Physicist Making Tea

A physicist went into the kitchen. He wanted to make a cup of tea so the first thing he does is opens the kettle and he thinks "I wonder how strong the current in the wire is that's being used to boil the water?" So he puts 250 g \SI{250}{\gram} of water into the kettle and finds that it takes 1 minute 35 seconds to heat the water to 100 ° C \SI{100}{\celsius} .

Can you calculate the current in the wire?

Details and Assumptions :

  • No heat energy is lost to the surrounding.
  • All heat energy from the wire is given to the water.
  • C C (the specific heat capacity of water) is equal to 4.2 J/g C 4.2\textrm{ J/g C} .
  • Voltage from the wall is 230 V 230\text{ V} .
  • The initial temperature of the water is zero but not frozen.

Give your answer to 3 decimal places.


The answer is 4.805.

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4 solutions

P = E t = C × 100 × 250 95 = V I P=\frac{E}{t}=\frac{C\times100\times250}{95}=VI Substituting in C = 4.2 , V = 230 C=4.2,V=230 , we can readily determine I I to be 4.805 Amperes.

Ashish Menon
Jul 11, 2016

We know that heat gainer is given by IVt \text{IVt} and mc t \text{mc}{\triangle}_{\text{t}} .
Equating both the equations, we get:-
mc t = IVt 250 × 4.2 × ( 100 0 ) = I × 230 × 95 I = 250 × 4.2 × 100 230 × 95 = 4.805 A \begin{aligned} \text{mc}{\triangle}_{\text{t}} & = \text{IVt}\\ 250 × 4.2 × (100-0) & = \text{I} × 230 × 95\\ \text{I} & = \dfrac{250×4.2×100}{230×95}\\ & = \color{#3D99F6}{\boxed{4.805 \text{A}}} \end{aligned}

Young Wolf
Jul 9, 2016

H H = = m m c c Δ \Delta θ \theta where Δ \Delta θ \theta is the change in the temperature; m m is the mass of water (in this case); H H is the amount of heat (basically energy) and c c is the specific heat capacity

Also, H H = = V V i i t t where V V is the voltage supplied; i i is the current passing through coil and t t is the time taken

Equating these two expressions and solving for i i we get i i \approx 4.805 4.805

Hung Woei Neoh
Jul 12, 2016

Q = P t m c Δ θ = I V t 250 ( 4.2 ) ( 100 0 ) = I ( 230 ) ( 1 × 60 + 35 ) I = 250 ( 4.2 ) ( 100 ) 230 ( 95 ) 4.805 A Q=Pt\\ mc\Delta\theta = IVt\\ 250(4.2)(100-0)=I(230)(1 \times 60 + 35)\\ I=\dfrac{250(4.2)(100)}{230(95)}\approx \boxed{4.805\text{A}}

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