eee1 lec1 basic concepts handout x4

1

EEE 1Essentials of Electrical

and Electronics Engineering

Lecture 1Charge, Current, Voltage

Energy and Power

Electrical and Electronics Engineering InstituteUniversity of the Philippines - Diliman

UVLe Information

• Look for EEE 1 1st Sem 2014-15– Enrollment key: eee1_lc2_1s2014

EEE 1 Essentials of Electrical and Electronics Engineering 2

Charge and Current• Charges (Q)

– Experience forces of repulsion and attraction among themselves:

F = (kQ1Q2)/d²– Can be positive or negative– Unit of measurement: Coulomb (C)

• Current (I)– Rate of flow of electric charge at a point (or area)

I = Q/t– Unit of measurement: Ampere (A) = 1 C/s– Positive current: direction of flow of positive charges

(by convention)


Direct and Alternating Currents• Current may or may not vary with time.

• Examples:– A constant current is termed direct current (DC).– Variables are written using capital letters.

– A current that vary sinusoidally is referred to as alternating current (AC).

– Time-varying variables are written using small letters.


2

• Current in a conductor

Label Arrow

(for direction flow)

Labeling Current

2A – 2A

equivalent


Consider a positive charge Q in free space.

Observations:1. Moving a 1-C test charge from infinity to point Y requires a

certain amount of energy from an external source.2. Potential energy is higher at distances closer to +Q.3. What happens when the test charge is allowed to move?

Energy is released.

Charge in an Electric Field

+Q electric field, EX ZY


Voltage• Voltage (V)

– Work in moving a unit-positive charge (1C) between two points in an electric field:

VXY = W/QX – final position; Y – initial position

– The term electric potential, or simply potential can be used when the initial position is at infinity: VX = VX∞

– Also referred to as the potential difference VXY = VX - VY

– Unit of measurement: Volt (V) = Joule/C


Notes:1. VX > VY > VZ

2. In circuit analysis, choosing a reference point or “ground” is the same as choosing the initial position.

3. The final and initial positions are indicated by the “+” and “-” signs, respectively.

4. Voltage at ground is zero.

Voltage

+Q electric field, EX ZY

points withequal voltagesVX

VYVZ


3

• Voltage across two terminals

Label Plus-Minus pair of signs

(for polarity)

Labeling Voltage

3V

–

+

– 3V

+

–

equivalent


Analogy to Gravitational Field

A

C

B


ground

Bdirection of movement

A A

EA > 0

EBA > 0 EAB < 0

EC < 0

gravitational field

Voltage Reference Points– Voltages (or potentials) of all points within a circuit

must be measured with respect to a reference pointor ground.

– Can opt not to put “+” and “-” signs. Labels VC and VDrepresent voltages that are assumed to be “+”. It is also understood that the “-” terminal is the ground.


Representing Voltages Across Devices

– Voltage (or potential difference) across a device or between any 2 points must be defined by assigning the “+” and “-” terminals (see slide 7).

– The ground is not necessarily the “-” terminal like VB.

+ +

_

_


4

Passive Sign Convention• Convention in assigning voltage and current of a

device• Passive – device is absorbing power


v–

+

i“Whenever the reference direction for

the current in an element is in the direction of the reference voltage drop across the element (as in figure), use a

positive sign in any expression that relates the voltage to the current. Otherwise, use a negative sign.”

Power and Energy• Energy

– W = VQ– Unit of energy: Joules (J) = VC = Ws

• Power– P = W/t = (VQ)/t = VI– Unit of power: Watt (W) = J/s = VA


Power using the Passive Sign Convention

3V

–

+

2A

P = (3V)*(2A) = 6W

3V

+

–

2A

P = – (3V)*(2A) = – 6W

3V

–

+

– 2A

P = (3V)*(– 2A) = – 6W

– 3V

–

+

– 2A

P = (– 3V)*(– 2A) = 6W


Power: Absorbed or Delivered

3V

–

+

2A

P = 6W (absorbed)

3V

+

–

2A

P = 6W (delivered)

3V

–

+

– 2A

P = 6W (delivered)

– 3V

–

+

– 2A

P = 6W (absorbed)


5

SI UnitsQuantity Quantity

symbolUnit Unit symbol

Capacitance C Farad FCharge Q Coulomb CCurrent I Ampere AElectromotive force E Volt VFrequency f Hertz HzInductance (self) L Henry HPeriod T Second sPotential difference V Volt VPower P Watt WResistance R Ohm ΩTemperature T Kelvin KTime t Second s


Common Prefixes

Prefix Name Meaning (multiply by)T Tera 1012

G Giga 109

M Mega 106

k kilo 103

m milli 10-3

micro 10-6

n nano 10-9

p pico 10-12


Example


Source: Nilsson and Riedel, Electric Circuits, 9th ed.

Example



6

Example



Example



eee1 lec1 basic concepts handout x4

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