Electromagnetism. Kevin Gaughan for. Bristol Myers Squibb

Transcription

1 Electromagnetism Kevin Gaughan for Bristol Myers Squibb

2 Contents Magnets and Ferromagnetism Domains Theory H, B and µ The links between electricity and Magnetism Electromagnets Induction Applications of Electromagnetism

3 Permanent Magnetic Materials have been known since antiquity. They attract magnetic materials Iron / Nickel They can attract or repel other magnets Every permanent magnet has two poles (North and South) Like Poles repel, Unlike Poles Attract Magnetism

4 Ferromagnetism Permanent magnetic materials are said to be ferromagnetic. Ferromagnetic materials become magnetised themselves in the presence of an external magnetic field and they retain some of that magnetism when the external field is removed. This is the cause of permanent magnets. Ferromagnetic material interact strongly with magnetic fields and the induced magnetism causes measureable forces of attraction. Ferromagnetic materials include Iron (Latin name Ferrous), Cobalt and Nickel

5 Other types of Magnetism Paramagnetic materials become magnetised in the presence of a magnetic field but do not retain that magnetism when the field is removed (Aluminium, Tungsten). Diamagnetic Materials actually resist an external magnetic field (Carbon, Copper, Water). NOTE: Paramagnetism and Diamagnetism are much much weaker effects than Ferromagnetism. They do not produce measurable forces. For our purposes these materials may be considered to be non-magnetic.

6 Poles and Flux Lines Flux Lines are Imaginary lines running from North Pole to South Pole of a magnet. They show the direction of magnet field. We say Magnetic Flux flows from North Pole to South pole. The stronger the magnetic field the higher the flux and the more closely spaced the flux lines. You can plot the path of the flux lines with a little compass or with iron filings but the lines are not physically real they only represent the direction of flow of magentic flux.

7 Unlike Poles Attract Like Poles Repel each other and unlike poles attract each other. One method of visualising this is to imagine that magnetic flux always tries to take the path of least resistance from a North Pole to South Pole. The magentic flux pulls unlike poles together in order to shorten the path but it pushes like poles apart as the fluxes coming from each magnet bash against one another.

8 Did you know that the North Pole is Really a South Pole? Image from This is a source of much confusion between Geographers and Physicists. Geographers even refer to the Magnetic North Pole when they mean the south magnetic pole. How do you think this confusion came about?

9 Distorting the field A piece of iron or other ferromagnetic material will distort the local magentic field. The flux will try to take the path of least resistance (through the iron). This fact is very useful is you want to control the path of a magentic field as in a motor or a transformer.

10 Domains Theory of Ferromagnetism Ferromagnetic materials have minute magentic domains within their atomic structure. When the ferromagnet is unmagnetised these domains are randomly oriented and there is no net magnetic field. When you apply an external magnetic field the domains allign and their fields add up increasing the total magnetic field through the ferromagnetic material.

11 Domains (cont) Permanent magnets arise when the domains are sticky they stay alligned and the material retains a net magnetic field even after the external field is removed. Hard magnetic materials retain a large permanent field. Soft magnetic materials retain less. Hard magnetic materials make good permanent magnets.

12 Demagnetising a Permanent Magnet You can demagnetise a permanent magnet by Hammering Heating Putting in in an external alternating magnetic field and slowly withdrawing it again. Why do the above methods work?

13 Ferromagnetic Saturation When all the domains are lined up a Ferromagnet is fully magnetised and is said to be saturated. Increasing the external magnetic field above the point of saturation can not allign any further domains so the resulting field will only increase slowly from then on. Magnetic saturation in iron alloys occurs at around 1.5 Tesla and this is the limiting magnetic flux density for many magnetic devices.

14 H B Just like electricity magnetism has a pressure property (like voltage) and a flow property (like current) The unit of magentic pressure is called Magnetic Field Strength H and is measured in Amps/Metre The unit of magnetic flow is called Magnetic Flux Density B and is measured in Tesla (Note sometimes it is useful to consider total magnetic flux which is B x Cross sectional area and is measured in Webers)

15 Magnetic Permeability µ Every material has a magentic permeability which represents how easily magnetic flux flows through it. This is called magnetic permeabiltiy µ. The reference permeability is that of a vacuum and the permeability of a vacuum is one of the fundamental scientific constants: µ 0 = 4π 10 7 Henries/ metre

16 Relative Permeabilty For any other material we say µ=µ r.u 0 Where µ r is the relative permeabililty of the material. All non magnetic materials have relative permeabilties very close to 1.0 They behave very like a vacuum. Ferromagnetic materials can have relative permeablities of 1000 or more. This means a ferromagnet is 1000 times better at carrying magnetic flux than a vacuum.

17 Mathematical Explanation of µ B = µh The bigger µ then the more magnetic flux you will get for a given magnetic field strength. For non magnetic materials that simple equation works. Ferromagnetic materials are very non linear however and the ratio between B and H changes.

18 B H curve of a Ferromagnetic Saturation Material NB NB The units of B are multiplied by 10 in this graph! For example Carbon Steel saturates at around 1.5 Tesla. Air Its actually even a bit more complicated that this because of.

19 Hysterisis Some B remains even after H is reduced to zero. This is called Remanenceand is responsible for permanent magnetism. Copied from bhcurve.com original source unknown.

20 Linking Electricity to Magnetism

21 Three fundamental principles of Electromagnetism How to create magnetism from electricity (Fundamental princple behind electric motors and transformers) How to create electricity from magnetism (Fundamental principle behind electric generators and transformers) How a current carrying wire experiences a force in a magnetic field. (Used in many electric motors)

22 Electricty -> Magnetism Every wire carrying current generates a small magnetic field The picture comes from

23 The Right Hand Grip Rule If you imagine gripping the wire in your right handwith thumb pointing in the direction of the current then your fingers trace the direction of the magnetic field. Notice how there is no obvious North or South Pole the magnetic field just goes around in a circle. Image from

24 The Solenoid: Its just a coil of wire. In practise one wire produces very little magnetism so we wrap many turns of wire into a coil often called a solenoid. The resulting coil acts like a bar magnet with North and South poles. The Magnetic Field in the middle of the coil is given by H = N L I

25 Electromagnets The current creats the H but the magnetic flux B also depends on the permeability of the material in the middle of the solenoid. If we wrap a coild of wire around an iron core (high permeability) we get a strong controllable magnet. We have just made an ELECTROMAGNET Notice how we can reverse the north and south poles by reversing the direction of current.

26 Magnetism to Electricity In order to go in the opposite direction you need a changing magnetic field. A changing magnetic field will induce a Voltage in a coil of wire. A changing magnetic field can mean a a stationary field which is growing stronger and or weaker (eg transformers). It can also mean a constant magnetic field which is moving (eg electric generators)

27 Electromagnetic Induction with a moving magnetic field

28 Inducing a voltage in a coil of wire The voltage generated in any one wire is small so you really need a coil of wire The induced voltage is given by V = N. d ( B dt A )

29 Michael Faradays Laws of Electromagnetic Induction

30 Force on a current in a Magnetic A wire carrying current in a magnetic field experiences a force which is proportional to the level of current (I), the Flux Density (B) and the length of the wire exposed to the field (L) Force =IxBxL (Newtons) Many electric machines utilise this principle to generate mechanical force from electricity Field

31 Force on a current another view Remember that every current carryign wire generates its own magnetic field. The force that the wire experiences in an external magnetic field is due to the interaction of the wires magnetic field with the external magnetic field.

32 Left Hand Rule The force on a current carying wire experiences a force that is perpendicular to the current and perpendicular to the original magnetic field. The Left Hand Rule allows you to predict the direction of the force.

33 A bit of Scientific History We have seen that electricty and magnetism are closely linked. In fact it is likely that all magnetic fields are generated by microscopic currents. The crowning achievement of 19 th century science came about when James Clerk Maxwell produced a unified theory of electricity and magnetism. This theory even allowed him to predict the existence of travelling electromagnetic waves (electromagnetic radiation). Electromagnetic radiation include radio waves, micro waves, x-rays, Ultra violet, Infra Red and even visible light. Maxwells equations allowed him to calculate the speed of light purely from physical constants.

34 Applications of Magnetism and Electromagnetism The Magnetic Compass was hugely important to historical navigation. A freely rotating magnetised needle will allign with the Earths magentic field so that the North Pole of the needle (usually coloured red) will point towards the geographic North Pole.

35 Magnetic Switches Commonly used in burglar alarms to detect opening windows and doors.

36 Electromagnetic Recording on Disk and Tape Magnetic Tape is not very popular any more but most hard disks still use magnetic recording. The resulting magnetised pattern is read using another coil to detect the magnetic field.

37 Solenoid Actuators Solenoid actuators are very commonly used in automation where linear movement is required. For example electric control of pneumatic and hydraulic valves.

38 Relays allow a small electric current in the coil to turn on and off a much larger current in the contacts circuit. Relays provide safety isolation. There is no direct contact between coil and contacts so a low voltage circuit may safely control a hazardous volatge circuit. Relays

39 Alternating current in the coil interacts with the permanet magnet to generate an oscillating force on the coil. The oscillating coil pushes the air back and forth and generates sound waves. A reversal of this principle can be used as a microphone. Loudspeakers

40 Generators and Motors ELectric motors and generators use electromagntism to either produce a force from electricity or to produce electricity by electromagnetic induction.

41 Transformers Transformers work because the primary winding generats an alternating magnetic field in the core which then induces a voltage in the secondary winding.

42 Leakage and Fringing Most electromagnetic devices use iron or other ferromagnetic material to force the magentic flux to go where it is wanted. Some useful flux is still lost. In the diagram we are using an iron core to try and focus the flux through an airgap. Some flux escapes the core altogether (a) This is called leakage. Also some flux spreads out at the airgap this is called fringing. In a typical electric machine up to one quarter of the flux may be lost through leakage.