14/06/2016 Physics Unit 2b

14/06/2016 P2.3.1 – Static Electricity

14/06/2016 Static Electricity An introduction – click here

14/06/2016 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:

14/06/2016 Static Electricity

14/06/2016 Gold Leaf Electroscopes Consider a gold-leaf electroscope… Now charge the top: This effect was seen because charges can easily move through conductors

14/06/2016 Van de Graaf generators

14/06/2016 P2.3.2 – Electrical Circuits

14/06/2016 Electric Current Electric current is a flow of negatively charged particles (i.e. electrons). Note that electrons go from negative to positive -+ e-e- e-e- By definition, current is “the rate of flow of charge”

14/06/2016 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)

14/06/2016 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?

14/06/2016 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

14/06/2016 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: W QV Energy transferred = charge x voltage (in J) (in C) (in V)

14/06/2016 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 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. 600J 200s 2A, 14,400J 900C, 5400J

14/06/2016 Circuit Symbols VA Battery Cell Fuse Resistor LDR Voltmeter Ammeter Variable resistor Diode Switch Bulb Thermistor

14/06/2016 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

14/06/2016 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

14/06/2016 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

14/06/2016 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)…

14/06/2016 Current in a parallel circuit If the current here is 6 amps The current here will be… And the current here will be…

14/06/2016 Some example questions… 3A6A 1A each 4A 2A

14/06/2016 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

14/06/2016 Voltage in a series circuit V V If the voltage across the battery is 6V… …what will the voltage across two bulbs be? 4V

14/06/2016 Voltage in a parallel circuit If the voltage across the batteries is 4V… What is the voltage here? And here? VV 4V

14/06/2016 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”

14/06/2016 An example question: V1V1 V2V2 6V 3A A2A2 A3A3 V3V3 A1A1 2A 1A 6V 3V

14/06/2016 Another example question: V1V1 V2V2 10V 3A A2A2 A3A3 V3V3 A1A1 1.2A 1.8A 6.7V 5V

14/06/2016 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

14/06/2016 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

14/06/2016 More examples… 12V 3A 6V 4V 2A 1A 2V What is the resistance of these bulbs?

14/06/2016 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 20  33 44 66

14/06/2016 Understanding Resistance When a voltage is applied it basically causes electrons to move towards the positive end of the battery: Notice that the ions were vibrating and getting in the way of the electrons – this is resistance. What would happen if we increased the temperature of the metal? IonsElectrons NegativePositive

14/06/2016 Resistors, bulbs and diodes

14/06/2016 Current-Voltage Graphs Voltage on powerpack/V Current/AVoltage/V … 0 …

14/06/2016 Current-voltage graphs I V I V I 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 due to increased vibrations in the ions in the filament A diode only lets current go in one _______ – it has very _____ resistance in the other direction Words – resistance, high, voltage, hotter, direction

14/06/2016LEDs “LED” stands for “Light Emitting Diode” – it’s basically a diode that emits light when current flows through in the right direction: Symbol for diode Symbol for LED LEDs use much less current than other forms of lighting so they are increasingly being used as sources of light:

14/06/2016 LDRs and Thermistors

14/06/2016 Two simple components: 2) Thermistor – resistance DECREASES when temperature INCREASES 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES Resistance Amount of light Resistance Temperature

14/06/2016 Using Thermistors and LDRs in circuits V A 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? 3)Try designing your own circuit that warns you when an intruder enters your home, maybe by standing between a source of light and an LDR

14/06/2016 P2.4.1 – Household electricity

14/06/2016 Using an oscilloscope Q. What is the voltage and frequency of this supply? This number tells you how many seconds each square on the horizontal axis represents This number tells you how many volts each square on the vertical axis represents 4V, 0.5Hz 3V, 1Hz

14/06/2016 Oscilloscope questions If time base = 2ms/div and each vertical square is 5V what is the voltage and frequency of each of these signals? 10V, 50Hz 15V, 200Hz

14/06/2016 DC and AC DC stands for “Direct Current” – the current only flows in one direction. Batteries supply DC. AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz). Household electricity is AC 1/50 th s 230V V V Time T

14/06/2016 Wiring a plug Earth wire Neutral wire Insulation Live wire Fuse 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 relative to the Earth

14/06/2016 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?

14/06/2016 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 _____. Fuses are designed to take different currents. Words – large, harm, safety, melt, live, circuit, fuse This is a 3A fuse – what would be different if it was a 13A fuse?

14/06/2016 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. Some appliances do not need earth wires as they are “double insulated” (i.e. have a plastic case). Words – fuse, fault, metal, surges, touch

14/06/2016 Circuit breakers Residual Current Circuit Breakers (RCCBs) are often used with fuses. They have some advantages over fuses: 1)They are safer – they don’t get hot 2)They react more quickly 3)They can be switched off for repairs 4)They are easy to reset 5)Each RCCB is attached to a certain circuit, so if one switches off you can see which circuit has a fault

14/06/2016 P2.4.2 – Current, Charge and Power

14/06/2016 Different types of bulb 25W “energy efficient” light bulb, £7.30 on Amazon 60W older bulb, roughly 70p, to be banned in the EU from The older style light bulbs wasted a lot of energy due to the filament getting hot during use. The newer bulbs don’t have a filament – very useful!

14/06/2016 Energy and Power revision The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second E TP E = Energy (in joules) P = Power (in watts) T = Time (in seconds)

14/06/2016 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)Rob 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)Jonny’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a 50 minute physics lesson? 6)Lloyd’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day? 60W 2KW 150J, 9KJ 120KJ 630MJ 0.2MW

14/06/2016 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).

14/06/2016 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 Stereo80230

14/06/2016 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 or 1 Stereo or 1

14/06/2016 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6 10A 600C 0.02A 0.06W 1.8KW

14/06/2016 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 13A 4.6KW 5A 10A

14/06/2016 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4 1A 5A 1500s, 8.7A, 13000C 40C