Computers – “From The Ground Up” Electricity, Binary Numbers, Logic Gates, Binary Adders, The Computer Power Supply

Electricity For our purposes “electricity” is the flowing of an electric charge. We can view that “flowing charge” as a rope passing through a pulley or as a stream of water passing through a pipe. When talking about electricity we use the terms “voltage”, “current”, “resistance” and “power”.

Current (I = amps): The amount of electrical flow across a conductor. Voltage (V = volts): The difference in electric potential between two points. Current (I = amps): The amount of electrical flow across a conductor. Power (P = watts): The amount of “work” done by an electric charge. P = I x V. Resistance (R = ohms): The amount of resistance to electrical flow from a given substance. Ohm’s Law: V = I x R Resistors, Conductors, Semi-conductors

Analogy: Consider the water pressure (voltage) required to pass a litre of water (current) through a copper pipe of a certain small diameter (resistance). Example: A 60 Watt light bulb hooked to a standard 120 Volt household circuit draws 0.5 amps of current (P = I * V) and thus the filament in the light bulb must be offering 240 ohms of resistance (V = I * R).

Electricity can be used to provide “work” in a variety of ways by: Heating a light bulb filament. Forcing a magnet to move inside an electric motor. “Flipping the switch” in a transistor.

Binary Numbers The binary number system works under the exact same principles as the decimal system, only it operates in base 2 rather than base 10. In other words, instead of columns being: | 10^2 | 10^1 | 10^0 | they are | 2^2 | 2^1 | 2^0 |

Boolean Logic Gates Binary numbers (0 and 1) can be used to represent whether or not an electrical circuit is off (0) or on (1). Boolean logic gates (built using transistors) can be used to conduct simple logical operations (such as addition).

And Gate X Y Output 0 0 0 0 1 0 1 0 0 1 1 1

Other Logic Gates NOT OR NAND NOR XAND XOR These gates can be put together to make logic circuits, like adders.

Binary Adders If you can add (and store numbers), you can do anything: http://woodgears.ca/marbleadd/

The Power Supply The power supply converts the alternating current (AC) power from the grid to the direct current (DC) needed by a personal computer. In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. It contains the power-cord receptacle and the cooling fan. Power supplies convert AC input to lower DC voltages. The typical voltages supplied are: * 3.3 volts * 5 volts * 12 volts The power supply is the one really “dangerous” part of a computer. NEVER EVER open it.

Elector Static Discharge (ESD) The human body can build up static charges of up to 25,000 volts.  ESD is not usually dangerous to humans because it lasts only microseconds. ESD poses a risk to some electronic circuitry, where even a micro-amp current discharge can damage miniature transistors. Make sure you “ground” yourself before touching electronic equipment and/or wear an anti-static wrist strap.

For more information: Electricity: Binary Numbers: http://www.swansontec.com/set.html Binary Numbers: http://www.math.grin.edu/~rebelsky/Courses/152/97F/Readings/student-binary Boolean Logic Gates & Binary Adders: http://computer.howstuffworks.com/boolean.htm The Binary Marble Adder (Info): http://www.exploringbinary.com/the-binary-marble-adding-machine/ The Computer Power Supply