Physics Unit 1 Direct Current Circuits

1) Circuit Rules

2) More about Resistance

3) Emf and internal Resistance

4) More Circuit Calculations

5) The Potential Divider

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Current Rules

At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction

The current entering a component is the same as the current leaving the component.

The current passing through 2 or more components in series is the same through each component.

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Potential Difference Rules

For 2 or more components in series, the total pd across all the components is equal to the sum of the potential differences across each component.

The pd across components in parallel is the same

For any complete loop of a circuit, the sum of the emfs round the loop is equal to the sum of the potential drops round the loop.

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Resistance equations

Resistors in series pass through the same current and the total pd is equal to the sum of the individual pds.

In series: Rt= R1+R2+R3+...

Resistors in parallel have the same pd. The current through a parallel combination of resistors is equal to the sum of the individual currents.

In parallel: 1/Rt= 1/R1 + 1/R2 + 1/R3 + ...

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Resistance hearting

The heating effet of an electric current in any component is due to the resistance.

The rate of heat transfer= I^2 R

If the component heats up, the temperature rise depends on the power supplied to it and the rate of heat transfer to the surroundings.

The energy transferred to the object by the electric current in time t= power x time

= I^2Rt

The energy transfer per second to the component does not depend on the direction of the current.

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Circuits with a single cell and one or more resist

1) sketch the circuit diagram if not drawn

2) Cell current= cell emf/ total circuit resistance

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

4) current through each resistor= pd across the parallel combination/ resistor's resistance

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Diodes in circuits

A froward pd of 0.6v whenever a current passes through it.

Infinite resistance in the reverse direction or at pds less than 0.6v in forward direction

Therefore, in a circuit with one or more diodes:

a pd of0.6v exists across a diode that is forward biased and passing through a current

a diode that is reverse biased has infinite resistance.

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The potential Divider

A potential divider consists of 2 or more resistors in series with each other and with a source of fixed pd. The potential difference of the source is divided between the components in the circuit as they are in series with each other.

A potential divider can be used:

1) to supply a pd which is fixed at any value before zero and the source pd

2) to supply a variable pd

3) to supply a pd that varies with a physical condition such as temperature or pressure

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Sensor circuits

A sensor circuit produces an output pd which changes as a result of a change of a physical variable such as temperature or light intensity.

1) A temperature sensor consists of a potential divider made using a thermistor and a variable resistor. If the temperature of the thermistor is constant then the source pd is divided between the thermistor and the variable resistor. The pd across the thermistor can be set to any value and when the temperature of the thermistor changes, its resistance changes so the pd across it changes

2) A light sensor uses an LDR and a variable resistor. The pd across the LDR changes when the incident light intensity on the LDR changes. If the light intensity increases then the resistance of the LDR and the pd decreases.

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