PH10-11

PH10-11

1. Magnetism

   • A Magnet can be Destroyed by Heating or by Hitting it with a Hammer
   • Every Magnet has Two Poles (North and South)
     • Like Poles Repel
     • Different Poles Attract
   • A PERMANENT Magnet produce their own magnetic field
     • An INDUCED magnet are magnetic materials that turn into a magnet when they're put into a magnetic field
       • Permanent and Induced Magnets always Attract each other

2. Electromagnets

   • A Moving charge Creates a Magnetic Field
     • The field is made up of Concentric Circles which are perpendicular to the wire, with the wire in the centre
       • Changing the Direction of the Current changes the direction of the magnetic field. You can use the Right THUMB rule to figure out which way it goes.
         • The strength of the magnetic field can change with distance from the wire or if the current increases
   • A Solenoid is a Coil of Wire
     • You can increase the strength of a magnetic field by wrapping the wire into a coil called a solenoid
       • This happens because the field lines around each loop of the wire line up with each other. This results in lots of field lines pointing in the same direction
         • You can increase the strength of a solenoid by adding more turns or by increasing the current
           • You can also add an iron core which increases the strength, this also turns it into an Electromagnet
   • Electromagnets are useful because they can be switched on and off
     • You can vary the strength
       • Used in RCD's, Relays and to pick up stuff in a scrapyward

3. Motor Effect

   • A current in a magnetic field experiences a force.
     • You can calculate the Size of the Force by Multiplying the Magnetic Flux Density, Length and Current
   • We can find the Direction of the force by using FLEMING'S LEFT HAND RULE
     • The Thumb represents Motion, First Finger represents Field and the Second Finger represents Current
   • A Current-Carrying Coil of Wire Rotates in a Magnetic Field
     • If you inverse the polarity of a DC Motor, it spins the otherway
   • Generator Effect
     • The Generator Effect induces a potential difference in a conductor
       • This can be done by two ways
         • Moving Magnet in a Coil of Wire
         • Moving Coil of Wire in a Magnet
     • Induced Current Opposes the Change that Made it (Lenz's Law)
     • Increasing the speed of movement or by increasing the strength of the magnetic field helps make a bigger PD
     • Alternators Produce Alternating Current. They use Slip Rings (2x) and brushes so the contacts don't swap every half turn
       • Dynamos Produce Direct Current. Instead of Slip Rings, they have a Split-Ring Commutator which swaps the Connection Every half turn to keep the current flowing in the same direction
         • Alternating Current is more useful to transfer over Longer Distances however when they get into your house, lots of your appliances have a Diode Bridge Rectifier which changes it into DC. This is because it's more useful to have a constant current for an electrical appliance.
           • It is a lot easier and cheaper to convert AC into DC. To do this, you just need 4 Diodes and a Capacitor. However, DC into AC is a lot harder to do and more expensive.
             • Finally, you can use Transformers to Change AC Voltage, which makes it easy to reduce the 400kV to 230V. Overhead cable lines to household voltage respectively.
               • Transformers
                 • Transformers Only work for Alternating Current
                   • When an Alternating PD is applied accross the primary coil, the iron core magnetises and demagnetises really quickly. This changing magnetic field induces an Alternating PD in the Secondary Coil
                 • They change the PD and Current in AC
                 • They all have two coils of wire, the Primary and Secondary Coil
                   • The Primary Coil is the Input and the Secondary Coil is the Output
                     • If there is more Coils on the Secondary Coil, compared to the Primary then it's known as a STEP-UP Transformer, stepping up the Voltage but decreasing the current
                       • If there is fewer Coils on the Secondary Coil, compared to the Primary then it's known as a STEP-DOWN Transformer, stepping Down the Voltage but increasing the current
                         • The Transformer Equation is the Following: Vp/Vs=np/ns
                           • Vp is PD across the Primary Coil (input). Vs is PD Across the Secondary Coil (Output). np is the number of turns across the Primary Coil and ns is the number of Turns Across the Secondary Coil.
                             • This Equation is a Ratio which means you can flip the WHOLE Equation Upside Down and still use it. You can't flip one side upside down but not the otherside. You must flip the whole thing
                               • Vs/Vp=ns/np
                               • We assume that Transformers are 100% efficient. However, NOTHING that's Electrical is 100% Efficient, some of it will be lost through heat.
                                 • Assuming it's 100% Efficient we use the Base Equation P=IV to calculate the PD and Current across the two coils.
                                   • VpxIp=VsxIs
                                     • This states that the Power of the Input MUST BE THE SAME as the Power on the Output
                                       • This has to be true as Energy Can't be CREATED or DESTROYED
                                         • This then Links to a Previous Statement that I made saying that If the Voltage Increases then the Current Must Decrease. This is because if the voltage increases and the current doesn't decrease, all of sudden you'll have more energy and you can't just randomly create energy. So to compensate for the Rise in Voltage the Current Must Decrease. To keep the energy levels the same on the input and output coils