Physics Notes Part 4

 v The magnetic force seems to come from two points called poles of the magnet.

v Magnetic material: It can be magnetised and is attracted to magnets. All strongly magnetic materials contain iron, nickel or cobalt. Strongly magnetic metals are ferromagnetics(ferrous). It is described as hard or soft.

v Hard magnetic materials such as steel and alloys called Alcomax, and Magnadur are difficult to magnetize but do not readily lose their magnetism. They are used for permanent magnets.

v Soft magnetic materials such as iron and mumetal are relatively easy to magnetize, but their magnetism is only temporary They are used in cores of electromagnets and transformers because their magnetic effect can be switched on or off easily.

v Non-magnetic materials include metals such as brass, copper, zinc, tin and aluminium, as well as non-metals. (Non-ferrous) Magnets exert force on non-magnetic materials.

v Magnet placed into the solenoid, with alternating current. Magnetic field changes direction as current flows forwards and backwards, throwing atoms in the magnet out of line, to demagnetize a magnet. If a magnet is hammered, it becomes demagnetised. Heating it to high temperature has the same effect.

v Scientific speaking, there’s a magnetic field around a magnet, and this exerts a force on the magnetic material in it.

v Magnetic fields can be investigated using a compass. When lines are drawn, the result is a magnetic field line.

v At a neutral point, the field from one magnet exactly cancels the field from the other, so the magnetic force on anything at this point is zero.

v Right-hand grip rule for field direction. A long coil is a solenoid. Increasing the current and increasing number of turns on the coil increases the strength of the field.

v Steel bar placed into the solenoid, where current is passed, to make a magnet.

v Unlike an ordinary magnet, an electromagnet can be switched on and off.

v In a simple electromagnet, a coil consists of several hundred turns of insulated copper wire and is wound around a core, usually of iron or mumetal.

v Force is increased by current increased, stronger magnet, length of wire increased.

v If a beam of charged particles (such as electrons) passes through a magnetic field, there’s a force on it, just as for current in a wire.

v The commutator or split ring, is fixed to the coil and rotates it. The brushes are two contacts which rub against the commutator and keep the coil connected to the battery. (DC. Motor)

v Increase the turning effect of the coil by increasing the area of the coil.

v Coils contain hundreds of turns of wire and are wound or a core called an armature.

v When the wire is moved across a magnetic field, a small voltage is generated in the wire. This effect is electromagnetic induction. Scientifically speaking emf is induced. If the wire forms part of a complete circuit, the EMF makes a current flow. This can be detected by a metre called a galvanometer.

v Emf and current can be increased by moving the wire faster, using a stronger magnet, and increasing the length of the wire. It is summed up as Faraday’s law of electromagnet.

v Lenz law: An induced current always flows in a direction that opposes the change that produced it.

v Direction of force can be predicted using Fleming’s left-hand rule. (FMC) If the straight wire in a complete circuit is moving at right angles to a magnetic field, the direction of induced current can be found using Fleming’s right-hand rule. (MFC)

v When current causes motion, the left-hand rule applies. When motion causes current, the right-hand rule applies.

v If an aluminium disk spins between the poles of a magnet, it stops almost immediately. This is because the disc is a good conductor and currents are induced in it as it moves through a magnetic field. These are eddy currents.

v Generators/dynamos use electromagnetic induction.  

v AC generators are called alternators. The slip rings are fixed to the coil and rotate with it. The brushes are two contacts which rub against the slip rings and keep the coil connected to the outside part of the circuit. (AC generator)

v With AC circuits giving voltage and current values are complicated by the fact that they vary all the time. To overcome this problem, a type of average called root mean square (RMS) value is used. It is equivalent to a steady voltage which would deliver energy at the same rate.

v Emf induced in a conductor is proportional to the rate at which magnetic field lines are cut by the conductor. Induced emf at switch on/ switch off increases by the number of turns on second coil increases, the core of electromagnet goes right through second coil. When coils are magnetically linked, so that a changing current in one causes an induced EMF in the other, is called mutual induction.

v AC voltages can be increased/ decreased using a transformer. When AC flows in the primary (input) coil, it sets up an alternating magnetic field in the core and therefore secondary (output) coil. This changing field induces an alternating voltage in the output coil. Provided all field lines pass through both coils and coils waste no energy because of heating effects.

v Step-up transformer: It has more turns on the output coil than on the input coil, so the output voltage is higher. A large step-up transformer is used in a power station to increase the voltage to levels needed for overhead power lines.

v Step-down transformer: It has more turns on the input coil than on the output coil, so the input voltage is higher. In battery chargers, computers and other electronic equipment reduce the voltage of AC mains to lower levels needed for other circuits.

v The transformers work with AC only. Power for AC mains is generated in power stations, transmitted through long-distance cables and then distributed to customers.

v Cables feed power to a nationwide supply network called a grid. Power from the grid is distributed by a series of substations.