Electricity N Bronks Basic ideas… Electric current is when electrons start to flow around a...
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Transcript of Electricity N Bronks Basic ideas… Electric current is when electrons start to flow around a...
ElectricityElectricity
N Bronks
Basic ideas…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
• Flow of electrons
timepoint a Passed ChargeCurrent
tQI
CurrentCurrent
Current in a series circuitCurrent in a series circuit
If the current here is 2 amps…
The current here will be…
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
2A
2A
Current in a parallel circuitCurrent 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)…
SummarySummary
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”
Advantages of parallel circuits…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 don’t all fail
Georg Simon Ohm 1789-1854
ResistanceResistance
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 Ohm’s Law:
Resistance = Voltage (in V)
(in ) Current (in A)
V
RI
An example question: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
VARIATION OF CURRENT (VARIATION OF CURRENT (II) WITH P.D. ) WITH P.D. ((VV))
A
V+
6 V-
Nichrome wire
VariationsVariations
(a) A METALLIC CONDUCTORWith 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 graphsCurrent-voltage graphsI
VI
V
I
V
1. Resistor 3. Diode
2. BulbCurrent 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 Factors affecting Resistance of a conductorResistance of a conductor
• Resistance depends on– Temperature– Material of conductor– Length – Cross-sectional area
TemperatureThe resistance of a metallic
conductor increases as the temperature increases e.g. copperThe resistance of a
semiconductor/insulator decreases as the temperature increases e.g. thermistor.
VARIATION OF THE RESISTANCE OF A VARIATION OF THE RESISTANCE OF A METALLIC CONDUCTOR WITH METALLIC CONDUCTOR WITH
TEMPERATURETEMPERATURE
Water Wire wound on frame
Glycerol
Heat source
10ºC
Digitalthermometer
Ω
10º C
Graph and PrecautionsGraph and Precautions
PrecautionsPrecautions
- Heat the water slowly so temperature does - Heat the water slowly so temperature does not rise at end of experimentnot rise at end of experiment
-Wait until glycerol is the same temperature -Wait until glycerol is the same temperature as water before taking a reading.as water before taking a reading.
R
Factors affectingFactors affecting Resistance of a conductor Resistance of a conductor
• Material, temperature, Area cross section and length
R = L = Resistivity A Unit: ohm meter m
RESISTIVITY OF THE MATERIAL OF A RESISTIVITY OF THE MATERIAL OF A WIREWIRE
Micrometer
Metre stick
l
Bench clamp
Stand
Nichrome wire
Crocodile clips
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
,Al
Rñ
4
2d
ρ
Resistors in series and Resistors in series and ParallelParallel
321 IIIIT
V1
I
I1
V
I2
IT
R1R1
R2 R3
R2
321 VVVVT
Resistors in series and Resistors in series and ParallelParallel
321 IRIRIRIRT
V1
I
I1
V
I2
IT
R1R1
R2 R3
R2
321 RRRRT
321 VVVVT
Resistors in series and Resistors in series and ParallelParallel
321 R
V
R
V
R
V
R
V
T
V1
I
I1
V
I2
IT
R1R1
R2 R3
R2
321
1111
RRRRT
321 IIIIT
Wheatstone BridgeWheatstone BridgeUses
– Temperature control– Fail-Safe Device (switch circuit
off)– Measure an unknown
resistance
– R1 = R3 (When it’s balanced)
R2 R4
Metre Bridge R1 = R2 (|AB|)
|BC|
I
r 1
r2
r 4
r3
AA CC
BB
DD
Effects of an Electric CurrentEffects of an Electric Current
•Heat•Chemical•Magnetic
Current-voltage graphsCurrent-voltage graphs
I
V
I
V
1. Active Electrodes
2. Inert Electrodes
e.g. Copper in Copper Sulphate
e.g. Platinum in Water
Current CarriersCurrent Carriers
Medium Carrier
Solid (Metal) Electrons
Liquid (Electrolyte) Ions
Gas Electrons and Ions
Resistance in Resistance in SemiconductorsSemiconductors
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 deviceFuse – 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)
2A5A
Other safety devices…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 SafetyElectrical 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 plugWiring a plug
Earth wire
Neutral wire
Insulation
Live wire
Fuse
1.
2.
3.
4.
5.
6. Cable grip
Charge & Charge & DischargeDischarge
Uses of Uses of CapacitorsCapacitors
• Storing charge for quick release – Camera Flash
• Charging and discharging at fixed intervals – Hazard Lights
• Smoothing rectified current – See Semiconductors
Parallel Plate CapacitorsParallel Plate Capacitors• The size of the capacitor depends on1. The Distance the plates are apart d
-
-
-
+
+
+
d
Parallel Plate CapacitorsParallel Plate Capacitors
2 /.The area of overlap A
-
-
-
+
+
+
A
Parallel Plate CapacitorsParallel Plate Capacitors
• 3/.The material between ()
-
-
-
+
+
+
High material
Called a
DIELECTRIC
--
--
++
++
EquationsEquations
C
d
A=
For the parallel plate capacitor
Distance in meters
Area In m2
Permitivity inFm-1
CapacitanceIn Farads
Example 1Example 1
0
C0.01m
0.04m2
=
The common area of the plates of an air capacitor is 400cm2 if the distance between the plates is 1cm and ε0=8.5x10-12Fm-1.
C
d
A=
8.5x10-12Fm-
1x=3.4x10-11F.
Capacitance experiment on the internet
EquationsEquations
C
V
Q=
Capacitance on any conductor
Potential Difference in volts
Charge in Coulombs
CapacitanceIn Farads
Placing a charge of 35μC on a conductor raises it's potential by 100 V. Calculate the capacitance of the conductor.
Info Q = 35μC and V = 100V find C=?
Using Q=VC or C = Q/V
= 35 x 10-6/100
= 35 x 10-8 Farads
EquationsEquations
C½Work Done
(V)2=
Energy stored on a capacitor
Voltage Squared
CapacitanceIn Farads
Energy Stored
Example 3Example 3Find the capacitance and energy stored of a
parallel plate capacitor with 2mm between the plates and 150cm2 overlap area and a dielectric of relative Permittivity of 3. The potential across the plates is 150V.
A = 150cm2=0.015m2, d = 2x10-3m,
ε = 3xε0 = 27x10-12Fm-1
As C = ε0A/d = 27x10-12 x 0.015/0.002 = 2.025x10-9 F
Energy stored = ½ C V2 = ½ x 2.025x10-9x (150)2
= 2.28x10-5 Joules
DC and ACDC 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
Vrms= Vpeak/ √2
1/50th s
240V
V
V
Time
T
The National GridThe National Grid
Power Transmitted is = P = V.I
JOULES LAW gives us the power turned into heat
Power Lost = I2R
So if we have a high voltage we only need a small current. We loss much less energy
Power stationStep up
transformerStep down
transformerHomes
Joules lawJoules law
Heating coil Lagging
Calorimeter Water
A
LidDigitalthermometer
10°C
Calculation and GraphCalculation and GraphRepeat 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. Joule’s law.
Electrical Power lost as Heat P I2 is Joules lawThe power lost (Rate at which heat is produced) is
proportional to the square of the current.
∆
I2
Coulomb's LawCoulomb's Law
• Force between two charged bodies
Force = f Q1.Q2
d2
Q1 Q2d
Put this as a sentence to get a law!
Coulomb CalculationsCoulomb Calculations
• We replace the proportional with a equals and a constant to get an equation
Force =f Q1.Q2
d2
Force = f = Q1.Q2
4d2
= permitivity as in capacitors
Coulomb's Law CalculationsCoulomb's Law Calculations
• Force between these bodies
Force = f = Q1.Q2
4d2
2C 4mCd=2m
= 3.4 x 10-11
Coulomb's Law CalculationsCoulomb's Law Calculations
• Force between these bodies
2C 4mCd=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
PrecipitatorPrecipitator
• Carbon and ash - can be removed from waste gases with the use of electrostatic precipitators
PhotocopierPhotocopier
• Charging:• Exposure: • Developing:• Transfer: • Fusing:• Cleaning:
Potential Difference (V)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)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 FieldCurrent in a Magnetic Field
N S N S
Current in a Magnetic FieldCurrent in a Magnetic Field
N S
Force
CurrentMagnetic Field
A conductor carrying a current in a magnetic field will always feel a force
The force is perpendicular to the current and the field. – This is THE MOTOR EFFECTTHE MOTOR EFFECT
Fleming’s Left Hand RuleFleming’s Left Hand Rule
I used my left hand to show the direction the wire would move
The Size of the ForceThe Size of the Force
Force = F = B.I.lForce = F = B.I.lWhere 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
The AmpereThe Ampere
• Basic unit of electricity
F=2x10-
7N/m
1m
The current flowing is 1A when the force between two infinitely long conductors 1m apart in a vacuum is 2x10-7N Per metre of length.
Moving ChargeMoving 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-Velocity
Force
e -
VelocityForce
e -
Velocity
Forcee-
e-
Moving ChargeMoving Charge
• A positive will move the other way
e-Velocity
Force
+
All charged All charged particles particles moving in moving in magnetic magnetic fields always fields always have a force have a force at right angles at right angles to their to their velocity so velocity so follow a follow a circular path circular path due to FLH due to FLH RuleRule
Force 0n a ParticleForce 0n a Particle
Force = F = B.q.vForce = F = B.q.vWhere B = Magnetic Field Density in Tesla (T)
q=charge on the particle (C) v=velocity of the particle…
ExampleExample What is the force acting on a particle travelling at What is the force acting on a particle travelling at 80m/s carrying a charge of 0.1C in a 10T magnetic field?80m/s carrying a charge of 0.1C in a 10T magnetic field?
UsingUsing Force = F = B.q.v= 10x.1x80
= 80N
InductionInductionis 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
ElectromagnetElectromagnetic inductionic 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
Faraday’s LawFaraday’s LawBasically
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
Lenz’s LawLenz’s LawThe 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 Mutual inductioninduction
• 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.
TransformersTransformersThis how A.C. changes voltage up or down
V In
V Out
Turns 2
Turns 1=
Self InductionSelf Induction
• property whereby an electromotive force (EMF) is induced in a circuit by a variation of current in the circuit its self
D.C. SourceCurrent Back EMF
Another example on LENZ’S LAW
Flux DensityFlux Density• Magnetic flux, represented by the Greek
letter Φ (phi), total magnetism produced by an object. The SI unit of magnetic flux is the Weber
• Magnetic field (B) is the flux through a square meter (the unit of magnetic field is the Weber per square meter, or Tesla.)
As the flux expands the density through any square meter decreases