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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 come
 Flowing electricity must
from a source.
 Flowing water encounters
resistance- water 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 more
 The more electrons, the
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 lowvoltage 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 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