Download Unit P2 - Physics for your Future 1

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Unit 2 –
Physics for your Future
(EdExcel)
N Smith
St. Aidan’s
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
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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
= =