Post on 19-Dec-2015
Arrange the following in the order of current, starting with the lowest. (A) 7 coulombs in 3 seconds. (B) 10 Volts across 5 Ohms (C) 15 Volts across 7 Ohms (D) 2 Amps
Question
If a laptop needs constantly needs 2 Amps current from a battery, how many electrons are drained from the battery in one hour?
1 Amp = 6.242 x 1018 electrons/second 2 Amp = 12.484 x 1018 electrons/second In one hour - > 3600 x 12.484 x 1018 electrons Answer is 4.49 x 1022 electrons
DC Voltage Supply
Resistance
R = ρ L/A ρ is the resistivity of
the material (units?)
Material ρ (10-8 Ohm-Metres)
Silver 1.645
Copper 1.723
Gold 2.443
Aluminum 2.825
Tungsten 5.485
Nickel 7.811
Iron 12.299
Tantalum 15.54
Nichrome 99.72
Tin Oxide 250
Carbon 3500
American Wire Gage (AWG) sizes
AWG # Diameter (in) Ω /1000ft.0000 0.46 0.0490
000 0.409 0.0618
0 0.325 0.0983
1 0.289 0.1240
2 0.257 0.1563
4 0.204 0.2485
10 0.102 0.9989
14 0.0640 2.525
28 0.0126 64.90
Color Coding
5 Bands of code (3 are mandatory) Bands 1 - 3 the value of the resistor Band 4 the range (tolerance) Band 5 the reliability
Color Code (Band 1-3)
Color Value
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9
Band 3 (special cases)
Gold = 0.1Red Blue Gold = 2.6 Ohm
Silver = 0.01Red Blue Silver = 0.26 Ohm
More BandsBand 4 Tolerance
Gold 5%
Silver 10%
None 20%
Band 5 Reliability (after 1000 Hrs of use)
Brown 1%
Red 0.1%
Orange 0.01%
Yellow 0.001%
Ohm’s Law
I = V/R V=IR R=V/I
Power
Power dissipated by charge flowing through a resistor P = VIP = V2/RP = I2R
Energy
Energy = Power x Time
Battery
Chemical Reactions to produce potential differenceAlkaline and lithium-iodine primary cellsLead Acid secondary cellNickel-Cadmium Secondary cellNickel-Hydrogen and Nickel-Metal Hydride
Secondary cells Solar Cells
Power Supply
Used very frequently in all devices. Transform the AC supply into a lower voltageRectify it (?)
Current Sources
Supplies a fixed amount of current It is the dual of the battery
In a battery voltage is constant, but current drains out
Ammeters
Device to measure current The wire in which current is to be
measured is broken up, and are joined via an ammeter.
What should be the resistance of the ammeter?
Voltmeters
Devices to measure voltage Connected in a parallel fashion across the
device where there is a need to measure potential difference.
What should be the resistance of a voltmeter?
Ohmmeters
Used to measure the resistance of a device.
Connected across the two pins of a resistor
Also used to check the continuity of networks.
Wattmeters
Used to measure the dissipation of power in a circuit element.
Includes both an ammeter and a voltmeter.
Displays the multiplication of both measurements.
Fuses and Circuit Breakers
The power supply to the homes is not ideal. If it goes above a specified level, it can burn the
devices. May result in Fire or Smoke. Fuse wires melt if they experience a large
current. In a breaker, a large current results in a large
enough strength in an inbuilt electromagnet to draw the switch open
Series Circuit
Two elements are in series ifThey have only one terminal in common. The common point in the two elements is not
connected to a third current carrying element.
Resistance
The resistance seen by the source
R=R1+R2
The two circuits on the right are equivalent
R1
R2
R1+R2
Voltage Drop?
The current through each resistor is calculated by the Ohm’s law=V1/R1
Where V1 is the voltage across the resistor.
=V/RT
Where RT is the total resistance in the circuit.
V1 = VxR1/RT
Power?
Power dissipated in each resistorP1 = V1
2/R1
P1 = (V2/RT2)x R1
Total power = V2/RT = P1 + P2 + …
Kirchhoff’s Voltage Law
The algebraic sum of the potential rises and drops around a closed loop is zero.
KVL
V + V1+V2 = 0 Can anyone prove
this mathematically?
R1
R2V
V1
V2
Voltage Divider Rule
In a series circuit the voltage across the resistive elements will divide as the magnitude of the resistors
Ground Terminal
This is not a loop. Or is it?
Ground terminal means that the two points are both connected to ground and are at a zero potential. So this is a loop.
Internal Resistances
Voltage and other sources have internal resistances, and they should be counted while solving circuits.
Resistance
R = ρ L/(A1+A2) Solving in terms of R1
and R2 gives 1/R = 1/R1 + 1/R2
The total value of the resistance is always smaller than the smallest resistance
Kirchhoff’s Current Law
KCL states that the algebraic sum of the currents entering and leaving a point or junction is zero.
i1+i2+i3+i4=0
i1
i2 i3 i4
Current Divider Rule
For parallel elements of different value the current will split with a ratio equal to the inverse of their resistor value
Reduce and Return Approach
Applicable to all single source circuits.
Currents in Loops (Remember KCL?)
i1i3i2
Ladder Networks
There are 2 approaches to solve this circuit Reduce Resistances using series parallel analysis Calculate current using current loops
No - Load
When is “no load” observed? When R2 is infinitely
large? Or when R2 is zero?
When R2 is infinite!!!
R1
R2
Voltage Divider Supply
We cannot calculate V2 and V3 unless we know what load is connected to them
The less the load, the closer they are to 9V.
R1
R2
R3
V3
V2
V1
Potentiometer
Voltage Sources
Two voltage sources of different ratings may not be connected in parallel.
Why?
Series operation, however, is permitted.
Current Sources
Two current sources of different rating may not be connected in series.
Parallel operation, however, is permitted.