3.3 Investigating the current - 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 >3400^oC, and glows brightly at 2500^oC, 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

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.

Need calculations for a non-ohmic conductor?

SITEMAP Website content © Dr Phil Brown 2000+. All copyrights reserved on Doc Brown's physics revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to GCSE science course specifications are unofficial.