1. Current and voltage are proportional;
Provided the temperature is the same.
2. I = V/R = 3 V ÷ 15 W = 0.2 A
3. If there is no current, there is no voltage
The gradient tells us the resistance.
4. The filament gets hot.
The resistance goes up.
The gradient gets steeper with increased resistance
5. R = V/I = 6 V ÷ 0.3 A = 18 ohms
When the students measure the filament's resistance with an ohm meter, the filament is cold, so the resistance is low. (In other words, when the bulb is on, the filament is hot, so the resistance is higher.)