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10/05/2015Electricity W Richards Worthing High School.

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1 10/05/2015Electricity W Richards Worthing High School

2 10/05/2015 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 ______, a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____.” (Words: volts, amps, ohms, voltage, ammeter, voltmeter)

3 10/05/2015 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

4 10/05/2015 Current in a series circuit If the current here is 2 amps… The current here will be… And the current here will be… In other words, the current in a series circuit is THE SAME at any point

5 10/05/2015 Current in a parallel circuit A PARALLEL circuit is one where the current has a “choice of routes” Here comes the current… And the rest will go down here… Half of the current will go down here (assuming the bulbs are the same)…

6 10/05/2015 Current in a parallel circuit If the current here is 6 amps The current here will be… And the current here will be…

7 10/05/2015 Voltage in a series circuit V VV If the voltage across the battery is 6V… …and these bulbs are all identical… …what will the voltage across each bulb be? 2V

8 10/05/2015 Voltage in a series circuit V V If the voltage across the battery is 6V… …what will the voltage across two bulbs be? 4V

9 10/05/2015 Voltage in a parallel circuit If the voltage across the batteries is 4V… What is the voltage here? And here? VV 4V

10 10/05/2015Summary 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”

11 10/05/2015 An example question: V1V1 V2V2 6V 3A A1A1 A2A2 V3V3 A3A3

12 10/05/2015 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

13 10/05/2015 Georg Simon Ohm Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using Ohm’s Law: Resistance = Voltage (in V) (in  )Current (in A) V RI

14 10/05/2015 An example question: V A 1)What is the resistance across this bulb? 2)Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter reads 10V Ammeter reads 2A

15 10/05/2015 More examples… 12V 3A 6V 4V 2A 1A 2V What is the resistance of these bulbs?

16 10/05/2015 Current-voltage graphs I V I V I V 1. Resistor 3. Diode 2. Bulb Explain the shape of each graph

17 10/05/2015 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

18 10/05/2015 Wiring a plug Earth wire Neutral wire Insulation Live wire Fuse Cable grip

19 10/05/2015 DC and AC DC stands for “Direct Current” – the current only flows in one direction: AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz) 1/50 th s 240V V V Time T

20 10/05/2015Fuses 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

21 10/05/2015 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

22 10/05/2015 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

23 10/05/2015 Power and fuses Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: P IV Power = voltage x current in W in V in A 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).

24 10/05/2015 Power and fuses Copy and complete the following table: AppliancePower rating (W) Voltage (V)Current needed (A) Fuse needed (3, 5 or 13A) Toaster Fire Hairdryer Hoover Computer Stereo80240

25 10/05/2015 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: Q TI Charge = current x time (in C) (in A) (in s)

26 10/05/2015 Example questions Charge (C)Current (A)Time (s) )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?

27 10/05/2015 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: E QV Energy transferred = charge x voltage (in J) (in C) (in V)

28 10/05/2015 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.

29 10/05/2015 Comparing magnets and solenoids N S Magnet: Solenoid:

30 10/05/2015Electromagnets 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)

31 10/05/2015 Examples of electromagnets 1. Speaker 2. Relay switch

32 10/05/2015 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

33 10/05/2015 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

34 10/05/2015 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 area of the coil

35 10/05/2015 The National Grid Electricity reaches our homes from power stations through the National Grid: 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 ____. Power station Step up transformer Step down transformer Homes Words – step down, high, power, low, high

36 10/05/2015Transformers 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. Voltage across primary (V p ) No. of turns on secondary (N s )Voltage across secondary (V s ) No. of turns on primary (N p ) Words – alternating, magnetic field, induce, step up, voltage We can work out how much a transformer will step up or step down a voltage:

37 10/05/2015 Some transformer questions Primary voltage V p Secondary voltage V s No. of turns on primary N p No. of turns on secondary N s Step up or step down? 12V24V100?? 400V200V20?? 25,000V50,000V1,000?? 23V230V150??

38 10/05/2015 Some example questions Primary voltage V p Secondary voltage V s No. of turns on primary N p No. of turns on secondary N s Step up or step down? 6V24V100?? 400,000V200V?1,000? 25,000V?20,00020? ?230V1501,500? 1)A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil? 2)A step-down transformer has twice as many turns on the primary coil than on the secondary coil. What will be the output (secondary) voltage if the input voltage is 50V?

39 10/05/2015 Static electricity Static electricity is when charge “build up” on an object and doesn’t move, e.g. rubbing a rod:

40 10/05/2015 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:

41 10/05/2015 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

42 10/05/2015 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

43 10/05/2015 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

44 10/05/2015 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.

45 10/05/2015Electrolysis Electrolysis is used to extract a HIGHLY REACTIVE metal. = chloride ion = copper ion 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!!!

46 10/05/2015 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?

47 10/05/2015 Numerical quiz 1)What is the resistance of a bulb with a voltage of 12V and a current of 2A through it? 2)This bulb transfers 100C of electrical charge into light energy. If the bulb is 100% efficient how much light energy was given out? 3)A step up transformer changes 24V into 240V. It has 100 turns on the primary coil. How many does it have on the secondary coil? 4)10g of copper is formed at the negative electrode during electrolysis. It took 5 minutes to do this. How much would you have after half an hour? 5)In this electrolysis experiment the current during the 5 minutes was 2A. How much charge flowed?

48 10/05/2015 Numerical quiz 6)A thermistor has a resistance of 200  when 20V is applied across it. What is the current through the thermistor? 7)The same thermistor is put in a warm water bath. The resistance drops to 120 . What is the current through it now? 8) A kettle runs on a power rating of 2.4kW. If it is plugged into a mains supply what current will it draw? 9)What fuse should this kettle have? 10)A transformer has 5 times as many turns on the secondary coil as it does on the primary coil. What will the voltage across the primary coil be if the secondary coil voltage is 250V?

49 10/05/2015 Numerical quiz 11)During electrolysis 0.1g of chlorine is released at the positive electrode. How much would have been released if the current was doubled? 12)A 150  resistor is placed across a 3V battery. What current will it draw? 13)How much power will this resistor use? 14) An electric fire uses 1200C of charge over 2 minutes. What current did it draw? 15)A laptop runs on 12V. A step down transformer is used so that it can be plugged in to a 240V mains supply. If the transformer has 200 turns on the primary coil how many will it have on the secondary coil?


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