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Unit 6 in four lessons

Unit 6 in four lessons. Resistors. Voltage current and resistance. p.d units = Volts Current = Amps Resistance = Ohms. Formula Voltage = Current * resistance V = I * R. A variable resistor is a length of very thin wire

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Unit 6 in four lessons

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  1. Unit 6 in four lessons Resistors

  2. Voltage current and resistance p.d units = Volts Current = Amps Resistance = Ohms Formula Voltage = Current * resistance V = I * R A variable resistor is a length of very thin wire It can be used to control the current The higher the resistance the lower the current Coil of thin wire Current travels only a little way along the coil (high current) Current travels a long way along the coil (low current)

  3. Ohmic and none Ohmic resistors 1. Ohmic resistors gave a constant resistance (as long as they don’t heat up) Voltage Resistance = change in Voltage change in current Current 2 Non Ohmic resistors have a variable resistance, i.e. a filament light bulb V The gradient increases because the resistance is increasing as the filament gets hotter I

  4. Potential divider (splits the voltage) The resistors are the same size so the voltage is split evenly 12V 6 ohms V = 6V 6 ohms 0V The larger one resistor is compared to the other the greater the voltage it will take 12V 12 ohms 6 ohms V = 4V 0V

  5. Formula to calculate the output voltage (Given in exam) V in R1 R2 V out 0V V out = x R2 V in (R1 + R2)

  6. Semi conductors Light dependent Resistor (LDR) As the light increases its resistance decreases Thermistor As the temperature increases its resistance decreases

  7. LDRs and thermistors affecting circuits As the light increases the Resistance if the LDR _______ decreases M increases So the current __________ And the motors speed increases As the temperature increases the Resistance if the thermistor _______ decreases M increases So the current __________ And the motors speed increases

  8. LDRs in potential divider circuits – automatic night lights V in V out – (to the light circuit) 0V WHY DOESN’T THE LIGHT COME ON DURING THE DAY? When it is light the LDR has a low resistance so does not take enough voltage to power the light circuit When it is darker the LDR’s resistance increases, eventually it takes enough voltage to power the light circiut

  9. Thermistors in potential divider circuits – temperature controls V in V out – (to the cut off switch) 0V When it is cold why doesn’t the cut off switch operate? When it is cold the thermistor has a high resistance so there is not enough voltage for the fixed resistor to operate the switch When it is warmer the thermistor resistance decreases so it takes less of the voltage, eventually the fixed resistor takes enough to operate the switch.

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