Transcript 1 - Pui Chor Wong

Fundamental Electrical
Theory
Definitions
• Current (I): flow of electric charges per unit time
or “flow rate”, measured in “amperes” or “amps”
(A)
• Electromotive Force (emf) (E): a potential
difference or “electric pressure” which drives the
flow of charges, measured in “volts” (V)
• Resistance (R): an electrical circuit’s opposition
to current flow, measured in “ohms” ()
• Conductor: a material which offers little
resistance to current flow, e.g. silver, copper,
iron, etc…
• Insulator: a material which offers high resistance
to current flow, e.g. wood, paper, plastic, etc...
Direct Current (DC)
• Current flow is unidirectional and of
constant magnitude (Batteries)
• Ohm’s Law: current in a circuit is
directly proportional to the applied
voltage and inversely proportional to
the circuit resistance
E=IR
P = I E = I2 R (W)
Basic Circuit Properties
• Electrons flow (-) to (+)
• Conventional Current flows (+) to (-)
• Kirchoff’s Law of Voltages (KVL)
– Sum of all voltages in a complete circuit is
zero
– Choose arbitrary loop of a circuit (CW or CCW)
– If encounters (+) terminal 1st then (-), then its a
voltage drop. (usually a resistor)
– If encounters (-) terminal 1st then (+) which is a
voltage increase. (usually a battery)
• Sum of current into and out of a node is
always zero and constant (KCL)
Basic Circuit Properties
• Series
– Current is constant (constant flow rate of e-)
– Voltage drops across each resistor (pressure)
– R = R1 + R2 + R3 + etc
• Parallel
– Voltage drop constant
– Iin = I1 + I2 + I3 +etc
– 1/R = 1/ R1 + 1/R2 + 1/R3 + etc
Example Problem #1
Determine V1, V2, V3, V4, and I.
10
10
5
20
V1
V2
V3
V4
90V
-
+
I
Meters, volt-meter and amp-meter
Example Problem #2
Determine I1, I2, I3, I4 and total circuit
resistance.
20
I4
30
I3
20
I2
75V
-
+
I1
Batteries
• Dry-cell batteries: cylindrical zinc
container, carbon electrode, and
ammonium chloride/water electrolyte
• Wet-cell batteries: lead-acid battery
is the most common, can be charged
by forcibly changing the direction of
electrical current
Voltage Kills
• It is the volume of the current that flows
that kills.
• 0.001 amps = 1 milliamp Tingles
• 0.01 amps = 10 milliamps Severe shock,
uncontrolled muscle spasms
• 0.1 amps = 100 milliamps DEATH!
If the current passes through vital organs
such as the heart.
FINSIHED FOR THE TIME BEING