Download Force, Mass and Momentum

Effects of an Electric Current and
Domestic Circuits
Chapter 24
Heating Effect of Electric Circuits
• It was James Watt who experimentally investigated
the effects of the heat, W, from a current carrying
wire. It was found that the following equation may
be used to find W:
W  I 2 Rt
• where W = amount of heat energy given
I = current through the wire
R = resistance of the conductor
t = time that the current flows for
• Joule’s law states that the rate at which heat is
produced in a conductor is directly proportional to
the square of the current provided its resistance is
constant:
P I2
• By dividing both sides of the heat effect equation
by ‘t’ we find:
P  I 2R
The Chemical Effect of an
Electric Current
• An electric current may cause a chemical
reaction when passed through a liquid, known
as electrolysis.
• The liquid in which the current is passed is
called the electrolyte, the plates that are in the
electrolyte are called electrodes, the positive
electrode is known as an anode, the negative
electrode is the cathode. The container,
electrodes and electrolyte are known as a
voltameter.
• Electrodes that are involved in the chemical
reaction are called active electrodes, those
that do not are called inactive electrodes.
• Examples of an electrolyte would be water
with an acid, base or salt in it (i.e. a
solution), or an ionic compound in it’s
molten state.
• An ion is an atom or molecule that has
gained or lost one or more electrons.
• The charge carriers in an electrolyte are
positive and negative ions.
Applications of the Chemical Effect
• Electroplating, covering one metal with a
thin plating of another.
• Extraction of minerals from their ores.
• In electrolyte capacitors electrolysis is used
• To purify metals.
Relationship between current and
voltage for different conductors
A metallic conductor
• Assuming we have constant
temperature, the resistance of
a conductor will not change as
current increases, so we get a
straight line graph through the
origin, obeying Ohm’s law.
• Charge carriers in a metal are
electrons.
A filament bulb
As the voltage across the filament
increases, so does the current and
in turn the temperature. Since
resistance α current, resistance
increases. So as the bulb gets
hotter a given increase in V doesn’t
produce an increase of I when it
was colder.
Charge carriers in a filament bulb
are negative electrons.
A semi conductor, e.g. a thermistor
As the p.d. across the
semiconductor is increased, the
current increases, and in so doing
the semiconductor gets hotter. This
produces more holes and electrons
for conduction and the resistance
drops. Thus a further increase in V
produces a larger increase of I
when it was cold.
Charge carriers are negative
electrons and positive holes.
Active electrodes
Inactive electrodes
Electrolytes/ionic solutions
As p.d. increases so will current.
Active electrode takes part in the
chemical reaction-also obey Ohm’s
law and the graph is linear through
the origin. If the electrodes are
inactive, the voltameter behaves
like a cell and has an emf that must
be overcome before current will
flow.
Charge carriers are positive and
negative ions.
E.g. neon lamps.
A Gas
An example would be a discharge
tube. In region OA the positive ions
in the tube are attracted to the
negative electrode and the electrons
move towards the positive electrode
once a p.d. is applied, as number of
ions crossing the tube increases so
does the current. In region AB all the
ions in the tube cross without
recombination so no increases in
current. Voltage increases to a stage
where collision between fast moving
ions and electrons produces more
ions, corresponding to region BC.
Charge carriers are positive ions,
negative electrons and a few negative
ions.
A Vacuum
No charge carriers in a
vacuum, however if the
cathode is heated sufficiently
electrons will be produced by
thermionic emission. A certain
voltage is reached where all
the electrons from the cathode
are carried across the tube and
the curve flattens out.
Domestic Electric Circuits
• Appliances that take a large current , such as an
electric cooker, electric shower or immersion heater
have a separate live and neutral wire coming from
the distribution box. Such a circuit is called a radial
circuit.
• In a ring circuit, the live terminals of each socket
are connected together. Power is thus fed along
both sides of the ring to each socket. The neutrals
are also connected together and connected back to
the neutral at the distribution box.
• In a plug the live
wire
is
brown,
neutral is blue and
earth
is
green/yellow.
• Bonding is when metal taps, metal water tanks, etc.
in a house are earthed in case they ever come into
contact with a live wire, thus nobody is
electrocuted.
• Earthing is where an appliance is earthed so that if
a fault develops where the live wire came in contact
with the outer metal casing, for example, nobody
would be electrocuted if they touched the casing.
• A fuse is a piece of wire within a
ceramic casing that is placed in
series with the live wire of an
electrical appliance, normally in
the plug. If a fault develops
within the appliance where it is
drawing too much current the
wire within the fuse would be
unable to maintain this extra
current and thus melt, thus
breaking the flow of electricity
and taking away the possibility
of electrocution.
• Miniature circuit
breakers(MCBs) are used
sometimes instead of fuses
in the distribution box. They
contain a bimetal strip and
an electromagnet. When the
current is larger than the
preset value, two contacts
are separated and thus
breaking the flow of
electricity. Bimetal strip is
used to trip small currents,
while the electromagnet is
used for larger currents.
• Residual current
devices(RCDs) operate by
detecting a difference
between the current in the
live and the neutral-which
could arise if somebody
came in contact with a live
wire. When the difference
between the two reaches a
preset value ( normally
30mA ), the RCD trips very
quickly and cutting off
electricity, whereas the fuse
and MCB would take
longer.
The Kilo-watt Hour
• The kilo-watt hour is the amount of energy
used by a 1000 W appliance in one hour.
Remembering that:
1000 W=1000 joules per second
and
energy=power x time.