Praveen's father noticed that, even after switching on the geyser, only cold water was coming out. Upon inspection, Praveen found that the connecting wire to the geyser had melted. 

His father asked him to repair it properly so that it would last longer. 

Wire Details: 

Praveen has recalled Joule’s Law of Heating, which states that the heat produced in a conductor is directly proportional to the square of the current, the resistance and time: 

He became confused because: 

To resolve this confusion, Praveen carefully studied the geyser circuit: 

He realised the wire and geyser coil are connected one after another in the same path, so the same current flows through both. 

He then planned experiments using 5 Ω and 1 Ω resistors connected in series to a 3 V battery to determine which resistor becomes less hot. Based on this, he can decide which wire to choose. 

Additional Data: 

Question: 

Based on the experiments he performed using 5Ω and 1Ω resistors, Praveen wants to identify the correct type of circuit that represents the real geyser situation and the heat generated in the connecting wire when it is connected to the actual geyser circuit (100Ω + wire). 

Praveen’s father noticed that, even after switching on the geyser, only cold water was coming out. Upon inspection, Praveen found that the connecting wire to the geyser had melted. 

His father asked him to repair it properly so that it would last longer. 

Praveen went to a shop to buy a replacement wire. The shopkeeper asked whether he wanted a thinner wire or a thicker wire. 

Praveen recalled the resistance and heating effect of current and remembered two formulas: 

Resistance (R) ∝ 1/A 

(Resistance is inversely proportional to cross-sectional area) 

So: 
Thin wire → high resistance 
Thick wire → low resistance 

He also recalled Joule’s Law of Heating, which states that the heat produced in a conductor is directly proportional to the square of the current, the resistance and time: 

Using Ohm’s Law (V = IR), this can also be written as: 

He became confused because: 

To resolve this confusion, Praveen carefully studied the geyser circuit: 

He realised the wire and geyser coil are connected one after another in the same path, so the same current flows through both. 

He then planned experiments using 5 Ω and 1 Ω resistors connected in series to a 3 V battery to determine which resistor becomes less hot. Based on this, he can decide which wire to choose. 

Question: 

Out of the following experiments, which one of the following represents the geyser circuit and ensures minimum heating in the connecting wire? 

Emma is working in a workshop with her mentor, Uncle Thomas. To help her understand rusting, his friend, Dr Elena Fischer, a materials scientist, sets up an experiment. 

She explains that rusting is an electrochemical reaction in which water acts as a conductor and oxygen causes iron to lose electrons, forming rust. 

Dr Fischer places identical iron rods (each weighing 50 g) in four different conditions. Each rod has an exposed surface area of 10 cm². Using a precision scale, she measures the mass of iron lost per hour. 

She shares the following data and rules with Emma and Uncle Thomas: 

“In plain water open to air, a bare iron surface loses about 0.8 milligrams (mg) per hour per cm² of exposed surface. Salt water triples this rate. Boiling the water first removes dissolved oxygen — this reduces the rate to nearly zero (we use 0.05 mg per hour per cm² as the residual rate from surface contact alone).  A thin oil film on iron in salt water reduces the rusting rate by 90%.  

Hint: If you know the rusting rate per square centimetre per hour, you can find the total mass lost by asking how many square centimetres are reacting, and for how many hours — then multiply all three numbers.” 

Each iron rod has an exposed surface area of 10 cm². 

Experimental Conditions 

Emma is visiting her uncle Thomas’s workshop. He shows her two iron nails — one rusty and one shiny — and asks why they look different. 

Uncle Thomas explains: 
Rusting happens when iron, water and oxygen are present together. Salt water makes rusting faster, while coatings such as paint protect iron and slow it down. 

To challenge her thinking, he sets a puzzle: 
 
“Instead of telling you what is inside each jar, I have used colours to represent the conditions. 
Decode the colours, identify each jar and then predict which nail rusted the most.” 
 
Colour Coding Rules 

Each condition is represented by a colour: 

Each jar is coloured using a combination of these colours ( patterned).