Faraday and Lenz’s Law

Updated website at: http://sciencehsc.com.au 

 

Faraday’s Law of induction states that “The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.”
i.e if there is relative motion (or change in flux) between a magnet and a conductor, then an electromotive force (emf) is induced.
Lenz’s Law says that if an induced current flows, its direction is always such that it will oppose the change which produced it.

The importance of Lenz’s law
Set up an experiment where the north pole of bar magnet is moving into a coil. As there is relative motion between the magnet and the conductor (the coils), an emf is induced by Faraday’s Law. Lenz’s law tells us that the induced emf will flow to oppose the change in flux that induced it, thus it would flow anticlockwise as viewed from the magnet. Now let us consider if the induced current flowed clockwise!

IF the induced current flowed clockwise then the coil would form a south pole (at the end the magnet is entering), and thus attracts the bar magnet, accelerating it. As the bar magnet increases in velocity, so does the magnitude of induced emf, and again it accelerates the bar magnet further. This clearly violates the law of conservation of energy as kinetic energy and electrical energy is being created. That is why induced emf MUST oppose the change in flux that induced it, otherwise it would violate one of the most fundamental laws of physics!

Example:

From the diagram, we can see a magnet moving towards a coil. By Faraday’s Law as there is a change in flux and relative motion between a magnet and a conductor, emf is induced. As it is a closed circuit, current is formed. This current will flow to oppose the change in flux that induced it by Lenz’s Law. Thus by using right hand grip rule, the current will flow anticlockwise as viewed from the magnet.

As the current will flow to oppose the change in flux that induced it, the current will induce a north pole at X, to repel the magnet. Using right hand grip rule, our thumbs point north and our fingers indicate the direction of current flow. 

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Heat of combustion of alkanols

Updated website at: http://sciencehsc.com.au 

 

Diagram:

Method:

1. Set up apparatus as shown.
2. Light the first spirit burner.
3. Adjust the height of the vessel so that the tip of the flame just touches the vessel.
4. Weigh the burner (initial mass) with its liquid contents and record.
5. Add 200 mL of cold water to the vessel using a measuring cylinder. Place the thermometer in the water and record its initial temperature.
6. Light the wick and stir the water gently (to ensure uniform heating).
7. When the temperature has risen 10C extinguish the flame by placing the cap.
8. Again weigh the burner (final mass). Remove any soot from the bottom of the vessel and replace the water before testing the next alcohol.

Methods to reducing error when determining heat of combustion

• Ensure the tip of the flame touches the vessel to minimise heat lost to the environment.
• Use a copper can or perform experiment in a bomb calorimeter to contain as much heat as possible.

Remember there will always be heat loss to the environment thus the energy absorbed by the water is less than the amount of energy released by the fuel combusting. This loss of energy can be considerably large and thus will result in large inaccuracies.

Practicals – reliability, accuracy, validity and errors

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ACCURACY (exactness)

Most measurements contain some uncertainty. Accuracy refers to the exactness of a measurement.
We can measure a small distance with a metre rule or with much greater accuracy using a micrometer.

RELIABILITY (dependability)

Reliability refers to the consistency with which we can confirm a result. Consistency is usually achieved by repetition.

VALIDITY (fairness)

A procedure is valid if it tests what it is supposed to be testing. A procedure is invalid if the method of the experiment is incorrect or partially incorrect.
In a valid experiment all variables are kept constant apart from those being investigated, all systematic errors have been eliminated and random errors have been reduced by taking multiple measurements.
In determining validity, students should consider the degree to which evidence supports the assertion or claim being evaluated. This may be done by making comparisons or conducting further experiments.

	first-hand information and data	secondary information and data
Accuracy	Instruments should be precise and calibrated.	Sources should be reputable?
Reliability	All tests should be repeated a significant number of times.	Information obtained should be consistent with information from other reputable sources.
Validity	Experiments should test the hypothesis that is proposed.The experimental method must be correct?All variables should be identified and controlled.	Information should be gathered in an unbiased and professional manner.Findings must relate to the hypothesis or problem.

ERRORS

The two different types of error that can occur in a measurement are:

1. Systematic error – this occurs to the same extent in each measurement. EG when the needle of a voltmeter is not correctly adjusted to zero when no voltage is present.

2. Random/Human error – this occurs in any measurement as a result of the variations in measurement technique. Eg parallax error, limit of reading.