MODELLING & MEASURING ELECTRICITY

TERMS TO DESCRIBE ELECTRICAL CURRENT  Voltage  Current  Resistance  Conductors  Cells  Electrons “flow” through conductors and resistors.

A MODEL USING WATER

COMPARING WATER & ELECTRICITY WaterElectricity  Flowing water must come from a source.  Flowing water encounters resistance- water smashing into rocks in rapids  Flowing electricity must come from a source.  Electricity encounters resistance- remember sometimes electrons (-) are bound tightly to the nuclei (+), so they resist movement.

COMPARING WATER & ELECTRICITY WaterElectricity  The more water, the more powerful the current in a river.  Source of water must be constantly replenished for flow to continue (melting snow).  The more electrons, the more powerful the current is in a conductor.  Source of electricity must be constantly replenished for flow to continue (generator or cell).

COMPARING WATER & ELECTRICITY WaterElectricity  If there is no change in elevation, water doesn’t flow- it just sits in a pool.  If you pump water up a hill, it gains gravitation potential energy, and then flows back down again.  Energy will not flow without a difference in electric potential (voltage)  A cell battery or generator “pumps” electrons to a point with a higher electrical potential (voltage).

COMPARING WATER & ELECTRICITY Electrical Potential Energy  The water in a garden fountain might be 1 metre from the ground, while a town’s water tower might be 50 metres above the ground…  The water from the water tower has much more gravitational potential energy and flows to the ground with greater force.

COMPARING WATER & ELECTRICITY Electrical Potential Energy  High voltage electrons from a generating station have more electrical potential energy than low- voltage electrons from a flashlight battery.

MODELLING RESISTANCE & CURRENT Flow of water in pipes:Flow of electricity in wire:  Imagine you are draining a pool- a pipe with a small diameter might be easier to hook up, but the longer and thinner the pipe, the greater the resistance to the flow of water.  A pipe with a bigger diameter has less resistance & allows for a greater flow of water.  For any given voltage, current decreases if you add resistance.  The least resistance is with a short, wide path with no obstructions- like with water.  The shorter & thicker the wire, the less resistance it creates for electrons.  Current is reduced if it has to pass through a resistor.

OHM’S LAW  German scientist who experimented with substances, and in 1826, proved the mathematical link between voltage (V), current (I), and Resistance (R).  Unit of resistance (the ohm) was named in his honour.

OHM’S LAW  States that as long as temperature stays the same:  The resistance of a conductor stays constant.  The current is directly proportional to the voltage applied

IN OTHER WORDS…  If you increase the voltage in a circuit, the current also increases.  If the voltage stays the same, but a resistor of greater value is used, then the current decreases.

VARIABLES IN MATH?  As long as 2 of the values are known, the 3 one can be calculated.  This means it is possible to calculate the value of an unknown resistor, or figure out the value of a resistor needed to obtain a particular current.

OHM’S LAW IS NOT PERFECT!  If the temperature of a resistor changes, its resistance changes as well.  Resistance is lowest when a conductor is cool.  As the temperature increases, the resistance increases.

LIGHT BULBS…  Sometimes blow when they are switched on because of the sudden temperature change and other forces caused the the large initial current.  In general, as the temperature increases (filament heats up as soon as light is switched on)- the filament’s resistance increases which reduces the current that flows through it.

USING TEST METERS  How to use a multimeter with some bad words AND YOU CANNOT MEASURE AC AT SCHOOL BECAUSE I DON’T WANT YOU TO DIE. How to use a multimeter with some bad words AND YOU CANNOT MEASURE AC AT SCHOOL BECAUSE I DON’T WANT YOU TO DIE.  Remember: You learned how a voltmeter is used to measure voltage in section 1.2

VOLTMETER  Remember: Voltage is the potential difference between 2 points.  To measure potential difference across a cell, battery, resistor, or other device in a circuit:  Each terminal of the device must be connected to the appropriate positive or negative terminal of a voltmeter.

VOLTAGE DROP  The potential difference across a resistor or device.  Millivoltmeters: are meters used to measure small voltages.

AMMETERS  Are used to measure electrical current in amperes.  Remember that current is the rate of flow of electricity in a circuit.  It is a measure of many electrons move past a point in a circuit in each second.  To measure this flow, an ammeter must be placed so that the current flows through it.

CIRCUIT- ONE CONTINUOUS LOOP  You can insert the ammeter between any two circuit components and still show the same reading.  The current is the same at every point in the loop.  Meters used to measure small currents are sometimes called galvanometers.

MULTIMETERS  Meters with several different measuring circuits in the same case.  By turning the selector switch you can set multimeters to measure voltage, current, or resistance in a circuit.  Be careful to select the right setting for the quantity you want to measure.

ESTIMATE ON THE PAT  Use the right scale.  If the needle falls between the numbers, estimate the last digit- if a needle rests between 2 & 3, but is slightly closer to 2, estimate the reading as 2.4 volts.  Obviously, our digital display does not require estimates.

TYPES OF RESISTORS

RESISTORS ARE USED A LOT IN ELECTRONICS  Radio, TV- contain dozens of different resistors.  Resistors are available with values covering the whole range between conductors (very low resistance) and insulators (very high resistance).

TECHNIQUES & MATERIALS  Most common: wire wound and carbon- composition (mixed with other materials).  The longer and thinner the wire, the higher the resistance.  Moulded carbon resistors are cheaper than wire- wound resistors, but less precise.

CHECK & REFLECT  Page 310, #s 1 & 2, 5-9