Transcript “Zip Zap”

“Zip Zap”
Electrical Safety
Charges
What is a Circuit?
• An electric circuit consists of:
– A source of electrical energy (battery or cell)
– A user of electrical energy (eg lamp)
– Connecting wires
Component Symbol
Function
Cell
An energy supply: converts chemical potential energy
to electrical energy
Battery
Cells connected together
Switch
Allows user to switch current on/off
Lamp
Converts electrical energy to light + heat
Resistor
Transforms electrical energy into heat energy (by
slowing down the passage of electrons)
Variable
Resistor
Allows user to control the amount of electrical energy
moving through the cricuit
Ammeter
Measures current
Voltmeter
Measures voltage
Wire
Conductor connecting up parts of the circuit
Crossing wires
(connected, not
connected)
Fuse
Breaks / melts when current is too high
Allows current to flow in one direction only
Name
Switch
Two way
switch
*Push button
switch
(normally off)
*Push button
switch
(normally on)
*Motor
*Buzzer
Symbol
Function
Rules for Drawing Circuit Diagrams
• Use a ruler
• No gaps / overlaps in wires
• Neat
• Organised
Eg:
3D Drawing
Circuit diagram
Conductors
Electric current is the flow of electrons
Conductors
• These are materials with electrons held loosely in
atoms
• When connected to a cell these electrons can be
made to move (electric current)
• Examples: all metals, graphite, salt water, acids
• Uses: where you want current to flow (power lines,
circuits)
• Static: conductors can’t get a build up of static
charges because the charges move (producing
current) as soon as they are separated.
Insulators
Insulators
• Are materials with electrons held tightly in atoms
• Connecting to a cell can’t make the electrons move
(no current)
• Examples: plastic, glass, rubber, wood
• Uses: where you don’t want current to flow, to protect
users against electric shock (sheath of wires, handles
of electrician’s tools)
• Static: insulators can get a build up of static charges
because when the charges are separated by friction
they stay there
This means:
Conductors are also poor insulators
Insulators are also poor conductors
Examples
Conductors
Insulators
Series Circuits
• In a series circuit the
components are
connected one after
another in a row.
• In series:
– lamps are dimmer,
– but the battery lasts
longer (less current is
drawn from it)
– if one bulb blows, the
others won’t work.
Parallel Circuits
• In a parallel circuit the
components are
connect in parallel
branches of the circuit
• In parallel:
– lamps are brighter,
– but the battery goes flat
more quickly (more
current is drawn from it).
– if one bulb blows, the
others will still go.
Problem Solving
• You have a stairway with a light in the middle. You want
to be able to control the light from the bottom and top of
the stairs
Design a circuit that will achieve this.
• Flipping either of the two way switches closes the circuit.
So you can install one switch at the bottom and one at
the top of a flight of stairs and both will turn on the light.
Problem Solving
• Design a circuit that will operate a doorbell
buzzer
Problem Solving
• Design a circuit that will turn off a fridge
light when the door closes
Voltage
• “A measure of how much energy can be given to
moving electrons in a circuit”
• When electrons go through a cell they gain
energy
• When electrons go through a user (eg lamp)
they lose energy.
• Voltage is measured in volts (V) using a
voltmeter.
• The voltmeter measures gain/loss of energy in
volts (V). * This is also called “potential
difference”
• 1V = 1000 millivolts (mV)
*Energy Gains & Losses
• A 9V battery gives an electron a parcel of
energy of 9 V
• When you measure voltage across a user
you are measuring the electrical energy
needed to get through that component; eg
5V across a lamp means each electron
gives up 5V of energy to get through that
lamp
Using a Voltmeter
• Connect in parallel
around a component: this
is so that it can measure
the amount of energy the
electrons use (eg in a
lamp) or gain (eg when
moving through a cell).
• Connect positive terminal
to positive terminal of
battery / cell (and
negative to negative)
+
-
Voltage and circuits
• An electron leaves the battery with a
package of energy and returns to the
battery with nothing.
Voltage in Series
• In series, the voltage
is shared amongst the
users and it adds up
to the total energy
that left the battery.
Voltage in Parallel
• In parallel branches
the voltage is the
same in each branch
because each
electron only goes
through one branch
and must drop off all
energy in that branch
and return with 0 volts
Current
• “A measure of the rate of flow of electrons
passing a point in a circuit each second”
• Current is measured in Amperes or Amps
(A) using an ammeter. 1A = 1000 mA.
• *1A = 6.25x1018 electrons per second
(6,250,000,000,000,000,000 per second!)
Using an Ammeter
• Connect in series with a
component: this is so that
it can measure the
number of electrons
going through the
component (eg lamp or
cell) per second
• Connect positive terminal
to positive terminal of
battery / cell (and
negative to negative)
+
-
Current and Circuits
• The amount of current that leaves the cell
also returns to the cell because this is the
number of electrons moving through the
circuit.
Current in Series
• Current stays the
same everywhere in a
series circuit
Current in Parallel
• Current in the parallel
parts of a circuit adds
up to the current in
the main branch.
Finding Lamp Power
Aim: To find out the power used by a lamp.
Method:
1 Set up the circuit (below)
2 Use P = VI to find the power used by the lamp.
Results:
Voltage
Current
Power used = V x I
=
=
Finding Lamp Resistance
Aim: To find the resistance of a lamp
Method:
1 Set up the circuit (below)
2 Use R = V / I to find the power used by the
lamp.
Results:
Voltage
Current
Power used = V / I
=
=
Resistances in Series and
Parallel - *Extension
The total resistance increases when resistors are added in series.
Rtotal = R1 + R2 + …
The total resistance decreases when resistors are added in
parallel.
1/Rtotal = 1/R1 + 1/R2 + …
Rtotal = total resistance of the arrangement of resistors, Ω
R1 = resistance of the first resistor, Ω.
Fuses
• Make a steel wool fuse
Power
Power is a measure of electrical energy transferred (gained or
used) per second. For example, a 60 Watt light bulb uses 60
Joules of energy every second.
Power has the symbol P and is measured in Watts, W.
P=VxI
P = power measure in watts, symbol: W
V = voltage across a component, measured in volts, symbol: V
I = current through that component, measured in amps, symbol: A
*Power could also be found using this equation: P = E / t (E is energy used or
supplied in joules, t is time the energy is used or supplied for in seconds). So, if a
lamp uses 60J in 1s it would have a wattage of 60W (same as above)
Energy Conservation & Changes
The law of the conservation of energy
states that “energy cannot be created or
destroyed it can only be transferred or
transformed from one form into another”
Example: a lamp – converts the electrical
energy it receives into heat and light (energy
in = energy out)
Lamp: electrical energy  heat + light
Billing Electricity
• You are billed in “units” what are they?
• 1 unit:
= 1kwh (one kilowatt hour)
= 1000 watts for an hour
= 1000 joules per second for an hour
= 1000 joules per second for 3600 s
= 3,600,000 joules
• Prices vary but a unit may cost between $0.21 and $0.25
• Why do they bill you in units, not joules?
• Power Company is a misnomer – what should they be called
instead?
Energy
The total energy used by a component
is the power of the component multiplied
by the time it was used for (in seconds).
E=Pxt
E = total energy used, J
P = power rating of the component, W
t = time the component was used for, s.
At What Cost?
10Mi are notorious for charging their laptops at
school. Is the cost significant to the school? If
so, should the school add an extra charge to
accounts to cover this?
Cost = power (in kW) x hours used x unit cost
10mi (2013) est of $763 for all laptops (based on 21.41c (actually probably 10c). Total bill
hostel + school inc heating $200,000