P6 - Electricity for gadgets

P6 - Electricity for gadgets

1. Electric circuits

   • Potential dividers consist of 2 resistors used to produce a specific voltage
   • A variable resistor is used to set the threshold level for an output voltage
   • A diode only allows a current to pass in one direction
   • Diodes are made from n-type and p-type semi-conductors joined together
   • Diodes can be arranged as a rectifier to change AC into DC
   • A capacitor stores electric charge
   • A capacitor smooths the output from a rectifier
   • Resistors control the current in a circuit
   • RESISTANCE = VOLTAGE / CURRENT
   • The resistance of a wire increases as the wire gets hotter
   • Longer wires have more resistance than smaller wires
   • The resistance of an ohmic conductor is found from the gradient of the voltage against current graph
   • The resistance of a light dependent resistor (LDR) decreases as the light intensity increases
   • The resistance of a thermistor decreases as the temperature increases
   • LDR's and thermistors can be used as one of the resistors in a potential divider

2. Logic circuits

   • In a transistor a small base current can switch on a greater current through the collector
   • 2 transistors can be combined to make an AND gate
   • Other logic gates can be made with different combinations of transistors
   • Transistor circuits always have a high value resistor in the base circuit to limit the current
   • The behavior of NOT, AND, OR, NAND and NOR gates can be described using truth tables
   • Inputs and outputs are described as high (1) and low (0)
   • Logic gates work at low voltages (5-6 volts)
   • Inputs to logic gates can be controlled by potential divider circuits
   • A relay uses a small current to switch a larger current
   • The output from a logic gate is a small current
   • A relay is often used to allow this to switch on a motor, heater etc.

3. Electromagnetism

   • A transformer changes the size of an AC voltage
   • If the transformer is 100% efficient
     • Power output = Power input
   • Electricity is transmitted at high voltages to reduce the current
     • Hence reduce energy loss in overhead power lines
       • POWER LOSS = CURRENT2 X RESISTANCE
   • In a generator a current is induced in a coil of wire when there is a changing magnetic field
   • The quicker the field changes, the greater the current
   • Slip rings allow AC to be generated
   • Brushes are contacts that touch the slip rings and complete the circuit
   • A current carrying wire has a magnetic field around it
   • A motor moves because of the interaction between the magnetic field
     • Due to current carrying coil and a permanent magnet
   • Flemmings left hand rule
     • Indicates the direction of the current
   • A commutator makes sure the motor keeps spinning in the same direction
   • A motor spins faster when
     • The magnetic field is stronger
     • The current is greater
     • There are more turns in the coil