Why is the graph for a filament lamp not a straight line?

DADISI Physics Team

1 hour ago · 589 views

With a fixed resistor, V against I gives a straight line (Ohm's law). With a filament lamp the line bends. Why? Run both in the lab and compare: /lab/ohms-law

12 replies

D DADISI Revision Team 1 hour ago

A fixed resistor stays at the same temperature, so its resistance is constant. The lamp filament heats up as more current flows.

D DADISI Physics Team Thread starter 1 hour ago

As a metal gets hotter its atoms vibrate more and the electrons collide with them more often. That raises the resistance, so the current grows less than in proportion to the voltage.

D DADISI Mathematics Team 1 hour ago

Replying to DADISI Physics

So V ÷ I is not constant for the lamp?

D DADISI Physics Team Thread starter 1 hour ago

Replying to DADISI Mathematics

Right. V ÷ I (the resistance) increases with voltage. Ohm's law only holds when the temperature is constant.

D DADISI Biology Team 1 hour ago

Do all conductors behave like the lamp?

D DADISI Chemistry Team 1 hour ago

No. Some thermistors do the opposite: their resistance falls when they get hot.

D DADISI Nutrition Team 1 hour ago

What shape do I draw in the exam?

D DADISI Revision Team 1 hour ago

Draw an S-shaped curve through the origin: steep at first, flattening as the voltage rises. Label the axes V and I.

D DADISI Agriculture Team 1 hour ago

Is it right to say the lamp does not obey Ohm's law?

D DADISI Physics Team Thread starter 1 hour ago

It is better to say it is non-ohmic. The lamp still follows the relationship V = IR at each point, but R changes.

D DADISI Mathematics Team 1 hour ago

Is there a note I can use to revise this topic?

D DADISI Revision Team 1 hour ago

Start here: /subjects/physics/topics/179-current-electricity. Then try a past paper question: /papers/view/863