Download Unit One Physics 2013 - Mill-Park

PHYSICS UNIT ONE
What is in Unit One?
 Nuclear Physics and Radioactivity
 Electricity
 One of …
 Astrophysics
 Astronomy
 Energy from the Nucleus…
What is required?
 Pass the SACs in the three areas of study
 Turn up to <90% of classes
 Complete all weekly homeworks
 Optional … 40+ club homework
What do I need?
 Yr 11 Physics Textbook
 Scientific Calculator (CAS not allowed in
exam)
 Workbook
NUCLEAR PHYSICS AND
RADIATION
What is an atom
Three key historic figures in discovery of what
an atom is:
1) Democritus – concept of atomos
2) J. J. Thompson – the plum pudding model
3) Lord Rutherford – the solar system model
Rutherford’s Gold foil
experiment
A small amount of the radiation is
deflected by very large angles
Fired radiation at a thin sheet of gold
(only 200 atoms thick)
Most radiation particles pass
through undeflected
Rutherford’s gold foil
experiment
 He concluded two things:
 1) The atom is mainly empty space
 2) It has a small, but dense and positive
nucleus.
Activity – work in groups
 Your teacher wants to make a scale model of
an atom for the classroom. We want to use a
tennis ball for the nucleus and marbles will be
the orbiting electrons. If the size of a nucleus
is r = 1x10-14m, and the electrons orbit at a
distance of 1 x 10-10m; how far away do my
electron marbles have to orbit the tennis ball?
Activity: Extension
 How small will the scale model nucleus have
to be such that the entire scale model atom
will fit in the classroom?
The atom
Nucleus: Made of
neutrons and protons
Electrons orbit the
nucleus
The atom
Name
Charge
Mass (atomic units: a.u.)
Proton
+1
1
Neutron
0
1
Electron
-1
0.0005
Particles that live in the nucleus are
called nucleons.
Protons and neutrons are nucleons.
Electromagnetic Force and
Nuclear Force
 Protons have the same charge, and just like
having two magnets with similar poles, protons
repel each other
 How does the nucleus stick together?
 There is a force called the strong nuclear force. It
only works at very small distances (when
nucleons are almost touching).
 Adding neutrons to a nucleus increases the
attractive strong nuclear force without
increasing the repulsive electromagnetic force
Electromagnetic Force and
Nuclear Force
 This is similar to adding a piece of metal
between “like” poles of two magnets. It helps
them stick!
Isotopes
 Isotopes are atoms of the same element that
differ in the number of NUETRONS they have.
 Isotopes have similar chemical properties but
different physical properties.
Example:
 Carbon has 3 isotopes: Carbon -12, 13, and 14.
All three isotopes have 6 protons.
 C-12 has 6 neutrons.
 C-13 has 7 neutrons.
 C-14 has 8 neutrons.
Describing an atom
Mass Number: Protons + Neutrons
Z
E
A
Symbol of element
Atomic Number: Protons
This is the isotopic symbol for an atom
Isotopic Symbol
 Why are there mass numbers with decimal
places on the periodic table? eg
 35.5
17𝐶𝑙
 How can you half a neutron?
 On the periodic table, the mass numbers are
called the relative atomic mass numbers.
 They are an average of all isotopes of that
element found on earth.
Questions: Complete the
table
Element
12
6𝐶
No. of protons No. of neutrons
143
85
37𝑅𝑏
53
3
1𝐻
72
Name
Uranium
Radioisotopes
 Sometimes the nucleus of a particular isotope




is unstable, and it may undergo a radioactive
decay or transmutation
This is known as a radioisotope
Radiation is emitted
Every element heavier than Bismuth (A = 83)
is radioactive.
Every element heavier than Uranium (A = 92)
has to be produced artificially.
Nuclear Radiation
 Three types of radiation can be emitted in
nuclear decay
 Alpha (α)
 Beta (β)
 Gamma (γ)
Radiation
Symbol
α
β
γ
Name
alpha
beta
Gamma
Made of
Two protons, two
neutrons (helium
nucleus)
electron
energy
+2
-1
Charge
Mass
Typical Energy
Heavy (4 a.u.)
5MeV
0
Light (0.0005 a.u.)
None
1MeV
0.1MeV
Radiation
α
β
γ
Range in air
Few cm
1 or 2 metres
Many metres
Shielded by
Sheet of paper
Ionizing ability
High
Speed
0.1c
Isotopic Symbol
4
2𝛼
cm of Al
Reasonable
0.9c
0
−1𝛽
c = the speed of light
Many cm of Pb
Poor
c
0
0𝛾
What is an electron volt?
(eV)
 We normally measure energy in the unit
Joules (J)
 Atoms and electrons often contain very small
amounts of energy if measured in Joules
(~10-19J)
 1eV = 1.6x10-19J
 It is the energy 1 electron gains after being
accelerated by 1V
Questions
 1) Convert into Joules
a) 5eV
b) 1keV
c) 3MeV
d)0.2keV
 2) Convert into eV
a) 3.2x10-19J
b) 6.4x10-14J
c) 10-15J
Prac: Detecting radiation
 Cloud chamber and glow-in-the-dark and
geiger
How radiation is detected
 1) Geiger counter
When it
touches the
anode, it
becomes an
electrical
pulse. The
counter
counts these
electrical
pulses
This collisions causes an
electron to be ejected. It is
attracted to the positive
anode
Incoming
radiation
strikes an
atom in the
Geiger
counter
How radiation is detected
 2) Cloud Chamber
Incoming radiation
The radiation
ionizes some of the
alcohol atoms
Super saturated
alcohol vapour
Clouds form
around ionized
alcohol atoms
How radiation is detected
 3) Fluorescent Materials – Some materials
(like glow in the dark stickers, or phosphorus
screens) will emit light when bombarded with
nuclear radiation.
Decay equations
Polonium-210 decays via alpha emission. What
product is formed?
210
84𝑃𝑜
→ 42𝛼 + 𝑦𝑥𝑧 ???
 The mass numbers must balance on both
sides of the equation
 The atomic numbers must balance on both
sides of the equation
x= 206
y= 82
z= Pb
Decay equations
 Carbon-14 decays via beta emission.
14
6𝐶
x= 14
y= 7
→
0
−1𝛽
+ 𝑦𝑥𝑧
z= N
→ −10𝛽 + 147𝑁 + ῡ
Note about beta decay: An antineutrino ( a
particle of very small mass, but no charge) is
also emitted. Symbol: ῡ
14
6𝐶
Decay equations
 The product nucleus is called the daughter
nucleus
 Any further decay will lead to a grandaughter
nucleus
Questions: Write equations
for the following:
 1) Uranium-238 decays via alpha emmision
 2) Francium-222 decays via beta emission
 3) Polonium-214 decays via alpha emission
 4) Carbon-12 emits gamma radiation
 5) A new isotope is formed by adding a
neutron to U-238
Decay equations prac
Half-life Prac
Half life
 It is random when a radio-isotope might
decay.
 However, with a large number of atoms, they
obey an exponential decay.
 Half the atoms have decayed in one half life.
 Half the remaining atoms have decayed in a
second half life.
Half Life
Activity is a measure of atomic decays.
 Units are Becquerels
 1 Bq = 1 decay per second
Half Life Questions
 From the following graph, estimate the half
life of the sample.
 Using this half-life, estimate at what time the
sample will have an activity of 100 kBq
Half Life Questions
 A hospital keeps a sample of Iodine-131 (I-131)
for use in radiation therapy. It is noted that
the activity of the sample has reduced by 75%
in 17 days. What is the half life of I-131?
Half-Life Question
 Carbon-14 is a radioisotope that is useful in
obtaining the age of carbon containing materials
(such as wood) up to about 50’000 years. Carbon14 is a naturally occurring radioisotope that
plants will absorb from the atmosphere, along
with the non-radioactive isotopes carbon-13 and
carbon-12. After the plant dies, it stops
absorbing carbon, and the carbon-14 within the
plant decays away. Carbon-14 decays through
beta decay with a half-life of 5730 years.
Half-Life Questions
 A team of archaeologists have discovered the
remains of what looks to be a fire-pit used by
primitive homo sapiens. They measure the
amount of carbon-14 in samples taken from
the fire-pit and find the measured ratio of
carbon-14 to be approximately 6.25% of the
ratio of atmospheric carbon-14.
 Approximately how many half-lives have
elapsed since this sample was alive?
 Approximately how old is the sample?
Half-Life Questions
Radiation
Isn’t always a bad thing! Has many uses
 Medical Imaging and treatment
 Carbon Dating
 Energy production (nuclear fission)
 Smoke alarms
 But in large quantities it can be harmful…
Which radiation is the most
harmful?
 Alpha radiation has the highest ionizing




ability.
Ionizing: The radiation removes electrons
from atoms in our cells/DNA. This causes the
cells/DNA to mutate/die.
However, alpha radiation is also the easiest to
shield
Our skin will effectively shield it.
However, if we breath it in, or eat
contaminated food = GAME OVER!
Which radiation is the most
harmful?
What radiation are these Fukishima workers
being protected from? How do you know?
Absorbed Dose
 The absorbed dose is the amount of
radiation energy that has been absorbed
per kilogram.
 𝐴𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝐷𝑜𝑠𝑒 =
𝐸𝑛𝑒𝑟𝑔𝑦 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑏𝑦 𝑡𝑖𝑠𝑠𝑢𝑒
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡𝑖𝑠𝑠𝑢𝑒
 Units: Gray (Gy). 1 Jkg-1 = 1 Gy
Dose Equivalent
 Different forms of radiation have
different ionising abilities, and so cause
varying amounts of damage to humans.
 As each type of radiation can affect tissue
differently, the Dose Equivalent is used
compare accurately radiation effects.
Dose Equivalent
 𝐷𝑜𝑠𝑒 𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 = 𝐴𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝐷𝑜𝑠𝑒 × 𝑄𝑢𝑎𝑙𝑖𝑡𝑦 𝐹𝑎𝑐𝑡𝑜𝑟
 Units: Sieverts (Sv)
Radiation
Quality Factor
Alpha
20
Beta
1
Gamma
1
Neutron
10
Effective Dose
 Different organs in the body have
different sensitivities to radiation doses.
eg. A person’s lungs would be more likely to
develop cancer than the liver if they were
both given the same amount of radiation.
 𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝐷𝑜𝑠𝑒 = Σ 𝐷𝑜𝑠𝑒 𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 × 𝑊
 W = Weighting
 Σ = “the sum of”
Effective Dose
Body Part
Weighting (W)
Ovaries/testes
0.2
Bone Marrow
0.12
Colon
0.12
Lung
0.12
Stomach
0.12
Bladder
0.05
Breast
0.05
Liver
0.05
Oesophagus
0.05
Thyroid
0.05
Rest of body
0.07
Total
1.00
Effects – Short Term
 Called Somatic effects
 Radiation ionizes molecules, causing cells to
mutate or die
 Immediate Sickness/Death
Effects – Long term
 Genetic Effects (mutates DNA in
testes/ovaries)
 Radiation causes genetic defects that show
up in offspring
Questions
1. An 80kg tourist absorbs a gamma radiation
dose of 200μGy during a return flight to
England. Calculate the amount of radiation
energy absorbed
2. Calculate the dose equivalent that has been
received
3. During a medical procedure a patient receives a
dose of gamma radiation. The organs which are
affected include a bladder (5000Sv) and the
ovaries (3000 Sv). Calculate the effective dose
of radiation to which this woman has been
exposed
Area of Study Two:
Electricity!
Electrical Circuits
 A circuit must be closed
 Must have a voltage source (EMF) – otherwise
no power!
 Must have a resistive load (bulb, resistor, bell
etc) – otherwise you get a short circuit!
Electrical Symbols
Device
Symbol
Device
Symbol
Wires crossed (not
joined)
Cell
Wires joined
Battery of cells
Resistor or other
load
AC supply
CC
VV
Resistor
Ammeter
A
Filament Lamp
Voltmeter
Diode
DC Supply
Earth or ground
Switch
V
Circuit Diagrams
Draw:
A) Two 1.5V cells connected to a lightbulb, and a
switch
B) A heating element (a resistor) connected to
an AC power supply with a switch
Circuit Diagrams
 Are the following circuits different?
 No! They are the same. One bulb, and a DC
power supply, connected to each other.
Circuit Diagrams
 Just because there are corners in a circuit
diagram, doesn’t mean there has to be
corners in the real circuit.
 This brings some confusion. Circuits that may
appear different as diagrams, may actually be
identical.
 These are called equivalent circuits.
Circuit Diagrams
 Which of the following are equivalent
circuits? Why?
Prac
 Simple Circuit.
 Part B. Using volt and ammeter
 Graph V/I
Ohm’s Law
 Ohm’s Law relates the voltage across a load




to the current it draws
V = IR
V = Voltage
Units: Volts (V)
I = Current
Units: Amps (A)
R = Resistance
Units: Ohms (Ω)
Ohm’s Law
 Ohmic Conductors obey Ohm’s Law
 They have linear relation between the
voltage and current (straight line graph)
Ohm’s Law
 Some devices don’t obey Ohm’s Law. Their
resistance is not constant
 Called non-ohmic devices
Ohm’s Law
 Examples of Ohmic Conductors: Resistors,
most bulbs
 Example of non-Ohmic Conductors: Diodes
Questions
 A resistor draws a current of 1mA and has a
voltage across it of 8V. Find the resisitance
 What current does a 1000Ω light bulb draw
from a 24V power supply
 You have an Ohmic resistor, which draws
75mA when connected to 12V power supply.
You decrease the voltage to 5V, what current
do you expect it to draw?
Resistance
Which has more resistance?
a. Fuse wire or power lines?
b. Copper wire or Iron wire?
Resistance
The more resistance something has, the less
current can go through it
 Resistance in a wire depends on:
 1) The length of the wire
 2) The cross sectional area of the wire
 3) What the wire is made of
Resistance
 𝑅=
𝜌𝐿
𝐴
 ρ = resistivity (depends on the material)
Unit: Ωm
 L = Length
Unit: m
 A = Cross sectional area.
Unit: m2
Resistance
 Example
 Normal household wire has a diameter of
1.8mm, and is made from copper (ρ =1.7x10-8)
 What is the resistance of a 10m long section?
𝜌 = 1.7 × 10−8 , 𝐷 = 1.8𝑚𝑚, 𝑙 = 10𝑚
𝐴 = 𝜋𝑟 2 = 𝜋 × 0.9 × 10−3 2 = 2.5 × 10−6 𝑚2
𝜌𝑙 1.7 × 108 × 10𝑚
𝑅= =
= 0.068Ω
−6
𝐴
2.5 × 10
Resistance
1) A 100m long section of telegraph wiring has
a diameter of 3.6mm and a resistivity of
2.8x10-8. What is the resistance?
2) You have to design a household’s wiring
system. Name three ways you could
decrease the resistance as much as possible.
3) Why don’t all electricians try and use really
thick wire all the time?
Electrical Power
 Power is how much energy is used per
second.
 Power = VI
 Measured in Watts (W)
 1W = 1Js-1
Questions
1) A bulb is connected to a 12V supply and draws
2)
3)
4)
5)
0.875A, what power is it using?
A toaster connected to the 240V mains supply uses
1kW of power. What current does it draw?
Which of the three identical bulbs in the picture
shown is using the most power?
Which of these two bulbs (connected to different
power supplies) is the brightest? Bulb 1 connected
to 10V supply draws 0.8A of current. Bulb 2
connected to a 15V supply draws 0.4A.
A 50Ω heating unit is connected to a 20V power
supply. What power does it use?
Questions
Electrical Energy
 If power is “energy per second”, then energy




is power multiplied by the time.
𝐸 = 𝑃𝑡
Measured in Joules (J)
kWh is another unit of energy (Note it isn’t
power!).
1kWh = 3.6MJ
Activity
The price for electricity supplied by the grid is about
20c per kWh. Find:
 The cost of heating something in the microwave for
5minutes
 How much a 1.5V battery should cost
 How much does it cost to boil a kettle (for 2 minutes)
 How much money are the solar panels on the
science block roof saving us per day (with approx 8
hours sun)
 A iphone charger left plugged in to the wall will still
use 1W. How much does this cost, if it is left plugged
in all year?
What is electricity?
 Electricity is moving charges carrying energy
 An example of a charge is an electron
 Charge is measured in Coulombs (C)
 1e- = 1.6x10-19C
 How many electrons in 1 C?
Current and Voltage
 What is current?
 Current is how much charge goes passed a
given point in 1 second
 𝐼=
𝑞
𝑡
 What is voltage?
 Voltage is how much energy each Coulomb of
charge carries
 𝑉=
𝐸
𝑞
Questions
1. How many electrons make up a charge of
1μC?
2. A) A hairdryer draws a current of 1.6A. What
charge flows through the hair dryer every
second?
B) How many electrons flow through the
hairdryer in 1 minute?
3. Use the formulae 𝑉 =
𝐸
𝑞
𝑞
𝑡
and 𝐼 = , and
substitute into P=VI, and simplify
Electrostatics
 Charges don’t always have to move
 They may just accumulate somewhere
 Law of conservation of charges: Charges
aren’t just created! To make some negative
charge you have to start with neutral atoms
and split into an equal negative charge
(electrons) and positive charge (remaining
ion)
Electrostatics
 Like charges repel
 Unlike charges attract (like a magnet)
 An excess of electrons on something gives it a
negative charge
 Deficit of electrons on something gives it a
positive charge
Prac: Charged Rods
Demo: Van de Graff generator
Van de Graaff Generator
 How does it work?
Charge
jumps!
Positive
charge left
on dome
Metal comb
touches belt,
causing
friction and
separating
positive and
negative
charges
Negative
charge is
taken to
bottom of
the
machine.
Conductors and Insulators
 Conductors: Allow the flow of electrons
 Insulators: Inhibit the flow of electrons
Coulomb's Law
 What caused the Al can to roll? A force!
 Charges exert a force on one another.
 𝐹=
𝑘𝑞1 𝑞2
𝑟2
 k = 9x109 Nm2C-2
 Positive force = repulsion ( a push )
 Negative force = attraction ( a pull )
Coulomb’s Law
 Example: Two charges, with a charge equal to
1μC, are held 2m apart from each other. What
is the force each charge exerts on each other?
Coulombs law
 Examiners like to ask proportionality
questions with Coulomb’ law
 Eg. Two charges, each with a charge of q, are
positioned a set distance apart from each
other. If the distance is doubled, how does
the force change?
Questions
1) A - 1μC and 6 μC are separated by 20cm.
Calculate the force.
2) Two 1μC are separated by an unknown distance
and experience a force of 3N. Calculate the
distance separating the two charges.
3) A charge of q, and another of 2q are separated
by a distance x. If the charge on the second is
doubled to 4q, and the distance between them
halves to x/2, how does the force change?
Electric Fields
 The electrostatic force is invisible. However, as
scientists, we have added lines around charges





to help visualise this force
Called “The Electric Field”
Shows where a positive charge would fly off to if
we dropped it into the field
A negative charge would head in the opposite
direction.
Where lines are close together, the field is strong
Where lines are far apart the field is weak
Electric Fields
 If we had a positive charge that couldn’t
move
Electric Fields
 And we dropped another positive charge just
north of it, which direction would the 2nd
charge fly off to?
Electric Fields
 So we put an arrow there
Electric Fields
 If we were to drop another positive charge to
the right of our main positive charge…
Electric Fields
 If we were to drop another positive charge to
the left of our main positive charge…
Electric Fields
 Continue putting field lines around our
charge…
Electric Fields
 Question: Where is the electric field the
strongest?
 Answer: Near the charge!
Electric Fields
Question: What would the field lines created by
a negative charge look like? Discuss with your
neighbour.
Electric Fields
 Some more complicated fields…
Electric Fields
 𝐹 = 𝐸𝑞
 E = Electric Field. Units: NC-1
Electric Fields - Questions
 Some small charged spheres are to be placed
in an electric field which points downwards
and has a strength of 5000NC-1. What force
would be experienced by a charge of +5 μC
and -0.6 μC ?
 Draw the electric field you would expect
around the three charges below
-q
+q
-q
Electric Fields Questions
 Draw the electric field you would expect
around the two charges below
+q
-2q
Back to Electric Circuits
 Draw two different ways you could place two
bulbs in a circuit with a battery
 One is called series
 One is called parallel
Kirchhoff’s Laws
 Law 1: The current law
 Law: “In any circuit the sum of all currents
flowing into a point equals the sum of all
currents flowing out of that point
 Analogy: River: The amount of water flowing
into a junction equals the sum of water
flowing out. [Draw this]
 What this means. Current is same at all places
on a series circuit. It splits up in a parallel
circuit.
Kirchhoff’s Laws
 Law 2: The voltage law
 Law: “The total potential drop around a
closed circuit must be equal to the total EMF
in the circuit
 Analogy: River. The pump (EMF) pumps water
to the top. It drops down as it goes through
devices [Draw this]
 What this means: Voltage is supplied by
battery, and used in devices. Voltage is the
same in parallel circuit.
Activity
 Smarties run.
 Questions?
Prac: Series and Parallel
Circuits
Resistors in Series
 𝑅𝑇 = 𝑅1 + 𝑅2 + …
 Eg Total resistance in the circuit?
6Ω
2Ω
 𝑅𝑇 = 6Ω + 2Ω + 3Ω = 11Ω
3Ω
Resistors in Parallel

1
𝑅𝑇
=
1
𝑅1
+
1
𝑅2
+ …
3Ω
6Ω


1
𝑅𝑇
1
1
3
= + =
3
6
6
6
𝑅𝑇 = = 2Ω
3
Resistors in Parallel
3Ω
6Ω
 The total resistance in a parallel circuit is
always less than the smallest resistor.
 Sometimes examiners ask for the equivalent
resistance (it’s the same thing as total
resistance)
Questions
 Find the equivalent resistance of the
following circuits:
1)
2)
1Ω
2Ω
4Ω
3Ω
3Ω
Total Resistance of Hard
Circuits!
 Rules: Work out one section at a time

: Work from the inside out
12Ω
6Ω
1Ω
Questions
3)
3Ω
4Ω
6Ω
4)
5Ω
12Ω
5Ω
3Ω
Questions
5)
3Ω
3Ω
6Ω
2Ω
6)
3Ω
3Ω
6Ω
10Ω
Questions
7) Draw a circuit with three resistors that has a
total resistance of 5Ω.
8) Draw the above circuit differently.
9) Using a 1Ω, 2Ω, 3Ω, 4Ω and 5Ω resistor draw
a circuit with total resistance = 6Ω
Final note about Voltages
and Current in circuits
 The total voltage in a circuit is set by the
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battery/powerpack.
However, the current drawn from this battery
depends ON WHAT IS IN THE CIRCUIT
Eg…POE
Draw two circuit diagram with two bulbs (series and
parallel).
Guess which will draw the most current
Measure total voltage in both, and total current.
Which draws most power?
Explain why one draws more current than the other
CUPS 9 and 10.
Prac: Which bulb is the
brightest
 Report required for this prac.
Demo: Internal Resistance
 Resistance within the powerpack/battery.
Uses some of the voltage up!
A
V
Internal Resistance
 Voltage without bulb =
 Voltage measured with bulb =
 Current =
 Voltage used in internal resistance
 Internal Resistance =
Other sources of EMF
 EMF = electro-motive force (what drives the


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
circuit)
Solar
20% efficient
2kW per square meter of sunlight energy
Don’t work at night!
Less power in winter. Why?
Household Electricity
 We have only been dealing with DC electricity
(Direct Current)
 Household electricity is AC (Alternating
Current)
 Why?
 We use AC because its relatively easily to
change between voltages
Household Electricity
 Power lines have a low resistance, but because
electricity has to travel a large distance, this
becomes an issue, and much power could be lost
 A high voltage has a lower current, and a lower
current loses less power!
 Transformers change between high voltage
(14000V) and low voltage (240V)
Household Electricity
Active:
240V
Neutral: 0V
Earth/Ground
 Current wants to flow from Active to Neutral
just like it flows from +ve to -ve in a battery.
Household Electricity
 Why do we have an earth wire?
 Imagine a fault in your toaster that touched
the active wire to the casing. Then you
touched the casing!
 The casing is safely connected to earth, so if a
fault did occur, high current would flow to
earth, and a fuse would blow.
Electric Shocks!
 Our muscles move when they get small
electrical shocks sent from our brain.
 Above 20mA of electric current, and we cant
let go.
 A shock across the heart of 50mA
for 5s = possible death
Prac: Build a buzzer game
with a bell