Transcript In series

Electricity
Electric charge
Conductors and insulators
Electric fields
Current, voltage, and resistance
Series and parallel circuits
Electrical power
Mains electricity
Calculating electrical energy
Bogota at night, it is so bright that it
can even be seen from space
Electric charge
• Electric charge or “electricity” can come from battery
and generators. But some materials become charged
when they are rubbed- electrostatic charge.
• Negative and positive charges- Experiments suggest
that there are two different and opposite types of el.
Charge: positive +, and negative -.
• Where charges come from?
• Atoms have electric charges inside them.
• A simple model of the atom: there is a central nucleus
made up of protons and neutrons, and electrons
orbiting around.
• Nb.el.=nb. Pr…… net charge=0
Conductors and insulators
• Conductors are materials that let electrons pass
through them. These free electrons also make
metals good thermal conductors. (metals)
• Insulators are materials that hardly conduct at all.
Their electrons are tightly held to atoms and are
not free to move.
• Semiconductors- These are ”in-between”
materials. They are poor conductor when cold,
but much better conductor when warm.
Attraction of uncharged objects
• A charged object will attract any uncharged
object close to it.
• Earthing- if enough charge builds up on
something, el may be pulled through the air and
cause sparks- it is dangerous. When earthing that
object( to the ground), charge flows away.
• Induced charges-Charges that appear on an
uncharged object because of a charged object
nearby.
Unit of charge
• The SI unit of charge is the coulomb (C).
• 1C=to charge on about 6 million million
million electrons.
• Often it is more convenient to measure charge
in microcoulombs
• 1microcoulomb (μC)=10^-6C
• 1electron charge=-1.6 x 10^-19C
• 1 proton charge=+1.6 x 10^-19C
Using electrostatic charge
• Electrostatic precipitators- are fitted to the
chimneys of some power stations and
factories.
• Industrial inkjet printers
• Photocopiers (p:173)
Questions
• 1.What charge does an electron have, + or -?
• 2.Would two electrons attract or repel one another?
• 3.Two identical metal spheres are placed close to one
another. One is given a large negative charge. The two
are then connected by a wire. Use the idea of electric
force to explain what happens next.
• 4.How many electrons make 1C of charge?
• Example:
• The charge on a rubbed polythene rod is, typically, only
about 0.005 μC
Electric field
• We have electric field around the charges.
• We use lines with arrows on them, to
represent electric fields.
• the field lines always point away from +,and
towards – (p: 174 pictures)
Curves, points, and ions
• When a conductor is charged up, like charges
repel, so they collect on the outside.
• The charges are most concentrated near the
sharpest curve, electric field is strongest and
the field lines are closest together.slike
• At this point electric field is strong enough to
ionize the air. Q p175
• If this point touches Van de Graaff generator,
charge immediately leaks away from it.
Van de Graaff generator
Ions
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Ions are electrically charged atoms, or group of atoms.
Atoms become ions if they lose or gain electrons.
Most of the molecules in air are uncharged, but not all.
Flames, air movement, natural radiation can all remove
electrons from molecules and ions are formed. Ions
can recombine with any free electrons around and
become neutral again.
• Normally air is neutral, but with ions air becomes a
conductor. In a thunderstorm, the concentrations of
different ions may be so great that a very high current
may flow through the air, causing a flash of lightning
Current in a simple circuit
• The conducting path through the bulb, wires,
switch, and battery is called a circuit.
• There must be a complete circuit for the
electrons to flow.
• Turning the switch OFF breaks the circuit and
stops the flow.
Measuring current
• A flow of charge is called an electric current.
• The higher the current, the greater the flow of
charge.
• SI unit of current is the ampere (A).
• Current can be measured by connecting an
ammeter into the circuit.
• For smaller current, a miliammeter is used.
• 1A=1000mA
Charge and current
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There is a link between charge and current
Charge=current x time
C=A x s
1C/s=1A
1C is the charge that passes when a current of 1A flows for
1s.
• The ampere is one of the SI base units.
• Current direction- conventional current direction from + to –
round the circuit.
• Being negatively charged, electrons are repelled by negative
charge, so are pushed out of the negative terminal of the
battery
Potential difference
• PD (voltage) across a cell-the cell has a voltage
marked on it. The higher its voltage, the more
energy it gives to the electrons pushed out.
• The scientific name for voltage is potential
difference. PD can be measured by connecting
a voltmeter.
• 1V=1J/1C…1V represents 1J of potential
energy given to each coulomb of charge.
Cells in series
• To produce a higher PD, several cells can be
connected in series.
• PDs around a circuit.
• The electrons flow through two bulbs, they lose
some of potential energy in the first bulb. The
rest in the second.
• Moving round a circuit, from one battery terminal
to the other, the sum of the PDs across the
components is equal to the PD across the battery.
Components in parallel
• There is another way to connect two lamps to a
cell. Place them side by side, you can see that
both lamps shine brightly.- they are in parallel
• When two lamps are connected in series, they
are dim.
• “in series”-”end-to- end”
• “in parallel”-”side- by- side”
• Current in parallel circuit, each lamp gets its own
share of the current. This shows that it is easier
for the current to flow.
Resistance
• Equation:
• Resistance(Ω)=PD across conductor (V)/current through conductor
(A)
• SI unit of resistance is the ohm (Ω)
• Some factors effecting resistance:
• 1)Length- doubling the length of a wire doubles its resistance
• 2)Cross-sectional area: Thin wire has more resistance then a thick
one.
• 3)Material: a nichrome wire has more resistance than a copper wire
of the same size.
• 4)Temperature: For metal conductors, resistance increases with
temperature.
• For semiconductors, it decreases with temperature.
Resistance components
• Resistors: In simple circuits, they reduce the current. In
more complicated circuits, such as those in radio, TV,
computers, they keep current and PDs at the same
level.
• Variable resistors (rheostats): used for varying current.
• Thermistors: have a high resistance when cold but
much lower resistance when hot.
• Light-dependent resistors (LDRs): have a high
resistance in the dark but a low resistance in the light
• Diodes: have an extremely high resistance in one
direction but low resistance in the other.
Resistance
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V, I, R equations:
R=V/I
R- resistance; V- PD; I-current
V=IR
I=V/R
Ohm’s law
The current is proportional to the PD.
Experiment results shows:
Graph of current against PD is a straight line through
the zero point
• If the PD doubles, the current doubles
• PD/current, always has the same value
More about resistance factors
• 1) A and B copper wires have the same cross-sectional
area and temperature, but
• B has2 x length of A
• B has 2 x resistance of A
• 2) C has 2 x cross-sectional area of B
• C has ½ x resistance of B (p:184)
• Proportionality problems:
• R=ρ x l/A ρ- constant for the material resistivity
• If wires A and B are made from the same material, ρ is
the same, and we can do:
• resistanceA x areaA/lengthA=res.B x ar.B/le.B
Series and parallel circuits
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(p:186)
Bulbs in series:
Bulbs share the PD-dimly
If the bulb is removed the other goes out- circuit
is broken
• Bulbs in parallel:
• Each gets the full PD
• If one bulb is removed, the other keeps workingit is part of unbroken circuit.
Basic circuit rules
• In series:
• Current through each of the component is the
same
• The total PD is the sum of the PDs across each of
them
• In parallel:
• PD across each of the components is the same.
• The total current in the circuit is the sum of the
currents in the branches.
Cell arrangements
• If cells are connected in series, the total PD is
the sum of the individual PDs
• PD across parallel cells is only the same as
from one cell, but together the cells can
deliver a higher current.
Combined resistance of resistors in
series and in parallel
• In series:
• R=R1+R2
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In parallel:
1/R=1/R1+1/R2
R=R1 x R2/R1+R2
(work p:189)
Electrical power
• Power=energy transformed/ time taken
• (Example: If the battery is supplying 5 joules of
energy every second, power is 5 watts.)
• In symbols: P=VI (equation)
• Units:
W=VA ( 1kW=1000watts)
• Power is the rate at which energy is transformed (
changed from one form to another)
• Appliances have a power rating marked on them.
• (Work p: 190, 191)
Power dissipated in a resistor
• When a current flows through a resistor, it has a heating
effect. Electrons lose potential energy, which is changed
into thermal energy.
• Scientifically speaking, energy is dissipated in the resistor.
• Another useful version of the electrical power equation:
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Power=PD x current
• But: PD= current x resistance
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power= current x resistance x current
P=I^2R
(resistors have resistance measured in ohms-Ω)
(work p:191)
Mains electricity (1)
DC (one-way; direct current)
• AC (alternating current; mains current)
• It flows backwards and forwards, backwards
and forwards…. 50 times per second, in some
countries. The mains frequency is 50Hz- hertz.
• In other countries, the mains frequency is
60Hz.
AC is easier to generate than DC ( like that from
a battery).
Live, neutral wire, switch, earth,
double insulation, plugs
• Live-or hot, or active wire, alternately negative and positive, making
the current flow backwards and forwards through the circuit.
• Neutral (or cold) wire- it is kept at zero voltage by the electricity
supply company.
• Switch- fitted in the live wire. Because, if it would be in neutral
wire, live wire would be live with switch off.
• Fuse- this is a thin piece of wire which overheats and melts if the
current is too high. Similar to the switch.(circuit breaker instead of a
fuse.)
• Earth- wire. This is a safety wire. It connects the metal body of
things, to earth.
• Double insulation-some appliances do not have an earth wire. Case
is made of plastic.
Plugs
• Plugs are safe and simple way of connecting appliances
to the mains.
• - plugs with two metal pins- live and neutral, earth
connection made by two metal contacts at the edge.
• - some plugs have a third pin for the earth connection
• A few countries use a three-pin plug with a fuse inside
• Fuse value:
• Example:
• Kettle: 2300W, 230V
• Current=power/voltage=2300W/230V=10A
• So a 13A fuse is needed
Mains electricity-2
• Circuits around the house
• The electricity supply company's cable into each house
contains a live and a neutral wire.
• In the consumer unit, these wires branch into several
parallel circuits; it also contains an earth wire.
• In the consumer unit (fuse box, breaker box) – circuit
breaker is an automatic switch when the current rises
above the specified value.
• For extra safety, some circuit can be fitted with a residual
current device (RCD)
• Mains sockets- is protected by its own fuse or circuit
breaker in the consumer unit (p:194)
• Two- way switches (p:195)
Safety first
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Old frayed wiring.
Long extension leads
Water in sockets or plugs
Accidentally cutting cables
• If an accident happens, and someone is
electrocuted, you must switch off at the socket
and pull out the plug before giving any help.
Electrical energy calculations
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Power=energy transformed/ time taken [W]=[J]/[s]
Power=PD x current
[W]=[V][A]
Calculating energy
Energy transformed=power x time
[J]=[W][s]
Energy transformed=PD x current x time
[J]=[V][A][s]
Measuring energy in kilowatt-hours
Energy supplied=power x time
[kWh]=[kW][h]
Example: 1kW is 1000W; 1h is 3600s
So if 1kW appliance is used for 1h
Energy supplied= 1000W x 3600s= 3 600 000 J
1kWh=3 600 000J
Central electricity
• The “electricity meter” in a house is an
energy meter. The more energy you take, the
more you have to pay.
• Example:
Present
Meter
reading
Previous
Meter
reading
Units used
Cost per unit
(cu)
INCL tax
Cost
(cu)
42935
41710
1225
10
12250
• energy supplied=42935kWh41710kWh=1225kWh=1225units
• Cost of energy supplied=1225 x 10=12250cu
• Example 1:
• If energy is 10cu per unit, what is the cost of
running a 2kW heater for 3 hours?
• Energy supplied=power x time=2kW x 3h=6 units
• Total cost=6 x 10cu= 60cu
• Example 2:
• If energy costs 10cu per unit, what is the cost of
running a 100W lamp for 30 minutes?
• 100W=0,1kW
• 30min=0,5h
• Energy supplied=power x time=0,1kW x 0,5h=0,05 unit
• Total cost= 0,05 x 10cu= 0,5cu
• Work (p:197,198,199)