SECTION 6: MAGNETISM AND ELECTROMAGNETISMChapter 21Magnetism and ElectromagnetismMagnets attract magnetic materials but not non-magnetic materials.Magnetism is a non-contact force (acts at a distance).Magnetic materials:ironsteelnickelcobaltThe region around a magnet where a magnetic force can be felt is called the magnetic field.The magnetic field is strongest at the poles of a magnet.north-seeking pole or north polesouth-seeking pole or south poleLike poles repelUnlike poles attractREPULSIONATTRACTIONREPULSIONATTRACTIONSSSSSSSSNNNNNNNNMagnetic field around a magnet has a specific shape.4
A magnetic field around a bar magnet has a shape and direction.The magnetic field is represented using magnetic field lines (lines of force, flux lines) that show the shape, direction and strength of the field.5Investigate the shape of the magnetic field between two bar magnets.ABCDNNNNNNNNSSSSSSSNSUNIFORM MAGNETIC FIELD.Magnetic field lines are parallel and equally spaced apart.7PERMANENT MAGNETSTEMPORARY MAGNETSMade from magnetically hard materials, such as steel.Retain their magnetism once they have been magnetised.Made from magnetically soft materials, such as iron.They are easy to magnetise but lose their magnetism easily.Magnetism can be induced in some materials when they are placed in a magnetic field.ElectromagnetismA wire carrying a current has a magnetic field around it.
The magnetic field around a current carrying wire has a circular shape.
9The right hand grip rule
Use the Right-hand grip rule to remember the direction of the current (conventional current + -)10The magnetic field around a flat coil
You can use your right hand to determine the direction of the magnetic field of a coil If you curl your fingers up so they are pointing in the direction in which current flows around the coil, your thumb will be pointing towards the north end of the coil.12
SolenoidsThe magnetic field around a solenoid has the same shape as the field around a bar magnet.The field inside the solenoid is very strong and uniform. It can be used to magnetise objects.To increase the strength of the magnetic field around a current-carrying wire we can: increase the current in the wire wrap the wire into a coil or solenoid
Polarity of a coil
15The strength of the magnetic field around a solenoid can be increased by:Increasing the current.Increasing the number of turns on the solenoid.Using a magnetically soft core such as iron.Bringing the poles together.
Advantages of using electromagnets: Can be switched on or off, strength can be controlled.Disadvantages: must be supplied with energy continuously.17The electric bell
An electromagnetic door lockThere is an iron bar keeping the door locked.If the security officer agrees to let you in, he presses a switch which allows a current to flow through the solenoid.What happens to the iron bar?The electromagnetic relay