

The current peaks (has its maximum) one quarter of a wave after the voltage when an inductor is connected to an alternating voltage.

Questions & Answers

how is ester formed
Aubrey Reply
how is n ester formed
Aubrey
Alcohol reacts with a carboxylic acid
Texas
an athlete with a mass of 70kg runs at a velocity of 45km . determine the athlete's momentum
Lesedi Reply
Is that a velocity or something else
msawenkosi
45km/h i guess
Texas
Change to m/s
Texas
45km/h = 12.5 m/s p=mv =70×12.5 =875 kg.m/s
Thato
what are the measures of the rates of reaction
Lesego Reply
Volume Concentration Temperature Pressure Surface Area
Thato
the principle of superposition of waves
Sfundo Reply
what is work
Kool Reply
is this a group chat
Nobuhle Reply
Hey can y'all define newton's 2nd law
mthebzification
If a resultant force act on an object...the object will accelerate in the direction of a resultant force,the acceleration of the object is directly proportional to the net force and inversely proportional to the mass of the object
mosa
how do you calculate tension force
Bulumko
use the formula Fnet=ma if there is tension connecting two objects
Sboniso
to calculate Tension, usually calculate acceleration first Draw separate free body diagrams for each body. Apply Fnet = ma to calculate Tension
Kevin
Hi people
Paul
how does temperature affect the equilibrium position
Blessing Reply
an increased temperature increases the average kinetic energy thus in turn increases the number of effective collisions........
Lwando
so...which reaction is favored between endothermic and exothermic .when temperature is increased..?
Blessing
exothermic reaction because energy is realised to the surroundings as heat and light energy ....graphical so much energy is realised as reactants to form product and because temperature is high rate of reaction is fast which means there is a successful collision
Code
INTEMENDO - INCREASE IN TEMPERATURE FAVOURS ENDOTHERMIC DETEMEXO - DECREASE IN TEMPERATURE FAVOURS EXOTHERMIC
Thato
an object will continue in a state of rest unless it is acted upon an unbalanced force
Junior Reply
Newton's Law 1
Code
First Newton's Law
Azola
Newton's first law
Surprise
newton first law
Thinavhuyo
Newton's first law
Blessing
when pressure is increased what happen to volume
Siphelo Reply
decreases
Code
care to explain?
Mpati
if pressure is applied to a pistol , the volume will decrease and particles will collide more frequently with the wall of a container .Each time they collide with the wall they exert a force on them .More collision means more force and the pressure will increase , that Boyle's Law
Code
Because the volume has decreased , the particle will collide more frequently with the wall of a container and each time they collide with the wall of a container they exert a force on them.More collision means more force so the pressure will increase , that Boyle's Law
Code
what is the difference between momentum and a change in momentum?
Chavonne Reply
How to name a branched molecule from right or left?
Vadin Reply
What's Free Fall
Senzo Reply
Free Fall means there is no acting force on that object.
Dingaletu
only gravitational force
Dingaletu
no external force acting on an object
Sphiwe
by only force of gravite
Sello
but gravitational force
Sphiwe
true
Lucky
a motion in which the only force acting is gravitational force
Blessing
and an object experiencing free fall is referred as a projectile
Blessing
Do polymers form restrictedly only if compound is saturated, only?
milani Reply
what is a free fall?
Beyanca Reply
is when The Only Force acting On an Object is Gravitational Force
Madman
Thats right
Beyanca
then Why ask when you Know the answer?
Madman
She's just helping those who forgot it...bro
Thato
guys I need help on Getting ready for a last minute test
Kenelioe Reply
what help you need
Neil
we'll I'm in grade 12 so we doing this topic about upac thing
Kenelioe
on What?
Madman
the organic molecule section
Kenelioe
IUPAC NAMING WHICH FUNCTIONAL GROUP YOU CANNOT NAME?SO I COULD HELP YOU
Madman
ester
Sboniso
you should also look at structural isomers. Its crucial that they might add that one. also try and write down the structural formula of all the given compounds on the table
milani

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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