VoltmetersThey measure the difference in potential between two points sothey are connected in parallel. Ideal Voltmeters They should have an infinite resistance so that no current is taken from the circuit. 2
Ammeters Ideal Ammeter This has zero resistance so that it doesn’t lower the current in the circuit. 3
Sensors Thermistor Light Dependent Strain Gauge Resistor As it gets hotter, more More light, releases As the strain gauge is charge carriers are more electrons, which stretched the wire inside itreleased by the material lowers the Resistance. gets thinner, increasinglowering the Resistance the Resistance.Temperature↑ R↓ Light↑ R↓ Strain↑ R↑ All of these sensors can be used with Variable Resistors and Potential Dividers to switch Output devices ON or OFF 4
Potential Dividers 20Ω 40Ω24V 24V V=12V V=16V M 20Ω 80Ω Because the resistors This time the With the thermistor are equal the potential is divided instead of a fixed potential difference into different resistor, the attached across each one is proportions. motor will go faster equal. when the thermistor gets colder. 5
Potential Divider EquationYou can either calculate Vout usingratios and your knowledge ofPotential Difference. ORYou can use an equation (whichdoes the same thing): 6
Automatic Light Switch How does it work? When light shines on the LDR the resistance is very low. This makes Vout low, so the switch is off. When it gets dark the LDR’s resistance increases. This makes Vout much larger and the switch comes on.What simple change would make the switch come on in the light? Swap R1 and the LDR 7
Fire AlarmWhen the temperature risesthe resistance of thethermistor falls.The potential difference acrossthe thermistor becomes low.Most of the potentialdifference is now across R2which turns on the ElectronicSwitch. The electronic switch can be used to ring a bell or turn on a sprinkler system 8
Strain GaugeAs the Strain gauge isstretched by heavy loadsit’s resistance rises.The pd across the straingauge becomes larger.The voltmeter can bemonitored.Low values mean largestrains in this circuit. 9
Using a Potential Divider to investigate Ohms Law This arrow shows that the potential divider can be varied – this is also called a Rheostat. The Potential Divider allow the Potential Difference across the bulb to be varied. The Ammeter measures the current through the bulb. The Voltmeter measures the PD across the bulb By collecting pairs of V and I, an Ohms Law graph can be plotted. 10
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