Presentation on theme: "Electricity N Bronks Basic ideas… Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured."— Presentation transcript:
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
Electrons are flowing from the negative to positive side of the battery through the wires Note current moves from positive to negative, however electrons are actually are moving in the opposite direction!
More basic ideas… Another battery means more current as there is a greater push on the electrons The extra resistance from the extra bulb means less current
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 2A
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)…
Current in a parallel circuit If the current here is 6 amps The current here will be… And the current here will be… 6A 2A
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 Voltmeter always in parallel
Voltage in a series circuit V V If the voltage across the battery is 6V… …what will the voltage across two bulbs be? 4V
Voltage in a parallel circuit If the voltage across the batteries is 4V… What is the voltage here? And here? VV 4V
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 eachstrand
An example question: V1V1 V2V2 6V 3A A1A1 A2A2 V3V3 A3A3
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) When one breaks they dont all fail
Georg Simon Ohm 1789-1854Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. That makes me so happy The resistance of a component can be calculated using Ohms Law: Resistance = Voltage (in V) (in )Current (in A) V RI
An example question: V A What is the resistance across this bulb? As R = volts / current = 10/2 = 5 Assuming all the bulbs are the same what is the total resistance in this circuit? Total R = 5 + 5 + 5 = 15 Voltmeter reads 10V Ammeter reads 2A
More examples… 12V 3A 6V 4V 2A 1A 2V What is the resistance of these bulbs?
Practice with Ohms Law OhmsVoltsAmps 410025 1515010 23015 9455 6488
VARIATION OF CURRENT (I) WITH P.D. (V) A V + 6 V - Nichrome wire
Method 1.Set up the circuit as shown and set the voltage supply at 6 V d.c. 2.Adjust the potential divider to obtain different values for the voltage V and hence for the current I. 3.Obtain at least six values for V and I using the voltmeter and the ammeter. 4.Plot a graph of V against I
Variations (a) A METALLIC CONDUCTOR With a wire (b) A FILAMENT BULB (c) COPPER SULFATE SOLUTION WITH COPPER ELECTRODES (d) SEMICONDUCTOR DIODE Done both ways with a milli-Ammeter and the a micro Ammeter
Current-voltage graphs I V I V I V 1. Resistor 3. Diode 2. Bulb Current increases in proportion to voltage As voltage increases the bulb gets hotter and resistance increases A diode only lets current go in one direction
Factors affecting Resistance of a conductor Resistance depends on –Temperature –Material of conductor –Length –Cross-sectional area Temperature The resistance of a metallic conductor increases as the temperature increases e.g. copper The resistance of a semiconductor/insulator decreases as the temperature increases e.g. thermistor.
VARIATION OF THE RESISTANCE OF A METALLIC CONDUCTOR WITH TEMPERATURE Water Wire wound on frame Glycerol Heat source 10 º C Digital thermometer Ω 10 º C
Method 1.Set up as shown. 2.Use the thermometer to note the temperature of the glycerol, which is also the temperature of the coil. 3.Record the resistance of the coil of wire using the ohmmeter. 4.Heat the beaker. 5.For each 10 C rise in temperature record the resistance and temperature using the ohmmeter and the thermometer. 6.Plot a graph of resistance against temperature.
Graph and Precautions Precautions - Heat the water slowly so temperature does not rise at end of experiment - Heat the water slowly so temperature does not rise at end of experiment -Wait until glycerol is the same temperature as water before taking a reading. R
Length Resistance of a uniform conductor is directly proportional to its length. i.e. R L Factors affecting Resistance of a conductor Cross-sectional area Resistance of a uniform conductor is inversely proportional to its cross- sectional area. i.e.R 1 A
Factors affecting Resistance of a conductor Material The material also affects the resistance of a conductor by a fixed amount for different materials. This is known as resistivity ( ). R = L = Resistivity A Unit: ohm meter m
RESISTIVITY OF THE MATERIAL OF A WIRE Micrometer Metre stick l Bench clamp Stand Nichrome wire Crocodile clips
Method 1.Note the resistance of the leads when the crocodile clips are connected together. Could also be precaution. 2. Stretch the wire enough to remove any kinks or slack in the wire. 3.Read the resistance of the leads plus the resistance of wire between the crocodile clips from the ohmmeter. Subtract the resistance of the leads to get R. 4.Measure the length l of the wire between the crocodile clips, with the metre stick. 5.Increase the distance between the crocodile clips. Measure the new values of R and l and tabulate the results. 6.Make a note of the zero error on the micrometer. Find the average value of the diameter d.
1.Calculate the resistivity where A = 2.Calculate the average value. Precautions Ensure wire is straight and has no kinks like.... Take the diameter of the wire at different angles ρ
Wheatstone Bridge Uses –Temperature control –Fail-Safe Device (switch circuit off) –Measure an unknown resistance –R 1 = R 3 (When its balanced) R 2 R 4 Metre Bridge R 1 = R 2 (|AB|) |BC| I r 1 r 2 r 4 r 3 AC B D
Effects of an Electric Current Heat Chemical Magnetic
Chemical Effects of an Electric Current Electrolysis is the chemical effect of an electric current Voltameter consists of electrodes, an electrolyte and a container Inactive electrodes are electrodes that dont take part in the chemical reaction e.g. platinum in H 2 SO 4 Active electrodes are electrodes that take part in the chemical reaction e.g. copper in CuSO 4
Chemical Effects Ion is an atom or molecule that has lost or gained 1 or more electrons Charge Carriers in an electrolyte are + and – ions Uses Electroplating to make metal look better, prevent corrosion Purifying metals Making electrolytic capacitors
Current-voltage graphs I V I V 1. Active Electrodes 2. Inert Electrodes e.g. Copper in Copper Sulphate e.g. Platinum in Water
Current Carriers MediumCarrier Solid (Metal)Electrons Liquid (Electrolyte)Ions GasElectrons and Ions
Resistance in Semiconductors 2) Thermistor – resistance DECREASES when temperature INCREASES – Due to more charge carriers being liberated by heat 1) Normal conductor like metal resistance increases as vibrating atoms slow the flow of electrons Resistance Temperature Resistance Temperature
Fuse – Safety device Fuses are designed to melt when too large a current tries to pass through them to protect devices. Prevent Fires Modern fuse boxes contain MCB (Miniature circuit breakers) that trip when too much current flows to protect the circuit 2A 5A
Which Fuse A i-pod charge uses 200W and is plugged into the mains at 230v. What fuse is in the plug? P=I.V 200=I.230 I = 200/230 = 0.87A is current used So the most the fuse should be is a 1A
Other safety devices… 1) Insulation and double insulation 2) Residual Current Circuit Breaker In some parts of Europe they have no earth wire just two layer of insulating material the sign is An RCCB (RCB) detects any difference in current between the live and neutral connectors and the earth it switches off the current when needed. They can also be easily reset.
Electrical Safety A combination of fuse and Earth A.C. Supply That Hurts! The fuse will melt to prevent electrocution and the electricity is carried to earth The casing touches the bare wire and it becomes live
Wiring a plug Earth wire Neutral wire Insulation Live wire Fuse 1. 2. 3. 4. 5. 6. Cable grip
Capacitors A device for storing charge. A pair of metal plates are separated by a narrow gap - - - - - - - + + + + + + --- electrons
Equations C d A = For the parallel plate capacitor Distance in meters Area In m 2 Permitivity in Fm -1 Capacitance In Farads
Example 1 0 C 0.01m 0.04m 2 = The common area of the plates of an air capacitor is 400cm 2 if the distance between the plates is 1cm and ε 0 =8.5x10 -12 Fm -1. C d A = 8.5x10 -12 Fm -1 x =3.4x10 -11 F.
Capacitance experiment on the internet Capacitance experiment on the internet
Equations C V Q = Capacitance on any conductor Potential Difference in volts Charge in Coulombs Capacitance In Farads
Placing a charge of 35 μ C on a conductor raises it's potential by 100 V. Calculate the capacitance of the conductor. Info Q = 35 μ C and V = 100V find C=? Using Q=VC or C = Q/V = 35 x 10 -6 /100 = 35 x 10 -8 Farads
Equations C ½Work Done (V) 2 = Energy stored on a capacitor Voltage Squared Capacitance In Farads Energy Stored
Example 3 Find the capacitance and energy stored of a parallel plate capacitor with 2mm between the plates and 150cm 2 overlap area and a dielectric of relative Permittivity of 3. The potential across the plates is 150V. A = 150cm 2 =0.015m 2,d = 2x10 -3 m, ε = 3x ε 0 = 27x10 -12 Fm -1 As C = ε 0 A/d = 27x10 -12 x 0.015/0.002 = 2.025x10 -9 F Energy stored = ½ C V 2 = ½ x 2.025x10 -9 x (150) 2 = 2.28x10 -5 Joules
Types of Batteries Type of BatteryContainsUses Wet cell rechargeable Lead and acidCars, industry Dry cell rechargeable Nickel, cadmium, lithium Mobile phones, power tools Dry cell non- rechargeable Zinc, carbon, manganese, lithium Torches, clocks, hearing aids Why use rechargeable batteries? Long long-term expense Can be used many times Less energy to produce Why use standard batteries? No need for charger Less expensive Rechargeables contain carcinogens
There are 2 types of currents: Direct Current (DC) – Where electrons flow in the same direction in a wire.
There are 2 types of currents: Alternating Current (AC) – electrons flow in different directions in a wire
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) Find Root Mean Square of voltage by V rms = V peak / 2 1/50 th s 240V V V Time T
The National Grid If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much energy lost by the time electricity reached our homes. This is explained by JOULES LAW Power station Step up transformer Step down transformer Homes
The National Grid Power Transmitted is = P = V.I JOULES LAW gives us the power turned into heat Power Lost = I 2 R So if we have a high voltage we only need a small current. We loss much less energy Power station Step up transformer Step down transformer Homes
Power loss in Transmission lines A power company wants to send 100000w of power by a line with a resistance of 12 ohms. If it uses 10A as the current Power transmitted = V. I 100000 = V. 10 So V=10000Volts But the loss is from Joules law = I 2 R = (10) 2.12 = 1200watts
Power loss in Transmission lines If we want the same power but use only 1A as the current Power transmitted = V. I 100000 = V. 1 So V=100000Volts (Another Zero) But the loss is from Joules law = I 2 R = (1) 2.12 = 12watts 100 times less!
Joules law Heating coil Lagging Calorimeter Water A Lid Digital thermometer 10°C
Method 1.Put sufficient water in a calorimeter to cover the heating coil. Set up the circuit as shown. 2.Note the temperature. 3.Switch on the power and simultaneously start the stopwatch. Allow a current of 0.5 A to flow for five minutes. Make sure the current stays constant throughout; adjust the rheostat if necessary. 4.Note the current, using the ammeter. 5.Note the time for which the current flowed. 6.Stir and note the highest temperature. Calculate the change in temperature.
Calculation and Graph Repeat the above procedure for increasing values of current I, taking care not to exceed the current rating marked on the rheostat or the power supply. Take at least six readings. Plot a graph of (Y-axis) against I 2 (X-axis). A straight-line graph through the origin verifies that I 2 i.e. Joules law. Electrical Power lost as Heat P I 2 is Joules law The power lost (Rate at which heat is produced) is proportional to the square of the current. I2I2
Experiment to Show shape of Electric Field The electrode is placed in the shallow glass dish containing a mixture of semolina and castor oil. The semolina aligns itself along the lines of the electric field.
The Electroscope + ++ + + + + + ---- The electroscope detects charge The Gold leaf and post repel each other
Coulomb's Law Force between two charged bodies Force = f Q1.Q2 d2d2 Q1Q2 d Put this as a sentence to get a law!
Coulomb Calculations We replace the proportional with a equals and a constant to get an equation Force =f Q1.Q2 d2d2 Force = f= Q1.Q2 4 d 2 = permitivity as in capacitors
Coulomb's Law Calculations Force between these bodies Force = f = Q1.Q2 4 d 2 2C4mC d=2m = 3.4 x 10 -11
Coulomb's Law Calculations Force between these bodies Force = f = 2 x 0.004 4 x3.4 x 10 -11 x 2 2 2C4mC d=2m
Coulomb's Law Calculations Force between these bodies Force = f = 7.49 x 10 -15 N 2C4mC d=2m
Coulomb's Law Calculations Force between these bodies 2C4mC d=2m Electric Field Strength = E = F/q Electric Field Strength = E = 7.49 x 10 -15 N /2C = 3.75 x 10 -15 N /C
Precipitator Carbon and ash - can be removed from waste gases with the use of electrostatic precipitators
Precipitator Dirt particles are charged then made to stick to oppositely charged plates
Potential Difference (V) Potential difference is the work done per unit charge to transfer a charge from one point to another (also Voltage) i.e V = W Q
Potential Difference (V) V = W Q Unit Volt V or J C -1 Volt is the p.d. between two points if one joule of work is done bringing one coulomb from one point to the other Potential at a point is the p.d. between a point and the Earth, where the Earth is at zero potential
Current in a Magnetic Field N S Force Current Magnetic Field A conductor carrying a current in a magnetic field will always feel a force THE MOTOR EFFECT The force is perpendicular to the current and the field. – This is THE MOTOR EFFECT
Flemings Left Hand Rule I used my left hand to show the direction the wire would move
The Size of the Force Force = F = B.I.l Where B = Magnetic Field Density in Tesla (T) I= Current in Amps (A)…………………………… L = length if the conductor in metres… Example What is the force acting on a conductor of length 80cm carrying a current of 3A in a 4.5T magnetic field? Using Force = F = B.I.l = 4.5x3x0.8 = 10.8N
Two Parallel Wires Wires also produce magnetic fields when a current flows Attraction
Two Parallel Wires The fields act like magnets when the current flows Repulsion
The Ampere Basic unit of electricity F=2x10 -7 N/m 1m The current flowing is 1A when the force between two infinitely long conductors 1m apart in a vacuum is 2x10 -7 N Per metre of length.
Moving Charge When any charged particle moves it is like a small current of electricity It feels the same force The crosses show a magnetic field into the screen e-e- Velocity Force e-e- Velocity Force e-e- Velocity Force e-e- e-e-
Moving Charge A positive will move the other way e-e- Velocity Force + All charged particles moving in magnetic fields always have a force at right angles to their velocity so follow a circular path due to FLH Rule
Force 0n a Particle Force = F = B.q.v Where B = Magnetic Field Density in Tesla (T) q=charge on the particle (C) v=velocity of the particle… Example What is the force acting on a particle travelling at 80m/s carrying a charge of 0.1C in a 10T magnetic field? Using Using Force = F = B.q.v = 10x.1x80 = 80N
Induction is where changes in the current flow in a circuit are caused by changes in an external field. N Moving Magnet Circuit turning off and on
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 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
Demo Coils and spot galvo Internethttp://phet.colorado.edu/en/ simulation/faradayhttp://phet.colorado.edu/en/ simulation/faraday
Generators (dynamos) 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
Faradays Law Basically 1.More turns (N) more EMF 2.Faster movement more EMF Rate of change of FLUX DENSITY is proportional to induced EMF Induced EMF = E = - Nd ( =B.A) dt
Lenzs Law The induced EMF always opposes the current/Motion You get ought for nought A version of Newton III and of energy conversion The induction always tries to stop the motion or change in the field. Aluminum Ring The ring moves away as the induced current is preventing more induction
Mutual induction Induction in a second circuit caused by changes in a first circuit Main use in a transformer As the current changes the field changes giving a EMF in the second circuit.
Transformers This how A.C. changes voltage up or down V In V Out Turns 2 Turns 1 =
Self Induction property whereby an electromotive force (EMF) is induced in a circuit by a variation of current in the circuit its self D.C. Source Current Back EMF Another example on LENZS LAW
Flux Density Magnetic flux, represented by the Greek letter Φ (phi), total magnetism produced by an object. The SI unit of magnetic flux is the WeberphiSIunitWeber Magnetic field (B) is the flux through a square meter (the unit of magnetic field is the Weber per square meter, or Tesla.)esla As the flux expands the density through any square meter decreases