Download RESISTANCE

Science 9
Electricity
Topic 3
RESISTANCE
RESISTANCE is a property of a substance that tries to slow down the movement of electric charge
and converts electric charge into other forms of energy.
Remember from the previous lesson that CURRENT (measured in amps) is how fast the charges are
moving, and VOLTAGE (measured in volts) is the pressure that gets those charges moving.
* Turn to page 279 of your textbook. Read the topic introduction.
In some applications, a type of conductor called a resistor is useful. A resistor allows electric
current to pass, but provides resistance to it. This limits the amount of current.
For any given voltage, more current flows through a resistor with a low resistance than through one
with a high resistance. Resistance is a measure of how difficult it is for electrons to flow through a
substance. It is measured in ohms. The symbol for ohm is Ω, the Greek letter omega.
Good conductors (such as copper) allow electrons (e-) to move easily, therefore, GOOD
CONDUCTORS HAVE LOW RESISTANCE.
Poor conductors (such as Nichrome) oppose the movement of e-, therefore, POOR CONDUCTORS
HAVE HIGH RESISTANCE.
One way to remember this is to compare it to golf; the better players have the lower score.
The more resistance a substance has, the more the substance gains energy from each electron that
passes through it. The energy gained by the substance is radiated to its surroundings as either heat
or light energy.
QUESTION 1:
What form(s) of energy does a light bulb filament convert electrical energy into?
QUESTION 2:
Complete the following sentences.
A good conductor has _______________ resistance. Poor conductors have _______________ resistance. An
_______________ is the standard unit for resistance, and resistance is measured with an _______________.
* Refer to the activity on page 280 of your textbook.
Compare the circuit diagram to the picture of the circuit. Note that wire made from Nichrome - an
alloy of iron and chromium with a nickel base - has a higher resistance to electron flow than copper
wire does. The longer the length of Nichrome included in the circuit’s path, the greater the
resistance in the circuit.
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Science 9
Electricity
Topic 3
Complete step 1 of “Procedure”. Use the data in the following data table.
QUESTION 3:
Answer question 1 of “What Did You Find Out?”.
* Turn to page 281 of your textbook and read “Calculating Resistance”.
German scientist Georg Simon Ohm made exciting electrical discoveries in the early 1800’s.
He experimented with many different substances, and in 1826, he was able to prove a mathematical
link between VOLTAGE (V), CURRENT (I), and RESISTANCE (R). The 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.
Ohm's law also covers changes in resistance. If the voltage stays the same, but a resistor of greater
value is used, then the current decreases. This table below shows how to use Ohm's law.
The mathematical equation for resistance is as follows:
R
=
V
I
resistance
=
voltage (potential difference)
current
This expression is known as OHM’s LAW, and the units for resistance are volts per ampere or
ohms.
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Science 9
Electricity
Topic 3
The “Model Problem” on page 282 of your textbook illustrates the GRASP method of problem
solving – GIVEN, REQUIRED, ANALYSIS, SOLUTION, AND PARAPHRASE. Use the GRASP method
for computational problems.
Let’s work on a question together.
What is the resistance of an electric heater, if a current of 12.5 A runs through it when it is
connected to a 120 V wall outlet.
GIVEN:
I = 12.5 A
V = 120 V
REQUIRED:
Resistance, R, in ohms Ω
ANALYSIS:
Use R = V
I
SOLUTION:
R=V
I
R = 120 V
12.5 A
R = 9.60 Ω
PARAPHRASE:
The resistance of the heater is 9.60 Ω.
QUESTION 4:
Do “Practice Problems” 1 and 2 on page 282. Use the GRASP method.
RESISTORS
Can you imagine going to a concert for your favourite singer and discovering that only one turnstile
works? Everyone must go through the same turnstile! And this is in an arena that seats several
thousand people! You may get in for the last song—if you’re lucky. Having many turnstiles open
will get everyone to the show on time. In this case, having many paths has an advantage over a
single pathway. Having many paths in an electric circuit also has advantages.
You have seen that resistance is used to convert electricity to light and heat. Resistance is also used
to control the flow of electricity in circuits. The next time you dim the lights, you’re using a variable
RESISTOR.
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Science 9
Electricity
Topic 3
* Turn to page 283 of your textbook and read “Resistors” and “Variable Resistors”.
Sometimes you want to change the current flow gradually in a circuit, rather than just turning it on
or off. For example, you may have a light switch in your house that’s used to dim the lights. This
type of control device is called a VARIABLE RESISTOR or RHEOSTAT.
Rheostats can increase or decrease the amount of current in a circuit by adjusting the portion of the
resistor that the current travels through. Examples of rheostats are volume controls on stereos,
foot – operated speed controls for sewing machines, and a dimmer switch. As you turn the knob or
press the pedal, you change the amount of current flowing through the circuit.
* Refer to the “Inquiry Investigation” on pages 284 and 285 of your textbook. When following the
steps of “Procedure”, the following data was collected.
Complete the “Calculated Resistance” column. Remember to convert mA to A. 1 A = 1000 mA.
Now use the collected data to answer the following questions.
QUESTION 5:
Complete question 1 of “Analyze” and questions 3, 4, and 6 of “Conclude and Apply”.
The next textbook reading will help you distinguish between PARALLEL CIRCUITS and SERIES
CIRCUITS.
* Turn to page 286 of the textbook and read “Types of Circuits”.
The circuit in which the current passes through each bulb in turn is called a SERIES CIRCUIT. In a
series circuit, there is only one pathway for the current, as shown in Figure 2.24. If that pathway is
interrupted, the whole circuit cannot function.
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Science 9
Electricity
Topic 3
The other problem with series circuits is that adding components increases the total resistance of
the circuit. This decreases the current. Thus, adding an extra bulb to a series string of lights makes
all the bulbs dimmer.
However, series circuits do have an important use. In household circuits, switches are wired in
series with other components (e.g., wall plugs, lights). This makes it possible to turn off all the
electricity in the circuit.
Many sets of decorative lights are not connected in series, but in parallel. PARALLEL CIRCUITS
have a separate current path for each section of the circuit (Figure 2.25). In a parallel – wired string
of lights, for example, each bulb has its own path to the current source. An interruption or break in
one pathway does not affect the rest of the pathways in the circuit. Similarly, adding a new
pathway with more resistors does not affect the resistance in any of the other pathways.
In fact, adding extra resistors in parallel decreases the total resistance of the circuit. This might
seem strange, but remember that adding more paths for the current to take means less total
resistance. Think about how much less resistance there is when you drink through two straws
instead of one.
* Turn to pages 289 and 291 of your textbook and read “House Wiring”.
You’ll find out why every light and appliance works independently of the others. As more lights and
appliances are turned on, the current becomes greater. Power cable must be designed to handle
this current flow without heating up.
Homes are wired in parallel, because turning on one appliance will not reduce the energy available
for other loads.
One problem does exist with parallel circuits: current through wires connected to the source
increases whenever another branch in the circuit is closed.
When you turn on any appliance in your home, the current in the wires closest to the source
increases. If you turn on too many appliances at one time, the current may increase too much and
may start a fire. This is why homes always include fuses or circuit breakers.
QUESTION 6:
Why does plugging in an electric kettle not dim the lights in the same room?
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Science 9
Electricity
Topic 3
KEY TERMS:
 RESISTOR: a device that has electrical resistance and that is used in an electric circuit for
current and voltage control
 VARIABLE RESISTOR: resistor whose resistance can be changed by adjusting the portion
of the resistor the current travels through
 RHEOSTAT: continuously variable resistor used to regulate electric current
 PARALLEL CIRCUIT: a circuit with more than one current path
 SERIES CIRCUIT: a circuit with only one current path
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