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Electronics > Theory 1 > Flashcards

Theory 1 Flashcards

1
Q

What is electric current ?

A

Electric current is the total charge that passes through some cross- sectional area A per unit time. 1A = 1 C/s

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2
Q

What is the charge on a single electron ?

A

Qelectron = (− e) = −1.602 × 10^(−19)C

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3
Q

What does volt unit mean ?

A
  • 1volt = 1joule/1coulomb = J/C (Energy definition)
  • 1.5 V battery is capable of moving 1 C of charge through a circuit while performing 1.5 J worth of work.
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4
Q

What is Power ? ( in circuits )

A
  • Power represents how much energy per second goes into powering a circuit.
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5
Q

What is Point Voltage ?

A
  • voltage measured between the ground reference and a specific point of interest within the circuit.
  • from ref to +’ve terminal = +’ve volt
  • from ref to -‘ve terminal = -‘ve volt
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6
Q

Memorise the voltage/ water analogy.

A
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7
Q

Find the voltage btw the various points.

A
  • a. VAC = 0, VBD = 0, VAD = 0, VBC = 0
  • b. VAC = 3 V, VBD = 0 V, VAD = 12 V, VBC = 9 V
  • c. VAC = 12 V, VBD = 9V. VAD = 21V, VBC = 0 V.
  • d. VAC = 3 V, VAB = 6 V, VCD = 1.5 V, VAD = 1.5 V, VBD = 4.5 V.

Volt btw similar battery terminals = subtract.

It’s like both ends = high pressure = repel

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8
Q

Find the point voltages (referenced to ground) at the various locations indicated in the following figures.

A

a. A = 3 V, B = −3 V, C = 3 V, D = 3 V, E = 3 V, F = 3 V, G = 6 V, H = 9 V.

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9
Q

Find the point voltages (referenced to ground) at the various locations indicated in the following figures.

A

b. A = 1.5 V, B = 0 V, C = 1.5 V, D = 1.5 V, E = −1.5 V, F = −3.0 V, G = 1.5 V, H = −1.5 V

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10
Q

How can drift velocity be slow and current measurement be instantaneous ?

A
  • Drift Velocity: Slow movement of individual electrons (cars).
  • Signal Propagation: Quick transmission of the electric field (traffic light signal).
  • Current Measurement: Overall flow rate of electrons (number of cars passing a point).
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11
Q

What effect does doubling the length and cross area of a wire have on resistance ?

A
  • double length = double resistance
  • double area = reduce resistance
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12
Q

What is the unit for resistivity ?

A

Ohm-meters

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13
Q

What is the unit of conductivity ?

A

Siemens = 1/(ohm-meters)

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14
Q

What do these energy bands represent ?

A
  • (a) Conductor—allowed band is only partially full, so electrons can be excited to nearby states.
  • (b) Insulator—forbidden band with large energy gap between the filled band and the next allowed band.
  • (c) Conductor—allowed bands overlap.
  • (d) Semiconductor—energy gap between the filled band and the next allowed band is very small, so some electrons are excited to the conduction band at normal temperatures, leaving holes in the valence band.
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15
Q

Does resistivity increase or decrease with increasing temperatures in conductors and semiconductors ?

A
  • conductors = increase = atoms vibrate, hence impede electron flow
  • semiconductors = decrease = temp provide enough energy for electrons to jump band gap
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16
Q

When can power across an element be treated as heat loss ?

A
  • When element is 100 % ohmic (resistor)
  • then can use ( V^2/R ) and ( I^2 × R ) to find loss
  • called ohmic heating / joule heating
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17
Q

Examples of ohmic elements that convert power to heat.

A
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18
Q

With an ammeter and voltmeter, you measure the current drawn by a computer to be 1.5 A and the voltage entering to be 117 V. How much power does the computer consume? Can we say how much power is lost to heat?

A

P = IV = (1.5 A)(117 V) = 176 W.

  • Knowing how much of the power is lost to heating is practically impossible to measure without taking the computer apart.
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19
Q

How is energy transferred in heating within gas, non-metals and metals ?

A
  • Gas = colliding gas molecules
  • Non-metals = vibrating lattices
  • Metals = vibrating lattices + mobile free electrons
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20
Q

What are heat sinks ?

A
  • Heat sinks are special devices that are used to draw heat away from temperature sensitive devices by increasing the radiating surface in air—which acts like a cooling fluid for conduction.
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21
Q

What is American Wire Gauge (AWG) ?

A
  • Standard for measuring the diameter of conducting wire
  • Australia equivalent = Brown & Sharp Gauge (B&S) and Metric Wire Gauge (MWG)
22
Q

Why shouldn’t you connect a wire across a voltage source? Eg. connect a 12- gauge wire directly across 120- V mains outlet. What will happen when you do this to a 12- V dc supply, or to a 1.5- V battery?

A
  • 120 V = draw huge current, CB trip
  • Battery = internal resistance cause heating.
23
Q

Where can the common return in a circuit be placed ?

A

Either at the negative terminal of a battery or any 0 reference point.

24
Q

What is earth ground?

A
  • a connection terminated at a rod driven into the earth to a depth of 8 ft or more.
25
Q

Where does the ground wire from a power cord connect to inside an equipment ?

A
  • connected internally to the equipment chassis
  • to the return portion of a channel that emanates from the interior circuitry.
26
Q

What does RF grounding do ?

A
  • Provides a low-impedance path for stray RF current caused by stray radiofrequency producing devices.
27
Q

What are some common dc power supply grounding error ?

A
  • connecting a load across +’ve and ground
  • must be across +’ve and -‘ve (floating load)
  • must connect -‘ve terminal to ground using jumber cable. (grounded load)
28
Q

How are 2 DC power supply connected for dual polarity voltages ?

A
29
Q

List the names of these ground symbols.

A
30
Q

List 3 types of return symbols

A
31
Q

What is this image trying to tell ?

A
  • Systems using chassis as return can have resistive leakage path
  • Shock hazard
  • Grounding chassis to place them at same potential.
32
Q

What is ground loops ?

A

A ground loop occurs when:

  • Multiple paths to ground exist between devices.
  • Different ground potentials cause currents to flow through these paths.
33
Q

As a rule of thumb, always select a resistor that has power rating of atleast …… ?

A

Twice the maximum value anticipated

34
Q

What happens to power/current when resistance increase/decrease ?

A
  • Increased resistance = decreased current/power
  • Decreased resistance = Increased current/power
35
Q
A
36
Q

What is the formula for finding total resistance in parallel circuits ?

A
37
Q

What is kirchoff’s current law ?

A
  • KCL states that the total current entering a junction (or node) in a circuit is equal to the total current leaving the junction
38
Q

What is kirchoff’s voltage law ?

A
  • KVL states that the sum of all voltages around any closed loop in a circuit is zero.
39
Q

Three resistors R1 = 1 kΩ, R2 = 2 kΩ, R3 = 4 kΩ are in parallel. Find the equivalent resistance. Also, if a 24- V battery is attached to the parallel circuit to complete a circuit, find the total current, individual currents through each of the resistors, total power loss, and individual resistor power losses.

A
40
Q

What is the voltage divider equation ?

A
41
Q

You have a 10- V supply, but a device that is to be connected to the supply is rated at 3 V and draws 9.1 mA. Create a voltage divider for the load device.

A
42
Q

You wish to create a multiple voltage divider that powers three loads:
load 1 (75 V, 30 mA), load 2 (50 V, 10 mA), and load 3 (25 V, 10 mA). Use the 10 percent rule and Fig. 2.49 to construct the voltage divider.

A
43
Q

What are the steps in designing multiple voltage dividers. ? (eg 3 Loads)

A
  • Calculate bleeder current and voltage of R4 using the 10% rule
  • i.e 10% * (I load1 + I load2 + I load 3)
  • Use ohms Law to find bleeder resistor. (R = V/I bleed)
  • Using KCL, I R3 = I R4 + I Rload3)
  • Find R3 using Ohm’s Law ( Diff in voltage/Current 3)
  • Repeat step 4 & 5 to find I2, R2, I1, R1
44
Q

When is internal battery resistance taken into account ? (DC voltage Source)

A
  • R load is smaller or closer to (r internal)
45
Q

What are the key diff btw electric and magnetic fields ?

A
  • SOURCE: Electric fields are generated by stationary charges, while magnetic fields are generated by moving charges or changing electric fields.
  • DIRECTION: Electric fields point away from positive charges and towards negative charges. Magnetic fields, on the other hand, circulate around current-carrying wires or changing electric fields.
  • EFFECT: Electric fields exert forces on charged particles, while magnetic fields exert forces on moving charges or changing electric fields.
  • RANGE: Electric fields decrease with distance (1/r^2), while magnetic fields decrease with distance (1/r^3).
46
Q

What is an ideal current source ?

A
  • Provides constant current regardless of load resistance changes
  • Terminal voltage changes with load resistance to maintain constant current
  • Practical current sources are usually made from transistors .
47
Q

What is the name of the internal resistance in current sources ?

A
  • Shunt (parallel) resistance
48
Q

What is the transformation formula btw ideal voltage and current sources.

A
49
Q
A
50
Q

What is the superposition theorem ?

A
  • The current in a branch of a linear circuit is equal to the sum of the currents produced by each source, with the other sources set equal to zero.
  • Voltage source set to zero by short circuit
  • Current source set to zero by open circuit.
51
Q

How is Thevenin’s theorem used to solve a voltage divider problem?

A
52
Q

How’s Norton’s Theorem used to solve voltage divider problem ?

A

Electronics (3 decks)

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