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Basics

##SI Prefixes The most common ones you'll see in this course are:

  • Mega (M) $$10^6$$
  • Kilo (k) $$10^3$$
  • Mili (m) $$10^{-3}$$
  • Micro ($$\mu$$) $$10^{-6}$$
  • Nano (n) $$10^{-9}$$
  • Pico (p) $$10^{-12}$$

Charge

The unit of charge is Columbs (C), which is equal to charge of $$6.24 10^{18}$$ electrons.

Charges cannot be created or destroyed, they are just transferred.

Current

Current is the rate of charge flow ($$i=\frac{dq}{dt}$$). 1 A equals to 1 Coulomb/seconds.

Video: Current and Amps: What are they?

Voltage

Voltage is related to the strenght of the electric field. It is analogous to pressure in a hydraulic circuit.

! Voltage is always measured between two points, that is why it is also called potential difference.

$$v = \frac{dw}{dq}$$

Voltage is equal to rate of work done by unit charge.

Video:Voltage/Volts: What is it?

Analogy to Hydraulic System

  • Voltage ~ Pressure
  • Electric Current ~ Water Current
  • Tap Resistance ~ Resistance

Power

Power is the rate of energy (expending or absorbing), denoted by $$P$$:

$$P=\frac{dw}{dt}$$

In electric circuits power is equal to multiplication of voltage and current.

$$Power = Voltage \times Current$$ $$P = VI$$

The unit of power is Watt.

Energy

Energy in the time integral of power. The unit of energy is Joules, which is equal to watt.seconds.

$$w=\int P(t) dt = \int V(t) I(t) dt$$

It is also common practice to express electrical energy in kilo-watt hours (kWh).

Example: Electricity bills(photo1, photo2) are calculated in terms of kWh.

Absorbing or Dissipating?

Circuit Symbols

Resistance

Limits the electric current. The unit is Ohm ($$\Omega$$)

$$Voltage = Current \times Resistance$$ $$V=IR$$

Voltage Source

An ideal voltage source produces a voltage regardless of the current absorbed or produced by the device. Can be a battery, DC voltage source or AC voltage source.

A lemon battery is also a DC voltage source (not an ideal one)

Current Sources

An ideal current source produces the specified current regardless of the voltage across its terminals.