# Chapter 5- Direct current circuits

Each section of chapter 5 summarised on a revision card.

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## 5.1 Circuit Rules

At a junction, the total current leaving it = the total current entering it.

In series:
The current entering a component=current leaving component
The current passing through 2+ components in series is the same through each component.
The total pd across all components=sum of potential differences across each component.

The pd across components in parallel is the same.

For complete loop of a circuit, the sum of emfs = sum of potential drops around the loop.

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Resistors in series pass the same current. Total pd = sum of individual pds.

In series, total resistance = sum of individual resistances. (R= R1+R2+R3+...)

In parallel, 1/R = 1/R1 + 1/R2 + 1/R3 + ...

Heating effect in any component is due to the resistance.

Rate of heat transfer= I^2 x R

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## 5.3 Emf and internal resistance

Internal resistance is due to opposition to flow of charge through the source.

Electromotive force (emf) is the electrical energy per unit charge produced by source. If electrical energy E is given to charge Q in the source,
=E/Q

Internal resistance of a source is the loss of pd per unit current in source when current passes through the source.

Cell of emf , internal resistance r, connected to external resistor resistance R, then
=IR +Ir

Power supplied by cell, I=I^2 R + I^2 r
Maximum power is delivered to the load when the load resistance is equal to internal resistance of source (when R=r)

Terminal pd V=-Ir

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## 5.4 More circuit calculations

Cell current= cell emf/total circuit resistance

Pd across each resistor in series with cell = current x resistance of each resistor

Current through each resistor = pd across the parallel combination/resistor's resistance

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## 5.5 The potential divider

Potential divider is 2+ resistors in series with each other and source of fixed pd. Can be used to supply fixed pd, supply variable pd or supply pd that varies with physical condition.

To supply fixed pd:

V1/V2 = R1/R2

The ratio of pds across each resistor is equalt to resistance ratio of the two resistors.

Sensor circuits:

Temperature sensor is potential divider using thermistor and cariable resistor.

Light sensor used LDR and variable resistor.

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