# Resistance

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We learned in Junior science that there are 2 different types of material when it comes to electricity , those that conduct and those that do not conduct. The 1st set of materials are known as conductors and the 2nd set are known as insulators.

However what we really have is different materials with different properties. These properties are due to the way that the atoms and molecules resist the motion of electrons. Some materials hang on to electons very tightly, these materials are considered to be Insulators as they do not allow the 'charge carriers' to move. Conductors do not have such a tight hold on their electrons.

This is all related to the bonding section in chemistry.

Resistance and resistivity

http://resources.schoolscience.co.uk/CDA/16plus/copelech2pg1.html

Georg Simon Ohm published the relationship that defined resistance in 1827, 98 years after Gray wrote about conduction.

Definition of resistance,

V = IR

unit

the unit of resistance is the ohm.

Ohm's law.

How to carry out the verification of Ohms Law,

Ohms Law demo

Resistors Magnitudes

▬▬▬▬ Silver

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Graphical Resistor Calculator

http://www.dannyg.com/examples/res2/resistor.htm

or here

http://www.csgnetwork.com/resistcolcalc.html

Resistance varies with length, cross-sectional area, and temperature.

Resistivity.

Resistance has two causes

1. defects like …impurities, grain boundaries, stresses.
2. lattice ion vibrations

the 2nd one is due to atoms/molecules getting in the path of the electrons. We can reduce the effect of this by cooling down the temperature. This may achieve a lower resistance but is not practical as lowering the temperature for most electrical circuits would use far more energy than the increased resistance would cause to be lost.

Series and Parallel

Resistors in series and parallel.

RT for resistors in series and in Parallel

ttp://www.aaroncake.net/electronics/math.htm

Combining Resistors with Walter

Another applet for combining resistors

http://www.lon-capa.org/~mmp/kap20/RR506a.htm

And yet another one here

http://vnatsci.ltu.edu/s_schneider/physlets/main/resistor1.shtml

Potentiometer by wally

Derivation of formulas.

Wheatstone bridge.

Wheatstone bridge more from Walter

Appropriate calculations.

Practical uses of Wheatstone bridge for temperature control and fail-safe device.

LDR – light-dependent resistor.

Thermistor.

Demonstration of LDR and thermistor.

Appropriate calculations.

Use of ohmmeter.

Appropriate calculations.

2. Measurement of the resistivity of the material of a wire.

Find the Resistance of Wire

• Zero a micrometer
• Place wire within the jaws
• Tighten Jaws and take reading
• Repeat at various points along wire
• Take average to find the diameter
• Divide diameter by 2 to determine radius.

Find the Length of WIre

3. To investigate the variation of the resistance of a metallic conductor with temperature.

4. To investigate the variation of the resistance of a thermistor with temperature.

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