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Electricity Section 3: 3.3
Investigating the current - voltage
characteristics of a metal filament lamp
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INDEX for
electricity section 3 notes on current, voltage, resistance, energy &
charge transfer in circuits
3.3
Investigating the current
The
voltage characteristics of a metal filament lamp
When electric charge flows through a high resistance, like the thin metal
filament of a lamp, it transfers some of the electrical energy to the
thermal energy store of the filament. The electric charge does work
against the resistance.
Circuit 45
shows how you can investigate the current - potential difference
characteristics of a
filament bulb.
The voltmeter is
wired in parallel with the thermistor, the p.d. V is measured in
volts (V).
The variable
resistor allows you to vary the p.d. and current flow.
The ammeter,
wired in series, gives you the current I reading in amps (A).
 The
passage of current heats up the filament and the rise in temperature
causes the resistance to increase. So a filament lamp is a non-ohmic
conductor.
This
'heating up effect' affects all resistors.
As the current increases, more heat energy is released and the
filament gets hotter and hotter, so further increase in temperature
further increases the resistance.
This decreases the rate at which the current increases with
increase in potential difference.
Therefore
the gradient of the I-V graph curve decreases and increasingly so
with increase in temperature - graph 2. Its a non-linear graph.
If the
gradient is changing, then the resistance is changing.
The
graph
(2)
is constructed on a crosswire axis. The top right half is your first
set of results, you then reverse the terminals on the power supply
and repeat the experiment giving the bottom left part of the graph.
The
phrase non-linear component may be used.
When the
current (A) is NOT
proportional to the p.d (V) the filament lamp is described as a
non-ohmic conductor
(doesn't obey Ohm's Law!).
You get
the same I-V shaped graph for a
thermistor.
 Theory
- with reference to the metallic structure diagram
A metal crystal lattice consists of immobile ions and freely moving
electrons between them. As the temperature increases, the metal ions
vibrate more strongly into which the electrons collide and this inhibits
the passage of electrons - reducing the flow of charge. As the current
increases, the vibrations increase causing more of the electrical energy
to be converted to heat - increasing the temperature AND the
resistance of the metallic filament, thereby lowering the current even
further.
So,
an increase
in temperature increases the resistance a filament lamp (or most other
resistors) and lowers the current flowing for a given p.d.
If a
resistor becomes too hot, almost no current will flow.
There is
one important exception to this 'rule', see notes on the
thermistor where the resistance actually falls with
increase in temperature.
The filament
bulb is just one of many examples were energy is transferred usefully,
BUT there is
always heat energy lost to the thermal energy store of the device and
the surroundings.
The filament
is often made of the metal tungsten that melts at >3400oC,
and glows brightly at 2500oC, but it still evaporates very
slowly. An inert gas such as argon or nitrogen is added to reduce this
evaporation - any evaporated tungsten atoms hit the unreactive (and so
non-oxidising) Ar or N2 molecules and hopefully condense back
on the filament.
See also
Conservation of energy,
energy transfers-conversions, efficiency - calculations
INDEX of
electricity section 3 notes on current, voltage, resistance, energy &
charge transfer in circuits including Ohm's Law investigations
Key points about electric circuits in physics courses:
I-V behaviour of a filament lamp resistor
Information
sources for Doc Brown's key points: IGCSE-GCSE physics are based on
textbooks & syllabus-specifications for students taking the UK AQA, Edexcel,
OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE
physics 9-1 level science examinations.
A detailed, exam-board-aligned
summary on filament bulb resistance and non-ohmic behaviour,
tailored for GCSE/IGCSE Physics students:
Resistance of a Filament Bulb – Summary
Revision Notes
Core Concept
A filament bulb contains a
thin wire (usually tungsten) that glows when current passes through it. As
the current increases, the wire heats up, and its resistance
increases. This is a classic example of a non-ohmic
conductor.
Key Physics Principles
| Concept |
Explanation |
| Non-ohmic behaviour |
Does not obey Ohm’s
Law: ( V proportional to I ) is not true |
| Temperature effect |
As current increases, filament
temperature rises → metal ions vibrate more |
| Increased collisions |
Hotter filament = more resistance due
to more frequent electron collisions |
| Graph shape (V–I) |
Curved graph: gradient increases →
resistance increases with voltage |
| Resistance versus Current |
Resistance increases
non-linearly with current |
Graphical Representation
- Ohmic conductor:
straight line through origin (constant resistance)
- Filament bulb:
S-shaped curve – flattens as current increases
- Gradient = resistance → steeper gradient =
higher resistance
Required Practical (All Boards)
Aim:
Investigate the current-voltage relationship for a filament bulb
Method Overview:
- Connect filament bulb in series with
ammeter and variable power supply
- Use voltmeter in parallel across bulb
- Vary voltage, record current
- Plot V versus I graph
Expected Result:
Non-linear curve showing increasing resistance
Typical Exam Board Specification Content
|
Key Focus |
| Required practical on V–I
characteristics; understanding non-ohmic behaviour |
| Emphasis on interpreting curved V–I
graphs and resistance changes |
| Includes filament bulb as a non-ohmic
component in practicals |
| Focus on circuit diagrams and
resistance variation with temperature |
| Investigates resistance changes in
filament bulbs and graph analysis |
| Strong emphasis on definitions,
graphical interpretation, and practical design |
Student Tips for Success
- Understand non-ohmic:
Know why filament bulbs don’t follow Ohm’s Law
- Sketch graphs:
Be able to draw and explain the curved V–I graph
- Explain temperature link:
Use particle model to describe increased resistance
- Use correct terms:
“Resistance increases due to increased ion vibrations”
- Revise practicals:
Be confident describing method, apparatus, and expected results
- Think real-world:
Filament bulbs are inefficient due to energy lost as heat
Keywords, phrases and learning objectives for the current-voltage
characteristics of metal filament bulb
Be able to describe
the circuit and method for investigating the electrical resistance
of metal filament lamp (old fashioned bulb).
Be able to describe
the current-voltage graph for a filament lamp bulb and calculations
of results for a non-ohmic conductor such as the metal filament
bulb.
Be able to explain why
you get an S shaped graph involving the disproportionate increase in
resistance with increase in the p.d. voltage, causing an increase in
temperature.
SITEMAP
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Based on the syllabus-specifications
for students taking the IGCSE/GCSE level physics examinations summary
revision notes and key points on investigating current-voltage character
of a filament bulb for students taking the AQA
igcse/gcse physics notes on investigating current-voltage character of a
filament bulb, Edexcel gcse
physics notes on investigating current-voltage character of a
filament bulb, OCR 21st century GCSE
physics notes on investigating current-voltage character of a
filament bulb, OCR gateway
GCSE physics notes on investigating current-voltage character of a
filament bulb, WJEC gcse physics notes on investigating
current-voltage character of a filament bulb, CCEA
gcse physics notes on investigating current-voltage character of a
filament bulb for students taking CIE Cambridge igcse
physics, or any other GCSE or IGCSE level physics exams notes on
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