2Electromagnetic Induction Practical and predict direction of the Iinduced
3Flemings right hand generator rule: First finger (Index) point to the direction of magnetic FieldThumb is the direction that the conductor is movingSeCond (Middle) finger=direction of the induction Current flow
9An airplane with a wing span of 30 An airplane with a wing span of 30.0 m flies parallel to the Earth’s surface at a location where the downward component of the Earth’s magnetic field is 0.60 ×10-4 T. Find the difference in potential between the wing tips is the speed of the plane is 250m/s。
12Electromagnetic induction When a magnet is pushed into a coil of wire an electric current is produced in the wire. We say that a current was induced in the wire.
13EM Induction explained The coils of wire are full of ‘free’ electrons randomly moving from atom to atom.When the magnetic field of the magnet moves past the coils of wire the magnetic field exerts a force on the ‘free’ electrons.We call this force an EMF ‘Electro Motive Force’.The force on the electrons causes them to move through the wire, this is an electric current.The changing magnetic field induces an electric voltage across the circuit.
14Moving up and downWhen we change the direction of the magnet we are changing the direction of the force on the ‘free’ electrons in the wire.The electric current is induced in the opposite direction.
16Stationary magnet = no current When the magnet is stationary there is no electric current.This is common sense we cannot generate electrical energy without some energy going into the system.Without the kinetic energy of the moving magnet the electrical energy of the current will not be created.
17Standard equipmentA suitable meter, either an a.c. meter or a galvanometer,A coil,Two connecting wires,A magnet.
19Increasing the speedMoving a magnet in and out at a faster speed will increase the voltage.The overall electrical energy from each movement will be the same but there will be more movements per second and hence more power. We will need to put in more kinetic energy every second.
20Increasing the magnetism If we increase the strength of the magnet passing into the coil there will be a more voltage.However, if this makes the magnet heavier we will need to provide more kinetic energy.
21Increasing the coilsIf we increase the number or area of the coils there will be more voltage.With more coils of wire we will have more ‘free’ electrons, there will be a greater current; but a longer wire also means more resistance – and something called Lenz’s law (not in examination).
22Increasing the voltage There are three mains ways to increase the size of the induced electric voltage.Use a stronger magnet,Use more coils of wire (sometimes called turns),Move the magnet quicker.
23What’s the conditions for creating induced current /e.m.f. ? Change of magnetic field in the loop
24Lenz’s lawThe direction of the induced e.m.f. is such as to cause effects to oppose the change producing it.The INDUCED current creates an INDUCED magnetic field of its own inside the conductor that opposes change of the original magnetic flux density.
25IInduced 1/φo point to downwards 2/φo increased , △φo>0 3/BInduced point to upwards4/IInduced Anti-clockwiseIInduced1/Direction of the original magnetic flux density2/Change of the original magnetic flux density3/Direction of the INDUCED magnetic field4/By using right hand grip rule, find out Iinduced.
27Any magnetic force on the coil? BCDBABCDFIinducedFIinducedBAny magnetic force on the coil?
28the current is always induced in such a direction that the magnetic force it produces opposes the relative motion between the conductor and the magnetic field.Magnitude of E?
29Metal bar ab keep in touching with the P and Q, Mass of ab is m， length of it is L，there is a resistor R in the circuit，the resistance in the wire is negligible ,the length of the P and Q is infinite , ab fall down in a uniform magnetic field, find out its maximum speed.bR× × × ×BamLPQ
34A microphone works when sound waves enter the filter of amicrophone. Inside the filter, adiaphragm is vibrated by thesound waves which in turn movesa coil of wire wrapped around amagnet. The movement of the wirein the magnetic field induces acurrent in the wire. Thus soundwaves can be turned intoelectronic signals and thenamplified through a speaker.
37A.C. and D.C.Direct current is present in one direction. The charge flows in one direction only, e.g. a battery.Alternating current is present in both directions. Charge flows first in one direction and then in the opposite direction. It has a typical frequency of 50 Hz (UK) or 60 Hz (Bermuda and the USA).There are advantages and disadvantages of both types of electrical power.37
38A.C. and D.C. Batteries always supply direct current. Mains electricity is always supplied as alternating current.38
39Alternating CurrentAn electric current is called an alternating current if the charge flows in one direction then changes to flow in the opposite direction.When looking at a diagram it is important to see if the line goes both above and below the zero line.ForwardsZero lineBackwards39
40a.c. sine wave0 Volts+-When electricity is generated in power stations it takes the form of a sine wave. Many electrical signals look like a sine wave.House current is 120 volts, 60 Hz in the US and Bermuda.House current is 240 volts, 50 Hz in the UK.4040
41Is this a.c. or d.c.+0 Volts-Although this electrical signal has a sine wave shape it is still a d.c. signal.The wave increases and decreases but never reverses direction. Think about a car moving along speeding up and then slowing down.4141
43The dynamo A dynamo has a rotating magnet inside a coil. Simple examples can be found in old bicycle lamps.43
44GeneratorRather than move a magnet inside of a coil we can turn the situation around.This time we will move a coil inside of a magnetic field.We call this arrangement a generator.Most generators produce a.c. electricity consequently they are sometimes called alternators.44
45a.c. generatorMoving a coil in a circle between two magnets produces an induced electric current in the coil.45
51Generator theory As the coil turns it cuts across the magnetic field, An EMF (Electro-Motive Force) pushes free electrons along the wire.This is an induced electric current.An a.c. is transmitted through the brushes to the rest of the circuit.51
57stronger magnetic field What are the four ways in which the (induced) current from an AC generator can be increased?1. _________________________2. _________________________3. _________________________4. _________________________faster movementstronger magnetic fieldmore coilslarger area of coils5757
58Summary - InductionWhen a conductor moves through a permanent magnetic field a voltage is induced in the conductor.When a magnet spins inside a coil of wire the magnetic field at any place in the coil is changing. The changing magnetic field induces a current in the wire. coil.Induced voltage can be increased by using a stronger magnetic field, using more coils of wire or by moving the generator faster.58