DC cuircits

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  • Created by: Jenna k
  • Created on: 01-03-14 08:04

Kirchhoff's Second Law

Kirchhoff's second law states: The sum of electomotive energy is equale to the sum of potential difference in a clossed loop. This is a cansequence of conservation of energy (what goes in must come out).

You need to be acble to apply this law to the circuits you use.

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Series circuits

Series Circuits rules

Resistance - resistance in a series ciruit is reprosented by the equation Rtotal=R1+R2+R3

Current - current is always maintained it is constant

Potential differance -  initial potential differance (EMF) = V1+V2+V3

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Parrellel Circuit

Parellel circuits

Resistance - This is slightly different form the series cuircuit as intead of just adding up the resistances to get the toatal reisitances you have to inverse them and then add them up. So 1/Rtotal = 1/R1 +1/R2 +1/R3

Current -  this is shown by the equation Itotal = I1 +I2+I3

Potential differance - the voltage read across each component will be the same as the intial voltage (EMF) so V1=V2=V3=E

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Internal resistance

Internal reisitance is that little fixed reistor with the small r next to the cell. This r is inside the cell and shows the resistance of the cells materials. Normally we would ingnore this but now you have to knwo how to work with it. The current is supplies stays constant most of its life. But this increases when it is nearing the end.

It can be reprosented by E=I(R+r) or E=IR+Ir which can be changed to E=V+Ir as V=IR

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Two Sources of Electromotive Energy

need some help on this

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Potential dividers

Potential dividers

When remembering potential dividers you need to know that the current in the circuit will never change. So this leads the potential differance across each resistor to be proportional to the resitance of the component.

Equations you need to knwo are : Vout = (R1/R1 + R2) x Vin this would work out the voltage for reistor 1 in the picture above. To work out the voltage across the resistor 2 in the picture above you would have to do Vout = (R2/R1 + R2) x Vin

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Variations of the potential divider

You may need to knwo how a fixed and variable reisitor work together. Well look at the picture above. If the voltage in was 9.0V and R1 was 2 ohms and R2 was 2 ohm what would the Vout be?

Well both the resitors are equal so the ratio of potential differance across them will also be the same. So Vout = 4.5V

But what if I decreased the resitance in the variable resistor(R2)? Well my Vout would also decrease because of the ratio of PD to resitance.

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Variations of potential dividers

Light dependant resistors have a very high reistance when it is dark. The resitance decreases when the amount of light falling on it increases. So when it is light the low reistance(light) means a low voltage output when there is a high reistance (dark) then there is a high voltage output.

Thermistors have switch on when they have a high voltage output or a high reistance (cold). When the tempriture rises the reistance drops and so does the voltage output. Thus causing the heating to switch off.

A data logger can be used with this to recode a continueous stream of tempritures.  

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