MODELLING MEASURING ELECTRICITY TERMS TO DESCRIBE ELECTRICAL CURRENT

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

COMPARING WATER & ELECTRICITY Water Electricity The more water, the The more electrons, the more powerful the current in a river. Source of water must be constantly replenished for flow to continue (melting snow). 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 Water Electricity If there is no change in Energy will not flow 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. 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: 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. Flow of electricity in wire: 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 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. 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 -9