1 / 13

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.

rona
Télécharger la présentation

LDRs & Thermistors

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related