Download Static and Current Electricity

Current Electricity
What do you know about Electricity?
Draw a Spider Diagram
Electricity
Electricity was first discovered in
600B.C. by Thales de Miletus a
Greek Philosopher.
He noticed that when amber, a hard
dry yellow substance, is rubbed
with wool or fur it attracted light
material e.g. hair or dust.
Facts about Electricity
Benjamin Franklin demonstrated that
lightning is electricity. He tied a key to a
kite string during a thunderstorm, and
proved that static electricity and lightning
were the same thing.
In 1879 Thomas Edison invented the first
long-lasting incandescent light bulb that
could be used for about 40 hours without
burning out. By 1880, his bulbs could be
used for 1,200 hours.
 Electricity travels at the speed of light - more
than 186,000 miles per second!
 A spark of static electricity can measure up to
three thousand (3,000) volts.
 A bolt of lightning can measure up to three
million (3,000,000) volts, and it lasts less than
one second!
 Electricity always tries to find the easiest path to
the ground.
 Electricity can be made from wind, water, the sun
and even animal poo.
 A 600 megawatt natural gas plant can power
220,000 homes.
Electrical conductors and insulators
• In some materials, like metals, charge can
move easily. In other materials, charge
cannot move easily.
• A conductor allows charge to move.
• An insulator holds a charge.
Insulators
• Insulators are substances which do not
normally allow charge to flow through
them.
• E.g. perspex, glass, polythene
Conductors
Conductors are substances which allow
charge to flow through them freely.
e.g. metals like copper
Current Electricity
The first battery (or cell) was discovered by Luigi Galvani. He was
dissecting a frog at the time.
He happened to be using two dissecting tools each of different metals. He
touched a nerve and the dead frog's leg twitched! He had accidentally
produced an electric current. This lead to the development of what we now
call the battery
Electrical circuits
• An electrical circuit is an arrangement of
conductors connected together so that
charge can move around in a loop.
Battery has a positive and
negative terminal
What is an electric current?
An electric current is a flow of microscopic particles
called electrons through wires and components.
+
-
In which direction does the current flow?
from the Negative terminal to the Positive terminal of a
cell.
Electric Current
A flow of electric charge is
called an electric current
Insulators & Electrons
In some atoms there is a strong force of
attraction between nucleus and outer
electrons.
These atoms do not tend to lose electrons
and have nearly no free electrons
These materials are known as insulators
Conductors & Electrons
Electrons are negatively charged.
Outer electrons in some atoms are only
loosely attached to nucleus
These electrons can easily escape from
the atoms
Materials containing free electrons like
these are known as conductors
Mandatory Experiment
To test if a substance is a
conductor or Insulator
Pg. 348
Method.
1. Make a list of all the materials you are going
to test
2. Divide them into two columns predicting
whether they are insulators or conductors
Table predicting insulators or
conductors
Material
Insulator
Conductor
Method
3. Set up the circuit as shown in diagram
Method
4. At the beginning there is a gap
between the crocodile clips X and Y.
The bulb does not light because the
circuit is not complete and electrical
charge cannot flow through it.
5. Now bridge the gap from X to Y with
one of the materials you want to test.
Method
Observe if the bulb lights up or does not light
up.
If the bulb lights up the material you are
testing is a
Conductor.
If the bulb does not light up the material you
are testing is an insulator.
Result
When _________ attached at clip X and
Clip Y the bulb lighted up and the circuit
was complete
When ___________ was attached at clip X
and Clip Y the bulb did not light and the
circuit was not complete
Conclusion
___________ are conductors and ______
are insultators
Current Electricity
Current Electricity is the flow of electrons
from the negative terminal of a battery to
the positive terminal
Simple Electrical Circuits
When charge moves as in current
electricity these charges flow around a
path called an electric circuit.
bulbs
Battery
wire
Points to remember about
electric circuits
1. Circuits must be complete for current to
flow if there is a gap or break in the circuit
current will not flow.
A switch provides
a gap in the
circuit. Current
flows when the
switch is closed
or “on”.
Current cannot
flow when the
switch is open
“off”.
2. Potential Difference
A circuit must have a source of potential
difference (p.d)
This is also called voltage (electron moving
force e.m.f.) This provides the energy to
push the electrons around the circuit.
Electrons cannot move around a circuit
without this energy.
Voltmeter
A voltmeter is used to measure potential
difference or voltage between two points in
a circuit
multimeter
3. Current
The potential difference or e.m.f. in a circuit
causes electrons to flow ie. a current
To measure electric current an Ammeter is
used. This measures the amount of charge
passing a particular point in one second
4. Resistance
Moving electrons may meet a part of a
circuit that is difficult to pass through e.g.
the thin wire in a bulb. The wire in the bulb
offers a resistance to the electrons – it
slows the electrons down.
In the bulb electrical energy is turned into
heat energy and light energy. Resistors are
often used in circuits to produce heat or
light or reduce the current flowing in the
circuit.
A rheostat is a resistor whose resistance
can be changed. They are also called
variable resistors. They can be used to
control the volume of the radio.
Ohmmeter
An ohmmeter is the instrument used to
measure the resistance of a resistor
Units used in Electricity
Different units are used in measuring
various quantities
Unit
Symbol of electricity
Information
Volt
V
Ampere
(AMP)
A
Unit of voltage or potential
difference. It is a measure
of push or strength of a
battery e.g. 1.5 V batteries,
6V batteries
Unit of Current. It gives a
measure of the amount of
charge flowing past a point
in a wire per second. A
current of 0.05 going
across your heart could kill
you.
Ohms
Symbol
Ω
Unit of resistance
It measures the
opposition of part
of a circuit to the
flow of charge.
The greater the
resistance the
smaller the
current.
Scientists
Alessandro Volta
from Italy invented
the battery.
Andre Marie Ampere
He devised a
method of
measuring current
and put forward a
theory to explain
magnetism
Circuit Vocabulary and diagrams
Symbols for a circuit
Relationship between Voltage,
Current and Resistance
You can use an electric circuit to show a
relationship between voltage, current and
resistance.
1. The resistance of a coil will be measured.
2. The voltage across the coil and the current
going through the coil will be measured
3. We will establish a ling between resistance,
voltage and current
Instead of using a battery we can use low
voltage power supply. This allows us to
vary the supply of voltage to the circuit.
We use a voltmeter to measure voltage
(potential difference) in a circuit. A
voltmeter must be connected to two
different points in the circuit. It is
connected in parallel in a circuit.
An ammeter is used in the circuit to
measure current. The ammeter is
connected in series in the circuit.
Simple Circuits
• Series circuit
– All in a row
– 1 path for electricity
– 1 light goes out and
the circuit is broken
• Parallel circuit
– Many paths for electricity
– 1 light goes out and the
others stay on
Ohm’s Law
• The ohm (symbol: Ω) is the SI derived unit
of electrical resistance, named after
German physicist Georg Simon Ohm.
Ohm’s Law
• The current decreases because the
resistance increases.
• Ohm’s Law says that I=V/R. The voltage
in the system is constant, resistance
increases.
The relationship between current,
potential difference and resistance
in metal conductors
• If we measure the current passed through a
metal conductor which has a certain resistance
and also measure the voltage (potential
difference) needed to push the current through
the resistor, we find that as the voltage is
increased, the current increases. We can write
this relationship in a formula.
• Voltage = resistance x current, or
• V = RI
Experiment to Measure the
Resistance of a coil or a resistor
(Mandatory Experiment)
Results Table
Voltage
(V)
Current
(A)
2
4
6
8
10
12
Ohms Law
Calculations based on Ohms Law
Relationship between potential difference,
current and resistance
Georg Ohm showed for a given conductor at
constant temperature the voltage and the
Current are proportional to each other
Ohms Law Triangle
Simple calculations on V=RI
• Calculate the voltage across a resistance
of 15 ohms when a current of 0.5 amps
flows through it.
• Solution: We are given R = 15 ohms and
I = 0.5 amps.
Using V = RI
V = (15)(0.5)=7.5 volts
Effects of an Electric Current
We cannot see electric charge moving.
We will examine 3 effects of electricity.
These are when electrical energy is
converted to (1) heat energy (2) magnetic
energy (3) chemical energy
1. Heating Effect
Some metals e.g. nichrome have a high
resistance to the flow of current. More
energy is need to force current through the
nichrome wire. As a result heat is given
off. That is why nichrome is used in the
element of an electric fire.
Nichrome Wire
Tungsten Wire
Tungsten wire is used in light bulbs.The bulb
is filled with argon or nitrogen gas.
Experiment to demonstrate the
heating effect of an electric
current
Expt 45.4 pg 354
Apparatus: Beaker, water, coil of nichrome
wire, low voltage power supply,
calorimeter, labquest & temperature
sensor, timer
Method: 1. set up apparatus as shown in
diagram
2. Take the temperature of the water
2. Turn on the power supply
Result:
Conclusion
Diagram:
Time
Temp of
water
0 mins
20.9
2 mins
22.8
4 mins
23.3
6mins
23.5
8 mins
24
To demonstrate simple series and
parallel circuits containing a switch
and bulb
• Date:
• Apparatus: Low voltage power supply,
switch, 2 bulbs, connecting wires,
crocodile clips
• Method:
• Diagrams
• Result
Simple series and parallel circuits
Result: a) Bulbs in series
b) Bulbs in Parallel
Conclusion
Note: Old Christmas tree lights are sometimes
arranged in series – if one bulb blows all the
lights go out.
Most lights in your house are arranged in parallel
the big advantage of this is that they can
individually be switched on and off.
Different ways of wiring a circuit
• There are two ways of wiring light bulbs in
a circuit.
• In series: The bulbs are connected so that
the current through one passes through
the next.
Different ways of wiring a circuit
• In parallel
• The current is
divided into different
branches.
• The advantage
is that if one bulb
blows, the others
remain lit
A Fuse
A fuse is thin piece of wire enclosed in a
porcelain case. It is a weak link which is
deliberately put into a circuit to make it
safer. If a current above the expected level
flows the fuse wire will get hot and melt
safely in its case and the circuit will be
broken.
A fuse
Draw diagram on pg. 355 fig 45.10
Everyday applications of
heating effect of an electric
current
1. Electric current producing heat in an
electric fire.
2. Metal coil in an electric kettle
2. Magnetic Effect
A compass needle moves if placed near
wire that had current flowing through it.
When a current flows in a wire that is wound
around a nail the nail becomes a strong
magnet called an electromagnet.
Everyday applications of the
magnetic effect of an electric
current
1. Electromagnets are used in electric bells
2. Electromagnets are used as circuit
breakers.
3. Chemical Effect
The chemical effect of an electric
current can be used to
separate water into hydrogen
and oxygen
This is called electrolysis of
water
Everyday Applications of the
Chemical Effect of an Electric
Current
1. Electrolysis can be used in beauty
treatments to remove hair from the body.
2. Electroplating is used in making cutlery
eg forks. A cheap metal is covered with a
layer of another metal to protect it from
corrosion and to make it look better.
Electroplating
Coating a cheap metal with a thin layer of
another more expensive metal. E.g.
cutlery
Direct and Alternating Current
1. Direct Current
Current that travels
in one direction
only is known as
direct current or
d.c. for short.
e.g. a battery pushes
current in one
direction only.
Alternating Current
The current from
the mains supply
changes direction
100 times per
second. This type
of current is
called alternating
current or a.c.
AC to DC
For most appliances it does not matter
whether the current is alternating or direct.
A bulb will work if the current is flowing
through it in one direction only or if it is
constantly changing direction.
Some appliances like a T.V. need a d.c.
supply. A device called a rectifier is
needed to convert a.c. to d.c.
Summary: Key points
• Charge exerts a force. There are two
types: positive and negative.
• Unlike charges attract, like charges repel.
• Conductors are materials that allow
charge to move through them. Insulators
are materials that hold a charge.
• All charge tries to go to earth. The earth is
a source and a sink for charge.
Summary (continued)
• Electrical current is the movement of charge
through a material.
• Potential difference is the energy needed to
move charge between points.
• When current moves through a device, there is
resistance to this movement.
• The relationship between current, potential
difference and resistance in a metal resistor is
V = RI.
• Devices can be connected in a circuit in series
or in parallel.