Transcript document

cell
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energy
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electron
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lamp
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Coulomb of charge (electrons)
Think of it as a “bag of electrons” (containing
6250000000000000000 electrons!)
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Current
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I’m counting
how many
coulombs of
electrons go
past me every
second
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The number of
Coulombs flowing
A
past a point in the
circuit every second.
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1 Amp = 1 coulomb per second
Voltage(emf)
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V
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I’m checking the
difference in
energy (per
coulomb) between
the 2 red arrows
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1 Volt = 1 Joule per coulomb
Voltage (p.d.)
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I’m checking the
difference in
energy (per
coulomb) before
and after the lamp
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V
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1 Volt = 1 Joule per coulomb
Draw the sentence
1. Electricity is the flow of electrons.
2. We measure the quantity of electrons in Coulombs.
3. One Amp means one Coulomb of electrons flows round
every second.
4. The electrons carry energy from the cell to the lamp.
5. The electrons then return to the cell to get more energy.
6. The electrons are NOT used by the lamp.
7. Voltage measures the amount of energy each coulomb
receives or gives.
8. Physics is the most important subject Science subject!
Cell and battery of cells
d.c. power supply
a.c. power supply
Lamp
Resistance of a filament lamp
• Resistance increases as the current
increases (because the wire gets hot)
Voltage
Current
open switch
Ammeter
Voltmeter
Resistor
Variable resistor (rheostat)
Bell/buzzer
Thermistor
• Resistance decreases with increasing
temperature
Resistance
Temp
Light-dependent resistor (LDR)
• High resistance in the dark but a low
resistance in light
Diode
• Allows current to flow in one direction only
(from + to – in the direction of the “arrow)
Current
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Can you
COPY
the blue
writing
please?
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I’m counting
how many
coulombs of
electrons go
past me every
second
☺
The number of
Coulombs flowing
A
past a point in the
circuit every second.
☺
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☺
1 Amp = 1 coulomb per second
Current
I = Q/t
Where I = current in Amps
Q = charge (# of electrons) in Coulombs
t = time in seconds
In a series circuit
Current is the same at any point in the circuit
2.5 A
2.5 A
2.5 A
2.5 A
In a parallel circuit
The current splits (total current stays the
same)
2.5 A
2.5 A
1.25 A
1.25 A
Milliamps!
1 A = 1000 mA
1 mA = 0.001 A
Current
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Can you copy
this please?
☺
☺
I’m counting
how many
coulombs of
electrons go
past me every
second
☺
The number of
Coulombs flowing
A
past a point in the
circuit every second.
I = Q/t
☺
☺
☺
☺
1 Amp = 1 coulomb per second
Voltage(emf)
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☺
☺
V
☺
☺
☺
I’m checking the
difference in
energy (per
coulomb) between
the 2 red arrows
☺
☺
☺
☺
☺
1 Volt = 1 Joule per coulomb
Voltage (p.d.)
☺
☺
I’m checking the
difference in
energy (per
coulomb) before
and after the lamp
☺
☺
☺
☺
V
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1 Volt = 1 Joule per coulomb
Voltage
Voltage is the amount of energy transferred
(changed) per coulomb of charge.
1 Volt = 1 Joule per coulomb
A voltmeter connected across the
component
In a series circuit
The sum of the voltages across the lamps
equals the voltage across the cells
9V
3V
3V
3V
In a parallel circuit
In a simple parallel circuit, voltage across
each lamp equals the voltage across the
cells
5V
5V
5V
Resistance
Measures how difficult it is for current to
flow. Measured in Ohms (Ω)
V
Resistance = voltage/current
A
R = V/I
Ohm’s Law
• V = IR
V
I
X
R
Resistance of a lamp
Vary the voltage and current using a variable resistor
(rheostat). Plot a graph of resistance against current
V
Resistance = voltage/current
A
R = V/I
Power
The amount of energy used by a device
per second, measured in Watts (Joules
per second)
A
V
Power = voltage x current
P = VI
Example
• A 200 W television is plugged into the
110V mains. What is the current in the
television?
• I = P/V = 200/110 = 1.8A
P
V I
X
Total energy
So the total energy transformed by a lamp
is the power (J/s) times the time the lamp
is on for in seconds,
E = VIt
E = energy transformed (J)
V = Voltage (also called p.d.)
I = current (A)
t = time (s)
Example
• A kettle uses 240V and 8A. What is its
power?
• P = VI = 240x8 = 1920W (=1.9kW)
• How much energy does the kettle use in 5
minutes?
• E = VIt = 240x8x300 = 576000 J
P
V I
X
Alternating current
• Current changes direction 50 times a
second.
Fuse
• Thin piece of wire which melts if the
current is too high
• If a fault develops, this thin piece of wire
melts if the current is too high
• Circuit breakers are sometimes used
instead
Earth wire
• Connects metal body of appliance to earth
• If a live wire comes loose, current flows to
earth and blows the fuse
Double insulation
• Some appliances have no earth wire
because they have a plastic outer case
• The plastic case acts as an extra layer of
insulation around the wires
Electrostatics
1. There are two types of electric charge
(positive and negative)
Hi!
Hola!
Electrostatics
2. Opposites attract
Electrostatics
3. Static charges can be produced by the
action of friction on an insulator
Plastic,
amber,
glass,
Perspex
Electrostatics
4. Electrons are negatively charged
Electrostatics
http://phet.colorado.edu/sims/balloons/balloons_en.jnlp
5. Objects are charged positive by the loss
of electrons
cloth
Electrostatics
6. Objects are charged negative by the gain
of electrons
Only electrons move,
that’s why they call it
electricity!
cloth
Electrostatics
YouTube - Refueling Fire Caused by Static Electricity
7. Static charges can be dangerous
Electrostatics
7. Static charges can be dangerousYouTube
- Gas Station Fire, Static Electricity
Starts a Flash Fire.
Electrostatics
8. Static charges can be useful
Electrostatics
8. Static charges can be useful
Electrostatics
YouTube - Bill Nye the Science Guy - 25: Static Electricity 1/3
YouTube - Mitsuba Systems - Sprayright Series 700 Powder Spray System
8. Static charges can be useful
Draw the sentence
1.
2.
3.
4.
5.
6.
7.
8.
There are positive and negative charges
Opposite charges attract and like charges repel
Static charges can be made by the action of friction on an
insulating material
Electrons are negatively charged
Objects are charged positively by the removal of electrons
Objects are charged negatively by the addition of electrons
Static electricity can be dangerous (aeroplane refueling and gas
stations)
Static electricity can be useful (painting, photocopiers and removal
of smoke from chimneys)