20.3 Resistance and Resistivity

The hand-to-foot resistance of a dry person is different from the human heart's.

Superconductors have no resistance at all, and a meter-long piece of large-diameter copper wire has a resistance of.
Resistance and Resistivity are related to the shape of an object and the material of which it is composed.

The phrase drop is often used.
It is similar to fluid pressure.
The flow of charge is caused by the pressure difference between the voltage source and the pump.
The Resistor is like a pipe that reduces pressure.
The consequences of energy conservativism here are important.
The source of energy is caused by an electric field and a current, and the Resistor converts it to another form.
In a simple circuit, the source's voltages are equal to the ones flowing through the resistors.
The energy supplied by the voltage source and the energy converted by the resistors are the same.

The output of the battery is equal to the drop across a Resistor in a simple circuit.

In an electrical circuit, the sole Resistor converts energy from the source into another form.

All of the energy supplied by the source is converted to another form by the Resistor alone.
The use of conserve energy is a powerful tool in circuit analysis.

We can gain insight into the resistance of more complicated shapes by analyzing the cylindrical resistor.
Similar to a pipe to fluid flow, the cylinder's electric resistance is proportional to its length.

Similar to the flow of fluid through a pipe, the greater the diameter of the cylinder, the more current it can carry.

Its resistance to the flow of current is similar to the resistance posed by a pipe.

The materials listed in the table are grouped into categories based on resistivities.
Conductors have the smallest resistivities, while insulators have the largest.
Most charges in insulators are bound to atoms and are not free to move, whereas conductors have varying but large free charge densities.
Semiconductors have less free charges than conductors, but have different properties that affect the number of free charges.
Modern electronics use these unique properties of semiconductors, as will be explored in later chapters.

Some become superconductors at very low temperatures.
Increasing temperature increases the resistivity of conductors.
Since the atoms vibrate more rapidly and over larger distances at higher temperatures, the electrons moving through a metal make more collisions, making the resistivity higher.

Positive for metals, that means their resistivity increases with temperature.
There are some alloys that have a small temperature dependence.
The resistivity of ganin, which is made of copper, manganese and nickel, is close to zero, and so it varies slightly with temperature.
For example, it's useful for making a temperatureindependent resistance standard.

At very low temperatures, the resistance of a sample of mercury is zero, but it increases with temperature.

The negative for the Semiconductor listed in Table 20.2 means that their resistivity decreases with increasing temperature.

The number of free charges available to carry current increases because they become better conductors at higher temperature.
The property of decreasing with temperature is related to the type and amount of impurities present.

If the cylinder does not change greatly with temperature, it will have the same temperature dependence.

The thermistor is a Semiconductor crystal with a strong temperature dependence and its resistance is measured to get its temperature.
The device is small and quick to come into equilibrium with the part of a person it touches.

The automated measurement of the thermistor's temperature- dependent resistance is what makes these familiar thermometers.

For large temperature changes, the equations work well, although caution must be used.

Since the original resistance of the filament was given, this is a straightforward application.