CarissaG
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natninja
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(Original post by CarissaG)
Followed a suggested experiment to test for the efficiency of a microwave oven. It was taking a measured amount of water at a recorded initial temperature containing a pinch of salt and heating it in a microwave oven for a measured amount of time, before then recording the final temperature. The experiment was repeated multiple times.

We've been asked to do the following:

a) calculate the heat input to the water using two methods:

i) using an approximate value for the specific heat of water,
ii) using the tabulated internal energy data for water

b) calculate the electrical energy used by the device using the manufacturers power consumption figures. Hence deduce the efficiency of the device.

The microwave input power is 1000W, and the energy output is 700W. The power supply is 230 V, and the required is 10 A.

For a) i), I'm assuming it is referring to the use of Q = mcΔT.

We used one litre of water, so that gives us a value of one kilogram for mass.

We know the specific heat capacity for water, so we can use that for c.

We know final and initial temperature readings, so can calculate ΔT.

For a) ii), do you think it is asking me to use my 'steam tables'? I'm really not sure what to do for this bit.

For b), what is(are) the equation(s) to calculate the electrical energy used by the device? And then what do you ratio to find efficiency?

Really just need nudging along the way on this, so help is much appreciated. Thanks.

for ai) you would be correct

for aii) I have no idea what you mean by steam tables but probably...
but dU=dQ+dW for internal energy where dW is the work term and dQ is the heat term so if you have a table with internal energy and temperature you can work out dQ as the change in internal energy as I think dW is zero as volume is constant and it is a PV system.

for b) you know the power of the device and hence know the energy it uses per second. (700W so 700J/s) and the ratio would be:

efficiency=total energy in/useful energy out. (multiply by 100 for percentage efficiency)
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Stonebridge
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(Original post by natninja)
for ai) you would be correct

for aii) I have no idea what you mean by steam tables but probably...
but dU=dQ+dW for internal energy where dW is the work term and dQ is the heat term so if you have a table with internal energy and temperature you can work out dQ as the change in internal energy as I think dW is zero as volume is constant and it is a PV system.

for b) you know the power of the device and hence know the energy it uses per second. (700W so 700J/s) and the ratio would be:

efficiency=total energy in/useful energy out. (multiply by 100 for percentage efficiency)
That should be the other way round

(useful energy out / energy in) x 100%
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Stonebridge
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(Original post by CarissaG)
Followed a suggested experiment to test for the efficiency of a microwave oven. It was taking a measured amount of water at a recorded initial temperature containing a pinch of salt and heating it in a microwave oven for a measured amount of time, before then recording the final temperature. The experiment was repeated multiple times.

We've been asked to do the following:

a) calculate the heat input to the water using two methods:

i) using an approximate value for the specific heat of water,
ii) using the tabulated internal energy data for water

b) calculate the electrical energy used by the device using the manufacturers power consumption figures. Hence deduce the efficiency of the device.

The microwave input power is 1000W, and the energy output is 700W. The power supply is 230 V, and the required is 10 A.

For a) i), I'm assuming it is referring to the use of Q = mcΔT.

We used one litre of water, so that gives us a value of one kilogram for mass.

We know the specific heat capacity for water, so we can use that for c.

We know final and initial temperature readings, so can calculate ΔT.

For a) ii), do you think it is asking me to use my 'steam tables'? I'm really not sure what to do for this bit.

For b), what is(are) the equation(s) to calculate the electrical energy used by the device? And then what do you ratio to find efficiency?

Really just need nudging along the way on this, so help is much appreciated. Thanks.


You measure the time it takes to heat the water from initial to final temperature. This gives the power supplied to heat the water, the useful energy output.
(Energy absorbed by water divided by time taken)
The efficiency is tricky because it can be thought of in two ways.
You say the machine has an input power of 1000W and is rated as 700W output. Well that's already a 70% efficiency for power out.
When you find the power supplied to the water to heat it, you have to decide whether you take this as a % of the 1000W input or the 700W rated output for the microwaves.
Ultimately it doesn't matter so long as you say clearly what you have measured.
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CarissaG
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