Ohm’s Law
Book page 85 – 87
Syllabus 2.10
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What’s wrong with this circuit diagram?
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Task 2
•
•
Sketch a simple series circuit containing a cell and a bulb.
On your circuit diagram, show an ammeter and
voltmeter being used to measure the current through the
bulb and the voltage across it.
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Task 3
• The two bulbs are identical.
• A1 is 4 A. So what are A2, A3 and A4?
1 A1
A2
2
A3
3
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A4
4
Task 4
• V2 is 4 V.
So what are V1 and V3?
V1
V2
V3
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Task 5
• The voltage across the battery is 6 V.
• The voltage across bulb B is 4 V. What are the
voltages across bulbs A and C?
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Aim
• Know how to graphically represent ohmic conductors
• Describe the effects of changing resistance in a circuit
• Know how to investigate resistance in a circuit
Key words
• Ohmic conductor
• Ohm’s Law
• Filament lamp
• Diode
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Have you ever wondered
The bigger picture
How can resistance be useful in electrical appliances?
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How is resistance useful?
• Adjustments in colour and volume in electrical
appliances are made by changing the current
and the voltage in the circuits of the appliance
• The results are achieved by altering the
resistance
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What affects resistance?
The resistance of a wire depends on several factors:
 material
 thickness
 length
 temperature
What experiments could be used to test how length and
thickness affect resistance?
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Measuring Resistance
• We make use of Ohm’s Law which states that the
𝑉
resistance is constant and that R = , providing
𝐼
the temperature is constant
AA
VV
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Planning Section of Your Write up
• Aim
– Investigate how the resistance of a wire changes with length.
• Hypothesis
– Explain with diagrams, the factors that affect resistance and how
they affect it. In particular make a hypothesis with a reason
about length.
• Method
– Begin your method, explaining how you will carry out the
experiment and, in particular how you will keep it a fair test, etc.
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Investigation
Length (cm)
Voltage (V)
Current (A)
Resistance (Ω)
5
10
20
30
• Plot a graph of length vs
resistance
• What is the relationship
between the two variables?
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How does length affect resistance?
What general pattern did your results show?
When the length of the wire increases, resistance increases.
Resistance is caused by electrons colliding with metal ions.
When the length of the wire is increased, the electrons have to
travel further, so more collisions will occur.
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The Effect of Thickness
• A thicker wire has more
paths, allowing more
electrons to pass
through without
collision
• So the thicker the wire,
the lower the resistance
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The Effect of Heating
• As a wire gets hotter, its atoms vibrate
more.
• This means that they create more of a
barrier to the flow of electrons
• So the hotter the wire, the larger the
resistance.
Cool
Hot
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Ohm’s Law
• The current that flows through a conductor is directly proportional to
the potential difference across its ends, provided its temperature
remains constant
Non ohmic conductor
Ohmic conductor
Can you match them?
Filament lamp
diode
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Resistance – true or false?
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Plenary
•
•
•
•
•
•
The resistance of a wire is 10W. What happens when:
The length of the wire is doubled? R is doubled
R is tripled
The length of the wire is tripled?
R is halved
The length of the wire is halved?
The cross-sectional area is doubled? R is halved
R is doubled
The cross-sectional area is halved?
• The length is tripled and the area is doubled at the same
3
R
is
tripled
x
R
is
halved
=
R
is
= 1.5 𝑅
time?
2
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Key words
• Ohmic conductor – components have a constant resistance. They
follow Ohm’s law
• Ohm’s Law -The current that flows through a conductor is directly
proportional to the potential difference across its ends, provided its
temperature remains constant
• Filament lamp – is an electric light with a wire filament heated to a
high temperature, by passing an electric current through it, until it
glows with visible light. It is an example for a non – ohmic conductor
• Diode - typically allowing the flow of current in one direction only. It
is a non ohmic conductor
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