10/11/2018 10/11/2018 Electricity AQA 2016 Physics topic 2

2.1 Current, Potential Difference and Resistance 10/11/2018 2.1 Current, Potential Difference and Resistance

Circuit Symbols Variable resistor Diode Switch Bulb A V Ammeter 10/11/2018 10/11/2018 Variable resistor Diode Switch Bulb A V Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery

Electric Current 10/11/2018 10/11/2018 Electric current is a flow of negatively charged particles (i.e. electrons). Note that electrons go from negative to positive + - e- By definition, current is “the rate of flow of charge” and it has the same value at any point in this circuit e-

You need to learn this equation!! Charge (Q) 10/11/2018 10/11/2018 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 T I Charge = current x time (in C) (in A) (in s) You need to learn this equation!!

Example questions Charge (C) Current (A) Time (s) 5 2 0.4 1 20 0.5 50 10/11/2018 10/11/2018 Charge (C) Current (A) Time (s) 5 2 0.4 1 20 0.5 50 250 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

Example questions Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 10/11/2018 10/11/2018 Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 40 50 0.2 250 180 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed? 90C 200s 300C

Words: volts, amps, ohms, voltage, ammeter, voltmeter Basic ideas… 10/11/2018 10/11/2018 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

More basic ideas… 10/11/2018 10/11/2018 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

You need to learn this equation!! Resistance Georg Simon Ohm 1789-1854 10/11/2018 10/11/2018 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: V R I Resistance = Voltage (in V) (in ) Current (in A) You need to learn this equation!!

Resistance Resistance is anything that opposes an electric current. 10/11/2018 10/11/2018 Resistance is anything that opposes an electric current. Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) What is the resistance of the following: A bulb with a voltage of 3V and a current of 1A. A resistor with a voltage of 12V and a current of 3A A diode with a voltage of 240V and a current of 40A A thermistor with a current of 0.5A and a voltage of 10V 3 4 6 20

Understanding Resistance 10/11/2018 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?

Using a battery to push a current 10/11/2018 Apply a voltage: Electrons Ions Negative 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.

Required Practical 6 – Factors affecting Resistance 10/11/2018 Task: Investigate how these factors affect resistance: The length of the wire The size (cross sectional area) of the wire The material the wire is made of (“resistivity”) 1) Increase length 2) Increase area 3) Decrease resistivity

Resistors, bulbs and diodes 10/11/2018 10/11/2018

Required Practical 7 – I-V graphs 10/11/2018 10/11/2018 Voltage on powerpack/V Current/A Voltage/V 12 10 8 6 4 2 -2 -4 -6 -8 -10 -12

Current-voltage graphs 10/11/2018 10/11/2018 I V I V I V 3. Diode 1. Resistor 2. Bulb A diode only lets current go in one _______ – it has very _____ resistance in the other direction Current increases in proportion to _______, provided the temperature doesn’t change As voltage increases the bulb gets ______ and _______ increases Words – resistance, high, voltage, hotter, direction

LDRs and Thermistors 10/11/2018 10/11/2018

Two simple components: 10/11/2018 10/11/2018 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES 2) Thermistor – resistance DECREASES when temperature INCREASES Resistance Amount of light Resistance Temperature

Using Thermistors in circuits 10/11/2018 V A What will happen to the resistance of the thermistor when it gets hotter? How will this affect the brightness of the bulb and the reading on the ammeter?

Using LDRs in circuits A 10/11/2018 V A 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.

2.2 Series and Parallel Circuits 10/11/2018

Circuit building practice 10/11/2018 1. 2. 3. 4. A 5. V

What happens to Current in a series circuit? 10/11/2018 10/11/2018 What happens to Current in a series circuit? The current here will be… If the current here is 2 amps… ?A ?A And the current here will be… ?A In other words, the current in a series circuit is THE SAME at any point

What happens to Current in a parallel circuit? 10/11/2018 10/11/2018 What happens to Current in a parallel circuit? A PARALLEL circuit is one where the current has a “choice of routes” ?A 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… In other words, the current in a parallel circuit SPLITS UP down each “strand”

Current in a parallel circuit 10/11/2018 10/11/2018 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…

Some example questions… 10/11/2018 10/11/2018 3A ?A 6A 4A 2A 1A each

Voltage in a series circuit 10/11/2018 10/11/2018 If the voltage across the battery is 4.5V… V …and these bulbs are all identical… V V …what will the voltage across each bulb be? 1.5V In other words, voltage in a series circuit “SPLITS UP” over each component

Voltage in a series circuit 10/11/2018 10/11/2018 If the voltage across the battery is 4.5V… V V …what will the voltage across two bulbs be? 1.5V

Voltage in a parallel circuit 10/11/2018 10/11/2018 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”

Summary Current is THE SAME at any point 10/11/2018 10/11/2018 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: 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V 10/11/2018 10/11/2018 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V

Another example question: 10/11/2018 10/11/2018 3A 10V A1 3A 1.2A A2 V1 6.7V A3 1.8A V2 V3 5V 5V

You need to learn this equation!! Resistance recap Georg Simon Ohm 1789-1854 10/11/2018 10/11/2018 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: V R I Resistance = Voltage (in V) (in ) Current (in A) You need to learn this equation!!

An example question: Ammeter reads 2A A 10/11/2018 10/11/2018 Ammeter reads 2A V A What is the resistance across this bulb? Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter reads 10V

What is the resistance of these bulbs? Example Questions 10/11/2018 10/11/2018 3A 6V 12V 4V 2A 1A 2V 3A What is the resistance of these bulbs?

Resistors in Series and Parallel 10/11/2018 3A 6V 12V 3A In SERIES, the total resistance is given by the sum of the individual resistances: RTotal = R1 + R2 What is the total resistance in this circuit? 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?

Equivalent Resistance 10/11/2018 9V 3A What is the resistance of this resistor? If all three resistors are the same, what is the total resistance? “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

2.3 Domestic Use and Safety 10/11/2018

DC and AC 10/11/2018 10/11/2018 V DC stands for “Direct Current” – the current only flows in one direction. Batteries supply DC. Time 1/50th s AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz). Household electricity is AC 230V T V

Wiring a plug 1. Earth wire 4. Live wire 5. Fuse 2. Neutral wire 6. 10/11/2018 10/11/2018 1. 2. 3. 4. 5. 6. Earth wire Live wire Fuse Neutral wire Cable grip Insulation 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

Different types of wire 10/11/2018 Two core – useful for lighting as no earth wire is needed Three core – useful for more powerful devices Some questions: What material is used for the wire and why? What material is used around the wire and why?

2.4 Energy Transfers 10/11/2018

You need to learn this equation!! Electrical Power 10/11/2018 10/11/2018 The power of an electrical device is given by this equation: P I V Power = voltage x current in W in V in A You need to learn this equation!!

Which devices transfer the most power and why? Example Questions 10/11/2018 10/11/2018 Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster 230 4 Fire 8 Hoover 460 2 Hairdryer 1150 Computer 100 Stereo 80 Which devices transfer the most power and why?

Example Questions Complete the following table: Appliance 10/11/2018 10/11/2018 Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster 920 230 4 Fire 1840 8 Hoover 460 2 Hairdryer 1150 5 Computer 100 0.43 Stereo 80 0.35

Another Equation for Power 10/11/2018 Here’s another way of calculating power – use this one if you’re given a value for resistance: Power = I2R You need to learn this equation!! A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor. A motor takes a current of 10A. If its resistance is 2.2MΩ calculate the power dissipated by the motor. A 2KW heater has a resistance of 20Ω. Calculate the current through it. 40W 220MW 10A

Energy and Power 10/11/2018 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 T P E = Energy (in joules) P = Power (in watts) T = Time (in seconds) You need to learn this equation!!

Some example questions 10/11/2018 What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds? What is the power of an electric fire that transfers 10,000J of energy in 5 seconds? Callum runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating? How much energy does a 150W light bulb transfer in a) one second, b) one minute? Curtis’ brain needs energy supplied to it at a rate of 40W. How much energy does it need during a 50 minute physics lesson? 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

Energy and charge 10/11/2018 10/11/2018 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) W Q V You need to learn this equation!!

Example questions (foundation) 10/11/2018 10/11/2018 In a radio circuit a voltage of 6V is applied and a charge of 100C flows. How much energy has been transferred? In another circuit a voltage of 10V is applied and 200C of charge flows around the circuit. How much energy is transferred? 48J of electrical work is done by a 6V battery. How much charge flowed around the circuit? Another battery transfers 20C of charge and does 100J of electrical work. What was the battery’s voltage? 600J 2000J 8C 5V

Example questions 10/11/2018 10/11/2018 In a radio circuit a voltage of 12V is applied and a charge of 100C flows. How much energy has been transferred? In the above circuit the radio drew a current of 0.5A. How long was it on for? 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 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

Numerical quiz 10/11/2018 10/11/2018 What is the resistance of a bulb with a voltage of 12V and a current of 2A through it? An electric fire uses 1200C of charge over 2 minutes. What current did it draw? What is the power rating of a kettle that runs on a voltage of 230V and draws a current of 8A? In an electrolysis experiment a 2A current flowed for 5 minutes. How much charge flowed? A 150 resistor is placed across a 3V battery. What current will it draw? How much power will this resistor transfer? 6 10A 1.8KW 600C 0.02A 0.06W

Numerical quiz 10/11/2018 10/11/2018 A thermistor has a resistance of 200 when 20V is applied across it. What is the current through the thermistor? The same thermistor is put in a warm water bath. The resistance drops to 120. What is the current through it now? A kettle runs on a power rating of 2.3kW. If it is plugged into a mains supply what current will it draw? What fuse should this kettle have? A 230V heater transfers 20C of charge. How much electrical energy has it converted? What fuse should a 230V, 920W microwave have in its plug? 0.1A 0.17A 10A 13A 4.6KJ 5A

Random questions 4 1A 40C 5A 1500s, 8.7A, 13000C 10/11/2018 10/11/2018 A battery has a voltage of 12V and it puts a current of 3A through a bulb. What is the bulb’s resistance? Another bulb transfers 120C of charge in 2 minutes. What was the current through it? A powerpack transfers 2,000J to a motor. If the motor ran on a voltage of 50V how much charge was transferred? A hairdryer runs on a 50Hz power supply. If it has a power rating of 800W what fuse should it have? 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

Words – step down, high, power, low, high The National Grid 10/11/2018 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

2.5 Static Electricity (Physics Only) 10/11/2018 2.5 Static Electricity (Physics Only)

Static Electricity + + - - + - + + + - - - + + - + - - 10/11/2018 10/11/2018 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: + + - - + - + + + - - - + + - + - -

Short Static Experiments 10/11/2018 10/11/2018 Try the following quick static electricity experiments: Rubbing a balloon on your jumper and “sticking” it to the wall Charging a plastic rod by rubbing it with a cloth and then holding it near the water from a smooth-running tap Charging a plastic rod and trying to pick up small pieces of paper (or someone else’s hair!) with it 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?

Static Electricity 10/11/2018 10/11/2018 + - - -

Gold Leaf Electroscopes 10/11/2018 10/11/2018 Consider a gold-leaf electroscope… Now charge the top: - - - - - - - - -

Van de Graaf generators 10/11/2018 10/11/2018 When a charge is neutralised by the movement of electrons either from the Earth or to the Earth we call this “earthing”

Van de Graaf generators and Electric Fields 10/11/2018 10/11/2018 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.

Electric fields around a point charge 10/11/2018 +Q -Q Strong field Strong field Weak field Weak field