LDRs & Thermistors Noadswood Science, 2012

LDRs & Thermistors To understand LDRs and thermistors Wednesday, April 19, 2017 LDRs & Thermistors To understand LDRs and thermistors

Resistance Look at the VI graph for a LDR and thermistor – what does this tell you about the resistance of the components? LDR Thermistor Bright light High temperature Current Current Dim light Low temperature Potential Difference Potential Difference

Resistance = Voltage ÷ Current LDR Resistance = Voltage ÷ Current At constant light intensity levels the line is straight, so the LDR resistance is constant If the light intensity increases the resistance decreases If the light intensity decreases the resistance increases LDR Bright light Current Dim light Potential Difference

Resistance = Voltage ÷ Current Thermistor Resistance = Voltage ÷ Current At constant temperature the line is straight, so the thermistor resistance is constant If the temperature increases the resistance decreases If the temperature decreases the resistance increases Thermistor High temperature Current Low temperature Potential Difference

Experiment - LDR Set up a circuit with a LDR in series with a cell and ammeter (place a voltmeter across the LDR) Record the current and potential difference with 5x different light intensities (place a lamp closer and closer to the LDR (use a fixed distance, i.e. 10cm closer each time)) Calculate the resistance change with increasing light intensities (R = V/I) + - A V

Experiment - thermistor Set up a circuit with a thermistor in series with a cell and ammeter (place a voltmeter across the thermistor) Record the current and potential difference with different temperatures (sensitive thermistors can have a significant change in resistance from just placing them between finger and thumb, warming them up) Calculate the resistance change with increasing temperature (R = V/I) + - A V

LDRs Light-dependent resistors (LDRs) are used to detect light levels, e.g. in automatic security lights, burglar detectors etc… As light levels increase the resistance decreases As light levels decrease the resistance increases (resistance is highest in darkness) Resistance (Ω) Dark Light Light Intensity

Thermistors Thermistors are used as temperature sensors, e.g. car engine sensors, fire alarm sensors, fridges etc… As temperature increases the resistance decreases As temperature decreases the resistance increases Resistance (Ω) Cold Hot Temperature

Summary Questions Complete the following using the key words: diode; filament lamp; resistor; and thermistor The resistance of a... decreases as its temperature increases The resistance of a… depends on which way round it is connected in a circuit The resistance of a… increases as the current through it increases The resistance of a… does not depend on the current through it

Summary Questions A thermistor connected in series with an ammeter and a 3V battery is shown At 15oC the current through the thermistor is 0.2A and the potential difference across it is 3V – calculate its resistance at this temperature State and explain what happens to the ammeter reading if the thermistor’s temperature is increased 3V + - A

Summary Answers The resistance of a thermistor decreases as its temperature increases The resistance of a diode depends on which way round it is connected in a circuit The resistance of a filament lamp increases as the current through it increases The resistance of a resistor does not depend on the current through it

Summary Questions Resistance = 15Ω (V/I) The ammeter reading increases because the resistance of the thermistor decreases 3V + - A