Current Electricity
Todays Lesson
• A=Voltmeter
• B=Ammeter
• C=Resister
• D=Bulb
• E=Battery or Cell
• F=Variable resistor
• G=Switch
7J Connecting circuits – Electrical symbols
What are conductors and
insulators?
What is a conductor?
• A conductor is something which allows electricity to
flow through.
• An example of a conductor is:
–
–
–
–
–
–
Scissors
Paper clip
Aluminum foil
Iron
Steel
Copper
What is a Insulator?
• An insulator is what makes electricity from not
flowing
• Examples of insulators are:
–
–
–
–
–
Cotton
Paper
Plastic
Glass
Rubber
These are pictures of conductors
and Insulators
Conductors
Insulators
Now its your turn…
Use of a circuit tester: To test whether the
material is a good electrical conductor.
Example of a circuit tester
Inquiring Minds Want to Know!
Conductor
or Insulator?
That
is the question!
Item
Insulator Did the
bulb light?
Conductor
Scissors
Plastic
Part
Paper clip
pencil
Rubber
Scissors
Metal part
Plastic
Metal
spatula
Step 1: Set up the circuit
tester as shown above.
Step 2: Place the object
between the open ends of the
wire.
If the bulb lights up, it
means the material allows
electric current to flow
through the closed circuit.
 The brighter the bulb, the
better the conductor of
electricity is the material.
Now write your conclusion based on
your investigation…
• What objects were conductors of electricity?
Why?
• What objects were insulators of electricity?
• Did any of the conductors have common
characteristics?
• What about the insulators?
• How would knowing the difference between
conductors and insulators help you in the real
world?
Conclusions…
Conductors allow electricity flow through
them. Conductors are materials that can carry
electricity - they conduct electricity. Metal materials
such as copper, iron, steel and aluminum are all good
conductors of electricity.
Insulators do not allow electricity to flow through
them. Materials such as wood, plastic, rubber and
glass do not carry electricity and are called insulators
– they don’t conduct electricity.
Insulator
• Keeps in charge
Conductor
• Allows charge to move
3 Main Concepts
• 1.Current
• 2.Voltage(potential difference)
• 3.Resistance
For current to flow we need two things:
1. There has to be a complete circuit.
2. There has to be a source of potential
difference (power supplies and batteries both
act as a source of potential difference).
Current
• Current is a flow of charge (the charge is
usually electrons)
• The unit of current is the amp (the symbol for
the amp is A).
• The symbol for current is I.
• Current is measured with an ammeter or
multimeter.
Electron flow in a wire
7J Connecting circuits – Making electrical objects work
All electrical objects need 3 things to make them work.
What are they?
1. A power supply
- like a cell (battery)
or a mains supply.
2. Wires
- provide a path for electrical charge to
pass through.
3. An electrical device
- like a bulb or a motor.
What must happen for the electrical device to work?
7J Connecting circuits – A simple circuit
An electrical device only works when all the components are connected to
make a complete circuit.
When the switch is turned on, the circuit is complete.
What happens to the bulb?
Electrical charge flows around a complete circuit.
The flow of electrical charge is called the current.
What happens to the current if there is a gap in the circuit or the switch is
turned off?
7J Connecting circuits – Switch on
Potential difference (commonly called
‘voltage’)
• Current will flow between two points if there
is a potential difference between the two
points.
• This is a bit like saying that water will flow
between two points if there is a height
difference between the two points.
• In an electric circuit current flows from the
positive end of the battery to the negative end
•
• Voltage provides the ‘push’ to move the
electrons around a circuit.
• The unit of potential difference is the volt (the
symbol for the volt is V)
• The symbol for potential difference is V.
• Potential difference is measured with a
voltmeter or multimeter.
Summary
Revision on concepts so far
• http://lgfl.skoool.co.uk/keystage3.aspx?id=93
Series Circuit
A series circuit
• This is where the two bulbs are connected one
after the other.
• All the current coming from the battery goes
through both bulbs.
• Advantage: Uses less electricity than if the
bulbs are connected in parallel.
• Disadvantage: If one bulb blows the circuit is
broken and so no current flows, e.g. lights in a
Christmas tree.
•
Parallel Circuits
A parallel circuit
• The current coming from the battery splits up
and some goes through each bulb.
• Advantage: If one bulb blows there will still be
a complete circuit through the other bulb so it
will remain lit.
• Light bulbs in a house are generally connected
in parallel for this reason.
• Disadvantage: It uses more electricity than if
connected in series.
Review Questions so far
Introduction to ohms law
• http://phet.colorado.edu/sims/ohmslaw/ohms-law_en.html
• http://www.thephysicsteacher.ie/lcphysics23r
esistance.html
• (last ohms law applet)
• http://www.absorblearning.com/media/attac
hment.action?quick=148&att=2885
• Ohms law graph
Ohms law.
• As the voltage increases so does the current.
• This means that current is proportional to
voltage.
• This is known as Ohms law.
• This is only true when the temperature stays
the same.
Resistance
• Resistance opposes the movement of
electrons around a circuit
• The unit of resistance is the ohm (the symbol
for the ohm is Ω).
• The symbol for resistance is R.
• Resistance is measured with an ohmmeter or
mulitmeter.
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
43
Ohms Law sample of results
Set up this circuit with a resistor and a variable resistor.
voltage
(V)
0
0.5
1.0
A
V
Slowly move the variable resistor
so that the voltage increases by
0.5 V and record the current for
each setting.
1.5
2.0
2.5
3.0
3.5
Plot a current-voltage graph of the results.
current
(A)
0
0.6
1.1
1.8
2.5
3.0
3.5
4.2
So if voltage doubles then the
current doubles and so on.
Current / A
The graph is a straight line so
the two quantities current and
voltage are proportional.
x
.
Current-voltage graph for a
resistor
x
x
x
x
x
x
x
Potential difference / V
.
Ohm’s Law
The current flowing through a wire is proportional
to the potential difference (voltage) across it
provided the temperature remains constant.
Ohms Law experiment
Electron flow in a wire
Electron flow and resistance
Electricity is the flow of electrons along a wire.
As the electrons move along the wire they collide with
the metal atoms.
These collisions make the atoms vibrate more…
…which makes the metal hotter.
Resistance is a measure of how much a material tries
to stop electricity passing through it.
All wires and devices have some resistance, which is why
electrical appliances always waste some energy as heat.
The resistance of a wire depends on several factors:
Factors that affect resistance
 Material – Different materials have different resistances
because some materials are better conductors. Nichrome
wire has a higher resistance than a copper wire of the
same size.
 Length – The longer a wire is the higher its resistance.
When electrons travel down a long wire they can collide
with more metal ions than in a short wire.
 Thickness – A thin wire has a higher resistance than a
thick wire.
 Temperature – The higher the temperature of a wire the
higher its resistance. Metal ions vibrate more at higher
temperatures and so collisions with electrons are more
likely to happen.
Using the resistance formula
triangle
Resistance calculations
Calculating the resistance of a
bulb
A filament bulb has a current of
20 A running through it, with a
potential difference of 100 V
across it.
What is the resistance of the
filament in the bulb?
V = IR
R = V
I
= 100 V
20 A
= 5
Revision Questions On Ohms Law
2013 Q
POWER
• Aidhm
To know how to calculate power.
To know how to calculate the cost of running an
appliance.
P=IxV
Power = Current x Voltage
Voltage = Potential Difference
Pee on the Ivy
P
=
I
x
V
Power and fuses
Power is “the rate of doing work”.
The amount of power being used in
an electrical circuit is given by:
Power = voltage x current
in W
in V
in A
P
V
I
Using this equation we can work out the fuse rating for any
appliance. For example, a 3kW (3000W) fire plugged into a
240V supply would need a current of _______ A, so a
_______ amp fuse would be used (fuse values are usually 3,
5 or 13A).
Power calculations
Power and fuses
Copy and complete the following table:
Appliance
Power rating
(kW)
Voltage (V)
Toaster
0.720
240
Fire
2
240
Hairdryer
0.300
240
Hoover
1
240
Computer
0.1
240
Stereo
0.08
240
Current
needed (A)
Fuse needed
(3, 5 or 13A)
Survey of Household appliances
•
•
•
•
•
•
•
•
•
Toaster 800W at 240V = 3.3A
Hoover 1300W at 240V= 5.4A
Power drill 600W at 240V= 2.5A
Kettle 2200W at 240V= 9.2A
Food mixer 300W at 240V=1.3A
Iron 1450W at 240V= 6A
Washing machine 1300W/240=5.4A
Cooker 3000W/240= 12.5A
Choose a fuse for each appliance
1,3,5,10 and 13 amp?
Power is...
...the rate of doing work
Power = Energy / Time
The Watt
1 Watt = 1 Joule transferred in 1 second
Power (W)= Energy (J) / Time (s)
The Kilowatt Hour
= the energy transferred when
a power of 1kilowatt (1000W)
is used for 1hour.
Number of Kilowatt hours used =
Energy used (J)
Energy in a kilowatt hour (J)
or
Power (kW) x Time (h)
Device
Electric
Cooker
Fridge
Freezer
Washing
Machine
Dishwas
her
Power
Rating
K/W
10
0.05
0.30
0.50
1.30
Time
(Hours a
day)
0.75
24
24
10
0.75
KWH
7.5
1.2
7.2
5
0.975
Cost at 9c
for day
68 C
11C
65C
45C
9C
Summary
 P = IV
 Power is the rate of doing work. P = Energy/Time
 1 Watt = 1 Joule per 1second
 1 kWh = 3600000J or the energy used by a 1000W device in 1 hour
 Number of Kilowatt hours used = Power (kW) x Time (h)
DC and AC
V
DC stands for “Direct
Current” – the current only
flows in one direction:
Time
1/50th s
AC stands for “Alternating
Current” – the current
changes direction 50 times
every second (frequency =
50Hz)
240V
T
V
AC/DC
What is AC?
Source of AC?
AC is alternating current, a
current which is constantly
changing in direction.
Mains electricity (50Hz)
What is DC?
Sources of DC?
DC is direct current, current
that is always flowing in
one direction.
Cells or batteries
AC/DC signal simulation
Use the animation below to recreate different a.c. or d.c. signals.
Safety when wiring plugs!!!!!!
• Copper is used as a material inside the cable as it is a
conductor of electricity .
• The pins on the plug are made from brass as brass is a good
conductor of electricity .
• This means that there has to be some type of insulators .
•
•
•
•
-Rubber case and plastic case
-A fuse
-Earthed pin
-A cable grip
Wiring a plug
Can you label the plug?
Live terminal
Neutral terminal
Earth terminal
Cable grip
Fuse
Fuse
• A fuse should always be the same as the one
recommended by the manufacturer of the
appliance .
• Appliances with a metal case should be
earthed .
• If there is a fault in an electrical circuit the
fuse acts as a safety device .
• If the current gets to high the fuse melts and
the breaks the circuit so it switches it off .
Fuses I
What is a fuse?
A fuse is a device that is used to protect
devices from a current which is too high.
How does a fuse work?
As current flows along a conductor there is friction between the electrons
and the atoms of the conductor. The more current, the more friction.
The friction causes heat, and the more current the hotter the fuse gets.
Fuses contain pieces of conductor that melt if the current is too high.
When the fuse melts, it breaks the circuit.
81
Circuit Breakers
Effects of electricity
• Aidhm
To know
-Magnetic
-Heat
-Chemical effect of electricity
Experiments
1. Magnetic effect of electricity
2. Heating effect of electricity
3. Chemical effect of an electric current
Hydrolysis of water
Chemical Effect of electricity
• http://www.youtube.com/watch?v=OTEX38b
Q-2w
Magnetic Effect of electricity
• http://www.youtube.com/watch?v=E69RSok2TQ
To Demonstrate the Magnetic Effect of an
Electric Current: The Electromagnet
• An electromagnet
consists of a soft iron
.When the current is
switched on the core
acts as a magnet and
can be used to pick up
paper clips.
Applications of the Magnetic Effect
of Current
• Giant electromagnet
• Telephone
• Electric motors
89
Heating Effect of a Current
• http://www.youtube.com/watch?v=E69RSok2TQ
• Wire heats up as a current passes through it
• Electrical energy is converted into heat
energy
• Heat produced depends on the resistance of
the wire
• Greater resistance, greater heat
90
Use a thermometer to measure the
heat generated by the wire
Heating Elements in Heating
Appliance
92