electricity

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Electricity

Milind Naik


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


Electric Charge

Measured in COULOMBS

Six million trillion electrons is about - 1 C.

Six million trillion protons is about + 1 C.


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!


•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


•Opposite charges(unlike) attract each other

•Similar charges repel each other


•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


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


Applications of Electrostatics


Electric Potential Energy



This spring has more PE when it is compressed



This spring has more SPE when it is compressed

Likewise, these charges will have more potential energy when they are pushed closer



PE equals the amount of work done to move a charge from one place to another.



Which will have the larger electric potential energy?



A – because it requires work to move it away from the negative charge.


Electric Potential (volts)

EP = work/charge



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?



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



What if the charge in the previous problem were now 100 C instead of 10? What would happen to the EP?



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



Electric potential is associated with LOCATION, not CHARGE


Electric Potential vs. Potential Energy

Which charge has more electric potential?

Which has more potential energy?


Is 5000 Volts Dangerous?


Is 5000 Volts Dangerous?

NOPE!

When 1 million electrons are added to a neutral balloon, it has an electric potential of 5000 volts


Potential Difference (voltage)

Difference in electric potential between two points.




Potential difference between A and B is 0 A B




Now there is a potential differenceA

B


Potential Difference

Batteries provide potential difference between one end of the circuit and the other



Batteries provide potential difference between one end of the circuit and the other

Charges flow from high to low electric potential



Why aren’t birds on power lines shocked?



Why aren’t birds on power lines shocked?

The Potential Difference between their feet is zero! (0 voltage)


From infinity

A B

1 J2 J 1V

+1C +1C+1C


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


•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.



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


•SI Unit of P.D. is Volt.

1 Joule

1 Volt = --------------------------

1 Coulomb


Current Electricity!(motion)

Electric charges are called electric current.

Free electrons continuously move to spaces where electrons are missing.


Conductors

A conductor is a material through which current can pass easily

An insulator is a material through which electricity cannot flow


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


The presence of “ free electrons” in a substance makes it conductor


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.


How much work is done in moving a charge of 2 coulombs from a point at 118 volts to a point at 128 volts?


How much energy is given to each coulomb of charge passing through a 6 V battery?


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


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


•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


•Some times smaller units are used

•Milli-ampere(mA)= 1/1000 Ampere

•Micro-ampere(A)= 1/1000000 Ampere


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.


An electric bulb draws a current of 0.25 A for 20 minutes. Calculate the amount of electric charge that flows through the circuit.


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


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.


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.


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.


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


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.


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


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


circuit diagramcircuit diagram

cell switch

lamp

wires

Scientists usually draw electric circuits using symbols;


Open and Closed Circuits

What will happen if the switch is lifted?


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


Symbols for electrical components


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


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.


Problem:Potential difference between two points of a wire carrying 2 ampere current is 0.1 volt. Calculate the resistance between these points


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.


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?


Graph between V and I

Current is directly proportional to potential difference


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


Experiment


Types of Circuits


types of circuittypes of circuit

There are two types of electrical circuits;

SERIES CIRCUITS PARALLEL CIRCUITS


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.


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.


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.


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.


measuring current

Electric current is measured in amps (A)

using an ammeter connected in series in

the circuit.

A


measuring current

A A

This is how we draw an ammeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT


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


copy the following circuits and fill in the missing ammeter readings.

?

?

4A

4A

4A

3A?

?

1A

?

3A

1A

1A


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


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.


measuring voltage

V

This is how we draw a voltmeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

V


V

measuring voltage

VV

V


series circuit

1.5V

• voltage is shared between the components

1.5V

3V


• voltage is the same in all parts of the circuit.

3V

parallel circuit

3V

3V


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.



V V

6V4A

A

A

a)a)



V

V

6V4A A

A

A

b)b)


answers

3V 3V

6V

4A 4A6V

6V

6V4A 4A

2A

2A

4A

a)a) b)b)


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?


Types of CurrentsDirect Current (DC) Alternating Current (AC)


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


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..


Standard Grade Science

Electrical Safety in the Home


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.


Electricity

Electricity can be supplied from either:

• A battery.

• The mains supply.


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.


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.


Electrical SafetyThe wires which carry electricity consist of two parts:

• The metal wires.

• The plastic coating round the metal wires.


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.


The PlugElectrical cables allow you to:

• Plug appliances into the mains supply.

• Draw electricity from the mains supply.


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.


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


Electrical Safety

It is very important that all plugs are wired correctly.

If they are not, then a number of things can go wrong!!!


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.


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


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.


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.


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.


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.


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.

electricity

Technology

electric potential volts

electric potential energy

quantity of electric

potential difference

charge q

si unit of electric

coulomb of charge

negative charge