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Electricity
Show video clip on introduction to electricity
Write down four things you learnt from the video
clip about electricity
Questions on Video clip
(1)What is electricity?
(2)What does a battery do?
(3)What is current and what is it measured in?
(4)What is meant by short circuit.
(5)What is a circuit?
(6)What happens the electic energy when a light bulb is in the
circuit?
(7)What happens the current when the light bulb is in the circuit?
(8)Are electrons big or small?
Keywords
Lesson objectives
To be able to clearly define the meaning of the terms “current”,
and “voltage / potential difference”
To be able to correctly use a voltmeter and an ammeter and an
ohmmeter.
Identify basic circuit symbols.
To understand what resistors and variable resistors are and how
they work.
To understand the relationship between potential difference and
current.
To preform calculations using ohm's law.
Lesson objectives
To measure resistance of various electronic components.
To distinguish between series and parallel circuits and their
effects ón light bulbs.
Lch. 2.
To know the three effects of electric
current
To distinguish between a.c and d.c.
INTRODUCTION TO ELECTRICITY
This is a brief
introduction to the
main facts of
electricity, potential
difference, current,
switches, parallel and
series circuits and
resistance.
In the diagram are
of the common
symbols for drawing
electric circuits.
ELECTRIC CIRCUITS
•An electric current is a flow of negatively charged
electrons.
•A circuit is a complete path along which a current
may flow.
•A circuit consists of materials which are conductors
(usually metals), which have “free” electrons, loosely
attached to atoms, which as they move from atom to
atom, carry an electric charge. This flow of charge is
the current.
•The electrons flow when there is a difference in
potential (voltage) across the wire.
Remember
to show
virtual
physics lab.
Demo.
In fact because electrons have a negative charge, current
actually flows from – to +, but it is the agreed convention
that currents shown in circuit diagrams flow from + to -. We
call this conventional current
Insulators are materials which will not carry currents; they
have no free electrons
How could you show if a material conducts electricity or
not?
What apparatus would you need?
How would you connect the test substance into the ciruit.
Circuit to test if a substance is a conductor or an
insulator.
Battery
bulb
test substance
Conductors and insulators
MATERIAL
Zinc
Lead
CONDUCTOR
INSULATOR
Conclusions
• All metals are conductor as because the
electrons are free to move.
• In non metals the electrons are held tightly
by the nucleus of the atom so cannot flow
we call these materials insulators.
• Graphite is one of the few non-metals
which can conduct electricity.
Voltage
Voltage is a measure of the energy carried by the current.
Voltage is causes the current to flow.
(Technically it is a measure of the difference in energy
between two points – hence the name potential difference).
Voltage is a measure of the energy carried by the charge.
Voltage is supplied by the battery (or power supply).
We say voltage across a component.
Voltage is measured in volts, V.
Voltage is measured with a voltmeter, connected in parallel.
The symbol V is used for voltage in equations.
Voltage / potential difference(p.d.)
Voltage is measured with
a voltmeter.
Voltmeters are always
placed in parallel in a
circuit.
Voltage is the driving
force that pushes the
current like an electric
pump.
Voltmeter in parallel
CURRENT
Current is the rate of flow of charge.
Current is not used up, only energy
Current is measured in amps (amperes), A.
Current is measured with an ammeter,
connected in series.
To connect in series you must break the
circuit and put the ammeter acoss the gap,
as shown in the diagram.
The symbol I is used for current in
equations.
Why is the letter I used for current? ...
please see FAQ.
Ammeter
in series
Measuring current flow
• The brightness of a lamp is an indicator of the
amount of current flowing through it. The
brighter the lamp the more current flowing.
• A better, and more accurate way of measuring
current is to use an AMMETER
symbol
A
A current measuring circuit:
Unit = AMPS (A)
Resistance
Resistance is anything in the circuit which slows the current
flow down.
Resistors use up the energy supplied by the battery and turn
it into other forms of energy for example
Light bulb – light and heat energy
Electric motor – kinetic and heat energy
VARIABLE RESISTOR
This is a component which can vary (change) the
resistance of a circuit.
This relies on the fact that longer wires have more
resistance, by changing the length you can control the
current in a circuit
Remember to show circuit board with variable resistor as
a dimmer switch.
Measuring RESISTANCE
Aim: To measure the resistance of
various components.
A multimeter set to
measure
Resistance in ohms (Ω) is
used.
Simply connect the
resistor accross]
The terminals and read
the resistance.
Learning Check
1)What is electric current?
2)Explain potential difference?
3)What is resistance?
4)What are conductors and insulators?
5)How could you show if a substance is a conductor or an
insulator?
6)What is a circuit?
7)Why must electric circuits be complete?
8)With what is (a) potential difference, (b) current (c) resistance
measured with?
9)What are the units of (a) potential difference, (b) current (c)
resistance
The ammeter and the voltmeter have to be
connected with their + terminals towards the
positive side of the battery.
Ohms Law
The potential difference is directly proportional to the current
flowing in a conductor for a constant temperature.
V = constant (called the resistance R) or V = I x R
I
Let's see how Ohm's law can be used to calculate the current,
voltage or resistance in a circuit.
If we know the values of any two of the three
quantities (voltage, current, and resistance) in
this circuit, we can use Ohm's Law to determine
the third.
calculate the amount of resistance (R) in a circuit, given values of
voltage (E) and current (I):
calculate the amount of voltage supplied by a battery, given values of
current (I) and resistance (R):
Showing the Relationship between current and
voltage – across a fixed resistor
Proving Ohm's Law
Set up this circuit and measure the current and voltage across the
resistor at different values – measure at least 5 values
Voltage (V) Current (A)
A
V
Use 470 resistors
Relationship between current and
voltage – across a fixed resistor
Now plot a graph of voltage against current for the fixed resistor
circuit.
Given what we observed using the virtual physics laboratory
when we changed the voltage or current what would you
expect the graph to look like?
Voltage,
V
Current, A
Series and Parallel Circuits
A. IN SERIES
The current flowing through a lamp makes it light. The more
current the brighter the lamp.
Series and Parallel Circuits
A. IN PARALLEL
All lamps are
normal
brightness
All lamps have the
same amount of
current flowing through
them.
SWITCHES
In a series circuit –
one switch at either
X or Y would
control the lamps.
In a parallel circuitEach lamp is
controlled
independently by
each switch.
Effects of Electric Current.
Three effects of an electric current.
Remembering what has been done in the lesson to date
and work we did in first year what effects of electric
current have you noticed?
(1) Heating effect. How would you show this?
(2) Magnetic effect. How would you show this?
(3)Chemical effect. How would you show this?
Heating effect of an Electric Current.
What would expect
to happen the
temperature?
Why?
What does it
demonstrate?
Uses of the heating
effect.
Kettle, oven, electric
fire, fuse, immersion
heater.
Magnetic effect of an electric current
What happens the compass
when the switch is closed?
Why?
Can you think of any other
way of demonstrating this
effect of electricity?
Uses of the Magnetic effect
Electric motors,
transformers, loudspeakers,
computers.
Chemical effect of an electric current
Here electricity is being
passed through water.
A chemical reaction takes
place and splits water into
hydrogen and oxygen.
This type of reaction is
called Electrolysis and is
an example of the chemical
effect of electricity.
Uses of the chemical effect
Electrolysis -extracting and
purifying metals.
Electroplating e.g silver
plating
Keywords
Electricity in the Home
Mains supply two cables
Live (brown) at ±230 V
Neutral (blue) at 0 V
Main fuse on live coming into
house.
Both wires go through electricity meter
recording the number of units used.
Both wires go through a distribution box with
circuit breakers.
Circuit Breakers and Fuses
Circuit breakers and fuses cut off the current if it gets to
high for safety, to prevent a fire etc.
Fuses melt if the current is too high (heating effect of an
electric current)
Fuses come in various current ratings e.g 1A, 5A, 13A.
Must be replaced if they blow.
Circuit breakers trip off if the current gets too high
(magnetic effect of an electric current), can be switched
back on when the fault is corrected.
Circuit breakers and fuses are always placed on the live
wire (why?).
Plugs
Most appliances are connected to the
electricity supply by pushing a
plug into a wall socket.
Three wires
Live (brown) with fuse connected
in it.
Neutral (blue)
Earth (green and yellow)
What are the safety features of a
plug? (3)
Earthing
The Earth wire in your house is
connected to a metal plate buried in
the ground.
It is a safety device.
If the live wire get damaged and results
in the metal body of an appliance
such as an electric kettle becoming
live then because it is earthed if you
touch it the current flows to earth
instead of through you.
Learning Check
(1) What is fuse? What effect of electricity does it show?
(2) How does a fuse work?
(3)What is circuit breaker? What effect of electricity does this
show?
(4)Why do we have an earth wire? How does it work?
(5)What are the colours for live, neutral and earth wires?
(6)What two advantages doe circuit breakers have over fuses?
The Cost of Electricity
Power = work done ÷ time measured in Watts (W)
The higher the power rating of an appliance the more energy
it uses per second.
Rem. 1000W = 1 kW.
Examples.
1500 W heater uses 1500 J of energy per second .
2kW cooker uses 2000 J of energy per second.
How much energy does a 8.5 kW electric shower use?
8500 J /s
The Cost of Electricity
Unit of electricity sold by ESB = the kilowatt-hour kW h
(the joule is too small)
A kilowatt-hour is the electrical energy converted in other
forms in 1 hour by a 1 kW appliance.
The number of kWh (units) = no. of kW x no. of hours
How many kWh are used by the following:
1) 8 kW shower for 20 minutes.
2) 1500 W iron for 1.5 hours.
3) 100 W bulb for 20 hrs.
4) 25 W i-pod for 40 hrs.
The Cost of Electricity
ANSWERS
The number of kWh (units) = no. of kW x no. of hours
1) Number of units = 8 kW x 0.33 hrs = 2.66
2) Number of units = 1.5 kW (1500W) x 1.5 hrs. = 2.25
3) Number of units = 0.1 kW (100W) x 20 hrs. = 2.0
4) Number of units = 0.025 kW (25 W) x 40 hrs. = 1
The Cost of Electricity
The cost of the electricity
Cost = number of kWh (units) x price per unit (kWh)
Example
What is the cost of running a 3kW electric fire for 5 hours if
the price per unit is 18 cent.?
ANSWER
Number of kWh = number of kW x number of hrs = 3 x 5=15
Cost = number of units x cost per unit
= 15 x 18 cent
= 270 cent or €2.70
The Cost of Electricity
Example 2
Find the cost of using the following appliances if a unit of
electricity costs 18 cent.
a) 2000 W heater for 3 hrs.
b) 1000 W iron for 10 mins.
c) 150 W freezer for 24 hrs.
d) 25 W light bulb for 40 hrs.
*** you must convert the W to kW first (how?)***
***then you must find the number of units (kWh)(how?)***
The Cost of Electricity
Answer
The number of kWh (units) = no. of kW x no. of hours
Number of units = 2 kW (2000 W) x 3 hrs. =
6
Number of units = 1 kW (1000W) x 0.167 hrs. = 0.167
Number of units = 0.15 kW (150W) x 24 hrs. = 3.6
Number of units = 0.025 kW (25W) x 40 hrs. = 1
Total number of units = 10.767
Cost = number of kWh (units) x price per unit (kWh)
Cost = 10.767 x 18 cent
=194 cent or €1.94
The Cost of Electricity
Working out the cost of your electricity from the bill.
Example
The final reading electricity bill ón a 2 months ago is 21185
and the new reading is now 21935. the price per unit is 18
cent. The standing charge is €16.89. V.A.T is charged @
13.5%. What is the cost of the electricity bill for 2 months?
Number of units used = new reading – previous reading
= 21935 – 21185
= 750
The Cost of Electricity
ANSWER (cont.)
Cost = number of kWh (units) x price per unit
= 750 x 18 cent
= €135.00
standing charge
= €16.89 (now add the standing charge)
Total before VAT = €151.89 now calculate the V.A.T.
VAT @ 13.5%
= €20.51
Total (including VAT)= €172.40
Electronics
Electronics is the branch of Physics dealing with the circuits
using small carefully controlled currents to operate them.
Devices such as computers, iPods, CD players, mobile
phones, digital cameras, detectors for smoke, heat,
moisture, light and sound all operate by small carefully
controlled currents in their circuits.
The Microchip made of silicon or germanium is at the heart
of the electronic revolution containing millions of tiny
components making up complex circuits to operate
electronic devices.
Electronics (Diodes)
A diode is a device which allows current to pass in one
direction only.
A diode has a silver band at one end which is called the
cathode (-). the opposite end is called the anode (+).
Uses of diodes
1) Rectifiers – change a.c to d.c current.
2) Protect appliances the use d.c only from damage if
connected the wrong way.
Electronics
Forward biased
When a diode is connected with it cathode to the negative of
the battery and anode to the positve of the battery it
conducts electricity and is said to be forward biased.
The diode is connected such that
current can flow, it is forward
biased, so the lamp lights.
Electronics
Reverse biased
When a diode is connected with it cathode to the positive of
the battery and anode to the negative of the battery it does
not conduct electricity and is said to be reverse biased.
The diode is connected such that
current can't flow, it is reverse
biased, so the lamp does not light.
Electronics
Light Emitting Diodes (LED)
This a special diode that gives out light when current passes
through them. The come in various colours, red, green,
blue and yellow.
The shorter cathode lead
is near the flat edge of the
base.
The diagram shows the
symbol for a LED.
Electronics
Light Emitting Diodes (LED)
Like diodes the current flows in one direction only,
forward biased.
Require only a very small current so must have a large
resistor in series with them to protect them from damage.
Uses of LED's
1) Indicator lamps.
2) Digital displays on clocks, calculators etc.
3) Very reliable, long lasting, use little current so now used in
traffic lights, bicycle lamps, torches.
Electronics
Use of LEDs to test the polarity of power supply or battery.
Depending on the polarity of the battery either the green or
red LED will light.
Electronics
The Light Dependent Resistor (LDR)
This is a variable resistor whose resistance depends ón the
amount of light shining on it. In dim light the resistance is
high and in bright light the resistance is low.
Uses of LDRs
1)Light controlled switches to
switch on/off street lights
automatically.
2)In light meters of cameras
and burglar alarms.
Learning Check
What is a diode?
Give two uses of diodes.
What is meant by reverse and forward bias?
What is a l.e.d.?
Give three uses of l.e.d.s.
What are the advantages of l.e.d.s over ordinary bulbs.
How does a LDR work.
How could you show how the resistance of an LDR changes with
brightness?