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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 flow of current can be seen as the reason for the expression for.
- 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.

- The equation form of Ohm's law relates to a simple circuit.
- If you want to see the current change, adjust the voltage and resistance.
- The symbols in the equation change in size to match the diagram.

- The shape and material of the object affect its resistance.
- 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.
- It is proportional to the cylinder's crosssectional area.

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

- The resistance depends on the material of the object.
- Different materials have different resistance to charge.
- Resistivity is a property of a material.

- Table 20.1 shows the values of.
- 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.

- A car headlight is made of a material that has a cold resistance.

- The equation can be rearranged to find the cross-sectional area.
- If it has a circular cross-section, its diameter can be found.

- The diameter is less than a millimeter.
- It is quoted to be two digits.

- The temperature is a factor in the resistivity of materials.
- 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.

- For larger temperature changes, a nonlinear equation may be needed.
- 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.

- The resistance of an object is proportional to the temperature.
- If the cylinder does not change greatly with temperature, it will have the same temperature dependence.

- The effect of temperature on resistance is the basis for many thermometers.
- 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.

- The headlight resistance example is consistent with this value.

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