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LDRs & Thermistors

LDRs & Thermistors. Noadswood Science, 2012. 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.

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LDRs & Thermistors

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  1. LDRs & Thermistors Noadswood Science, 2012

  2. LDRs & Thermistors • To understand LDRs and thermistors

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

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

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