What are the physical phenomena which occur when a voltage is pulled towards a source level?

Question

Wikipedia's entry on pull-up resistors states that

A pull-up resistor pulls the voltage of the signal it is connected to towards its voltage source level.

Being that voltage is a measurement and not a physical object, the use of the word "pull" seems to be metaphorical.

However, the sentence also appears to be describing a specific physical phenomenon beyond simply an inexplicable change in a signal's logic level.

What is the physical phenomenon to which the metaphor of "pulling" refers? (If it is a series of events, a description of each event would be most informative.)

Answer 1

When current flows into a resistor, a voltage difference develops across it, according to Ohm's law. This means that one lead can be at a different potential from the other one.

If current does not flow into the resistor (or if very little current flow, due to the high impedance of the subsequent stage) both leads are at the same potential. You have transferred the potential at one lead to the other lead. So, if the lead with a definite potential is at Vcc, you 'pulled up' the potential; if it is at ground, you 'pulled down' the potential.

The switch in the Wikipedia figure is there to change the state from "no current flows" (hence both leads are at the same potential) to current flowing (hence, the leads can be at different potential). When you close the switch you pass from one state to the other.

By forcing one definite state (usually it's either Vcc or ground), you avoid floating inputs (they are bad because they tend to pick up noise). Also, as Tony pointed out in its answer, you can use an open collector stage as the switch.

But you want to know why, when there is no current flowing through R, both leads are at the same potential. If Ohm's law does not give you a satisfying answer, I guess you have to consider that a conductor where no current is flowing must be equipotential.

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Edited to change "other" with "at a definite potential"

Answer 2

Complementary Voltage drivers are called Push Pull drivers.

Since voltage logic normally uses positive voltages,

- PUSH means "away" from ground (0V ="0") and towards (+Vcc ="1" )
- PULL means "towards" ground 

In the case of open collector or open drain outputs the NPN or N-ch FET switches are active switches that PULL towards 0V only and open circuit without any PUSH driver so the positive bias towards Vcc (+) is called PULL-UP ( which is a passive bias R to Vcc or some valid output voltage if it is a high voltage output type. So Active PULL-DOWN or sink and passive PULL-UP or source.

It also applies to high impedance inputs where a logic "1" is needed a pullup to Vcc may be used for many inputs that are not needed to be isolated for test purposes.

Historically logic in TTL used active low for set or reset inputs, so pullup R's were common to disable this feature and up to 10 TTL inputs could be share a Pullup R of 10K. For reasons of avoiding secondary breakdown of reverse biased emitters if V+ happened to exceed max rated Vin on inputs going high these Pullup R's were preferred to limit current. But with CMOS being very high impedance these requirements changed so a direct connection or a pullup R could be used to Vcc if desired or a Pulldown R to 0V with a manual switch to V+ is desired.

Answer 3

The metaphor maps electrical quantities and laws to mechanical quantities and laws:

• voltage ⇆ position
• current ⇆ force
• conductance (inverse of resistance) ⇆ spring constant

The relevant laws in both fields are

• Hooke's law in mechanics: D = F/x
• Ohm's law (as Sredni Vashtar mentioned) in electricity: R = V/I

According to the metaphor

• a voltage source is mapped to an upper pixed Point,
• a resistor connected to this voltage source (e.g. Vcc) is mapped to a spring connected to the fixed point,
• another resistor connected to another voltage source (e.g. GND) is mapped to another spring fixed to another lower fixed position.

Now depending on spring constant and position of the fixed points both springs pull their common connection point to a certain position.

The same happens electically with the voltage level that is "pulled" up or down by resistors.

Answer 4

In steady state, each signal X can be modeled by a voltage source \$V_X\$ with an internal resistance \$R_X\$. When you add a pull-up resistor \$R_{PU}\$ between X and the power line, the voltage will shift from \$V_X\$ towards \$V_{CC}\$:

^{simulate this circuit – Schematic created using CircuitLab}

$$V = \frac{V_X*R_{PU}+V_{CC}*R_X}{R_{PU}+R_X}$$

Since \$V_{CC}\$ is higher than any signal voltage, adding a pull-up will result in \$V\$ going up, hence the name.

Note that I assumed \$R_X\$ being positive here, which holds true in the vast majority of cases.