Download Unit D Topic 3 VL

SCIENCE 9
UNIT D TOPIC 3
RESISTANCE &
CIRCUITS
NAME: ____________
DATE DUE: _________
MARK /30:____
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Resistance
Resistance is a property that every substance has. It is the amount that a
substance converts energy held in the electrons to other forms of energy.
For example, in a light bulb, the tungsten filament resists the flow of
electrons causing heat and light to be produced. Copper wire has a
much lower resistance to electron flow than tungsten so it does not
get as hot and does not light up.
o Good conductors have a low resistance
o Poor conductors have a high resistance
Remember the analogy of resistance being like an American
gladiator in a hallway. Resistance opposes the movement of
charge
Resistance of a substance is expressed in ohms, shown by the
Greek letter omega (Ω) Resistance can be measured with an
ohmmeter.
Resistance affects the amount of current flowing through a circuit.
Resistance can actually be calculated from the voltage and current flowing
through a circuit. Georg Ohm discovered experimentally that there was a
relationship between current, voltage and resistance of a circuit. This relationship
is described by the equation:
V
R
I
R= Resistance (Ω)
V= Voltage (V)
I= Current (A)
Using this law any one of these three variables can be determined if 2 variables
are known.
Using this law it can be found that the unit 1Ω is equal to
1Volt
1Amp
Ohm’s law can also take the forms:
V
I
R
V  IR
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Example Problems
1) An electric motor is connected to a 9.0V battery. If a current of 0.20A is flowing
through the motor, what is the resistance of the motor?
V  9.0V
I  0.20 A
R
R?
V
I
R
9.0V
 45
0.20 A
2) An individual with a resistance of 10kΩ sticks a fork into a power outlet
causing 0.0110A to run through him. What voltage is the outlet? (note: Do not try
this… It could kill you!)
V ?
I  0.0110 A
R  10k  10000
V  IR
V  (0.0110 A)(10000)  110V
Resistors
Providing resistance in a circuit can be a desirable thing as it can be used to
control current or voltage in a circuit to meet the needs of devices.
For example, if attaching an LED to a power supply, a resistor might be
used to limit the current travelling through the LED to ensure that it does
not burn out.
Resistors convert energy from electrons into heat energy. They often
contain a heat-conducting core to absorb the heat generated from the
conducting material.
Variable Resistors
Variable resistors can be used to control current and voltage easily.
Dimmer switches for lights are variable resistors. This allows the user to
control the amount of current travelling through a device.
Digital weight scales often use a variable resistor. It contains a
resistor that changes resistance as pressure is added to the material. This
change in current flow is measured by the scale which gives the weight of
the object being weighed
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Series and Parallel
There are two ways you can attach circuit components, in series or in parallel.
Series Circuit:
o One single loop of conductors and electrical devices.
o Only one pathway for the electrons to flow.
o This means that all electrons go through one device.
Parallel Circuits:
o Multiple loops of conductors
o There is more than one pathway for the electrons to flow
o Different electrons go through each device
o The voltage is equal in every branch
House Wiring:
Houses are wired so that if one device is turned off, other devices will
still work. This means that devices must be connected in parallel.
In a parallel circuit, the voltage (energy of the electrons) is the same in
all the branches. However, when additional devices are turned on,
overall current from the power supply increases.
Circuit breakers are added as a master switch that turns of all branches when
open. This breaker ensures that if current gets too high, the breaker switch will
open which cuts power to all loads.
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1) Explain what resistance is in your own words (1 mark)
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2) a) Would you expect an electric heater or a copper wire to have
greater resistance? (1 mark)
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b) Explain your answer (1 mark)
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3) a) Which is a better conductor? 1 meter of Iron wire which has a resistance of
8.1 mΩ or 1 meter of silver wire which has a resistance of 2.7 mΩ? (1 mark)
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b) Which one of these wires would you expect to produce more heat if
connected to a power supply? (1 mark)
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You are trying to pimp your car by adding some
underglow LEDs. The LEDs can only take a current
of 0.200 Amps without burning out; however, your
car battery is delivering 2.50 Amps of electricity to
the LED.
4) a) What circuit component could you add in order to decrease the current? (1
mark) ________________________________________________________
_____________________________________________________________
b) Draw a proper schematic of the LED wiring including the LED (use the light
bulb symbol), the battery, a switch and the component in part a. (3 marks)
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5) What is the symbol and unit name for resistance? (2 marks)
________________________________________________________________
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6) A regular iPod power supply delivers 3.7 V to the ipod and it delivers about
0.125 A off current. Calculate the average resistance of the circuit within
the ipod? (2 marks)
7) A hot water kettle is attached to a house outlet and has a resistance of 250Ω
and a current running through it of 0.440A. What voltage is the house power
outlet supplying? (2 marks)
8) What is the resistance of your tongue if when touching it with a
9.0V battery a current of 20 mA is running through your tongue?
(2 marks note: 1 mA = 0.001 A).
9) What energy conversion takes place within resistors? (2 marks)
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10) If you are building an electric circuit for a model car and you want to be able
to control the speed of the car, what kind of resistor would you put in the
circuit? (1 mark)
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11) If you were making Christmas tree lights, would you want those lights to be in
series or parallel? Explain your answer. (2 marks)
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12) Draw schematics for a circuit that includes two light bulbs, a power supply
and a switch where the 2 light bulbs are: (remember to use a ruler!)
In Series (2 marks)
In Parallel (2 marks)
13) If a contractor wired a new house so that all of the electrical outlets were in
series, explain what problems a family might experience if they moved into
this house? (2 marks)
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14) What is responsible for stopping power in a house if too many devices are
turned on at once? How is this a useful safeguard? (2 marks)
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