P5.

P5.

1. Electricity

   • Potential Difference
     • Measures the difference in energy between two points in a circuit, measured by a Voltmeter
       • same as voltage, measured in V
       • Usually measured before and after a bulb or resistor
   • Static
     • Electrons move from one object to another and this makes the charges change
     • E.g. rubbing hair on balloon means that negative particles/electrons are passed to the hair. Hair becomes negatively charged and repel each other. Hair is attracted to positively charged balloon.
   • Current
     • a.c.= Alternating current
     • d.c.=Direct current
     • Measured in Amps
     • Moving electric charges
   • Resistance
     • Measured in Ohms
     • resistance = Voltage (V) / Current (A)
     • Resistors get hot when current flows through them (e.g. lamp filament is caused to glow)
     • 2 resistors in series have more resistance than one on its own
   • Power
     • P (W) = current (A) x voltage (V)

2. Circuits

   • Series
     • The current will be the same at all points in the circuit
     • p.d. across all components added together = p.d. across battery
     • p.d. is biggest across the component with the most resistence
     • changing the resistance of one component changes p.d. across all other components
   • Parallel
     • current halves when the wire splits to go down two possible routes
     • Current is largest through the component with smallest resistance
     • Battery pushes more current through a component with a smaller resistance (because it is easier)
     • p.d. across all components added together = p.d. across battery

3. Generating Electricity

   • Electromagnetic Induction
     • When a magnet is moved in and out of a coil of wire
     • Voltage is induced at ends of coils of wire
     • Produces d.c.
   • Electric Motors
     • Convert electrical to kinetic energy
     • A coil of wire is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves.
     • The direction of the current is reversed every half turn using a split ring or commutator.
     • left hand rule
   • Generators
     • right hand rule
     • Increasing induced current
       • amount of turns in coil
       • speed of coil rotation
       • strength of magnets
       • Iron core
     • Induce a current by spinning a coil of wire inside a magnetic field, or by spinning a magnet inside a coil of wire
     • Makes a.c.

4. Transformers

   • Changes the voltage of an AC supply.
     • Only works with a.c.
     • Vp/Vs = Np/Ns
       • Vp/Vs = voltage of primary coil/secondary coil Np/Ns = Number of turns in the primary/secondary coil
         • Step-up = more turns on the secondary coil
         • Step-down = fewer turns on the secondary coil
   • A pair of coils wound on an iron core. The AC in primary coil produces a changing current and therefore changing magnetic field, inducing a voltage in the secondary coil.
   • No electrical connection between the primary and the secondary coils.

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