Charge can
also be present on insulators
and because these materials do not allow the charge to flow,
this is called electrostatic charge
(static meaning it "stays still").
and because these materials do not allow the charge to flow,
this is called electrostatic charge
(static meaning it "stays still").
Insulators can transfer charge by friction.
When the surface of one insulator rubs against another,
electrons can be transferred.
When the surface of one insulator rubs against another,
electrons can be transferred.
The insulator which gains electrons will get a negative charge,
the insulator which loses electrons will get a positive charge.
the insulator which loses electrons will get a positive charge.
It is most
important to know that
it is only the negative electrons which can move.
Positive charges (protons) cannot move
because they are stuck inside the nuclei of the atoms of the material.
it is only the negative electrons which can move.
Positive charges (protons) cannot move
because they are stuck inside the nuclei of the atoms of the material.
For example, if polythene (a type of plastic) is rubbed with a dry cloth,
electrons are transferred from the cloth to the polythene.
The polythene gains electrons and becomes negatively charged,
the cloth loses electrons and becomes positively charged.
electrons are transferred from the cloth to the polythene.
The polythene gains electrons and becomes negatively charged,
the cloth loses electrons and becomes positively charged.
It is not
possible to predict in advance
which way the electrons will go for a certain material.
The same cloth, when rubbed against acetate (a different type of plastic)
will gain electrons and become negatively charged,
leaving the acetate with a positive charge.
which way the electrons will go for a certain material.
The same cloth, when rubbed against acetate (a different type of plastic)
will gain electrons and become negatively charged,
leaving the acetate with a positive charge.
Attraction and Repulsion. Opposite charges attract (pull towards each other),
Like charges repel (push away from each other).
Like charges repel (push away from each other).
This means that two positively charged things will repel each other,
two negatively charged things will repel each other.
One positively charged thing
and one negatively charged thing will attract each other.
two negatively charged things will repel each other.
One positively charged thing
and one negatively charged thing will attract each other.
The further apart the charged things are,
the weaker the forces of attraction and repulsion are.
Gold Leaf Electroscope.
The electroscope is a very
very thin piece of gold
foil (called gold leaf)
fixed at the top to a piece of copper.
The copper has a large round top, called the cap.
The whole thing is put inside a glass case,
to stop air blowing the delicate gold leaf around.
fixed at the top to a piece of copper.
The copper has a large round top, called the cap.
The whole thing is put inside a glass case,
to stop air blowing the delicate gold leaf around.
The piece of copper goes through insulation in the top of the glass case,
so that any charge on the gold leaf cannot escape.
so that any charge on the gold leaf cannot escape.
Charge can
be transferred to the electroscope
by wiping the charged object across the cap.
The charge flows over the conducting copper and gold,
and the gold leaf rises as it is repelled
by having the same charge as the copper.
by wiping the charged object across the cap.
The charge flows over the conducting copper and gold,
and the gold leaf rises as it is repelled
by having the same charge as the copper.
Neutral Objects.
We know that like charges repel and unlike charges attract
but what about neutral (uncharged) objects?
but what about neutral (uncharged) objects?
t is found that a charged object (whether positive or negative)
may attract uncharged objects,
for example a charged plastic comb will pick up small pieces of paper.
You can try this yourself.
Just charge the comb by combing your hair! (hair is a good insulator).
What is happening?
may attract uncharged objects,
for example a charged plastic comb will pick up small pieces of paper.
You can try this yourself.
Just charge the comb by combing your hair! (hair is a good insulator).
What is happening?
It is thought that when a negatively charged object
gets close to an uncharged one,
electrons in the uncharged object are repelled,
leaving the positive charges behind.
These positive charges are then attracted to the negatively charged object.
gets close to an uncharged one,
electrons in the uncharged object are repelled,
leaving the positive charges behind.
These positive charges are then attracted to the negatively charged object.
If the rod was positively charged,
then it would attract electrons in the neutral object
and so the two would still attract each other
(just reverse the + and - in the picture above).
then it would attract electrons in the neutral object
and so the two would still attract each other
(just reverse the + and - in the picture above).
The small
number of charges shown is an oversimplification
since in reality there are millions and millions of atoms
in a tiny piece of paper, each with its own electrons and protons.
When we draw the rod with a few negative charges,
it means that the rod has a few more negative than positive charges.
since in reality there are millions and millions of atoms
in a tiny piece of paper, each with its own electrons and protons.
When we draw the rod with a few negative charges,
it means that the rod has a few more negative than positive charges.
Electrostatic Shock.
If a high
amount of charge builds up on an insulator,
it can escape through a small distance in air
to a neutral or oppositely charged object.
it can escape through a small distance in air
to a neutral or oppositely charged object.
You may have noticed that pulling off a jumper or shirt
over you head can cause crackling.
If the clothing is made from a synthetic fibre
(a plastic material which is a good insulator)
then charge is transferred as it rubs against your hair (also a good insulator).
The crackling is the sound made by the charge
jumping between the clothing and your hair through the air.
over you head can cause crackling.
If the clothing is made from a synthetic fibre
(a plastic material which is a good insulator)
then charge is transferred as it rubs against your hair (also a good insulator).
The crackling is the sound made by the charge
jumping between the clothing and your hair through the air.
Touching a car
door or a radiator can sometimes give you a shock.
If both the car seat and your clothing are made from a synthetic fibre,
then one rubs against the other transferring charge as you step out of the car.
The charge then jumps the small air gap between your finger and the car
as you go to close the car door. This can feel unpleasant.
Similarly, if both the carpet and your shoes are made from synthetic materials,
then charge is transferred as you walk around.
Touching a radiator will cause the charge to jump
the small air gap between your finger and the radiator.
If both the car seat and your clothing are made from a synthetic fibre,
then one rubs against the other transferring charge as you step out of the car.
The charge then jumps the small air gap between your finger and the car
as you go to close the car door. This can feel unpleasant.
Similarly, if both the carpet and your shoes are made from synthetic materials,
then charge is transferred as you walk around.
Touching a radiator will cause the charge to jump
the small air gap between your finger and the radiator.
If your clothing, carpet, shoes etc. are made from natural fibres
then you are much less likely to get a shock.
Natural fibres such as wool and cotton
attract a small amount of moisture (water) to their surface
and this moisture allows the material to conduct a little
so the charge escapes before it can build up enough to jump through air.
If the air itself is moist, the charge will also escape and no shock will occur.
then you are much less likely to get a shock.
Natural fibres such as wool and cotton
attract a small amount of moisture (water) to their surface
and this moisture allows the material to conduct a little
so the charge escapes before it can build up enough to jump through air.
If the air itself is moist, the charge will also escape and no shock will occur.
When charge jumps across a small air gap
it causes a spark which can be dangerous.
Lightning is a natural example of a huge charge
jumping across a very large air gap,
and we know how dangerous lightning can be.
it causes a spark which can be dangerous.
Lightning is a natural example of a huge charge
jumping across a very large air gap,
and we know how dangerous lightning can be.
Dangers.
When charge jumps across an air gap it causes a spark.
The spark can ignite (set fire to) flammable liquid vapours
and powders in pipes.
The spark can ignite (set fire to) flammable liquid vapours
and powders in pipes.
Care must be taken to avoid sparks when putting fuel in cars or aircraft.
The fuel itself is an insulator (a hydrocarbon)
and charge can be transferred as it flows through a pipe
if the pipe is also an insulator.
This happens because there is friction between the fuel and the pipe.
As the nozzle (the end) of the pipe is brought close to the fuel tank,
a spark can jump between the two igniting the fuel.
This can cause a serious explosion,
particularly with aircraft which are filled at very high speed.
The fuel itself is an insulator (a hydrocarbon)
and charge can be transferred as it flows through a pipe
if the pipe is also an insulator.
This happens because there is friction between the fuel and the pipe.
As the nozzle (the end) of the pipe is brought close to the fuel tank,
a spark can jump between the two igniting the fuel.
This can cause a serious explosion,
particularly with aircraft which are filled at very high speed.
The spark can be avoided if the pipe nozzle
is made to conduct by connecting an earthing strap to it
and so any charge can be safely conducted away.
An earthing strap connects the pipe to the ground (the Earth).
In addition, a cable can connect the pipe to the fuel tank,
so that there can be no difference in charge between them.
is made to conduct by connecting an earthing strap to it
and so any charge can be safely conducted away.
An earthing strap connects the pipe to the ground (the Earth).
In addition, a cable can connect the pipe to the fuel tank,
so that there can be no difference in charge between them.
There is a very
similar situation with powders in pipes.
If the powder is an insulator
then charge is transferred in the same way as fuel in pipes.
A spark can ignite a powder and cause an explosion
just like a flammable liquid vapour.
The solution is the same.
Use earthing straps between the pipe and earth.
If the powder is an insulator
then charge is transferred in the same way as fuel in pipes.
A spark can ignite a powder and cause an explosion
just like a flammable liquid vapour.
The solution is the same.
Use earthing straps between the pipe and earth.
Uses.
Electrostatic charge is used in paint spraying, insecticide spraying,
inkjet printers, photocopiers and the removal of pollution from industrial chimneys.
inkjet printers, photocopiers and the removal of pollution from industrial chimneys.
Millions of cars are
made each year
and the car bodies must all be painted to prevent them from going rusty.
The paint is sprayed onto the car bodies
and the process is made more efficient by using electrostatic charge.
and the car bodies must all be painted to prevent them from going rusty.
The paint is sprayed onto the car bodies
and the process is made more efficient by using electrostatic charge.
The paint spray goes past a high voltage positive needle
as it leaves the spray gun
and the tiny droplets of paint pick up a positive charge.
Remember, they do this by losing negative electrons.
It is only the electrons which can move.
The car body is then given a high voltage negative charge
which attracts the positively charged paint droplets.
as it leaves the spray gun
and the tiny droplets of paint pick up a positive charge.
Remember, they do this by losing negative electrons.
It is only the electrons which can move.
The car body is then given a high voltage negative charge
which attracts the positively charged paint droplets.
This is good for two
reasons.
Firstly, the paint droplets spread out more as they leave the gun.
This happens because they all get the same positive charge
and so they all repel each other.
This is better than coming straight out of the gun
as the paint will cover a wider area more evenly.
Firstly, the paint droplets spread out more as they leave the gun.
This happens because they all get the same positive charge
and so they all repel each other.
This is better than coming straight out of the gun
as the paint will cover a wider area more evenly.
Secondly, the paint droplets are attracted to the negative metal car body,
and so less paint will be wasted on the floor or the walls of the paint shop.
and so less paint will be wasted on the floor or the walls of the paint shop.
What is Electricity?
Electricity is
a flow of charged
particles, which may be electrons or ions.
In chemistry, ions which
are free to move will conduct
electricity
during electrolysis.
In physics, we are dealing with electricity as a flow of electrons.
A cell uses chemical reactions to make electricity.
during electrolysis.
In physics, we are dealing with electricity as a flow of electrons.
A cell uses chemical reactions to make electricity.
In the circuit below, electricity will flow from the cell (or battery),
through the lamp (light bulb) and back to the cell.
through the lamp (light bulb) and back to the cell.
There is a difference between a cell and
a battery.
In every-day life, we use the word "battery".
In physics, one "battery" on its own is called a cell.
Two or more cells which are joined together are called a battery.
In every-day life, we use the word "battery".
In physics, one "battery" on its own is called a cell.
Two or more cells which are joined together are called a battery.
The cells of a battery are joined
together in series.
The positive side of one cell is joined to the negative side of the next cell.
The positive side of one cell is joined to the negative side of the next cell.
The word "battery" is used to mean
"collection".
A collection of cells is called a battery of cells.
A collection of cells is called a battery of cells.
Conventional Current. A cell is drawn with a long line and a shorter line.
The long line is the positive side (remember plus means more).
The short line is the negative side (remember minus means less).
The long line is the positive side (remember plus means more).
The short line is the negative side (remember minus means less).
All electrical
circuits are drawn
as though electricity flows from positive to negative.
This is called conventional current.
as though electricity flows from positive to negative.
This is called conventional current.
In reality, electricity is a flow of electrons
and electrons are negatively charged.
They must therefore flow from negative to positive,
since they are repelled from the negative side of the cell
and attracted to the positive side.
This is called real current.
and electrons are negatively charged.
They must therefore flow from negative to positive,
since they are repelled from the negative side of the cell
and attracted to the positive side.
This is called real current.
Why are electric
circuits deliberately drawn using conventional current,
when we know that this is wrong?
when we know that this is wrong?
Electricity flowed from one
side of a cell to the other
but this was long before atomic theory had advanced to the level
of knowing about electrons and protons.
but this was long before atomic theory had advanced to the level
of knowing about electrons and protons.
André guessed
that electricity was a flow of positive charge
that went from plus to minus.
He got it wrong but by the time this was discovered,
a large number of electrical circuits had already been drawn
and since it makes no practical difference,
it was decided to keep the conventional direction of current flow.
that went from plus to minus.
He got it wrong but by the time this was discovered,
a large number of electrical circuits had already been drawn
and since it makes no practical difference,
it was decided to keep the conventional direction of current flow.
Coulombs
Electrons are very small.
In physics, we take a very very large number of electrons
as 1 unit of charge - called a Coulomb.
Charge is given the symbol Q.
In physics, we take a very very large number of electrons
as 1 unit of charge - called a Coulomb.
Charge is given the symbol Q.
1 Coulomb = 6.2 x 1018 electrons.
(This is 6.2 million million million electrons).
Such a large number of electrons can do useful things like light a lamp.
(This is 6.2 million million million electrons).
Such a large number of electrons can do useful things like light a lamp.
Think of Coulombs as though they are busses,
taking a large number of electrons (like passengers)
from one side of the cell, through all the components in the circuit,
and back to the other side of the cell.
The electrons are not used up but keep flowing around the circuit.
taking a large number of electrons (like passengers)
from one side of the cell, through all the components in the circuit,
and back to the other side of the cell.
The electrons are not used up but keep flowing around the circuit.
We need to know the rate of Coulombs flowing around the circuit
(how many Coulombs per second)
and how much energy each Coulomb has (how many Joules per Coulomb).
(how many Coulombs per second)
and how much energy each Coulomb has (how many Joules per Coulomb).
1 Amp = 1 Coulomb per second.
The word "per" means "divided
by",
so current = charge ÷ time.
so current = charge ÷ time.
Current, which is given the symbol I, is shown using an ammeter.
The ammeter,
shown as a circle with the letter A inside,
is always connected in series with a component.
is always connected in series with a component.
If the ammeter reads 1 Amp,
then the current (I) = 1 Amp at that point in the circuit.
I = 1 Amp = 1 Coulomb per second.
then the current (I) = 1 Amp at that point in the circuit.
I = 1 Amp = 1 Coulomb per second.
If the ammeter reads 6 Amps,
then I = 6 Amps = 6 Coulombs per second.
then I = 6 Amps = 6 Coulombs per second.
So current = charge ÷ time.
I = Q ÷ t
I = Q ÷ t
This can be rearranged to give
Q = I x t,
or, charge = current x time
Q = I x t,
or, charge = current x time
Electricity
P = V x I. Power = Voltage x Current.
Q = I x t. Charge = Current x time.
E = V x Q. Energy = Voltage x Charge.
Total
Cost = Number of Units x Cost per Unit.
Volts
The power
supply (the cell or battery) gives an amount
of energy
to each Coulomb going around an electric circuit.
A 6 Volt cell gives 6 Joules of energy to each Coulomb.
to each Coulomb going around an electric circuit.
A 6 Volt cell gives 6 Joules of energy to each Coulomb.
1 Volt = 1 Joule per Coulomb.
The word "per" means "divided
by",
so Voltage = Energy ÷ Charge.
so Voltage = Energy ÷ Charge.
This can be rearranged to give
Energy = Voltage x Charge. E = V x Q.
Energy = Voltage x Charge. E = V x Q.
Since Q = I x t, if we write I x t instead
of Q in the above equation we get
E = V x I x t. Energy = Voltage x current x time. (see equations).
E = V x I x t. Energy = Voltage x current x time. (see equations).
We can also write Work instead of Energy,
so you might see one of the
above equations written as Work = Voltage x Charge. W = V x Q.
above equations written as Work = Voltage x Charge. W = V x Q.
Voltage (which
is also called potential
difference, or p.d.)
is an electrical pressure pushing current around a circuit.
Doubling the voltage will double the current.
is an electrical pressure pushing current around a circuit.
Doubling the voltage will double the current.
Voltage is
measured using a voltmeter.
The voltmeter,
shown as a circle with the letter V inside,
is always connected in parallel with the component.
(The voltmeter is said to be connected across the component,
where the word "across" means "in parallel with").
The circuit on the left would show the voltage of the cell.
is always connected in parallel with the component.
(The voltmeter is said to be connected across the component,
where the word "across" means "in parallel with").
The circuit on the left would show the voltage of the cell.
The circuit on the right shows the voltmeter connected across a lamp.
This will tell you how many Joules of energy are being converted
from electrical energy into light energy (+heat)
for each Coulomb which passes through it.
This will tell you how many Joules of energy are being converted
from electrical energy into light energy (+heat)
for each Coulomb which passes through it.
A reading of 6 Volts tells you that 6 Joules of energy
are being converted for each Coulomb passing through the lamp.
are being converted for each Coulomb passing through the lamp.
A reading of 10 Volts tells you that 10 Joules of energy
are being converted for each Coulomb passing through the lamp.
are being converted for each Coulomb passing through the lamp.
1 Watt = 1 Joule per second.
then Watts = Volts x Amps,
or Power = Voltage x Current
or Power = Voltage x Current
P = V x I
This equation is very
important!
On the next page we shall see how to calculate the power of a lamp.
On the next page we shall see how to calculate the power of a lamp.
Since from the above, power = energy ÷ time,
then energy = voltage x current x time,
E = V x I x t.
E = V x I x t.
To calculate the power of a lamp. Firstly, we need to measure the current flowing through the lamp,
and the voltage across the lamp.
The circuit above shows where to place an ammeter and a voltmeter.
and the voltage across the lamp.
The circuit above shows where to place an ammeter and a voltmeter.
If the ammeter reads 2 A, and the voltmeter reads 6 V,
Resistance
Resistance is
measured in Ohms (symbol ).
Resistance is a measure of how much the current is slowed down.
The bigger the resistance, the smaller the current.
Resistance is a measure of how much the current is slowed down.
The bigger the resistance, the smaller the current.
The very
important equation
is an expression of Ohm's Law.
If the resistance of a component is constant (stays the same)
for different values of V and I,
then a plot (graph) of V against I will be a straight line.
The gradient (slope) of the line shows how big the resistance is.
for different values of V and I,
then a plot (graph) of V against I will be a straight line.
The gradient (slope) of the line shows how big the resistance is.
Components which
obey Ohm's Law are Wires and Resistors.
A component will only obey Ohm's Law at constant temperature
(meaning that the temperature must not change).
A component will only obey Ohm's Law at constant temperature
(meaning that the temperature must not change).
In reality,
an increase in current through a component
will change its temperature (the temperature usually goes up),
and so Ohm's Law is only an approximation
but it works quite well for many components.
The next page shows plots for components which don't obey Ohm's Law.
will change its temperature (the temperature usually goes up),
and so Ohm's Law is only an approximation
but it works quite well for many components.
The next page shows plots for components which don't obey Ohm's Law.
1. The thin wire (filament) inside the light bulb
gets very hot when a current flows through it and it glows brightly.
This rise in temperature causes an increase in resistance of the filament,
and so the gradient (slope) of the plot is seen to increase.
gets very hot when a current flows through it and it glows brightly.
This rise in temperature causes an increase in resistance of the filament,
and so the gradient (slope) of the plot is seen to increase.
2. A thermistor is a special
type of resistor
which has been deliberately manufactured so that its
resistance decreases as its temperature rises.
which has been deliberately manufactured so that its
resistance decreases as its temperature rises.
To calculate the resistance of a resistor.
(A resistor converts electrical energy into heat, see resistance of wires). Firstly, we need to measure the current flowing through the resistor,
and the voltage across the resistor.
(A resistor converts electrical energy into heat, see resistance of wires). Firstly, we need to measure the current flowing through the resistor,
and the voltage across the resistor.
If the ammeter reads 2 A, and the voltmeter reads 6 V,
R = V I
= 6 2
= 3 Ohms.
= 6 2
= 3 Ohms.
Test Circuit for a Component.
Anything in
an electric circuit (lamp, resistor, motor, diode etc.)
is called a component.
Each component has its own circuit symbol.
is called a component.
Each component has its own circuit symbol.
A test
circuit can be used to find the characteristics of a component.
A variable
resistor
(sometimes called a rheostat when placed in series in a circuit)
can change the amount of current flowing through the component,
and the voltage across it it.
(sometimes called a rheostat when placed in series in a circuit)
can change the amount of current flowing through the component,
and the voltage across it it.
Values obtained
from the voltmeter and ammeter
are then used to plot the graphs shown on the previous pages.
The shape of the graph describes the characteristics of the component.
are then used to plot the graphs shown on the previous pages.
The shape of the graph describes the characteristics of the component.
Series and Parallel
So far we have looked at only one component
in a circuit with meters.
When more than one component is used in a circuit,
there are two different ways of arranging them
and these are called series or parallel.
in a circuit with meters.
When more than one component is used in a circuit,
there are two different ways of arranging them
and these are called series or parallel.
There are different rules for series and parallel circuits
and you must know these rules.
and you must know these rules.
Rules for
a Series Circuit.
When components are connected one following another in a ring,
the components are said to be in series with each other
and the circuit is called a series circuit.
the components are said to be in series with each other
and the circuit is called a series circuit.
Below is a series circuit shown with three different resistors.
Current in a Series Circuit. The current in a series circuit is the same everywhere.
An ammeter placed anywhere in a series circuit
always gives the same reading.
In the circuit above, A1 = A2 = A3 = A4.
always gives the same reading.
In the circuit above, A1 = A2 = A3 = A4.
If an identical
cell (battery) is placed in series with the original cell
the current doubles because the total voltage of the circuit doubles.
However, two cells together provide electricity for only
the same amount of time as one cell before they both run out.
the current doubles because the total voltage of the circuit doubles.
However, two cells together provide electricity for only
the same amount of time as one cell before they both run out.
Voltage in
a Series Circuit.
To calculate the voltages
below,
we need to know the total resistance of the circuit,
and the current flowing through it.
we need to know the total resistance of the circuit,
and the current flowing through it.
2. The voltage across all of
the components
adds up to the supply voltage from the cell (or battery).
In energy terms, the work done by the cell on each coulomb of charge
equals the work done on the components of the circuit.
adds up to the supply voltage from the cell (or battery).
In energy terms, the work done by the cell on each coulomb of charge
equals the work done on the components of the circuit.
Vsup = V1 + V2 + V3.
The supply voltage is divided (shared) between the components.
If there is a change in the resistance of one component
then the voltage across all of the components will change.
The supply voltage is divided (shared) between the components.
If there is a change in the resistance of one component
then the voltage across all of the components will change.
If more cells (batteries) are connected together in series
the total voltage is the sum of the individual voltages for each cell
(provided they are connected the right way round, plus to minus).
the total voltage is the sum of the individual voltages for each cell
(provided they are connected the right way round, plus to minus).
If an identical
cell is placed in series with the original cell
in the circuit above, then the voltage doubles.
However, two cells together provide electricity for only
the same amount of time as one cell before they both run out.
in the circuit above, then the voltage doubles.
However, two cells together provide electricity for only
the same amount of time as one cell before they both run out.
Resistance in a Series Circuit. You can calculate the total resistance of a series circuit
by adding up the resistance of each component.
by adding up the resistance of each component.
Rtotal = R1 + R2 + R3.
In the above circuit,
Rtotal = 2 + 3 + 4
= 9 Ohms.
Calculation of Voltages and Current in a Series Circuit. If the supply
voltage (from the cell) is 12 Volts,
what are the voltages across each resistor?
what are the voltages across each resistor?
I = V R
= 12 9
= 1·333 Amps.
= 12 9
= 1·333 Amps.
V1 = 1·333 x 2
= 2·667 Volts.
= 2·667 Volts.
V2 = 1·333 x 3
= 4·000 Volts
= 4·000 Volts
V3 = 1·333 x 4
= 5·333 Volts
= 5·333 Volts
You must always
say what the units are at the end of the calculation.
If you write V3 = 5·333 without putting the word "Volts" afterwards,
you will lose a mark in the exam.
If you write V3 = 5·333 without putting the word "Volts" afterwards,
you will lose a mark in the exam.
We can see that the largest resistor (4 Ohms) has the largest voltage (5·333 Volts)
and the smallest resistor (2 Ohms) has the smallest voltage (2·667 Volts) across it.
In energy terms, the largest amount of work is done by the charge
moving through the largest resistance.
and the smallest resistor (2 Ohms) has the smallest voltage (2·667 Volts) across it.
In energy terms, the largest amount of work is done by the charge
moving through the largest resistance.
V1 + V2 + V3 = 2·667 + 4·000 + 5·333
= 12 Volts.
= 12 Volts.
Switches and Lamps in Series Circuits. An open switch in a series circuit will turn everything off,
because the circuit will be disconnected from the cell.
because the circuit will be disconnected from the cell.
When lamps are connected
in series,
the more lamps in the circuit the dimmer they get,
because the voltage is divided between them.
the more lamps in the circuit the dimmer they get,
because the voltage is divided between them.
If one
lamp in a series circuit breaks or fails,
all the others will go out with it.
For this reason, lamps are always connected in parallel
(except Christmas Tree Lights or Fairy Lights,
where the large mains voltage is conveniently divided between the lamps).
all the others will go out with it.
For this reason, lamps are always connected in parallel
(except Christmas Tree Lights or Fairy Lights,
where the large mains voltage is conveniently divided between the lamps).
Rules for a Parallel Circuit. Below is a parallel circuit shown with three different resistors.
Current in a Parallel Circuit. 1. The current in a parallel circuit depends on the resistance of the branch.
2. The total
current flowing in to
the branches
is equal to the total current flowing out of the branches.
A1 = A5
is equal to the total current flowing out of the branches.
A1 = A5
I = V R
= 12 2
= 6 Amps.
You would get the same answer for the 2 Ohm resistor,
whether or not the other resistors are connected in the circuit.
For parallel circuits, each component behaves
as if it is connected independently to the cell,
and is unaware of the other components - see Lamps
(continued on the next page).
whether or not the other resistors are connected in the circuit.
For parallel circuits, each component behaves
as if it is connected independently to the cell,
and is unaware of the other components - see Lamps
(continued on the next page).
If an identical
cell (battery) is placed in parallel with the original cell
the current stays the same because the total voltage of the circuit is the same.
The two cells together provide electricity for twice as long
before they both run out. See also cells in series.
the current stays the same because the total voltage of the circuit is the same.
The two cells together provide electricity for twice as long
before they both run out. See also cells in series.
The current
A3 flowing through the 3 Ohm resistor is
I = V R
= 12 3
= 4 Amps.
The current
A4 flowing through the 4 Ohm resistor is
I = V R
= 12 4
= 3 Amps.
A1 = A2 + A3 + A4
= 6 + 4 + 3
= 13
Amps.
This is much
larger than the current of 1·333
Amps
which flows through a series circuit
with the same resistors and supply voltage
which flows through a series circuit
with the same resistors and supply voltage
V1 = V2 = V3.
2. The voltage for each
branch is the same as the supply voltage.
V1 = V2 = V3= Vsup.
If an identical
cell (battery) is placed in parallel with the original cell
the voltage stays the same.
The two cells together provide electricity for twice as long
before they run out.
the voltage stays the same.
The two cells together provide electricity for twice as long
before they run out.
resistance in a Parallel Circuit. The total resistance of a parallel circuit is calculated using the
formula
1/R = 1/R1 + 1/R2 + 1/R3 ·····
In the above circuit,
1/R = 1/2 + 1/3 + 1/4
= 6/12 + 4/12 + 3/12
= 13/12
R = 12/13
= 0·92 Ohms.
Notice that this is a much smaller resistance
than you get in the series circuit using the same resistors.
than you get in the series circuit using the same resistors.
It is even
smaller than the smallest resistor in the parallel circuit,
which is 2 Ohms.
Putting more resistors in the parallel circuit decreases the total resistance
because the electricity has additional branches to flow along
and so the total current flowing increases.
which is 2 Ohms.
Putting more resistors in the parallel circuit decreases the total resistance
because the electricity has additional branches to flow along
and so the total current flowing increases.
Switches and Lamps in a Parallel Circuit. A switch at S1 or S5 will switch all the lamps off and on together
(assume that all the other switches are "closed" which means "on").
(assume that all the other switches are "closed" which means "on").
The switch at S2 will only
light the lamp at L1.
This is very useful because it means that we can switch the lamp
on and off without affecting the other lamps.
The brightness of the lamp does not change
as other lamps in parallel are switched on or off.
For this reason lamps are always connected in parallel (except Fairy Lights).
This is very useful because it means that we can switch the lamp
on and off without affecting the other lamps.
The brightness of the lamp does not change
as other lamps in parallel are switched on or off.
For this reason lamps are always connected in parallel (except Fairy Lights).
Similarly, the switch at S3 will only
light the lamp at L2.
The switch at S4 will only light the lamp at L3.
The switch at S4 will only light the lamp at L3.
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