Download - Electricity
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Electricity
Milind Naik
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Static Electricity!(stationary/rest)
Charge that stays on an object
When something gives up or gains electrons
Does not move along a wire or other conductor
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Electric Charge
Measured in COULOMBS
Six million trillion electrons is about - 1 C.
Six million trillion protons is about + 1 C.
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Atoms have protons, neutrons, and electrons.Same number of protons and electrons, it is balanced and neutralSometimes electrons get knocked out of place – free electronsIf they go to where other electrons are missing, this creates a current of electricity!
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•If we bring a plastic comb near some very tiny pieces of paper, it will not have any effect on them.
•If we rub a plastic comb with dry hair and then brought near some very tiny pieces of paper, we find that the comb attracts the pieces of paper towards itself
•These observations are explained by saying that initially the comb is electrically neutral so it has no effect on tiny pieces of paper
•When the comb is rubbed with dry hair then it gets electric charge
•Other examples: silk cloth and glass rod, ebonite rod woolen cloth
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•Opposite charges(unlike) attract each other
•Similar charges repel each other
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•The SI unit of electric charge is coulomb which is denoted by C
•One coulomb is that quantity of electric charge which exerts a force of 9 X 109 Newton on an equal charge placed at a distance of 1 m from it.
q1q2
F= Q ------------
r2
• A proton possesses a positive charge of 1.6 x 10 –19 C whereas an electron possesses a negative charge of 1.6 x 10 –19 C
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Calculate the number of electrons constituting one coulomb of charge.
We know that the charge of an electron is 1.6 X 10 –19 C
Now if charge is 1.6 x 10 –19 C, No. of electrons = 1
1
If charge is 1 C, No. of electrons = ----------------------- x 1
1.6 x 10 –19 C
= 6.25 x 10 18
THUS 1C IS EQUIVALENT TO CHARGE ON 6.25 x 10 18 ELECTRONS
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Applications of Electrostatics
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Electric Potential Energy
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Electric Potential Energy
This spring has more PE when it is compressed
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Electric Potential Energy
This spring has more SPE when it is compressed
Likewise, these charges will have more potential energy when they are pushed closer
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Electric Potential Energy
PE equals the amount of work done to move a charge from one place to another.
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Electric Potential Energy
Which will have the larger electric potential energy?
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Electric Potential Energy
A – because it requires work to move it away from the negative charge.
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Electric Potential (volts)
EP = work/charge
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Electric Potential (volts)
Example: 1000 joules of work is done tomove the charge q from far away to the place indicated.
If q = 10 C, what is the electric potentialat the new location?
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Electric Potential (volts)
Example: 1000 joules of work is done tomove the charge q from far away to the placeindicated.
If q = 10 C, what is the electric potentialat the new location?
Voltage = work/charge
V = 1000 J/10C = 100 Volts
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Electric Potential (volts)
What if the charge in the previous problem were now 100 C instead of 10? What would happen to the EP?
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Electric Potential (volts)
What if the charge in the previous problem were now 100 C instead of 10? What would happen to the EP?
IT WOULD BE THE SAME!!!
Work would also increase
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Electric Potential (volts)
Electric potential is associated with LOCATION, not CHARGE
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Electric Potential vs. Potential Energy
Which charge has more electric potential?
Which has more potential energy?
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Is 5000 Volts Dangerous?
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Is 5000 Volts Dangerous?
NOPE!
When 1 million electrons are added to a neutral balloon, it has an electric potential of 5000 volts
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Potential Difference (voltage)
Difference in electric potential between two points.
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Potential Difference (voltage)
Difference in electric potential between two points.
Potential difference between A and B is 0 A B
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Potential Difference (voltage)
Difference in electric potential between two points.
Now there is a potential differenceA
B
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Potential Difference
Batteries provide potential difference between one end of the circuit and the other
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Potential Difference
Batteries provide potential difference between one end of the circuit and the other
Charges flow from high to low electric potential
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Potential Difference (voltage)
Why aren’t birds on power lines shocked?
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Potential Difference (voltage)
Why aren’t birds on power lines shocked?
The Potential Difference between their feet is zero! (0 voltage)
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From infinity
A B
1 J2 J 1V
+1C +1C+1C
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Electric Potential
The electric potential at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point
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•Potential is denoted by the symbol V & its unit is Volt.
A potential of 1 Volt at a point means that 1 Joule of work is done in moving 1 unit positive charge from infinity to that point.
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Potential Difference
The difference in electric potential between two points is known as potential difference
The potential difference between two points in an electric circuit is defined as the amount of work done in moving a unit positive charge from one point to the other point
Work done
Potential difference = --------------------------------
Quantity of charge moved
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•SI Unit of P.D. is Volt.
1 Joule
1 Volt = --------------------------
1 Coulomb
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Current Electricity!(motion)
Electric charges are called electric current.
Free electrons continuously move to spaces where electrons are missing.
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Conductors
A conductor is a material through which current can pass easily
An insulator is a material through which electricity cannot flow
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Metal alloys such as Nichrome, Manganin, and Constantan which are used for making heating elements of electrical appliances are also conductors but their electrical conductivity is much less than that of pure metals
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The presence of “ free electrons” in a substance makes it conductor
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The potential difference(Voltage) is measured by means of an instrument called voltmeter
• The voltmeter is always connected in parallel across the points where the potential is to be measured
•Voltmeter has a high resistance so that it takes a negligible current from the circuit.
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How much work is done in moving a charge of 2 coulombs from a point at 118 volts to a point at 128 volts?
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How much energy is given to each coulomb of charge passing through a 6 V battery?
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Electric Current
When two charged bodies at different electric potentials are connected by a metal wire then electric charges will flow from the body at higher potential to the one at lower potential till they acquire the same potential.
This flow of charges in the metal wire constitutes an electric current
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The magnitude of electric current in a conductor is the amount of electric charge passing through a given point of the conductor in one second.
Q
I = ------
t
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•The SI unit of electric current is ampere (A)
•When 1 coulomb of charge flows through any cross section of a conductor in 1 second, the electric current flowing through it is said to be 1 ampere.
1 coulomb
1 ampere = ---------------------
1 second
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•Some times smaller units are used
•Milli-ampere(mA)= 1/1000 Ampere
•Micro-ampere(A)= 1/1000000 Ampere
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The current (I) is measured by means of an instrument called ammeter
• The ammeter is always connected in series with the points where the current is to be measured
•Ammeter has a low resistance so that it may not change the value of current from the circuit.
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An electric bulb draws a current of 0.25 A for 20 minutes. Calculate the amount of electric charge that flows through the circuit.
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How to get continuous flow of electric current
•The simplest way is to connect the cell or a battery.
•Due to the chemical reactions going on inside the cell or battery, a potential difference is maintained between its terminals
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The CELLThe CELL
The cell stores chemical energy and transfers
it to electrical energy when a circuit is
connected. When two or more cells are
connected together we call
this a Battery.
The cells chemical energy is
used up pushing a current
round a circuit.
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What is an electric current?
An electric current is a flow of microscopic
particles called electrons flowing through
wires and components.
+ -
In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell.
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Direction of electric current
•The conventional direction of electric current is from positive terminal of a cell to the negative terminal through the circuit•The actual flow of electrons is however from negative terminal topositive terminal of a cell.
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How current flows in a wireWhen the metal wire has not been connected to a source of electricity like a cell or a battery, then the electrons present in it move at random in all the directions between the atoms of the metal wire as shown in figure
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When a source of electricity like a cell or a battery is connected between the ends of the metal wire, then an electric force acts on the electrons present in the wire and start moving from negative end to positive end of the wire.
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Electric circuits A continuous conducting path consisting of wires and other resistances and a switch between the two terminals of a cell along which an electric current flows is called a circuit
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simple circuitssimple circuits
Here is a simple electric circuit. It has a cell, a
lamp and a switch.
To make the circuit, these components are
connected together with metal connecting wires.
cell
lamp
switch
wires
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circuit diagramcircuit diagram
cell switch
lamp
wires
Scientists usually draw electric circuits using symbols;
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Open and Closed Circuits
What will happen if the switch is lifted?
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Circuit diagrams A diagram which indicates how different components in a circuit have been connected by using the electrical symbols for the components is called a circuit diagram
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Symbols for electrical components
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Ohms law
At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends
• I V• V= IR• V/I = R
Current is directly proportional to PDCurrent is inversely proportional to R
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Resistance of a conductor
The property of a conductor due to which it opposes the flow of current through it is called resistance
1 ohm is the resistance of a conductor such that when a potential difference of 1 volt is applied to its ends, a current of 1 ampere flows through it.
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Problem:Potential difference between two points of a wire carrying 2 ampere current is 0.1 volt. Calculate the resistance between these points
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A simple electric circuit has a 24 V battery and resistor of 60 ohms. What will be the current in the circuit? The resistance of the connecting wires is negligible.
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An electric iron draws a current of 3.4 A from the 220 V supply line. What current will this electric iron draw when connected to 110V supply line?
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Graph between V and I
Current is directly proportional to potential difference
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The values of current I flowing through a coil for the corresponding values of the pd V across the coil are shown below:
I(A) 0.05 0.10 0.20 0.30 0.40
V(V) 0.85 1.70 3.5 5.0 6.8
Plot a graph between V & I and calculate the resistance of the coil
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Experiment
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Types of Circuits
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types of circuittypes of circuit
There are two types of electrical circuits;
SERIES CIRCUITS PARALLEL CIRCUITS
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Series CircuitSeries Circuit
A circuit that only A circuit that only has one path for has one path for current to flow current to flow through is called a through is called a series circuitseries circuit. . If the path is broken, no current flows through the circuit.
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Parallel Circuit
A type of circuit that has more than one path for current is called a parallel circuit. If the path is broken, the current continues to flow through the circuit.
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The components are connected end-to-end, one after the other.
They make a simple loop for the current to flow round.
SERIES CIRCUITS
If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out.
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PARALLEL CIRCUITS
The current has a choice of routes.
The components are connected side by side.
If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.
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measuring current
Electric current is measured in amps (A)
using an ammeter connected in series in
the circuit.
A
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measuring current
A A
This is how we draw an ammeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT
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measuring currentSERIES CIRCUIT
PARALLEL CIRCUIT
• current is the same
at all points in the
circuit.
2A 2A
2A
• current is shared between the components
2A2A
1A
1A
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copy the following circuits and fill in the missing ammeter readings.
?
?
4A
4A
4A
3A?
?
1A
?
3A
1A
1A
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measuring voltage
The ‘electrical push’ which the cell gives to the
current is called the voltage. It is measured in
volts (V) on a voltmeter
V
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Different cells produce different voltages. The
bigger the voltage supplied by the cell, the bigger
the current.
measuring voltage
Unlike an ammeter a voltmeter is connected
across the components
Scientist usually use the term Potential
Difference (pd) when they talk about voltage.
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measuring voltage
V
This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT
V
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V
measuring voltage
VV
V
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series circuit
1.5V
• voltage is shared between the components
1.5V
3V
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• voltage is the same in all parts of the circuit.
3V
parallel circuit
3V
3V
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measuring current & voltage
copy the following circuits on the next two slides.
complete the missing current and voltage readings.
remember the rules for current and voltage in series and parallel circuits.
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measuring current & voltage
V V
6V4A
A
A
a)a)
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measuring current & voltage
V
V
6V4A A
A
A
b)b)
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answers
3V 3V
6V
4A 4A6V
6V
6V4A 4A
2A
2A
4A
a)a) b)b)
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Combination Circuit
What will happen to the circuit if one of the motors fails to function?What will happen if one of the bulbs fails to function?
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Types of CurrentsDirect Current (DC) Alternating Current (AC)
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AC DC
Electrical Power
Sources
Direction of electrons reverses
Electricity is generated by coal, water, fossil fuels, or nuclear reaction at a generating station
Electrons flow in one direction
Electricity is generated
by stored chemicals,
radiation, wind, or fuel
Can be portable
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Alternating CurrentThe power that comes from The power that comes from a power plant is called a power plant is called alternating currentalternating current (AC). (AC). The direction of the current The direction of the current reverses, or alternates, 60 reverses, or alternates, 60 times per second (in the times per second (in the U.S.) or 50 times per U.S.) or 50 times per second (in Europe/ India, for second (in Europe/ India, for example). The power that is example). The power that is available at a wall socket in available at a wall socket in the United States is the United States is 120-120-volt, 60-cycle AC powervolt, 60-cycle AC power..
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Standard Grade Science
Electrical Safety in the Home
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From this lesson you will learn:
The effects of electric shocks.
How a plug should be wired correctly.
How a plug works.
How electrical safety devices work.
The various faults that can develop.
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Electricity
Electricity can be supplied from either:
• A battery.
• The mains supply.
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Electricity
The main advantage of electrical energy is that it can be transported over large distances to our homes.
Wires carry electricity from power stations to our homes/schools/offices.
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Electric Shocks
Electricity is dangerous and can kill!!
An electric shock can:
You can get an electric shock from anything which is plugged in or connected to the mains supply.
• Affect your muscles.
• Affect your nerves.
• Paralyse you.
• Stop your heart beating.
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Electrical SafetyThe wires which carry electricity consist of two parts:
• The metal wires.
• The plastic coating round the metal wires.
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Electrical Safety
The metal wires act as a conductor of electricity.
The plastic coating acts as an insulator which prevents people from being electrocuted.
Conductors are materials which allow electricity to pass through them easily.
Insulators are materials which do not allow electricity to pass through them easily.
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The PlugElectrical cables allow you to:
• Plug appliances into the mains supply.
• Draw electricity from the mains supply.
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The PlugMost electric cables normally contain three wires:
Each of these three wire have different colours of insulation.
• The live wire.
• The neutral wire.
• The earth wire.
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The PlugYou have to remember the names and colours of each of these three wires. You also have to remember the position of each of these three wires in a standard plug.
Wire Insulation Colour
Location
LIVE BRown BR – bottom right
NEUTRAL BLue BL – bottom left
EARTH Green/Yellow Centre
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Electrical Safety
It is very important that all plugs are wired correctly.
If they are not, then a number of things can go wrong!!!
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Electrical Safety
Appliances have three safety devices:
• The fuse.
• The earth wire.
• The switch.
These devices are designed to stop you from being electrocuted and to prevent the appliance from being damaged.
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The Fuse
A fuse is a thin piece of wire which will melt when too much current passes through it.
A 3A fuse will melt when the current is greater than 3A.
When a fuse has blown, the wire inside it has melted.
Current is a flow of electricity and is measured in amps (A).
Example
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The Fuse
When the wire melts, all current is stopped from reaching the appliance and switches it off.
The fuse stops a large current from flowing through the appliance which could cause wires to overheat, melt or catch fire.
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The SwitchTogether, the fuse and switch control the amount of electricity that enters an appliance.
The fuse and the switch are connected to the live wire.
Electricity comes in through the live wire.
If the fuse is blown, or the switch is off, then the electricity cannot reach the appliance.
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The Earth Wire
Plugs have 3 pins on them.
The live and neutral pins allow electricity to pass through the appliance and cause it to work.
The earth wire does nothing unless something goes wrong.
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What can go wrong!The most dangerous thing that can happen is that the live wire can become loose inside an appliance and touches the casing.
This makes the casing LIVE!
If you touch the LIVE casing, you could be electrocuted as you are providing a path for the electricity to flow.
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PreventionTo prevent electrocution, the earth wire is connected to the casing of the appliance.
If the live wire touches the casing, the earth wire provides a path for the electricity to flow.
This blows the fuse and switches the appliance off, leaving you safe.