P3 - Electricity and Magnetism

P3 - Electricity and Magnetism

1. Magnets

   • Any material that is attracted or repelled by another magnet
   • Magnetic Material
     • Any material that is attracted or repelled by another magnet Eg : iron, cobalt, nickel
   • Properties of Magnets
     • Dipole - N and S pole Like poles repel, unlike poles attract Strength of magnet lies at poles and gets weaker at middle When a magnet is broken, each piece behaves like a magnet
   • Magnetic Field
     • An area around a magnet where another magnet experiences a force
     • Magnetic Field Lines
       • Pictoral representation of a magnetic field, vary according to the poles, always go from north to south 
   • Induced Magnetism
     • The process where a magnet causes a magnetic material to start behaving like a magnet when brought in close proximity
   • Types of Magnets
     • Permanent
       • Always a magnet  Eg : bar magnet
     • Temporary Magnet
       • Only a magnet in certain conditions, due to external factors Eg : electromagnet
     • Soft and Hard Magnets
       • Soft magnetic materials are materials that can have a magnetic field induced very easily, but they dont stay magnetised for long Eg : Iron Hard magnetic materials are materials which take longer to be magnetised but stay magnetised for longer Eg : Steel

2. Electricity

   • Charge
     • When a neutral atom loses or gains electrons, it gains a charge. Either positive or Negative, Represented as Q, with units C (coloumb)
     • Properties of Charge
       • Like charges repel Opposite charges attract
   • Conductors and Insulators
     • Conductors
       • Allows easy flow of electrons through material Eg : Metals, due to delocalised electrons allowing flow of electricity 
       • Charging a Conductor
         • Induction
           • Physical process where a charge is induced without the source and object touching. 
           • Induction Experiment
             • 1. Put two objects together  2. Place a negative charge carrying object near the objects - the electrons will move away from the object and only the positively charged particles will be left 3. Separate the two objects while keeping the object close 4. Remove the charge carrying object - each object will now have a charge
     • Insulators
       • Doesn't allow flow of electrons through Eg : rubber, plastic
       • Charging an Insulator
         • Only charges the surface by rubbing two insulators and causing electrons to jump from one object to another, thereby causing the surface of both objects to have a charge Direction of movement depends on nature of material
         • Experiment
           • Rubbing a glass rod with a silk cloth - the electrons jump from the rod to the cloth, causing the rod to have a positive charge and the cloth to have a negative charge
     • Discharging an Object
       • Sometimes you don't want an object to have a charge, but that charge must go somewhere
       • Grounding an Object
         • The earth has zero potential - will both give and take away electrons with no preference Grounding a negatively charged object - remaining electrons will flow into ground Grounding a positively charged object - electrons will flow from ground into object until neutral  Must require a pathway for the electrons to flow

3. Current Electricity

   • Current
     • Rate of flow of charge - Represented as I, but units are Amps or A I = Q/t (current = charge flowing/time)
   • Voltage
     • Potential difference = Energy (J)/ charge (C) V = E/Q or W/Q (work done/charge)Units and representation - V or Volts
     • Potential Difference Vs EMF
       • Potential difference is the amount of electrical energy converted to other forms of nergy between two points in a circuit  EMF is potential difference for the whole circuit - from one side of the cell/battery to the other
     • Measuring Amps and Volts
       • Ammeter
         • Device used to measure current, applied in series, not connected in parallel as resistance is low and would cause short circuit 
       • Voltmeter
         • Voltmeter - device used to measure voltage, applied in parallel, high resistance so electrons don't flow through voltmeter
   • Resistance
     • Difficulty offered to a flowing electron/difficuly electron faces while moving - usually applies to components. Represented as R, but units are Ohms and greek omega sign
     • Factors Affecting Resistance
       • Resistance is directly proportional to length of a wire Resistance is inversely proportional to thickness or diameter of wire R = L/A resistance = length/area
   • Conventional or Electron Flow
     • Conventional flow : natural convention of physics, what people thought the flow of electrons was, from positive to negative Electron flow : actual flow of electrons, from negative to positive
   • Ohm's Law
     • States that the voltage of a current carrying wire is proportional to the current if external factors (heat, humidity) remain constant V = IR  R is the constant in place, for resistance in the circuit, due to the requirement for a constant in a proportionality relationship

4. Series and Parallel

   • Series
     • Current has only one path to flow through OR each component is connected next to each other
     • Effect on Factors
       • Current - remains the same throughout the circuit Voltage - divided equally between components (if one component has more resistance, it gains more voltage) Resistance = total resistance is equal to sum of individual resistances
   • Parallel
     • Current has more than one path to flow OR each component is connected one line below another 
     • Effect on Factors
       • Current - divided equally across each branch  Voltage - stays the same across each branch  Resistance = 1/resistance of object, so total resistance is the sum of the inverse of the resistances  1/R = 1/R1 + 1/R2 and total resistance is lower than individual resistances
     • Advantages of Parallel
       • Each branch gets a full voltage supply If one branch goes out, the others can keep working, Each branch is operated independently 

5. Electrical Componenets

   • Anything that functions due to an electric current Symbols in divesh notes
   • Transducers
     • Devices which convert non-electrical energy into electrical energy 
     • LDR
       • Light dependant resistor - inversely proportional to light intensity, so as light intensity gets higher, resistance gets lower Application : automatic street lights
     • Thermistors
       • Same as LDR, but for temperature Resistance inversely proportional to temperature Application : fire alarms
   • Relays
     • Uses one circuit in order to trigger another circuit in some way. Eg : using a small switch/circuit in order to trigger a bigger, more powerful circuit

6. Dangers of Electricity

   • Damaged Insulation
     • Damaged insulation causes exposure to metal wires and electrical conductors Can result in electrical shocks, or impeding electrical signals in body, causing heart attacks or electric burns
   • Damp Environment
     • Using electrical equipment in the presence of water, as impure water with mineral ions is a good conductor of electricity 
   • Overheating of Cables
     • Overheating causeed by current flowing for a long time, resulting in malfunctioning of equipment Caused due to improper use of cables or overloading of socket
     • Heat Loss through Current
       • P = I^2R Higher current leads to more power loss - transmission cables minimise current to stop this
   • Protective Measures
     • Circuit Breakers
       • Electromagnetic switch that disables a circuit after a certain current/electrical threshold Can be switched off and on 
     • Fuses
       • Component with extremely thin wire inside, which melts if current is too high. Must be replaced and only usually found in appliances, not circuits Fuse tolerance should be kept slighly higher than appliance limit, due to variation in power supply. 

7. Electric Field

   • Space or region around a charge where another charge experiences a force, similar to magnetic field Always moves from North to South 

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