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Physics for Industrial Design with Peter Eyland

Lecture 3 Magnetism

In this Lecture:

• Compasses and bar magnets are introduced,

• magnetic fields are mapped,

• the relationship between electric currents and magnetic fields is demonstrated,

• the various uses of electromagnets are described, and

• the force on a current in a magnetic field is seen in various devices.

 

Introduction

The word magnet comes from "Magnesia stone" which is an iron oxide stone that the Ancient Greeks (circa 600 BCE) discovered attracted other pieces of iron.

Only Iron, Cobalt and Nickel do this naturally.

 

In Europe, it was noticed that when pieces of Magnesia stone were suspended so they could rotate, they moved around to line up in the direction of the Pole Star.

It was found that by rubbing a small iron needle with a Magnesia stone the direction-finding property was transferred to the needle.

Because they moved to point North, suspended needles were called Compasses (from the Latin to "step around") and were used for navigation in Europe from the 11th Century.

The Chinese report that their compass (Si Nan) was described by Hanfucious about 280 - 233 BCE.

Diagram from W.H.Campbell "Introduction to Geomagnetic Fields".

As shown, the Chinese compass was spoon shaped with a narrow handle that pointed South.

 

Magnetic Poles

When it was found that small iron bars with the direction-finding property both attracted and repelled each other, they were called "magnets".

The attraction and repulsion were concentrated at the ends, so it led to the idea of North and South "magnetic poles".

Unlike poles attract and like poles repel.

 

If you cut a bar magnet in two you do not get separate North and South monopoles because you end up with two lots of North and South poles.

Steel bars make permament magnets because Steel is more difficult to magnetise than Iron and so it doesn't lose its magnetism like Iron does.

 

Geomagnetism

Gilbert's book of 1600 ACE shows the idea of magnetic "dip" where compass needles are inclined to the horizontal at different places on the Earth.

This indicates that the Earth has a big magnet inside it, and compasses work by their North Pole aligning with the Earth's South Magnetic Pole.

(The Earth's Geographic North Pole is near the Magnetic South Pole.)

 

Magnetic Fields

Since magnets produce and experience forces at a distance, they have a magnetic field of influence around them.

The magnetic field pattern can be revealed by a simple experiment.

Place a bar magnet underneath a piece of paper that has iron filings sprinkled on it.

Tap the paper gently, and the filings move to make a pattern that is concentrated at the ends as shown below.

The filings map out the shape of the magnetic field.

 

Using a compass needle instead of the iron filings gives magnetic field lines and directions as shown in the diagram below.

If the lines are continued inside the magnet, they are closed loops as shown below.

Notice how the lines change direction and and are more concentrated inside the magnetised material.

 

• Horseshoe magnets

A horseshoe magnet is a bent permament magnet where the field lines outside the material between the Poles form an approximately uniform magnetic field.

(If the lines are continued inside the material, they still form closed loops.)

 

Comparing Electric and Magnetic fields

Electric fields

Magnetic fields

field lines start on positives and end on negatives

field lines are closed loops: out from N and into S

a tangent to a field line gives the direction of the force on a positive charge

a tangent to a field line gives the direction a compass needle will point

charges can exist singley as plus or minus

magnetic poles always exist in pairs

 

Moving charges cause magnetic fields

Hans Christian Oersted (1777 - 1851) in 1820 wanted to show his students that there was no connection between electricity and magnetism.

He set up a large current in a horizontal wire and placed a compass needle nearby wanting to show no effect when a current passed through the wire.

It was a demonstration that went very wrong because the compass needle moved to line up at right angles to the current and jumpped 1800 when the current was reversed.

(Diagram from the UNSW Physics Demonstrations Unit)

Oersted is now famous for demonstrating that magnetic fields are caused by the movement of electric charges!

Using iron filings on paper threaded through a current carrying wire, circular patterns are found.

With current going into the page (away from you), a compass needle gives clockwise field lines.

 

In three dimensions these circles translate into cylindrical surfaces.

 

Solenoids

Winding a wire into a helix and sending a current through the wire will produce a magnetic field that looks just like a bar magnet.

The diagram below shows a South pole on the Left and North pole on the right.

Such coiled tubes are called solenoids (from the Greek solen = "tube")

Since the magnetism depends on the current size and direction, the solenoid acts as a variable magnet that can be switched on and off, or reversed.

 

Electromagnets

When a solenoid is combined with an iron core that concentrates the magnetic field, the magnetic effect can be multiplied some 2000 times.

Such Iron cored solenoids are called electromagnets.

Electromagnets are used in a wide variety of places, e.g. scrap metal businesses to lift car bodies, electric bells, relays and circuit breakers.

 

• Electric bell

Depressing a momentary switch will complete a circuit that powers the electromagnet.

This pulls an armature (a "covering") across to cover the end of the iron core. As the armature moves, the clapper (fixed on the end of the armature), hits the bell and the current stops as the "make & break" switch opens.

The armature is pulled back by a spring (not shown) which makes the circuit complete again and powers the electromagnet.

The cycle is repeated as long as the momentary switch is depressed.

Note that the core is Iron (easily magnetised), the windings are Copper (low resistance) and the make & break contacts are often Platinum (resists damage from sparks).

 

• Electric Relay

A relay is a remote switch. It is often used to turn high-current devices on and off remotely, with a low current in the relay circuit.

 

• Circuit breakers

Circuit breakers protect circuits and devices from too much current.

They can be simple bimetallic strips which respond to heat by bending and breaking the circuit, or electromagnetic (shown below) which respond to a sudden rise in current.

The trip switch will stay open after it has been activated.

It has to be reset manually after the cause of excess current has been fixed.

 

• Ground Fault Circuit Interupters

In Australia the domestic electrical supply has three wires.

The Active is the high potential wire.

The Neutral is the return wire and Earthed at the distant supply.

The Earth wire is the local earth to keep the outer shell of appliances at local Earth.

If current flows along the Earth wire, there is some fault in the system and the currents in the Active and Neutral will be different.

Ground Fault Circuit Interupters (also called "Earth Leak Trips" and "Safety Switches") have a trip switch that cuts the power when this happens.

GFCIs have two coils wound in opposite directions. One coil has the Active wire and the other has the Neutral wire.

If the currents are the same then there is no net magnetic field, but if the currents differ by more than 30 mA then the resultant magnetic field will trip the switch.

 

Force on a current in a magnetic field

As shown in the devices above, a current in a wire has a magnetic field around it that can attract the easily magnetised iron.

When the magnetic field from a current carrying wire suddenly interacts with another magnetic field then a force appears whose direction depends on how the field lines can adapt.

 

Example

When the cylindrical magnetic field of a straight wire carrying a current, interacts with a uniform magnetic field (from a "horse-shoe" type magnet), the following emerges:

Magnetic field lines do not merge, so when opposed arrows met, one of the lines must jump to a new position.

In the case above, the circles cannot jump to anywhere but the downwardly bent lines can.

As the bottom lines jump above the wire, a downward force on the wire appears.

This force is the basis of the operation of electric motors, meters and loudspeakers etc.

 

• Motors

From Cutnell & Johnson Physics CD (recommended)

• Moving coil meters

From Halliday, Resnick, Walker & Christmas CD Physics (recommended)

 

• Loudspeakers

From Cutnell & Johnson Physics CD (recommended)

 

Summarising:

Compasses are magnetised suspended needles.

Bar magnets have North and South Poles.

Like Poles repel and unlike Poles attract.

The Earth acts as if it has a big magnet inside it.

Electric fields

Magnetic fields

field lines start on positives and end on negatives

field lines are closed loops: out from N and into S

a tangent to a field line gives the direction of the force on a positive charge

a tangent to a field line gives the direction a compass needle will point

charges can exist singley as plus or minus

magnetic poles always exist in pairs

An electric current has a magnetic field associated with it.

Winding a wire into a helix forms the coiled tube called a solenoid.

An electromagnet is a solenoid with an Iron core.

Electromagnets are used for bells, relays and circuit breakers etc.

The force on a current in a magnetic field can be used to make motors, meters and loudspeakers etc.

 

 


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