# Resistance M2

This involves Module 2 Resistance

• Created by: Jenna k
• Created on: 26-02-14 12:41

## Circuit Symbols

Most of the symbols you should know, including junctions, wires, switches, cells, batteries, filiment bulb, fixed resistor, terminals, ammetre, volt meter and a crossing connection.

The new ones inculed variable resistors, potential divider, fuse, heater, LED, Light depending resistor, termistor and  diode.

You need to know and be able to draw all of these components. You need to also be able to put these all in order to make a circuit.

They will come up!

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## Potential differance

Potential differance- is the energy transformed from elctrical energy into another type of energy. It can also be defined by the equation V=W/Q which stands for Potential differenace (V) = energy (J) / Electrical charge (Q).

V=W/Q is also equale to power x time / current x time (V=P x t / I x t) and power over current (V=P/I)

So if you look at the units used and what the volt is therfore equale to them you'd find that 1 volt = 1 coulomb per second (Cs-1) and 1 V = Watt per ampere (WA-1).

Volt - voltage is measured in volts.

Voltmeters are used a lot. Thay alaways have to be set up in parrellel with the cuircit. The volt metre measures the amount of volts that are being lost. EMF is the starting voltage. This is the maximum it will ever be. When this travels from the negitive to positive terminals it will go through components. If these components have resistance then energy will be expended in them. This causes a use of energy and therefore a use in Volts. So the overall vots decreases as it crosses resitive components. (This will come up later in more detail)

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## Electromotive force

EMF - is the energy tranfered per unit charge, when one type of energy is converted into elctrical energy. Eg. Chemical energy in a battrey into electrical energy in a cuircit.

It is also defined by EMF= electrical energy transferred / charge.

EMF is meausered in joules per coulomb (JC-1) which you will remember is equale to 1 volt.

(OFF SPEC)

EMF is the starting amount of energy in a cuircit. It is used in batteries, cells and cell packs. This will drop when resistive compnents are put accross it.

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## Resistance and Ohms law

Resistance - a property of a component that regulates the electrical current through it.

It is more often difined with the equation R= V/I resistance (ohms) = Potential voltage (Volts) / Current (Amps)

Ohms Law

In the 1820's Mr Georg Ohm stated that the current in a conductor is proportional to the potential voltage across it, providing physical conditions are maintained.

Resistance is measured in ohms. Ohms  is reprosented by the almega symbole and is equal to Volts per ampre (VA-1)

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## IV charactoristics

IV charactoristics is at it sounds. It's a graph that shows current (x) against potential differance (y).You will need to know about the IV charactoristics for and wire, fliment light bulb and an LED.

First thing is how to conduct and experient to find such charactoristics. Well it's really simple just but the componet in series with an ammetre and put a volt meter in parrellel. This would give you the readings in amps(A) and volts(V) which you can then plot and see the correnlation between the two variables.                                                                                                                          For a plain wire this should show an exact possitive correlation, which would conenside with ohms law as he said that the current through a componet is directly proportional to the potential voltage across the compent.                                                                                          For a filiment bulb you should have a gradual curve that eventually plato's out. This is due to the tempriture change. As the tempriture changes the Potential differance increases and so does the resistance causeing the the incline in the tempriture. As you know ohms law does not applie when the physical conditions aren't maintained.                                                                           For a LED the graph is very different. A diode only allows current to flow one way through it so the graph will have no potential differenace reading until suddenly it will increase steadily and then plato. Research the graphs to see for yourself what they look like.

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## LED's

You also need to knwo about the benifits of using LED's.

- They switch on instantly

- They are robust

-They are very versitile

-They work on a low potential diffenace

-They have a very long life span

Amoung others these are the resons my book gives.

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## Resistivity

Resisivity is a poperty of a material that is ratio of the product of resistance, crossectional area and length or a componet.

It can be defined by the equation p=RA/L which stands for rho (unit of resistivity) = the reistance (ohms) and cross-sectional area (menters squared) divided by the length of the wire(metres) . So the units or resitivity is ohmics metres.

What you will need to know and practice is what is proportional to what etc. when you dont have any values to substitue into the formula. For example if you start with a lenght of uniform wire with the resistance of 2 ohms. Calculate the resistance of an indentical wire that is twice as long.

Rearange the formula to get R=rho X length / Cross-sectional area. This may or may not help.

Length X2 = Resistancex2  Resistance is proportional to length.

If you double the diamiter of the wire then you have to divide the resistance by the square of the scale factor. eg R=2 ohms and its doubled (X2) so 2/2 squared so it eqaule 0.5 ohms. However if you double the cross-sectional area you have to divide the resistance by the scale factor. (2 / 2 = 1 ohm)

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## Resistivity continued

Resistance is directally proportional to the tempriture meausred in Kelvin.

Metals

In a metal the tempricture can increase. This is not due to the increase in potential voltage passing through the wire but becasue the internal energy has increased causeing an increase in kenetic energy and more movement of the conduction electrons. (Think carpet burn and friction) However the length and Cross-sectional area should not change.

Semiconductor (Thermistors)

Instead of the tempriture increasing with resistance, like it does in a conductor, resistance decreases with an increase in tempriture in a semiconductor. This is called the negitive tempricture coefictiant (NTC) as tempriture inversly proportional resistance.

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## Power

Power - the rate of energy transferal form one type of enery to another measured in watts(W).

This can be defined by the equation  P=VI power(W) = potential differance(V) X Current (A)

However we also know that V=IR so we can sub that in to get P=RI squared. Which can also be written as P=V squared /R (Don't ask me how that came about I don't know)

Another equation you will need to be able to use is W=IVt which stands for workdone (J)= current (A) x potential voltage (V) x time taken (s). This will help if you need to work out energy transfer or work done.

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## Fuses and Kilowatts

Fuses are very clever devices. They consist of a copper wire in a ceramic casing. They are designed to melt when the current gets to high, whcih increases the tempriture. Fuses should be higher than the maxium amount of current draw form the device otherwise it would instantly melt and our tea would be cold.

Killwatt hour - is the unit of energy measued an hour.

This is what we use in our dometic lives. You have to knwo how to use this o charge for energy use, so its basically a tarrif that is independent to different organisations which is worked out per Kilowatt-hour used.

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