Download Unit P2 - Physics For Your Future

22/05/2017
Unit 2 –
Physics for your Future
(EdExcel)
W Richards
The Weald School
Topic 1 – Static and Current
Electricity
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The structure of the atom
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ELECTRON –
negative, mass
nearly nothing
NEUTRON –
neutral, same
mass as
proton (“1”)
PROTON –
positive, same
mass as
neutron (“1”)
The structure of the atom
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Particle
Proton
Relative Mass
1
Relative Charge
+1
Neutron
Electron
1
0
0
-1
MASS NUMBER = number of
protons + number of neutrons
SYMBOL
PROTON NUMBER = number of
protons (obviously)
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Static Electricity
Static electricity is when charge “builds up” on an object and
then stays “static”. How the charge builds up depends on what
materials are used and the insulator can be charged up by
friction by “transferring electrons”:
-
+
-
+
-
+
+
-
-
+
-
+
-
+
-
+
+
-
-
+
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Static Electricity
+
+
-
-
+
-
-
-
-
-
-
-
Short Static Experiments
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Try the following quick static electricity experiments:
1) Rubbing a balloon on your jumper and “sticking” it to the
wall
2) Charging a plastic rod by rubbing it with a cloth and then
holding it near the water from a smooth-running tap
3) Charging a plastic rod and trying to pick up small pieces of
paper (or someone else’s hair!) with it
4) Rubbing a balloon on someone else’s head – you might want
to ask their permission first…
Can you explain what you saw in each of these experiments?
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Static Electricity in Lightning
e-
e-
e-
e-
Van de Graaf generators
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When a charge is neutralised by
the movement of electrons either
from the Earth or to the Earth
we call this “earthing”
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Using Static in Paint Sprayers
Connected to
negative voltage
Connected to positive
voltage
1) Why is the paint sprayer given a negative charge?
2) Why is the car given a positive charge?
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Dangers of Static – fuelling lines
Electric Current
Electric current is a flow
of negatively charged
particles (i.e. electrons).
+
-
e-
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Note that
electrons go
from negative
to positive
By definition, current is “the
rate of flow of charge”
Notice that the electrons from this battery only went in one
direction around the circuit
e- – this is called “direct current”
(d.c.).
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Charge (Q)
As we said, electricity is when electrons move around a
circuit and carry energy with them. Each electron has a
negative CHARGE. Charge is measured in Coulombs (C).
We can work out how much charge flows in a circuit using
the equation:
Charge = current x time
(in C)
(in A)
Q
(in s)
I
T
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Example questions
Charge (C)
Current (A)
Time (s)
5
2
0.4
1
20
0.5
50
250
3
60
1) A circuit is switched on for 30s with a current of 3A. How
much charge flowed?
2) During electrolysis 6A was passed through some copper
chloride and a charge of 1200C flowed. How long was the
experiment on for?
3) A bed lamp is switched on for 10 minutes. It works on a
current of 0.5A. How much charge flowed?
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Example questions
Charge (C)
Current (A)
Time (s)
10
5
2
0.4
1
0.4
20
0.5
40
50
0.2
250
180
3
60
1) A circuit is switched on for 30s with a current of 3A. How
much charge flowed?
90C
2) During electrolysis 6A was passed through some copper
chloride and a charge of 1200C flowed. How long was the
experiment on for?
200s
3) A bed lamp is switched on for 10 minutes. It works on a
current of 0.5A. How much charge flowed?
300C
Topic 2 – Controlling and Using
Electric Current
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Circuit Symbols
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Variable
resistor
Diode
Switch
Bulb
A
V
Ammeter
Voltmeter
LDR
Resistor
Cell
Fuse
Thermistor
Battery
Basic ideas…
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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
More basic ideas…
If a battery is added
the current will
________ because
there is a greater
_____ on the
electrons caused by a
greater potential
difference
If a bulb is added
the current will
_______ because
there is greater
________ in the
circuit
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Current in a series circuit
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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
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Current in a parallel circuit
A PARALLEL circuit is one where the current has a “choice of
routes”. Notice how current is “conserved” at each junction:
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…
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Current in a parallel circuit
If the
current
here is 6
amps
And the
current here
will be…
The current
here will be…
The current
here will be…
The current
here will be…
Some example questions…
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3A
6A
4A
2A
1A each
Voltage in a series circuit
If the voltage
across the
battery is 6V…
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V
…and these
bulbs are all
identical…
…what will the
voltage across
each bulb be?
V
V
2V
Voltage in a series circuit
If the voltage
across the
battery is 6V…
…what will the
voltage across
two bulbs be?
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V
V
4V
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Voltage in a parallel circuit
If the voltage across
the batteries is 4V…
What is the
voltage here?
4V
V
And here?
V
4V
Summary
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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”
An example question:
3A
6V
3A
A1
6V
A2
V1
2A
1A
A3
3V
V2
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V3
3V
Another example question:
3A
10V
A1
1.2A
3A
A2
V1
6.7V
A3
5V
V2
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1.8A
V3
5V
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Energy and charge
The amount of energy that flows in a circuit will depend on
the amount of charge carried by the electrons and the
voltage pushing the charge around:
Energy transferred = charge x voltage
(in J)
(in C)
(in V)
By definition then, voltage means
“energy transferred per unit
charge” and 1V = 1J/C
W
V
Q
Example questions
1) In a radio circuit a voltage of 6V is applied and a
charge of 100C flows. How much energy has been
transferred?
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600J
2) In the above circuit the radio drew a current of
0.5A. How long was it on for?
200s
3) A motor operates at 6V and draws a current of
3A. The motor is used for 5 minutes. Calculate:
a) the charge flowing through it, b) the energy
supplied to it
900C,
5400J
4) A lamp is attached to a 12V circuit and a charge
of 1200C flows through it. If the lamp is on for
10 minutes calculate a) the current, b) the energy
supplied to the bulb.
2A,
14,400J
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Resistance
Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
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:
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Ammeter
reads 2A
A
V
Voltmeter
reads 10V
1) What is the resistance across
this bulb?
2) Assuming all the bulbs are the
same what is the total resistance
in this circuit?
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More examples…
3A
6V
12V
3A
2A
4V
2V
1A
What is the
resistance of
these bulbs?
Resistance
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Resistance is anything that opposes an electric current.
Resistance (Ohms, ) =
Potential Difference (volts, V)
Current (amps, A)
What is the resistance of the following:
1) A bulb with a voltage of 3V and a current of 1A.
3
2) A resistor with a voltage of 12V and a current
of 3A
4
3) A diode with a voltage of 240V and a current of
40A
6
4) A thermistor with a current of 0.5A and a
voltage of 10V
20
Varying Resistance
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Recall our earlier
idea that if you
increase the number
of bulbs in a circuit
you increase the
resistance and
therefore decrease
the current:
The same effect is seen when using a variable resistor:
Increase
the
resistance:
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Resistors, bulbs and diodes
Current-Voltage Graphs
Voltage on
powerpack/V
12
10
…
0
…
-10
-12
Current/A
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Voltage/V
I
Current-voltage graphs
I
I
V
1. Resistor
Current increases
in proportion to
_______,
provided the
temperature
doesn’t change
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V
V
2. Bulb
As voltage increases the
bulb gets ______ and
_______ increases due to
increased vibrations in the
ions in the filament
3. Diode
A diode only lets
current go in one
_______ – it has
very _____
resistance in the
other direction
Words – resistance, high, voltage, hotter, direction
LDRs and Thermistors
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Two simple components:
1) Light dependant
resistor – resistance
DECREASES when light
intensity INCREASES
Resistance
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2) Thermistor –
resistance DECREASES
when temperature
INCREASES
Resistance
Amount of light
Temperature
Understanding Resistance
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When a voltage is applied it basically causes electrons to move
towards the positive end of the battery:
Negative
Electrons
Ions
Positive
Notice that the ions were vibrating and getting in the way of
the electrons – this is resistance. This effect causes the
metal to heat up.
Using this heating effect
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This heating effect can have its advantages and its
disadvantages. For example, consider an old-fashioned light
bulb:
This heating
effect causes the
filament to emit
light…
…but it also causes
a lot of energy to
be wasted to the
environment
Electrical Power revision
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Power is defined as “the rate of transferring energy” and is
measured in units called “Watts” (W).
The amount of power being transferred in
an electrical device is given by:
Power = voltage x current
in W
in V
in A
P
V
I
1) How much power is transferred by a 230V fire that runs
on a current of 10A?
2) An electric motor has a power rating of 24W. If it runs
on a 12V battery what current does it draw?
3) An average light bulb in a home has a power rating of
60W and works on 230V. What current does it draw?
Energy and Power
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The POWER RATING of an appliance is simply how much
energy it uses every second.
In other words, 1 Watt = 1 Joule per second
Energy transferred (J) = power (W) x time (s)
OR
Energy (J) = current (A) x voltage (V) x time (s)
Some example questions
1) A battery gives out a current of 0.2A and has
a voltage of 1.5V. If it is used for 30
seconds how much energy has it transferred?
2) An electric fire runs at a voltage of 230V
and a current of 8A. If it is left on for 2
hours how much electrical energy has it
transferred?
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9J
13.2MJ
3) A toaster transfers 20,000J of electrical
energy. If it runs at a voltage of 230V and a
current of 2A how long was it on for?
43.5s
4) A light bulb is left on overnight for 8 hours.
If it transfers 1,000,000J of energy and
runs on a voltage of 230V what current did it
draw?
0.15A
Topic 3 – Motion and Forces
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Some subtle differences…
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“Distance” is how far you have gone, “displacement” is how far
you are from a point and can be positive or negative:
Distance =
Distance =
Displacement =
Displacement =
Start
-1 metre
1 metre
Distance
Distance
= =
Displacement
Displacement
= =
Some subtle differences…
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“Speed” means “how fast you are going”, “velocity” means “how
far you are going in a certain direction”. If the following
journeys take 1 second then work out:
Speed =
Speed =
Velocity =
Velocity =
Start
-1 metre
1 metre
Speed
Speed
= =
Velocity
Velocity
= =
Speed vs. Velocity
1) Is this car travelling at constant speed?
2) Is this car travelling at constant velocity?
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Vector vs. scalar
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Scalar quantities have size (“magnitude”) only and no direction.
Vector quantities have both size and direction.
Scalar or vector???
Scalar
Vector
2. Distance10. Acceleration
1. Mass
6. Energy
7. Time
3. Displacement
4. Speed
9. Force
8. Current
5. Velocity
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Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
T
1) Freddie walks 200 metres in 40 seconds. What is his
speed?
5m/s
2) Hayley covers 2km in 1,000 seconds. What is her
speed?
2m/s
3) How long would it take Lauren to run 100 metres if she
runs at 10m/s?
4) Jake travels at 50m/s for 20s. How far does he go?
5) Izzy drives her car at 85mph (about 40m/s). How long
does it take her to drive 20km?
10s
1000m
500s
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Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
T
1) Sarah walks 2000m in 50 minutes. What is her speed in
m/s?
0.67m/s
2) Jack tries to walk the same distance at a speed of 5m/s.
How long does he take?
400s
3) James drives at 60mph (about 100km/h) for 3 hours. How
far has he gone?
4) The speed of sound in air is 330m/s. Molly shouts at a
mountain and hears the echo 3 seconds later. How far
away is the mountain? (Careful!)
300km
495m
Distance-time graphs
2) Horizontal line =
40
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4) Diagonal line
downwards =
30
Distance
(metres)
20
10
0
Time/s
20
1) Diagonal line =
40
60
80
100
3) Steeper diagonal line =
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40
Distance
(metres)
30
20
10
0
20
40
60
80
1) What is the speed during the first 20 seconds?
100
Time/s
0.5m/s
2) How far is the object from the start after 60 seconds?
40m
3) What is the speed during the last 40 seconds?
1m/s
4) When was the object travelling the fastest?
40-60s
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Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
1) A cyclist accelerates from 0 to 10m/s in 5 seconds.
What is her acceleration?
T
2m/s2
2) A ball is dropped and accelerates downwards at a rate of
10m/s2 for 12 seconds. How much will the ball’s velocity
increase by?
120m/s
3) A car accelerates from 10 to 20m/s with an acceleration
of 2m/s2. How long did this take?
5s
4) A rocket accelerates from 1,000m/s to 5,000m/s in 2
seconds. What is its acceleration?
2000m/s2
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Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
T
1) Will accelerates from standstill to 50m/s in 25 seconds.
What is his acceleration?
2m/s2
2) Pierre accelerates at 5m/s2 for 5 seconds. He started at
10m/s. What is his new speed?
35m/s
3) Elliott is in trouble with the police. He is driving up the A29
and sees a police car and brakes from 50m/s to a standstill.
His deceleration was 10m/s2. How long did he brake for?
5s
4) Another boy racer brakes at the same deceleration but only
for 3 seconds. What speed did he slow down to?
20m/s
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Velocity-time graphs
1) Upwards line =
80
Velocity
m/s
4) Downward line =
60
40
20
0
10
2) Horizontal line =
20
30
40
50
3) Upwards line =
T/s
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80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
50
1) How fast was the object going after 10 seconds?
40m/s
2) What is the acceleration from 20 to 30 seconds?
2m/s2
3) What was the deceleration from 30 to 50s?
3m/s2
4) How far did the object travel altogether?
1700m
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80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
50
1) How fast was the object going after 10 seconds?
10m/s
2) What is the acceleration from 20 to 30 seconds?
4m/s2
3) What was the deceleration from 40 to 50s?
6m/s2
4) How far did the object travel altogether?
1500m
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80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
50
This velocity-time graph shows Coryn’s journey to school.
How far away does she live?
2500m
Introduction to Forces
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A force is a “push” or a “pull”. Some common examples:
Weight (mg) – pulls
things towards the
centre of the Earth
Friction – a contact force
that acts against anything
moving
Air resistance/drag – a contact
force that acts against anything
moving through air or liquid
Upthrust – keeps things afloat
Free body force diagrams
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The Earth pulls Newton down with a
gravitational force of 700N.
direction
what
on
what
type
size
Newton pulls the Earth up with a
gravitational force of 700N.
Action and reaction are equal
and opposite!!
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Balanced and unbalanced forces
Consider a camel standing on a road.
What forces are acting on it?
Reaction
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
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Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
Air Resistance
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Air resistance is a force that opposes motion through air. The
quicker you travel, the bigger the air resistance:
The same applies to a body falling through a liquid (called
“drag” or “upthrust”).
Balanced and unbalanced forces
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Balanced and unbalanced forces
1) This animal is either
________ or moving
with _______ _____…
2) This animal is getting
________…
3) This animal is getting
_______….
4) This animal is also
either _______ or moving
with ________ ______..
Words - Stationary, faster, slower or constant speed?
Summary
Complete these sentences…
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If an object is stationary and has NO resultant force on it the object
will…
If an object is stationary and a resultant force acts on it the object will…
If an object is already moving and NO resultant force acts on it the
object will…
If an object is already moving and a resultant force acts on it the object
will…
…accelerate in the direction of the
resultant force
…continue to move at the same
speed and the same direction
…continue to stay stationary
…accelerate in the direction of the
resultant force
Resultant Force
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Calculate the resultant force of the following:
500N
100N
700N
600N
50N
700N
700N
200N
800N
800N
100N
Force and acceleration
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If the forces acting on an object
are unbalanced then the object will
accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
A
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Force, mass and acceleration
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
much does it accelerate by?
F
M
A
2m/s2
2000kg
2500N
0.5m/s2
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Weight vs. Mass
Earth’s Gravitational Field Strength is 10N/kg. In other
words, a 1kg mass is pulled downwards by a force of 10N.
W
Weight = Mass x Gravitational Field Strength
(in N)
(in kg)
(in N/kg)
M
1) What is the weight on Earth of a book with mass 2kg?
2) What is the weight on Earth of an apple with mass 100g?
g
20N
1N
3) James weighs 700N on the Earth. What is his mass?
70kg
4) On the moon the gravitational field strength is 1.6N/kg.
What will James weigh if he stands on the moon?
112N
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
resistance is _______ so he
_______ downwards.
2) As his speed increases his air
resistance will _______
3) Eventually the air resistance will be
big enough to _______ the
skydiver’s weight. At this point
the forces are balanced so his
speed becomes ________ - this is
called TERMINAL VELOCITY
Words – increase, small,
constant, balance, accelerates
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Terminal Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly ________,
causing him to start _____ ____.
5) Because he is slowing down his air
resistance will _______ again until
it balances his _________. The
skydiver has now reached a new,
lower ________ _______.
Words – slowing down, decrease,
increases, terminal velocity, weight
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Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
Topic 4 – Momentum, Energy,
Work and Power
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Stopping a car…
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What two things must the driver of the car do in order to stop
in time?
Tiredness
Stopping a car…
Thinking
distance
Too many
drugs
Heavy
vehicle
Tyres/brakes
worn out
(reaction time)
Braking
distance
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Too much
alcohol
Poor
visibility
Wet/icy
roads
Driving too
fast
Total Stopping Distance = Thinking Distance + Braking Distance
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Momentum
Any object that has both mass and
velocity has MOMENTUM. Momentum
(symbol “p”) is simply given by the formula:
P
Momentum = Mass x Velocity
(in kgm/s)
(in kg)
(in m/s)
M
V
What is the momentum of the following?
1) A 1kg football travelling at 10m/s
2) A 1000kg Ford Capri travelling at 30m/s
3) A 20g pen being thrown across the room at 5m/s
4) A 70kg bungi-jumper falling at 40m/s
10kgm/s
30,000kgm/s
0.1kgm/s
2800kgm/s
Conservation of Momentum
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In any collision or explosion momentum is conserved (provided that there
are no external forces have an effect). Example question:
Two cars are racing around the M25. Car A collides with the back of car B
and the cars stick together. What speed do they move at after the
collision?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Mass = 1800kg
Speed = ??m/s
Momentum before = momentum after…
…so 1000 x 50 + 800 x 20 = 1800 x V…
…V = 36.7m/s
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Momentum in different directions
What happens if the bodies are moving in opposite directions?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Momentum is a VECTOR quantity, so the momentum of the
second car is negative…
Total momentum = 1000 x 50 – 800 x 20 = 34000 kgm/s
Speed after collision = 34000 kgm/s / 1800 = 18.9m/s
Another example
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Consider the nuclear decay of Americium-241:
237
93
Np
241
95
Am
If the new neptunium atom moves away at
a speed of 5x105 m/s what was the speed
of the alpha particle?
2.96x107 m/s
4
2
α
More questions…
1.
A car of mass 1000kg heading up the M1 at 50m/s collides
with a stationary truck of mass 8000kg and sticks to it.
What velocity does the wreckage move forward at?
2. A defender running away from a goalkeeper at 5m/s is hit
in the back of his head by the goal kick. The ball stops
dead and the player’s speed increases to 5.5m/s. If the
ball had a mass of 500g and the player had a mass of 70kg
how fast was the ball moving?
3. A white snooker ball moving at 5m/s strikes a red ball and
pots it. Both balls have a mass of 1kg. If the white ball
continued in the same direction at 2m/s what was the
velocity of the red ball?
4. A gun has a recoil speed of 2m/s when firing. If the gun
has a mass of 2kg and the bullet has a mass of 10g what
speed does the bullet come out at?
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5.6m/s
70m/s
3m/s
400m/s
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Recap question on momentum
1. Matt and Dan are racing against each other over 400m at
Sports Day. Matt is running at 8m/s and catches up with
Dan who is running at 6m/s. After the collision Matt stops
and Dan moves slightly faster. If Matt’s mass is 60kg and
Dan’s is 70kg calculate how fast Dan moves after the
collision.
12.9m/s
2. Bobbie is driving her 5kg toy car around. It is travelling at
10m/s when it hits the back of Heather’s (stationary) leg
and sticks to it. Assuming Heather’s leg can move freely
and has a mass of 10kg calculate how fast it will move after
the collision.
3.3m/s
Safety features
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How do air bags and crumple zones work?
Basically:
1) The change in momentum is the same with or without an
airbag
2) But having an airbag increases the time of the collision and
therefore reduces the “rate of change of momentum”
3) Therefore the force is reduced
Force and momentum
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Newton’s second law of motion says that the force acting on an
object is that object’s rate of change of momentum. In other
words…
Force = Change in momentum (in kgm/s)
(in N)
mv-mu
Time (in s)
Also called “impulse”
F
T
For example, Ronaldo takes a free kick by kicking a stationary football with
a force of 40N. If the ball has a mass of 0.5kg and his foot is in
contact with the ball for 0.1s calculate:
1) The change in momentum of the ball (its impulse),
2) The speed the ball moves away with
Example questions
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1) Paddy likes playing golf. He strikes a golf ball with a force
of 80N. If the ball has a mass of 200g and the club is in
contact with it for 0.2s calculate a) the change in
momentum of the golf ball, b) its speed.
16Kgm/s,
80m/s
2) Courtney thinks it’s funny to hit tennis balls at Kit. She
strikes a serve with a force of 30N. If the ball has a
mass of 250g and the racket is in contact with it for 0.15s
calculate the ball’s change in momentum and its speed.
4.5Kgm/s,
18m/s
3) Tom takes a dropkick by kicking a 0.4kg rugby ball away at
10m/s. If his foot was in contact with the ball for 0.1
seconds calculate the force he applied to the ball.
40N
4) Jenny strikes a 200g golf ball away at 50m/s. If she
applied a force of 50N calculate how long her club was in
contact with the ball for.
0.2s
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Work done
When any object is moved around work will need to be
done on it to get it to move (obviously).
We can work out the amount of work done in moving an
object using the formula:
Work done = Force x distance moved
in J
in N
E
in m
F
D
Example questions
1.
Hannah pushes a book 5m along the table with a force of
5N. She gets tired and decides to call it a day. How much
work did he do?
2. Courtney lifts a laptop 2m into the air with a force of 10N.
How much work does she do? What type of energy did the
book gain?
3. Tom does 200J of work by pushing a wheelbarrow with a
force of 50N. How far did he push it? What type of
energy did the wheelbarrow gain?
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25J
20J,
GPE
4m, KE
4. Dan cuddles his cat and lifts it 1.5m in the air. If he did
75J of work how much force did he use?
50N
5. Simon drives his car 1000m. If the engine was producing a
driving force of 2000N how much work did the car do?
2MJ
Stopping a car…
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Recall our earlier situation regarding stopping distances…
Energy and Power
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The POWER RATING of an appliance is simply how much
energy it uses every second.
In other words, 1 Watt = 1 Joule per second
E
E = Energy (in joules)
P = Power (in watts)
T = Time (in seconds)
P
T
Some example questions
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1) What is the power rating of a light bulb that transfers
120 joules of energy in 2 seconds?
60W
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
2KW
3) Rob runs up the stairs in 5 seconds. If he transfers
1,000,000J of energy in this time what is his power
rating?
0.2MW
4) How much energy does a 150W light bulb transfer in a)
one second, b) one minute?
150J,
9KJ
5) Jonny’s brain needs energy supplied to it at a rate of
40W. How much energy does it need during a 50 minute
physics lesson?
120KJ
6) Lloyd’s brain, being more intelligent, only needs energy at
a rate of about 20W. How much energy would his brain
use in a normal day?
1.73MJ
Gravitational Potential Energy
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To work out how much gravitational potential energy
(GPE) an object gains when it is lifted up we would use
the simple equation…
GPE
(Joules)
=
Weight
(newtons)
x
Change in height
(metres)
GPE
(Remember - W=mg)
mg
H
Some example questions…
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How much gravitational potential energy have the following
objects gained?:
1.
A brick that weighs 10N lifted to the top of a house
(10m),
100J
2. A 1,000kg car lifted by a ramp up to a height of 2m,
20KJ
3. A 70kg person lifted up 50cm by a friend.
350J
How much GPE have the following objects lost?:
1.
A 2N football dropping out of the air after being kicked
up 30m,
60J
2. A 0.5N egg falling 10m out of a bird nest,
5J
3. A 1,000kg car falling off its 200cm ramp.
20KJ
Kinetic energy
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Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found using
the formula:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
KE =
½
in m/s
mv2
Example questions
1) Bex drives her car at a speed of 30m/s. If
the combined mass of her and the car is
1000kg what is her kinetic energy?
2) Emma rides her bike at a speed of 10m/s. If
the combined mass of Emma and her bike is
80kg what is her kinetic energy?
3) Rob is running and has a kinetic energy of
750J. If his mass is 60kg how fast is he
running?
4) Josh is walking to town. If he has a kinetic
energy of 150J and he’s walking at a pace of
2m/s what is his mass?
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450,000J
4000J
5m/s
75kg
Stopping a car…
What happens inside the car when it stops?
In order to stop this car the
brakes must “do work”. This work
is used to reduce the kinetic
energy of the vehicle and the
brakes will warm up.
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An example question…
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This car can apply a maximum braking
force of 10,000N. If the car’s mass
is 1000Kg how far is its stopping
distance when it is travelling at a
speed of 15m/s (roughly 30mph) and
30m/s (roughly 60mph)?
15m/s = 11.25m stopping distance
30m/s = 45m stopping distance (4 times greater)
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A Practical Example of Doing Work
Consider a rocket re-entering the Earth’s atmosphere:
The rocket would initially have
a very high _______ energy.
This energy would then _____
due to friction caused by
collisions with _______ in the
atmosphere. These collisions
would cause the rocket to ____
up (_____ is “being done” on
the rocket). To help deal with
this, rockets have special
materials that are designed to
lose heat quickly.
Words – work, kinetic,
particles, heat, decrease
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Energy Changes in Roller Coasters
1) Electrical energy is
transferred into gravitational
potential energy
3) Kinetic energy is
transferred back
into gravitational
potential energy
2) Gravitational potential
energy is transferred into
kinetic energy
Test questions…
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1)
Julia tries to run 100m in 12 seconds and succeeds. How fast did
she run?
8.3m/s
2)
Isabelle accelerates at a rate of 2m/s2 for 3 seconds. If she
started at 10m/s what was her final speed?
16m/s
3)
Jake decides to lift his book up into the air. His book has a mass
of 100g and he lifts it 50cm. Calculate the work done.
0.5J
4)
Jamie accelerates from 0 to 10m/s in 5 seconds. If her mass is
60kg how much force did her legs apply?
120N
5)
Lily rides 1km at a speed of 20m/s. How long did the journey
take?
6)
Rob thinks it’s funny to push Jack with a force of 140N. If Jack
has a mass of 70kg calculate his acceleration.
2m/s2
7)
Vicky slams on the brakes on her bike and her brakes do
20,000J of work. If the combined mass is 100kg what speed
was she travelling at?
20m/s
8)
Paddy has a mass of 75kg. If he accelerates from 10 to 20m/s
in 2s how much force did he apply?
375N
50s
9)
Test questions…
Bex amuses herself by throwing things at Kit. If she throws a ball
with a speed of 20m/s and the distance between her and Kit is 5m
how long will it take to reach him?
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0.25s
10) Dave throws calculators around the room with a force of 20N. If
each calculator has a mass of 200g calculate the acceleration.
100m/s2
11) Max has a mass of 70kg. What is his weight on Earth, where the
gravitational field strength is 10N/kg?
700N
12) Kathryn does some work by pushing a box around with a force of
1N. She does 5J of work and decides to call it a day. How far did
she push it?
5m
13) On the moon Jake might weigh 112N. If the gravitational field
strength on the moon is 1.6N/kg what is his mass?
70kg
14) Heather likes bird watching. She sees a bird fly 100m in 20s. How
fast was it flying?
5m/s
15) How much kinetic energy would Simon have if he travelled at a
speed of 5m/s and has a mass of 70kg?
875J
Topic 5 – Nuclear Fission and
Nuclear Fusion
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The structure of the atom
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ELECTRON –
negative, mass
nearly nothing
NEUTRON –
neutral, same
mass as
proton (“1”)
PROTON –
positive, same
mass as
neutron (“1”)
The structure of the atom
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Particle
Proton
Relative Mass
1
Relative Charge
+1
Neutron
Electron
1
1/2000 (i.e. 0)
0
-1
NUCLEON/MASS NUMBER =
number of protons + number of
neutrons
SYMBOL
ATOMIC/PROTON NUMBER =
number of protons (obviously)
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Introduction to Radioactivity
Some substances are classed as “radioactive” – this means that
they are unstable and continuously give out radiation at
random intervals:
Radiation
The nucleus is more stable after emitting some radiation – this
is called “radioactive decay”. This process is NOT affected by
temperature or other physical conditions.
Ionisation
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Radiation is dangerous because it “ionises” atoms – in other
words, it turns them into ions by “knocking off” electrons:
Types of radiation
Unstable
nucleus
New
nucleus
Alpha
particle
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1) Alpha () – an atom decays into a new
atom and emits an alpha particle (2
protons and 2 ______ – the nucleus of a
______ atom)
2) Beta () – an atom decays into a new
atom by changing a neutron into a
_______ and electron. The fast moving,
Beta high energy electron is called a _____
particle particle.
Unstable
nucleus
New
nucleus
Unstable
nucleus
New
nucleus
3) Gamma – after  or  decay surplus
______ is sometimes emitted. This is
called gamma radiation and has a very
high ______ with short wavelength.
The atom is not changed.
Gamma
radiation
Words – frequency, proton,
energy, neutrons, helium, beta
Blocking Radiation
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Each type of radiation can be blocked by different materials:



Sheet of
paper (or
6cm of air
will do)
Few mm of
aluminium
Few cm of
lead
Summary
Property
Charge
Mass
Penetration
ability
Range in air
What is it?
Ionising ability
Alpha
Beta
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Gamma
Nuclear power stations
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Nuclear fission reactions can be a source of energy, like in a
nuclear power station:
Nuclear fission
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More
neutrons
Neutron
Uranium or
plutonium
nucleus
Unstable
nucleus
New nuclei
(e.g. barium
and krypton)
Chain reactions
Each fission reaction releases
neutrons that are used in
further reactions.
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Nuclear power stations
22/05/2017
Notice that the heat from these reactions is used to heat
water and turn it into steam, which then drives turbines.
Fission in Nuclear power stations
How are control rods used to control the rate of these reactions?
These fission reactions occur in the fuel
rods and they become very hot. Water
cools the rods (which then turns to steam)
and the control rods (made of boron) are
moved in and out to control the amount of
fission reactions taking place. This is
called a Pressurised Water Reactor (PWR)
Nuclear Fusion in stars
Proton
22/05/2017
Neutron
Nuclear fusion basically combines smaller nuclei to make
larger nuclei. It happens in stars but it’s not possible to use
it in power stations yet as it needs temperatures of around
10,000,000OC. At lower temperatures, electrostatic
repulsion of protons occurs (i.e. they repel each other due to
their positive charges).
Cold Fusion
Stanley Pons and Martin
Fleishmann
22/05/2017
In 1989 we claimed that we
had enabled “cold fusion”,
i.e. we had created fusion
reactions in lab
temperatures. However,
no one else could verify our
findings so our theories
have not been accepted.
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Topic 6 – Advantages and Disadvantages of
using Radioactive Materials
Background Radiation
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13% are
man-made
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
Notice that the amount of radon gas in the atmosphere varies
according to location so some areas in the UK are more
radioactive than others!
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Background Radiation by Location
In 1986 an explosion occurred at the Chernobyl nuclear power
plant. Here is a “radiation map” showing the background
radiation immediately after the event:
Other “risky” areas could be mining underground, being in a
plane, working in an x-ray department etc
Uses of radioactivity 1
Sterilising medical instruments
Gamma rays can be used to kill and sterilise
germs without the need for heating. The
same technique can be used to kill microbes
in food so that it lasts longer.
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Uses of radioactivity 2 - Tracers
A tracer is a small amount of radioactive material used to
detect things, e.g. a leak in a pipe:
Gamma
source
The radiation from the radioactive source is picked up above
the ground, enabling the leak in the pipe to be detected.
Tracers can also be used in medicine to detect
tumours:
For medicinal tracers, you would probably use a beta source
with a short half life – why?
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Uses of radioactivity 3 – Smoke Detectors
Smoke detectors
Alpha
emitter
+ve electrode
-ve electrode
Alarm
Ionised air particles
If smoke enters here a
current no longer flows
Uses of radioactivity 4 – Determining thickness
Beta
detector
Rollers
Paper
Beta
emitter
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Uses of Radioactivity 5 - Treating Cancer
High energy gamma radiation can be used to kill cancerous
cells. However, care must be taken in order to enure that the
gamma radiation does not affect normal tissue as well.
Radioactive iodine can be used to treat thyroid cancer. Iodine
is needed by the thyroid so it naturally collects there.
Radioactive iodine will then give out beta radiation and kill
cancerous cells.
A radioactive decay graph
Activity (Bq)
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“1 Becquerel” means “1
radioactive count per second”
Time
Half life
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The decay of radioisotopes can be used to measure the
material’s age. The HALF-LIFE of an atom is the time
taken for HALF of the radioisotopes in a sample to decay…
= radioisotope
At start
there are 16
radioisotopes
After 1 half
life half have
decayed
(that’s 8)
= new atom formed
After 2 half
lives another
half have
decayed (12
altogether)
After 3 half
lives another
2 have
decayed (14
altogether)
A radioactive decay graph
22/05/2017
Count
1 half
life
1 half
life
1 half
life
Time
Dating materials using half-lives
22/05/2017
Question: Uranium decays into lead. The half life of uranium is
4,000,000,000 years. A sample of radioactive rock contains 7 times as
much lead as it does uranium. Calculate the age of the sample.
Answer: The sample was originally completely uranium…
1 half life
later…
1 half life
later…
1 half life
later…
8
8
4
8
2
8
1
…of the
sample was
uranium
Now only 4/8 of
the uranium
remains – the
other 4/8 is lead
Now only 2/8 of
uranium remains
– the other 6/8
is lead
Now only 1/8 of
uranium remains
– the other 7/8
is lead
8
So it must have taken 3 half lives for the sample to decay until only 1/8
remained (which means that there is 7 times as much lead). Each half
life is 4,000,000,000 years so the sample is 12,000,000,000 years old.
An exam question…
22/05/2017
Potassium decays into argon. The half life of potassium is
1.3 billion years. A sample of rock from Mars is found to
contain three argon atoms for every atom of potassium.
How old is the rock?
(3 marks)
The rock must be 2 half lives old – 2.6 billion years
Ionisation
22/05/2017
Radiation is dangerous because it “ionises” atoms – in other
words, it turns them into ions by “knocking off” electrons:
Alpha radiation is the most ionising (basically, because it’s the
biggest). Ionisation causes cells in living tissue to mutate,
usually causing cancer.
Understanding Radioactivity over History
Task: Find out about the work of Marie Curie, including:
1) Which elements she discovered
2) Brief details of the work she did
3) What prizes she won
4) How her work eventually caused her death
5) How our understanding of radioactivity has changed due to
her work
Disposing of radioactive waste
The key to dealing with radioactive waste is
to IMMOBILISE it. There are a number of
ways of doing this depending on how
__________ the waste is:
High level waste is immobilised by mixing with
____ making ingredients, melting and pouring
the glass into steel containers.
Intermediate waste is set in
cement in _____ drums.
The containers are then kept
in stores, often _________.
Words – glass, steel, underground, radioactive
Nuclear Power Stations
Advantages
Disadvantages
Don’t produce
greenhouse gases
Low levels of
waste
Low fuel costs
More jobs for
local people
Risk of accident
Why use nuclear power?
Radioactive
waste
Visual pollution
More traffic