Electricity. AQA Physics topic 2

Transcription

1 Electricity AQA Physics topic 2

2 Identify circuit components from their symbols. Draw and interpret simple circuit diagrams. Construct a simple electrical circuit.

3 State that resistance restricts the size of a current in a circuit. State Ohm s law and describe its conditions. Measure the current and potential difference in a circuit to determine the resistance.

4 Identify the key characteristics of electrical devices. Identify components from simple I V graphs. State the operation of a diode in simple terms.

5 " Identify the key characteristics of electrical devices. Identify components from simple I V graphs. State the operation of a diode in simple terms."

6 " State that the current in any part of a series circuit is the same. Calculate the potential difference provided by cell combinations. Calculate the total resistance of two resistors placed in series."

7 " Identify parallel sections in circuit diagrams. State the effect of adding resistors in parallel on the size of the current in a circuit. State that the p.d. across parallel sections of a circuit is the same."

8 2.1 Current, Potential Difference and Resistance

9 Circuit Symbols Variable resistor A V Diode Switch Bulb Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery

10 Electric current is a flow of negatively charged particles (i.e. electrons). Electric Current + - e - Note that electrons go from negative to positive By definition, current is the rate of flow of charge and it has the same value at any point in this circuit e -

11 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 (in C) (in A) (in s) Q You need to learn this equation!! I T

12 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?

13 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? 90C 200s 300C

14 Basic ideas Electric current is when electrons start to flow around a circuit. We use an to measure it and it is measured in, named after Andre Ampere. 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 Alessandro Volta. Resistance is anything that resists an electric current. It is measured in, named after Georg Ohm. Words: volts, amps, ohms, voltage, ammeter, voltmeter

15 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

16 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: Georg Simon Ohm Resistance = Voltage (in V) (in ) Current (in A) You need to learn this equation!! I V R

17 Resistance Resistance is anything that opposes an electric current. Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) What is the resistance of the following: 1) A bulb with a voltage of 3V and a current of 1A. 2) A resistor with a voltage of 12V and a current of 3A 3) A diode with a voltage of 240V and a current of 40A 4) A thermistor with a current of 0.5A and a voltage of 10V

18 Understanding Resistance What happens inside a conducting material? The following model of a metal wire could help: Electrons Ions At normal temperatures, with no current flowing, electrons hurtle around continuously. What happens if you connect a battery?

19 Using a battery to push a current Apply a voltage: Negative Electrons Ions Positive This time we can see that the electrons are accelerated from negative to positive. This is an electric current and happens because the voltage pushes the electrons.

20 Required Practical 6 Factors affecting Resistance Task: Investigate how these factors affect resistance: 1) The length of the wire 2) The size (cross sectional area) of the wire 3) The material the wire is made of ( resistivity ) 1) Increase length 2) Increase area 3) Decrease resistivity

21 Resistors, bulbs and diodes

22 Required Practical 7 I-V graphs Voltage on powerpack/v Current/A Voltage/V

23 I Current-voltage graphs I I V V V 1. Resistor 3. Diode 2. Bulb Current increases in proportion to, provided the temperature doesn t change As voltage increases the bulb gets and increases A diode only lets current go in one it has very resistance in the other direction Words resistance, high, voltage, hotter, direction

24 LDRs and Thermistors

25 Two simple components: 1) Light dependant resistor resistance DECREASES when light intensity INCREASES Resistance 2) Thermistor resistance DECREASES when temperature INCREASES Resistance Amount of light Temperature

26 Using Thermistors in circuits A V 1) What will happen to the resistance of the thermistor when it gets hotter? 2) How will this affect the brightness of the bulb and the reading on the ammeter?

27 Using LDRs in circuits A V Here s a circuit that has been left in a room. Will the lamp come on when the room is light or dark? Explain your answer.

28 2.2 Series and Parallel Circuits

29 Circuit building practice A 5. A V

30 What happens to Current in a series circuit? If the current here is 2 amps?a?a?a 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

31 What happens to Current in a parallel circuit? A PARALLEL circuit is one where the current has a choice of routes Here comes the current?a?a?a Half of the current will go down here (assuming the bulbs are the same) And the rest will go down here In other words, the current in a parallel circuit SPLITS UP down each strand

32 Current in a parallel circuit If the current here is 6 amps And the current here will be The current here will be The current here will be The current here will be

33 Some example questions 3A 6A?A?A 4A?A?A 2A?A 1A each

34 Voltage in a series circuit If the voltage across the battery is 4.5V V and these bulbs are all identical what will the V V voltage across each bulb be? 1.5V In other words, voltage in a series circuit SPLITS UP over each component

35 Voltage in a series circuit If the voltage across the battery is 4.5V V what will the voltage across two bulbs be? V 1.5V

36 Voltage in a parallel circuit If the voltage across the batteries is 4V What is the voltage here? 4V V And here? 4V V In other words, the voltage in a parallel circuit STAYS THE SAME across each strand

37 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

38 An example question: 6V 3A 3A 6V A 1 V 1 A 2 2A 1A A 3 3V V 2 V 3 3V

39 Another example question: 10V 3A 3A 1.2A A 1 A 2 V 1 6.7V A 3 1.8A 5V V 2 V 3 5V

40 Resistance recap 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: Georg Simon Ohm Resistance = Voltage (in V) (in ) Current (in A) You need to learn this equation!! I V R

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

42 Example Questions 3A 6V 12V 3A 4V 2A 1A 2V What is the resistance of these bulbs?

43 Resistors in Series and Parallel 3A 6V 12V In SERIES, the total resistance is given by the sum of the individual resistances: R Total = R 1 + R 2 What is the total resistance in this circuit? 3A In PARALLEL the total resistance is less than the resistance of the individual resistors as the current has more routes to flow through. What is the highest value the resistance of this circuit could be?

44 Equivalent Resistance What is the resistance of this resistor? If all three resistors are the same, what is the total resistance? 9V 3A Equivalent resistance is the idea of replacing all of the resistors in a circuit with one resistor of the same value. The above circuit becomes this: 9Ω

45 2.3 Domestic Use and Safety

46 DC stands for Direct Current the current only flows in one direction. Batteries supply DC. DC and AC V Time AC stands for Alternating Current the current changes direction 50 times every second (frequency = 50Hz). Household electricity is AC 1/50 th s V 230V T

47 Wiring a plug 1. Earth wire 4. Live wire 2. Neutral wire 5. Fuse 3. Insulation 6. Cable grip The live wire of a plug alternates between positive and negative potential relative to the Earth The neutral wire of a plug stays at a potential close to zero and is used to complete the circuit

48 Different types of wire Two core useful for lighting as no earth wire is needed Three core useful for more powerful devices Some questions: 1) What material is used for the wire and why? 2) What material is used around the wire and why?

49 2.4 Energy Transfers

50 Electrical Power The power of an electrical device is given by this equation: P Power = voltage x current in W in V in A V I You need to learn this equation!!

51 Example Questions Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster Fire Hoover Hairdryer Computer Stereo 80 Which devices transfer the most power and why?

52 Example Questions Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster Fire Hoover Hairdryer Computer Stereo

53 Another Equation for Power Here s another way of calculating power use this one if you re given a value for resistance: You need to learn this equation!! Power = I 2 R 1) A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor. 2) A motor takes a current of 10A. If its resistance is 2.2MΩ calculate the power dissipated by the motor. 3) A 2KW heater has a resistance of 20Ω. Calculate the current through it. 40W 220MW 10A

54 Energy and Power The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second Energy = Power x Time E E = Energy (in joules) P = Power (in watts) T = Time (in seconds) P T You need to learn this equation!!

55 Some example questions 1) What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds? 2) What is the power of an electric fire that transfers 10,000J of energy in 5 seconds? 3) Callum runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating? 4) How much energy does a 150W light bulb transfer in a) one second, b) one minute? 5) Curtis brain needs energy supplied to it at a rate of 40W. How much energy does it need during a 50 minute physics lesson? 6) Fergal s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a 24 hour day? 60W 2KW 0.2MW 150J, 9KJ 120KJ 1.73MJ

56 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) You need to learn this equation!! W V Q

57 Example questions (foundation) 1) In a radio circuit a voltage of 6V is applied and a charge of 100C flows. How much energy has been transferred? 600J 2) In another circuit a voltage of 10V is applied and 200C of charge flows around the circuit. How much energy is transferred? 3) 48J of electrical work is done by a 6V battery. How much charge flowed around the circuit? 4) Another battery transfers 20C of charge and does 100J of electrical work. What was the battery s voltage? 2000J 8C 5V

58 Example questions 1) In a radio circuit a voltage of 12V is applied and a charge of 100C flows. How much energy has been transferred? 2) In the above 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 charge flowing through it, b) 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 energy supplied to the bulb. 1200J 200s 900C, 5400J 2A, 14,400J

59 Numerical quiz 1) What is the resistance of a bulb with a voltage of 12V and a current of 2A through it? 2) An electric fire uses 1200C of charge over 2 minutes. What current did it draw? 3) What is the power rating of a kettle that runs on a voltage of 230V and draws a current of 8A? 4) In an electrolysis experiment a 2A current flowed for 5 minutes. How much charge flowed? 5) A 150 resistor is placed across a 3V battery. What current will it draw? 6) How much power will this resistor transfer? 6 10A 1.8KW 600C 0.02A 0.06W

60 Numerical quiz 7) A thermistor has a resistance of 200 when 20V is applied across it. What is the current through the thermistor? 8) The same thermistor is put in a warm water bath. The resistance drops to 120. What is the current through it now? 9) A kettle runs on a power rating of 2.3kW. If it is plugged into a mains supply what current will it draw? 10)What fuse should this kettle have? 11) A 230V heater transfers 20C of charge. How much electrical energy has it converted? 12)What fuse should a 230V, 920W microwave have in its plug? 0.1A 0.17A 10A 13A 4.6KJ 5A

61 Random questions 1) A battery has a voltage of 12V and it puts a current of 3A through a bulb. What is the bulb s resistance? 2) Another bulb transfers 120C of charge in 2 minutes. What was the current through it? 3) A powerpack transfers 2,000J to a motor. If the motor ran on a voltage of 50V how much charge was transferred? 4) A hairdryer runs on a 50Hz power supply. If it has a power rating of 800W what fuse should it have? 5) An electric fire transfers 3MJ of energy. If it has a power rating of 2KW calculate how long it was on for, the current it ran on (assuming it was connected to the mains supply) and the amount of charge it transferred. 4 1A 40C 5A 1500s, 8.7A, 13000C

62 The National Grid Electricity reaches our homes from power stations through the National Grid, a system of power cables and transformers connecting us to power stations: Power station Step up transformer Step down transformer Homes 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

63 2.5 Static Electricity (Physics Only)

64 Static Electricity Static electricity is when charge builds up on an object and then stays static. How the charge builds up depends on what materials are used:

65 Short Static Experiments Try the following quick static electricity experiments: 1) Rubbing a balloon on your jumper and sticking it to the wall 2) Charging a plastic rod by rubbing it with a cloth and then holding it near the water from a smooth-running tap 3) Charging a plastic rod and trying to pick up small pieces of paper (or someone else s hair!) with it 4) Rubbing a balloon on someone else s head you might want to ask their permission first Can you explain what you saw in each of these experiments?

66 Static Electricity

67 Gold Leaf Electroscopes Consider a gold-leaf electroscope Now charge the top:

68 Van de Graaf generators When a charge is neutralised by the movement of electrons either from the Earth or to the Earth we call this earthing

69 Van de Graaf generators and Electric Fields Here s a charged VDG: I can really feel the electric field here I can t feel it over here! Clearly, the electric field gets weaker the further out you get.

70 Electric fields around a point charge +Q -Q Strong field Strong field Weak field Weak field