# Physics P6

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## ELECTRIC CIRCUITS

• A potential divider consists of two resistors used to produce a specific voltage
• A variable resistor is used to set the threshold level for an output voltage
• The resistance of a light dependant resistor decreases as 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
• A diode only allows 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
• 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 shorter wires
• The resistance of an ohmic conductor is found from the gradient of the voltage against current graph
• A capacitor stores electric charge
• A capacitor smoothes the output from a rectifier
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## LOGIC CIRCUITS

• The behaviour of NOT, AND, OR, NAND, and NOR gates can be describes using truth tables
• Inputs and outputs are describes as high (1) and low (0)
• Logic gates work at low voltages (about 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 and a relay is often used to allow this to switch on a motor, heater ect..
• In a transistor a small base current can switch on a greater current through the collecor
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## LOGIC CIRCUITS CONTINUED

• Two 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

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## ELECTROMAGNETISM

•A current carrying wire has a magnetic field around it •A motor moves because of the interaction between the magnetic field due to a current carrying coil and a permanent magnet •Fleming's left hand rule allows you to predict the direction in which a motor turns •A commutator makes sure the motor keeps spinning in the same direction •Motors spin faster when : 1) the magnetic field is stronger. 2) the current is greater. 3) there are more turns on the coil •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 transformer changes the size of an AC voltage •Electricity is transmitted at high voltages to reduce the current and hence reduce the energy loss in the overhead power lines

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