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Investigation of determination of flux density of magnets(plane)

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Investigation of determination of flux density of magnets


A magnet is a material or object that produces a magnetic field. A low-tech means to detect a magnetic field is to scatter iron filings and observe their pattern, as in the accompanying figure. A ‘hard’ or ‘permanent’ magnet is one that stays magnetized, such as a magnet used to hold notes on a refrigerator door. Permanent magnets occur naturally in some rocks, particularly lodestone, but are now more commonly manufactured. A ‘soft’ or ‘impermanent’ magnet is one that loses its memory of previous magnetizations. ‘soft’ magnetic materials are often used in electromagnets to enhance the magnetic field of a wire that carries an electrical current and is wrapped around the magnet; the field of the ‘soft’ magnet increases with the current.

As current flows through a wire, the wire is magnetized and it generates a magnetic field around it, and this magnetic field will interact with any other magnetic field to produce a magnetic force. We can predict the direction of force by using Fleming’s left-hand rule. There are three

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The size of the magnetic force depends on:

The magnetic field strength, B

The current I, flowing at right angles to the field

The length l of the conductor I the magnetic field. ( in this case, the length of the conductor will be the length of the magnet)

Sum up these three factor, we can get an equation for the force:

F = BIl

We can see from this equation that the force F, is proportional to each of these quantities.

we can arrange the formula:

B = F/Il

A magnetic field has a strength of one tesla ( 1T ) , if it exerts a force of 1N on a conductor of length 1m, carrying a current of 1A at right angles to the field. Thus the equation b = F/Il defines the quantity magnetic field strength( flux density) B, and its unit, the tesla: 1T = 1NA-1m-1

As the force of magnetic is equal to the restoring weight, the length of the conductor can be measured by measuring the length of magnet, and the current can be measured by an ammeter. Therefore the magnetic flux density can be calculated easily.

There are few factors which I think would affect my experiment:

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An electronic balance

As it is electronic, and it will give the value up to 2 decimal places, so that produce more accurate results.

Copper wires

Variable resistor

This is used to vary the resistance in the circuit to give different current.

An ammeter

This is used to measure the amount of current


Wearing goggles

Watching out the current, if it gets to large, the wire could be burnt or get to hot. Make sure do not touch the wire if it gets too hot.


  1. set up equipments like the diagram
  1. switch on the power supply and vary the variable resistance, record the current shown on the ammeter and adding paper pieces on to the frame
  1. as soon as the current balance goes to equilibrium which can be seen by comparing the level of two frame on each side, to see if they are at the same level, and switch of the power supply
  1. carefully moving the stored paper on to the balance and record the mass shown on the screen
  1. changing different current by varying the variable resistor and do the same steps as above
  1. repeat the experiment with the same current
  1. calculate the average mass of paper for each current and use the average to plot a weight—current graph
  1. tidy up the equipments

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