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Current Voltage Characteristics

A resistor at constant temperature (ohmic conductor) resistor

Current is directly proportional to potential difference. Doubling the potential difference doubles the current in the circuit. The resistance remains the same. Plotting a graph of potential difference against current gives a straight line passing through the origin (0,0). 

VI graph for resistor 

Ohm’s Law 

“The electrical current in  a conductor is proportional to the potential difference applied to it provided the temperature remains the same.” 

 resistance equation

V = IR

Potential difference =        current         x            resistance 

       (V, volts V)                  (I, amps A)                     (R, ohms W

Measuring current and potential difference 

Current is measure with an ammeter, ammeters are always connected in series with the component of interest. 

Potential differences are measured using a voltmeter, voltmeters are connected in parallel with the component of interest. 

By measuring the current and potential difference you can calculate the resistance. 

 simple circuit diagram 

A Filament Lamp  filament bulb

Here the graph curves because as the filament heats it’s resistance goes up (the resistance of the filament is changing).  

VI graph for filament lamp 

A diode  diode

A diode only allows current to flow in one direction through it (forward biased), when the current tries to flow the other way (reverse biased) no current is allowed to flow through the diode.  

 VI graph for filament diode  

 When the diode is reversed biased if we keep increasing the potential difference the diode will eventually begin to conduct in the reverse direction, this is called the break down voltage.

break down diode

Thermistor  thermistor

The resistance of a thermistor decreases as it’s temperature increases.  

Thermistors can be used as thermostats, the thermistor is used in circuits which monitor and control the temperature of rooms, freezers & fridges etc. 

 thermistor graph  

Thermistors can have a positive or a negative temperature coefficient. A negative temperature coefficient means that its resistance decreases with an increase in temperature, this is caused by the release of extra charge carriers in the thermistor.

LDR – Light Dependant Resistor  LDR

The resistance of an LDR decreases as the light intensity falling on it  increases.  

LDR’s are used in circuits which automatically switch on lights when it gets dark, for example street lighting. 

LDR graph