http://www.s-cool.co.uk/a-level/physics/magnetic-fields/revise-it/effect-of-ferrous-cores-and-the-relationship-between-curre  http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/magnets_electric_effects/revision/4/  http://www.howmagnetswork.com/Electromagnetism.html
 http://www.wisegeek.com/what-is-a-solenoid-door-lock.htm  http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html
The shape of a magnetic field around a solenoid
How to strengthen a magnetic field around / in a solenoid
http://www.s-cool.co.uk/a-level/physics/magnetic-fields/revise-it/effect-of-ferrous-cores-and-the-relationship-between-curre Electromagnets all consists of a current carrying conductor, however the strength of the magnetic field produced also depends on the surrounding substance. Air is not particularly good at ‘carrying’ a magnetic field and so the field around a wire is quite weak. By placing a core inside of a coil the strength of the magnetic field can be increased.
By putting a ferrous (iron) core inside a solenoid, the field lines are concentrated. This has the effect of strengthening the field. Inside the core, the field lines are much closer together → stronger field. Iron is a particularly good core for solenoids as it is easily magnetised and demagnetised. This means that when the current is switched off the iron does not stay magnetic. Iron is said to be magnetically soft. http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/magnets_electric_effects/revision/4/ Making an electromagnet stronger
We can make an electromagnet stronger by doing these things: wrapping the coil around an iron core
adding more turns to the coil
Increasing the current flowing through the coil.
The magnetic field of an electromagnet
The magnetic field around an electromagnet is just the same as the one around a bar magnet. It can, however, be reversed by turning the battery around. Unlike bar magnets, which are permanent magnets, the magnetism of electromagnets can be turned on and off just by closing or opening the switch.
Solenoids as magnets
How a solenoid makes a magnet that can be switched on or off http://www.howmagnetswork.com/Electromagnetism.html
The solenoid is a long coil containing a large number of close turns of insulated copper wire. The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet.
The lines of magnetic force pass through the solenoid and return to the other end. If a current carrying solenoid is suspended freely, it comes to rest pointing North and South like a suspended magnetic needle. One end of the solenoid acts like a N-pole and the other end a S-pole. Since the current in each circular turn of the solenoid flows in the same direction, the magnetic field produced by each turn of the solenoid adds up, giving a strong resultant magnetic field inside the solenoid.
A solenoid is used for making electromagnets. The strength of magnetic field produced by a current carrying solenoid is: 1. Directly proportional to the number of turns in the solenoid 2. Directly proportional to the strength of current in the solenoid 3. Dependent on the nature of “core material” used in making the solenoid. The use of soft iron rod as core in a solenoid produces the strongest magnetism.
How a solenoid can be used to secure a door
A solenoid door lock is a remote door locking mechanism that latches or opens by means of an electromagnetic solenoid. In most cases, the actual locking mechanism of a solenoid door lock will be identical to a conventional key-operated example. The only difference between the two is the inclusion of a low-voltage solenoid in the mechanism, which pulls the latch back into the door when a push button or other controller is activated.
The latch will then be retained in the door for as long as the button is pushed, or, in the case of a latching solenoid, indefinitely until the button or controller is activated again. These types of door locks are used extensively in remote security access and automotive doors. As solenoids are much like electromagnets, they can be turned on and off.
The use of this is that a strong magnet could be used to keep a door shut but when a ‘key’ is activated, the current flowing to the solenoid is cut off, allowing the door to open. As the door is shut again, the circuit is closed/completed again and the magnet holds the door in place again. While the locking solenoid will keep the device locked, it is not technically on when in locking mode, because no power is being used.
The solenoid only needs power when unlocking and, because most devices are consistently locked, very little energy is ever required by the solenoid. For this reason, most solenoids run on battery power, and the battery rarely needs to be changed.
The majority of these solenoids are made to be small, because they fit in small devices, but there are some that are large and exhibit powerful electromagnetic forces. Heavy-duty solenoids are used in construction equipment, such as forklifts, to lock the mast when the machine is off or not operating. Unlike smaller solenoids that exhibit the force of about 45 Newtons, these larger solenoids use about 1,000 Newtons to lock the equipment in place.
When a door or device locks via a solenoid, it is using basic electromagnetic forces to control the lock. The solenoid fits in the locking mechanism and, when locked, will expand so the device cannot be unlocked by sheer force. An electromagnetic force, such as a keycard, is needed to tell the solenoid to move, allowing the device to unlock and open. Convert:
45 newton’s=4.588723 kilograms force. This means 4.588723 kg is applied to keep the door shut. This relatively low locking force is actually rather