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Charging – Revision Pack (P6) Diode Characteristics: You would set up the diode circuit as you would for a resistor or bulb. Most diodes need a voltage of 0.6V before they start to work. When a diode is connected so current can pass (voltage is high enough), it is forward biased. When a diode is connected so that current CANNOT pass (voltage is too low), it is reverse biased. An AC supply to a single diode produces a halfwave rectified output. REMEMBER – AC input means DC output A resistor is always used in series with a diode to protect the diode. How a Diode Works: A diode consists of an n-type semiconductor and a p-type semiconductor joined together. The n-type has an excess of electrons, while the p-type has a shortage of electrons – the gaps are called ‘holes’. The space either side of the junction (between the two semiconductors) has NO electrons or holes. If you were going to connect the positive terminal of a power supply to the n-type semiconductor, then the space in the junction would widen and NO current passes. If you were going to connect the positive terminal of a power supply to the p-type semiconductor, them the space in the junction would narrow, eventually disappear and a current passes. Rectifier Circuits: Four diodes can be arranged to make a bridge circuit. The addition of a capacitor makes the output smoother. Charging – Revision Pack (P6) B C A D Rectification: In the bridge circuit, during the positive half-cycle, the current passes from A to B to the external circuit, to D then C, then back to the AC supply. During the negative half-cycle, the current passes from C to B, to the external circuit, to D then A then back to the AC supply. This makes sure that the DC output is always positive at B and negative at D. Storing Charge: The chemicals in a battery continue to produce energy for a battery to use. When the chemicals are all used up, the battery no longer works. The capacitor stores electrical energy. There is NO continual energy source. Smoothing: A smoothing capacitor acts as a reservoir. When the DC voltage from the rectifier circuit falls, the capacitor supplies current to the input. The capacitor charges near the peak value of the varying DC (just as it’s running out of steam). Past Papers: PPQ(1): Charging – Revision Pack (P6) PPQ(2): Continued on next page... Charging – Revision Pack (P6) PPQ(3): Charging – Revision Pack (P6) PPQ(4): Charging – Revision Pack (P6) PPQ(5): Continued on next page... Charging – Revision Pack (P6) PPQ(6): Charging – Revision Pack (P6) Mark Schemes: PPQ(1): PPQ(2): PPQ(3): PPQ(4): PPQ(5): Charging – Revision Pack (P6) PPQ(6): Truth Tables: When there are several logic gates combined together, truth tables can be used as a simple way to work out what happens within the system; for example: A 0 1 0 1 1 1 0 0 Input Signals B C 0 0 0 1 1 1 1 1 1 1 0 1 1 1 0 0 D 0 0 0 0 1 1 1 1 Output Z 0 0 0 0 1 1 1 0 REMEMBER – work step-by-step! Switching Logic Gates: Switches, LDR’s and thermistors are all used in a potential divider circuit to give logic gates a high input. Using the switch as an example, when it is open, he input to the logic gate is low (0). However, when the switch is closed, the input is connected directly to the 5V supply – here, the input is high (1). Thermistors and LDR’s work in the same way; just use ideas about resistance rather than it being open or closed. NOTE – if a variable resistor was used in place of a fixed resistor, a threshold level can be set. Charging – Revision Pack (P6) How a Relay Works: STEP 1 - When a small current passes through a coil, the iron armature is attracted STEP 2 – the armature pivots and pushes an insulating bar against the central contact STEP 3 – the central contact moves, opening the normally closed contacts and closing the normally open contacts. Normally open contacts Common Normally Closed contacts This represents a coil of wire Current output from logic gates is low BUT can be passed through a relay to switch on a larger current needed for motors, heaters etc. Furthermore, the relay also isolates the logic circuit from the HIGH current to avoid damage to the logic gates. Charging – Revision Pack (P6) Past Papers: PPQ(1): Charging – Revision Pack (P6) Continued on next page... Charging – Revision Pack (P6) PPQ(2): PPQ(3): Charging – Revision Pack (P6) PPQ(4): Charging – Revision Pack (P6) PPQ(5): PPQ(6): Continued on next page... Charging – Revision Pack (P6) Charging – Revision Pack (P6) Mark Schemes: PPQ(1): PPQ(2): PPQ(3): PPQ(4): Charging – Revision Pack (P6) PPQ(5): PPQ(6): Inducing Voltages: When a wire is held stationary between the poles of a magnet, there is NO current in the wire. If the wire is moved upwards, or the magnet is moved downwards, a voltage is induced in the wire and current flows. When the wire moves downwards, or the magnet moves upwards, the induced voltage is reversed and the current flows in the opposite direction. Charging – Revision Pack (P6) If you reverse the direction of the magnetic field, you will also reverse the direction of the induced voltage and current. Whenever a magnetic field changes, a voltage is ALWAYS created. You can control the size of the induced voltage; it is dependent on the rate at which the magnetic field changes: - If the wire is moved really quickly, then the induced voltage will be higher If the wire is moved slowly, the induced voltage will be lower AC Generators: In a power station generator, the magnetic field inside the coil is produced by electromagnets. The electromagnetic is made from coils of wire (rotor coils) which are turned by the turbine. An alternating current is generated in the stator coils which surround the rotor coils. If you increase the speed at which the electromagnet rotates: - The current induced in the stator coils will also increase The frequency of the generated voltage is also increased If you increase the number of turns on the electromagnet: - You increase the magnetic field (and as such) induce a larger voltage in the stator coils Some AC generators have a coil rotating between the poles of a magnet. As the coil rotates, the direction of the current reverses every half turn to ensure that the coil rotates in the same direction. Slip rings are connected to the ends of the coil to allow the coil to spin without winding the wire around itself. The brushes are contacts that the touch the slip rings to complete the circuit. Charging – Revision Pack (P6) Past Papers: PPQ(1): Charging – Revision Pack (P6) PPQ(2): Charging – Revision Pack (P6) Mark Schemes: PPQ(1): Charging – Revision Pack (P6) PPQ(2): Transistors: In a transistor, a small base current can switch on a GREATER current through the collector. To work out the current coming out of the emitter: Ie = I b + Ic Logic Gates: Type of Gate AND OR NOT Symbol Explanation To give a high output, both A and B must have a high output or be on. AND gates are like two switches in series; the circuit can’t work if one switch is off. To give a high output, one of A or B must have a high output or be on. OR gates are like two switches in parallel; only one switch needs to be on for the circuit to work. To give a high output, the input must be low or completely off. NOT gates will give the opposite of the (single) input – the output is not what the input is. Charging – Revision Pack (P6) Making an AND gate: Two transistors can be combined to make an AND gate. 1) A small input at A will switch on the upper transistor. 2) A small input at B will switch on the lower transistor. 3) A current at A and at B are needed to switch on the 6V supply. The other logic gates can be made by using different combinations of transistors. Transistor circuits ALWAYS have a high value resistor in the base circuit to limit the current. NAND and NOR gates: Type of Gate NAND Symbol NOR Explanation It behaves like an AND gate followed by a NOT gate. It behaves like an OR gate followed by a NOT gate. Truth Tables: Truth tables show how logic gates behave. - When there is some input (or output), a 1 is entered in the table When there is NO input (or output), a 0 is entered in the table AND Gate: Input A 0 1 0 1 OR Gate: Input B 0 0 1 1 Output 0 0 0 1 NAND Gate: Input A 0 1 0 1 Input B 0 0 1 1 Input A 0 1 0 1 Input B 0 0 1 1 NOR Gate: Output 1 1 1 0 Output 0 1 1 1 Input A 0 1 Charging – Revision Pack (P6) 0 1 NOT Gate: Input A 0 1 Output 1 0 Past Papers: PPQ(1): PPQ(2): Input B 0 0 1 1 Output 1 0 0 0 Charging – Revision Pack (P6) PPQ(3): PPQ(4): Charging – Revision Pack (P6) Mark Schemes: PPQ(1): PPQ(2): PPQ(3): PPQ(4): Charging – Revision Pack (P6) Magnetic Fields: A magnetic field can be created if an electric current moves through a coil of wire. The direction of a field around a wire can be found using the right hand grip rule. You should imagine you’re wrapping your RIGHT hand around a wire. Your thumb should point towards the direction of the current. Your other fingers will then give the direction of the magnetic field (see left). The field pattern of a long coil of wire is similar to that of a regular bar magnet; opposite forces attract, and like-forces repel. When a wire is placed between the poles of a magnet, the wire moves out of the gap when the current is switched on. The X shows us that the current is flowing into the paper; it looks like the back of an arrow that we’ve just fired. The dot (.) shows us that the current is flowing away from the paper; it looks like an arrow is coming towards us. Motor Direction: Fleming’s left hand rule can be used to predict the direction in which a motor turns. Your Thumb = Motion Your First Finger = Field Your Second Finger = Current The direction of current, magnetic field and motion are all at right angles to each other. Charging – Revision Pack (P6) Turning Coils: When a current passes through a coil, placed between the poles of a magnet, there is a force on each side of the coil. If you use Fleming’s Left Hand Rule, you can see that the motion (or forces) on each side of the coil are opposite in direction. As one side is forced up, the other is forced down – this means that the coil starts to spin. As you can see, because the direction of current changes on each side, the force or motion also changes, making the coil spin. You can make the coil spin faster by: - Increasing the number of turns on the coil Increasing the size of the current Increasing the strength if the magnetic field Practical Motors: The job of a commutator is to make the coil continue to spin. The direction of current in the coil is revered every half turn; this ensures that the force on the coil is ALWAYS in the same direction. The magnets in practical motors have curved poles to give a radial magnetic field. A radial field increases the force on the coil and keeps it constant as the motor turns. It also ensures that the field lines are always 90o to the coil. Additional NOTES: If the direction of the current is reversed, then the wire moves in the opposite direction – this is the same for reversing the direction of the magnetic field. Charging – Revision Pack (P6) Past Papers: PPQ(1): Charging – Revision Pack (P6) PPQ(2): PPQ(3): Charging – Revision Pack (P6) PPQ(4): Continued on next page... Charging – Revision Pack (P6) PPQ(5): Charging – Revision Pack (P6) PPQ(6): PPQ(7): Charging – Revision Pack (P6) Mark Schemes: PPQ(1): Charging – Revision Pack (P6) PPQ(2): PPQ(3): PPQ(4): PPQ(5): Charging – Revision Pack (P6) PPQ(6): PPQ(7): Resistance and Current: Higher resistance means lower current (flow of electrons). The higher the current, the brighter the bulb is in the circuit. The higher the current, the faster a motor turns. NOTE – voltage is needed to make the current flow through a circuit – it pushes the current along (as seen to the left). Resistance (R) = Voltage (V) / Current (I) R = Ohms, V = Volts, I = Amps What affects resistance? The resistance of a wire is dependent upon its length. A variable resistor (symbol to the left) works by changing the lengths of wire in the circuit; the longer the wire, the greater the resistance. Ohmic and Non-Ohmic Devices: Charging – Revision Pack (P6) Ohm’s law is the idea that the current travelling through something is directly proportional to the voltage across it; if you double the voltage, then the current will also double. Resistors, at a constant temperature, follow Ohm’s law – the graph is a straight line passing through the origin. The steeper the gradient of the graph (left), the greater the resistance – this is because if the gradient is steeper, the current will be less and the voltage more. The same graph for a bulb will NOT be a straight line passing through the origin. The gradient of the line increases as current increases (it curves positively) – it does NOT follow Ohm’s Law. The resistance of a filament lamp increases as current increases. When electrons collide with the filament (a thread-like fibre), it makes the atoms vibrate more. The increased vibration means two things: - An increased number of collisions, which leads to more resistance An increase in the temperature of the filament In a fixed resistor, the gradient of the graph is equal to the resistance. The gradient of a voltage-current graph for a filament bulb will increase as resistance increases. To find the resistance, instantaneous results from the graph must be used. A non-Ohmic device shows a curve with an increasing gradient, which means an increased resistance. If the voltage supplied across a non-Ohmic device is increased, the electrons carry more energy. When these electrons collide with the metal atoms in the conductor, more energy is transferred and this increases the resistance and the temperature (this can be seen to the left). NOTE – the cause of electrical resistance is the electrons (charge carriers) colliding with the metal atoms as they flow through a metal conductor, like a wire. Charging – Revision Pack (P6) Past Papers: PPQ(1): Charging – Revision Pack (P6) Continued on next page... Charging – Revision Pack (P6) PPQ(2): Continued on next page... Charging – Revision Pack (P6) (This circuit had a bulb) Continued on next page... Charging – Revision Pack (P6) PPQ(3): Charging – Revision Pack (P6) PPQ(4): Charging – Revision Pack (P6) PPQ(5): Charging – Revision Pack (P6) Mark Schemes: PPQ(1): Charging – Revision Pack (P6) PPQ(2): PPQ(3): PPQ(4): PPQ(5): Charging – Revision Pack (P6) Potential Divider Circuits: A potential divider is a simple circuit which uses resistors (as well as LDR’s and thermistors) to supply a variable potential difference (or output voltage). Some of these circuits have just fixed resistors – in this instance, the output voltage is a fixed proportion to the supply voltage; it’s a bit like ratios. If your R1 and R2 values are the same, then the output value will be half of the input voltage – you’ll understand why when you see the equation below. Calculating Output Voltage: Some of these circuits have one variable resistor and one fixed resistor – the output voltage can be altered using this. VOUT = VIN x (R2 / R1 + R2) When your R2 value is much greater than your R1 value, the output voltage will be approximately the same as the input voltage. When your R2 value is much less than your R1 value, the output voltage will be approximately zero. Some electrical components will only start working at a threshold voltage – when you use a variable resistor as part of a potential divider, you can set this threshold voltage. Resistors in parallel: We can arrange resistors in parallel and this will decrease the total resistance; the equation to work out total resistance is shown below: 1/RT = 1/ R1 + 1/ R2 + 1 / R3 LDR’s and Thermistors: Charging – Revision Pack (P6) The resistance of an LDR decreases as light intensity increases. The resistance of a thermistor decreases as temperature increases. Uses of Potential Divider Circuits: Street lights use LDR’s in potential divider circuits this is placed in the R2 position (as seen to the left). When it’s bright, the resistance of the LDR (R2) is very low. The R1 fixed resistor is made to have a higher resistance than the LDR – this will mean that the output voltage will be very low and the street light will NOT be on. When it gets dark, the resistance of the LDR will increase. It will increase so that it is more than the R1 value. This will mean that the output voltage will increase, and the street light will turn on. A thermistor can be used to switch on a heater when the temperature falls too much in a home – it uses the above principal. Charging – Revision Pack (P6) Past Papers: PPQ(1): Charging – Revision Pack (P6) PPQ(2): Charging – Revision Pack (P6) PPQ(3): Continued on next page... Charging – Revision Pack (P6) Charging – Revision Pack (P6) PPQ(4): PPQ(5): Charging – Revision Pack (P6) PPQ(6): Charging – Revision Pack (P6) Mark Schemes: PPQ(1): PPQ(2): PPQ(3): PPQ(4): PPQ(5): PPQ(6): Charging – Revision Pack (P6) Transformer Design: A transformer consists of two coils of wire wound round an iron core. The input AC voltage is connected to the primary coil, while the output AC voltage is obtained from the secondary coil (see diagram). Step-down transformers have MORE turns on the primary coil. Step-up transformers have MORE turns on the secondary coil. How transformers work: The changing current in the primary coil creates a changing magnetic field in the iron core. The changing magnetic field in the iron core induces a changing voltage in the secondary coil: Voltage across primary (Vp) = Number of turns on primary coil (Np) Voltage across secondary (Vs) Number of turns on secondary coil (Ns) An isolating transformer has THE SAME number of turns on the primary and secondary coil. The mains supply is hidden in an isolating transformer. The output terminals are NOT live so there is no danger of you getting electrocuted if you touch them with wet hands. An ordinary circuit is not used because it would be live and could electrocute you if you touched it with a wet hand AND the water and steam in bathrooms could cause damage to the house’s wiring system. Energy Loss: When a current passes through a wire, the wire gets hot. The transformer coils and overhead power lines of the national grid get hot and lose energy to the surroundings. Reducing Transmission Loses: Power is a measure of how fast energy is used up. Power Loss = (current2 x resistance) The electrical power supplied to the primary coil of a transformer is dependent on the input voltage and current: Charging – Revision Pack (P6) Pp = Vp x Ip Similarly, the output power of the secondary coil is given by: Ps = Vs x Is If the transformer is 100% efficient, then the input voltage = the output voltage. In a step-up transformer, increasing the voltage leads to a decrease in current by the same factor. This assumes that the transformer is 100% efficient; e.g. If the voltage was stepped up by a factor of 16 to 400kV, then the current in the overhead cables would reduce to 1/16 of what it would be if electricity was transmitted at 25,000V. (25,000 x 16 = 400,000V or 400kV). Past Papers: PPQ(1): Charging – Revision Pack (P6) PPQ(2): Charging – Revision Pack (P6) Charging – Revision Pack (P6) PPQ(3): Charging – Revision Pack (P6) PPQ(4): PPQ(5): Charging – Revision Pack (P6) Mark Schemes: Charging – Revision Pack (P6) PPQ(1): PPQ(2): PPQ(3): PPQ(4): Charging – Revision Pack (P6) PPQ(5): PPQ(6)