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Potential Dividers You will be familiar with the use of a variable resistor to vary current.

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M The variable resistor acts as a control over the flow of current. It is being used as a rheostat. The result is a control over the speed of the motor. more resistance less current slower motor The next circuit is very different..

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R1R1 R2R2 V1V1 V2V2 V V does not change V = V 1 + V 2 The two resistors are dividing up the potential (or voltage) - (the Potential Divider)

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What happens if: R 1 increases and R 2 is unchanged? R 1 decreases and R 2 remains the same? The values of both R 1 and R 2 are doubled? The values of both R 1 and R 2 are halved? The supply voltage V is trebled? V1V1 V2V2 V x 3 3V V V V V V1V1 V1V1 V2V2 V2V2

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Uses of the potential divider To supply a variable voltage To make an input sensor from other components - many detectors will switch on as a voltage goes above or below a certain threshold (such as in a thermostat).

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Consider the thermistor -its resistance decreases as the temperature increases R1R1 R2R2 V1V1 V2V2 V R 1 is a thermistor. Suppose a heater switches on if V 2 “went high”. As it gets hot, R 1 xxcreases and so V 1 xxxxs. This means V 2 must xxxx. ie it “goes ?” The thermostat turns on. Falls decreases Rise Could you explain what would happen if you put the thermistor where R 2 is?

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Now using the original circuit as in the diagram, what would be the effect of making R 2 a variable resistor? R1R1 R2R2 V1V1 V2V2 V It would effectively vary the switching temperature.

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An LDR (light dependent resistor) has a resistance which decreases with increased illumination. Sketch a circuit to show how you could use a power supply, voltmeter, LDR and a fixed resistor to measure light intensities. What would be the point in replacing the fixed resistor with a variable resistor? How would you modify your circuit to measure temperature instead of light intensity?

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R1R1 R2R2 V1V1 V2V2 V Replace with an LDR

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What would be the point in replacing the fixed resistor with a variable resistor? How would you modify your circuit to measure temperature instead of light intensity? Change the lighting level at which it switched Use a thermistor

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Measuring PD with a potential divider. V S AB 2 volt Moving the slider S from A to B will make the voltmeter read from zero up to the supply voltage - in this case from 0V to 2V. The resistance AB could be 1m of “resistance wire”. If AS was 25cm, what would the voltmeter read? 2Vx(25cm/100cm) = 0.5V If SB was 20cm, what would the voltmeter read? 2V((100cm-20cm)/100cm) = 1.6V We don’t really need the voltmeter to know what that voltage is!

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This is only because the current through the wire AB is the same along its length and we assume none flows through the voltmeter. S AB 2 volt Centre Zero galvanometer - a very sensitive ammeter. V For practical purposes a resistor is frequently placed in series with the galvo to protect it from high currents.

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Suppose the voltage from A to S is the same as the emf of the second cell. S AB 2 volt Centre Zero galvanometer - a very sensitive ammeter. V The voltage drop from A to S due to the 2 V cell will match the emf from the other cell so no current will flow through the galvo. If we move S to the left, V AS falls, so V will drive current through the galvo. If we move S to the right, V AS rises and forces current back through the galvo and cell, the other way. We now know that V = V AS.

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Question If the wire AB is 1m long and a “balance point” is reached when AS = 20cm, What is the value of V ? V = 2volt x (20cm/100cm) V = 0.4volt

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Why might you do the following? S AB 2 volt V Add this resistor here? S AB 2 volt V Add this switch and resistor here? This type of circuit is frequently used to measure very small emfs such as those generated by thermocouples - what are they? It is called a null method as a reading is made when the galvo is reading zero. It is more accurate to look for the balance point when making the connection at S produces no deflection of the galvo - why?

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IP2.18.3 Circuit symbols © Oxford University Press 2011 Circuit symbols.

IP2.18.3 Circuit symbols © Oxford University Press 2011 Circuit symbols.

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