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Electricity W Richards Worthing High School Basic ideas… Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____. Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______. Resistance is anything that resists an electric current. It is measured in _____.” (Words: volts, amps, ohms, voltage, ammeter, voltmeter) More basic ideas… If a battery is added the current will ________ because there is a greater _____ on the electrons If a bulb is added the current will _______ because there is greater ________ in the circuit Current in a series circuit If the current The here is 2 current amps… here will be… The current And the here will current be… here will be… In other words, the current in a series circuit is THE SAME at any point Current in a parallel circuit A PARALLEL circuit is one where the current has a “choice of routes” Here comes the current… Half of the current will go down here (assuming the bulbs are the same)… And the rest will go down here… Current in a parallel circuit If the current here is 6 And the amps current here will be… The current here will be… The current The current here will be… here will be… Voltage in a series circuit If the voltage V across the battery is 6V… …and these bulbs are all identical… …what will the V V voltage across each bulb be? 2V Voltage in a series circuit If the voltage V across the battery is 6V… …what will the voltage across V two bulbs be? 4V Voltage in a parallel circuit If the voltage across the batteries is 4V… What is the voltage here? 4V V And here? 4V V Summary In a SERIES circuit: Current is THE SAME at any point Voltage SPLITS UP over each component In a PARALLEL circuit: Current SPLITS UP down each “strand” Voltage is THE SAME across each”strand” An example question: 6V A3 3A A1 V1 A2 V2 V3 Advantages of parallel circuits… There are two main reasons why parallel circuits are used more commonly than series circuits: 1) Extra appliances (like bulbs) can be added without affecting the output of the others 2) If one appliance breaks it won’t affect the others either Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. Georg Simon Ohm 1789-1854 The resistance of a component can be calculated using Ohm’s Law: V Resistance = Voltage (in V) (in ) Current (in A) I R An example question: Ammeter reads 2A A V 1) What is the resistance across this bulb? 2) Assuming all the bulbs are the Voltmeter same what is the total resistance reads 10V in this circuit? Current-voltage graphs I I 2. Bulb V V I 1. Resistor 3. Diode V Explain the shape of each graph Three simple components: 1) Diode – only lets current flow in one direction 2) Light dependant resistor – resistance DECREASES when light intensity INCREASES 3) Thermistor – resistance DECREASES when temperature INCREASES Wiring a plug DC and AC V DC stands for “Direct Current” – the current only flows in one direction: Time 1/50th s AC stands for “Alternating 240V Current” – the current changes direction 50 times every second (frequency = T 50Hz) V Fuses Fuses are _______ devices. If there is a fault in an appliance which causes the ____ and neutral (or earth) wire to cross then a ______ current will flow through the _____ and cause it to _____. This will break the _______ and protect the appliance and user from further _____. Words – large, harm, safety, melt, live, circuit, fuse Circuit breakers If the current becomes too high the __________ is activated. This will ______ the iron and the contact will be _______. This will break the circuit. Circuit breakers have two main advantages over fuses: they work ______ and can easily be ______. Words – electromagnet, broken, attract, reset, quicker Earth wires Earth wires are always used if an appliance has a _____ case. If there is a _____ in the appliance, causing the live wire to ______ the case, the current “_______” down the earth wire and the ______ blows. Words – fuse, fault, metal, surges, touch Power and fuses Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: P Power = voltage x current in W in V in A V I Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A). Power and fuses Copy and complete the following table: Appliance Power rating Voltage (V) Current Fuse needed (W) needed (A) (3, 5 or 13A) Toaster 720 240 Fire 2000 240 Hairdryer 300 240 Hoover 1000 240 Computer 100 240 Stereo 80 240 Charge (Q) As we said, electricity is when electrons move around a circuit and carry energy with them. Each electron has a negative CHARGE. Charge is measured in Coulombs (C). We can work out how much charge flows in a circuit using the equation: Charge = current x time Q (in C) (in A) (in s) I T Example questions Charge (C) Current (A) Time (s) 5 2 0.4 1 20 0.5 50 250 3 60 1) A circuit is switched on for 30s with a current of 3A. How much charge flowed? 2) During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? 3) A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed? Energy and charge The amount of energy that flows in a circuit will depend on the amount of charge carried by the electrons and the voltage pushing the charge around: Energy transferred = charge x voltage (in J) (in C) (in V) E V Q Example questions 1) In a radio circuit a voltage of 6V is applied and a charge of 100C flows. How much energy has been transferred? 2) In this circuit the radio drew a current of 0.5A. How long was it on for? 3) A motor operates at 6V and draws a current of 3A. The motor is used for 5 minutes. Calculate: a) The motor’s resistance, b) the charge flowing through it, c) the energy supplied to it 4) A lamp is attached to a 12V circuit and a charge of 1200C flows through it. If the lamp is on for 10 minutes calculate a) the current, b) the resistance, c) the energy supplied to the bulb. Comparing magnets and solenoids Magnet: N S Solenoid: Electromagnets The strength of an electromagnet can be increased by doing three things: 1) Increasing the voltage 2) Increasing the number of coils 3) Inserting an iron core (The poles can be reversed by reversing the direction of the current) Electromagnetic induction The direction of the induced current is reversed if… 1) The magnet is moved in the opposite direction 2) The other pole is inserted first Electromagnetic induction The size of the induced current can be increased by: 1) Increasing the speed of movement 2) Increasing the magnet strength 3) Increasing the number of turns on the coil AC Generator Induced current can be increased in 4 ways: 1) Increasing the speed of movement 2) Increasing the magnetic field strength 3) Increasing the number of turns on the coil 4) Increasing the are of the coil The National Grid Electricity reaches our homes from power stations through the National Grid: Step up Step down Power station Homes transformer transformer If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much ______ loss by the time electricity reaches our homes. This is because the current is ___. To overcome this they use devices called transformers to “step up” the voltage onto the power lines. They then “____ ____” the voltage at the end of the power lines before it reaches our homes. This way the voltage is _____ and the current and power loss are both ____. Words – step down, high, power, low, high Transformers Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil. Words – alternating, magnetic field, induce, step up, voltage We can work out how much a transformer will step up or step down a voltage: Voltage across primary (Vp) No. of turns on primary (Np) Voltage across secondary (Vs) No. of turns on secondary (Ns) Static electricity Static electricity is when charge “build up” on an object and doesn’t move, e.g. rubbing a rod: Two different rods will attract each other if they have different charge: Two rods made of the same material will repel each other due to having the same charge: Van de Graff generators A charge builds up on the dome due to electrons being “______ off” by the belt. If a big enough ______ is built up then the voltage becomes high enough to _____ the air molecules and the electrons “___” down to Earth – this is an electric _________. Words – charge, jump, current, rubbed, ionise Use of static 1 - Photocopiers Photocopiers use static electricity. They work by: 1) Copying an ______ of the page onto a ______ plate, 2) Light then causes the charge to ____ away, leaving an “electrostatic impression” of the page, 3) The charges left on the plate ______ small drops of black powder, 4) The powder is transferred from the plate onto the _____, 5) The paper is _____ to “fix” the powder. Words – heated, leak, paper, image, charged, attract Use of static 2 - Printers Inkjet printers work by spraying charged drops of ink onto a page. The droplets can be directed using two oppositely charged plates. The voltage on these plates can be easily swapped or varied. The inkjet cartridge can also moved across the page by the printer Dangers of static During refuelling the fuel gains electrons from the pipe, making the pipe positive and the fuel negative. The resulting voltage may cause a spark – bad news! Solution: Either earth the fuel tank with a copper rod or connect the tanker to the plane by a copper conductor. Electrolysis Electrolysis is used to extract a HIGHLY REACTIVE metal. When we electrolysed copper chloride the negative chloride ions moved to the positive electrode and the positive copper ions moved to the negative electrode – OPPOSITES ATTRACT!!! = chloride ion = copper ion Some example questions 1) A current of 2A flows through some copper chloride for 1 minute and 0.01g of copper is deposited at the negative electrode. a) How much would be deposited if the current was increased to 6A? b) How much would be deposited if the current was kept at 6A and the experiment was left for another minute? c) How much charge flowed in question (b) above? 2) A current of 0.05A flows through some copper chloride for 500 seconds and 0.05g of chlorine is released at the positive electrode. a) How much would be deposited if the current was increased to 0.1A? b) How much would be deposited if the current was kept at 0.1A and the experiment was left for 250 seconds instead? c) How much charge flowed in question (b) above?
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