Resistance

Question File Number 06



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Resistor Colour Code

Go here for some practice exercises  Resistor Circuits

Ohm's Law Revision

This study is about the application of Ohm's Law to examples using more than one resistor. For revision click here and go to:  Ohm's Law

You will recall Ohm's law:  Where E = Volts; I = current in amperes and R = resistance in ohms. By transposing we get the following: 

Ohm's Law

Remember to cover up the value you seek and the formula to get it using the two remaining values is given.

Resistors in Series

This is easy. To INCREASE resistance just add-up the value of each of the resistors in series. 

Example: a 10k, 47k and a 56 k resistor are in series. Total = 113k. (This answer is a nominal value). 

Resistors in SERIES: Remember - you ADD their values up.  

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Resistors in Parallel

This is a little more complicated - but there are shortcuts in practice! 

Resistors in parallel must always have a resultant value that is less than the smallest of any of the component resistors. The current divides between the parallel resistors. The SMALLER resistor will carry the LARGER current. The total current will be the sum of the currents in each leg of the network. 

Remember:  Where the component resistors are different values, the resultant parallel value must be less than the smallest component value alone. 

TWO resistors of the same value in parallel will act the same as one resistor of HALF that value. The wattage rating will be TWICE that of one of the component resistors. 

For example: Two 10k resistors in parallel = 5 k. 

THREE resistors of the same value in parallel will be ONE-THIRD of the value of a single resistor (but three times the wattage rating). 

Example: Three 10k resistors in parallel = 3.3k 

...and so on. 

Networks of Resistors

The resulting value of a network of resistors can sometimes be solved without any great skill being required. Look at this example:

Network

The 30 ohm and the 15 ohm in series, together could be replaced with one (30 + 15) = 45 ohm resistor. Happy so far?

The 20 and 60 ohm resistors in parallel can be solved with a little thinking. 

The 20-ohm could be replaced with three 60-ohm resistors in parallel. So a 20-ohm and 60-ohm in parallel could be replaced with FOUR 60-ohm resistors in parallel. 

So the resulting resistance of the two parallel resistors is one-quarter of 60, i.e. = 15 ohm. 

So the resulting value of this whole network is (45 + 15) = 60 ohm. 

Sketch The Circuit

It is important in all network problems to be able to visualise the circuit. Sketch the circuit, then place the value of each component alongside it. Study it carefully

Put all the information you are given on to your diagram. Determine what it is that you are expected to find. 

Careful consideration of the components in a network will often lead to an easy evaluation. 

If you are asked for the voltage across PART of a circuit, remember that two EQUAL resistors in SERIES will have HALF of the applied voltage across each resistor. 

If you are asked about the current in a network containing two resistors of EQUAL value in PARALLEL, remember that the current will DIVIDE EQUALLY through each resistor. 

Internal Resistance

Batteries, and power-supply substitutes for batteries, exhibit internal resistance. It is this characteristic that causes the voltage from any source to droop or sag, that is, drop or decrease when a heavy load current is drawn from it.

This is considered in  Power Supplies.

Go here for some practice exercises Resistor Circuits



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Compiled Mon Nov 29 2010 at 8:11:49pm


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