Topic 12 Physics Magnetic Fields

• the shape of a magnetic field can be found using plotting compasses - the field is strongest where the lines are closest together
• unlike magnetic poles attract and like magnetic poles repel
• iron cobalt nickel and steel are permanently magnetic can be used for fridge magnets, speakers/headphones, electric motors
• a permanent magnet is always magnetic

  induced magnet - magnetic material in a magnetic field and becomes a magnet field itself
• current travelling along a wire produces a magnetic field, the field is stronger nearest the wire and when the current is greater
• a solenoid is an example of an electromagnet, field lines around individual wires merge to form a strong uniform field inside and a weaker field outside where the lines are further apart
• forces can be used to cause a flat coil of wire to rotate, using a commutator these forces can be continuously applied this is the principle of an electric motor
• a current carrying conductor placed near a magnet experiences a force and that an equal and opposite force acts on the magnet
• Magnetic forces are due to interactions between magnetic fields
• Plotting compasses are used to find magnetic field lines. You can find the shape and direction. You put the magnet on the piece of paper and draw around it, then draw two dots at each end of the needle, keep moving the compass to get field lines.
• When there isn't a magnet, compasses point to the north as the earth generates its own magnetic field towards the iron core in the earth.
• When a current flows through a long, straight conductor like a wire, a magnetic field is created around it. It's made up of circles perpendicular to the conductor. This is how a current can create a magnetic field.
• The strength the field depends on the size of the current and the distance from the long straight conductor.
• A solenoid is an example of an electromagnet (turned on and off by current). You can increase the strength of a magnetic field by wrapping the wire into a coil called a solenoid. Inside the solenoid, lots of field lines point in the same direction so the field is strong and almost uniform. Outside of the solenoid, a lot of the overlapping field lines cancel out so the field is weak at the end of it.
• A current carrying a conductor placed near a magnet experiences a force and that an equal and opposite forces acts on the magnet.
• Forces between magnets are caused by interacting magnetic fields. When 2 poles are near each other, the magnetic lines overlap and cancel each other out. The reaming fields push the 2 poles apart. This is when they repel. When they attract, a uniform field is created which pulls the two poles together.
• Fleming's left-hand rule - represents relative directions of force, current and magnetic field:

  
• Force on a conductor at right angles to a magnetic field carrying a current (N) = magnetic flux density (T) × current (A) × length (m)