Transcript Electricity revision

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 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
Current
• Flow of electrons
Current 
Charge Passed a point
time
I
Q
t
Current in a series circuit
If the current
here is 2
amps…
The current
here will
be…
2A
The
current
here will
be… 2A
And the
current
here will
be…
2A
In other words, the current in a series
circuit is THE SAME at any point
Current in a parallel circuit
A PARALLEL circuit is one where the current has a “choice
of routes”
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…
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”
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
Resistance
Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
That makes me so happy
Georg Simon Ohm
1789-1854
The resistance of a component can be
calculated using Ohm’s Law:
Resistance
(in )
=
V
Voltage (in V)
Current (in A)
I
R
An example question:
Ammeter
reads 2A
A
What is the resistance across this bulb?
V
As R = volts / current = 10/2 = 5
Assuming all the bulbs are the same what is
the total resistance in this circuit?
Voltmeter
reads 10V
Total R = 5 + 5 + 5 = 15 
VARIATION OF CURRENT (I) WITH P.D.
(V)
A
+
6V
-
V
Nichrome
wire
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
I
I
V
V
V
1. Resistor
Current
increases in
proportion
to voltage
2. Bulb
As voltage increases
the bulb gets hotter
and resistance
increases
3. Diode
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
10º C
10ºC
Ω
Digital
thermometer
Wire wound
on frame
Water
Glycerol
Heat source
Graph and Precautions
R

Precautions
- 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.
Factors affecting
Resistance of a conductor
• Material, temperature, Area cross section and length
R = L
A
 = Resistivity
Unit: ohm meter  m
RESISTIVITY OF THE MATERIAL OF A
WIRE
Micrometer
Nichrome
wire
Crocodile clips
l

Metre stick
Bench
clamp
Stand
 R
1.
Calculate the resistivity ρñ    A,
2

d
l
where A =
4
2.
Calculate the average value.
Precautions Ensure wire is straight
and has no kinks like ....
Take the diameter of the wire at
different angles
Resistors in series and Parallel
I
IT
R1
V1
R2
R3
VT  V1  V2  V3
V
I2
R1
I1
R2
IT  I1  I 2  I 3
Resistors in series and Parallel
I
IT
R1
V1
R2
R3
VT  V1  V2  V3
IRT  IR1  IR2  IR3
RT  R1  R2  R3
V
I2
R1
I1
R2
Resistors in series and Parallel
I
IT
R1
V1
R2
R3
IT  I1  I 2  I 3
V
V V
V



R T R1 R 2 R 3
1
1
1
1
 

RT R1 R2 R3
V
I2
R1
I1
R2
Wheatstone Bridge
Uses
– Temperature control
– Fail-Safe Device (switch
circuit off)
– Measure an unknown resistance
B
A
I
C
D
– R1 = R3 (When it’s balanced)
R2 R4
Metre Bridge
R1 = R2 (|AB|)
|BC|
Effects of an Electric Current
• Heat
• Chemical
• Magnetic
Current-voltage graphs
I
I
V
V
1. Active
Electrodes
2. Inert Electrodes
e.g. Copper in
Copper Sulphate
e.g. Platinum in
Water
Current Carriers
Medium
Carrier
Solid (Metal)
Electrons
Liquid (Electrolyte)
Ions
Gas
Electrons and Ions
Resistance in Semiconductors
1) Normal conductor like
metal resistance increases
as vibrating atoms slow
the flow of electrons
Resistance
2) Thermistor – resistance
DECREASES when
temperature INCREASES –
Due to more charge carriers
being liberated by heat
Resistance
Temperature
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)
2A
5A
Other safety devices…
1)
Insulation and double insulation
In some parts of Europe they have no
earth wire just two layer of insulating
material the sign is
2)
Residual Current Circuit Breaker
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
The casing touches the bare wire and it becomes live
The fuse will melt to
prevent electrocution
and the electricity is
carried to earth
A.C. Supply
That
Hurts!
Wiring a plug
1. Earth
wire
4. Live
wire
5. Fuse
2. Neutral
wire
3. Insulation
6. Cable
grip
Charge & Discharge
Uses of
Capacitors
• Storing charge for quick
release – Camera Flash
• Charging and discharging
at fixed intervals –
Hazard Lights
• Smoothing rectified
current – See
Semiconductors
Parallel Plate Capacitors
•
1.
The size of the capacitor depends on
The Distance the plates are apart d
-
+
-
+
-
+
d
Parallel Plate Capacitors
2 /.The area of overlap A
-
+
+
+
A
Parallel Plate Capacitors
•
3/.The material between ()
-
-
-
+
+
+
+
+
+
+
High  material
Called a
DIELECTRIC
Equations
For the parallel plate capacitor
Capacitance
In Farads
Permitivity in
Fm-1
C = A
d
Distance in
meters
Area
In m2
Example 1
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 = 0 A
d
C=
8.5x10-12Fm-1x 0.04m2 =3.4x10-11F.
0.01m
Capacitance experiment on the
internet
Equations
Capacitance on any conductor
Capacitance
In Farads
C = Q
V
Potential
Difference
in volts
Charge in
Coulombs
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
Equations
Energy stored on a capacitor
Energy
Stored
Capacitance
In Farads
2
C
(V)
Work Done = ½
Voltage
Squared
Example 3
Find 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 AC
V
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)
Time
1/50th s
240V
Find Root Mean Square of
voltage by
T
Vrms= Vpeak/ √2
V
The National Grid
Power station
Step up
transformer
Step down
transformer
Homes
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
Joules law
10°C
A
Digital
thermometer
Calorimeter
Heating coil
Lid
Water
Lagging
Calculation and Graph
∆
I2
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. Joule’s law.
Electrical Power lost as Heat P  I2 is Joules law
The power lost (Rate at which heat is produced) is
proportional to the square of the current.
Coulomb's Law
• Force between two charged bodies
Q1
Force = f
d
Q2
 Q1.Q2
d2
Put this as a sentence to get a law!
Coulomb Calculations
Force =f
 Q1.Q2
d2
• We replace the proportional with a
equals and a constant to get an
equation
Force =
f
=
Q1.Q2
4d2
 = permitivity as in capacitors
Coulomb's Law Calculations
• Force between these bodies
d=2m
2C
Force
=
f
4mC
=
Q1.Q2
4d2
 = 3.4 x 10-11
Coulomb's Law Calculations
• Force between these bodies
2C
d=2m
4mC
Electric Field Strength
Electric Field Strength
=
E = F/q
=
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
Photocopier
•
•
•
•
•
•
Charging:
Exposure:
Developing:
Transfer:
Fusing:
Cleaning:
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
N
S
Current in a Magnetic Field
A conductor carrying a current in a
magnetic field will always feel a force
Current
N
S
Magnetic
Field
Force
The force is perpendicular to the current and the
field. – This is THE MOTOR EFFECT
Fleming’s 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
The Ampere
• Basic unit of electricity
1m
F=2x10-7N/m
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 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-
Force
Velocity
e-
e-
Moving Charge
• A positive will move the other way
+
e-
Force
Velocity
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
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.
Moving Magnet
N
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
Faraday’s 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
Lenz’s 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.
The ring
moves away
as the
induced
current is
Aluminum
preventing
Ring
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
Current
Back
EMF
D.C. Source
Another example on LENZ’S 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 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