# Resistor - Resistance

The resistance (or ideal resistor) of videomodels represents in 3D the behavior of the linear circuit theory resistance.

The circuit theory allows to model each real component by a set of basic ideal components (or elements ) interconnected.
A two terminals component is called dipole . It imposes a relationship linking the voltage between its terminals and the current flowing therethrough.
In the important case of linear circuits, the dipole elements are only five:
element : at all times, the element imposes only :
resistance proportionality between voltage U and current I
capacitance proportionality between voltage U variation and current I
inductance proportionality between voltage U and current I variation
voltage source the voltage U (for any current)
current source the current I (for any voltage)

The resistance is characterized at all times by the Ohm law :

$$U = R \cdot I$$

This element is involved in the modeling of many real components, especially resistor which is the real component whose behavior is supposed to match the best than the resistance.

Very often R coefficient, also called resistance is simply considered as a constant independent of U and I . We say then that the resistance is linear because its voltage-current characteristics is a straight.

The symbol of the resistance is (like resistor) a zigzag or rectangle (see figure below).

The resistance is here represented in 3D: or by a green parallelepiped, or by a green zigzag. The parallelepiped is preferred because it shows more inclination of the resistor (which reveals its voltage) by relative to the horizontal conductors.

On the interactive figures below, you can :

• Change the voltage V1 clicking its text :
- on the right to increase its value
- on the left to decrease
Possible values are -5V, -1V, 0V, 2V, 4V
• Similarly modify voltage V2
Possible values are 0V, 200mV, 500mV, 2V, 4V, 6V, 10V
• Modify resistance clicking it
Possible values are 10kΩ, 1kΩ, 330Ω, 100Ω, 33Ω, 1Ω, 330mΩ
• Check that in all cases $$U = R \cdot I$$

In order to verify the law $$U = R \cdot I$$
you can use the calculator clicking icon Cal=
to the left on top of the 3D images.
Enter expressions in the left rectangle of the calculator, then click the right rectangle to see the result.
Example: copy the following lines in the left rectangle and click outside the left rectangle :
V1 = 2; V
V2 = 500m; V
R = 33; ohm
U = V1-V2; V
I = U / R; A

Although Ohm's Law expresses a very simple behavior, many students do not have the reflex to associate it directly to resistance circuits they are studying. The videomodels can help.

We can see on videomodels that at any time, in a resistance, the current flows in the direction of descent (from the highest to the lowest potential). From then, if the voltage is reversed, the current also. In addition, if twice the current, voltage as well (provided that the coefficient R is constant, which is assumed in most time).

The term resistor means essentially a real component available on the market and very widespread under the most diverse forms.

Enough to be convinced to look at supply vendors as RS (http://be01.rs-online.com/web/c/passive-components/fixed-resistors/) or Farnell (http://uk.farnell.com/resistors-fixed-value).

The real component heats according to the power it dissipates, so that power can not be overlooked without destroying the component during use if the power rating is exceeded.

Power is indicated on videomodels above. It is the product of voltage and current across the resistance. If we wanted to achieve in practice the different cases available in these videomodels, could we use a power resistor rated 0.125W ? or 250 mW ?

Plenty of information available on the Internet for resistance.

## References

Resistor. Wikipedia.
http://en.wikipedia.org/wiki/Resistor

Wikipedia.
http://en.wikipedia.org/wiki/Electrical_element