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 = I R

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 #1Determine V1, V2, V3, V4, and I.

V1 V2 V4V3

I

90V+-

2051010

Meters, volt-meter and amp-meter

Example Problem #2

Determine I1, I2, I3, I4 and total circuit resistance.

I1

75V+-

I4

I3

I2

20

30

20

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