Chapter 2 : Electricity
1. 2. 2.1 Electric Field and Charge Flow2.1.1 Electric charge,
Q
1. Two types of charge: ___________, ___________2. An electron :
a negative charge (_____________________)3. A proton : a positive
charge (_____________________)4. SI unit of charge is the coulomb
or C.5. A body is :*neutral (equal number of positive and negative
charges)*charged positive (an atom loses electrons)*charged
negative (an atom gains electrons)6. Like charges repel, unlike
charges attract.Total charge, Q =A small body has an initial charge
of + 3.0 C. What is its final charge ifa) it acquires 1.875 1019
electrons from another body,b) it loses 1.25 1019 electrons?
2.1.2 Static electricity (Electrostatics charges)*electric
charges which do not move*can be produced by rubbing two materials
together (rubbing polythene rod with a dry woolen cloth)*electrons
are pulled from woolen cloth to the polythene rod*polythene rod
gains electrons ___________charged Woolen cloth loses electrons
___________ charged
Detecting and testing small charge (____________________)*When a
charged object touches the metal cap at the top of electroscope,
some of the charges are transferred to the gold leaf and the metal
plate. (giving charge to the electroscope)*Similar charges on the
gold leaf and metal plate __________ each other > the leaf
rises
*When a positively charged rod is brought near the metal cap of
a positively charged electroscope, the gold leaf rises even
moreReason: Electrons are attracted to the metal cap by the
positive charges on the rod. The gold leaf and the metal plate have
more positive charge. The gold leaf is repelled further by a
stronger force.Effects of static electricity
1. Rubbing plastic comb on your shirt sleeve and use it to pick
up small pieces of tissue paper.
2. When a negatively charged polythene rod is brought near a
fine stream of water, the stream of running water is bent towards
the rod because of the force of attraction between opposite
charges.
3. Carpet attracting dust
*A carpet surface becomes charged by the rubbing of shoes.*Each
dust particle carries equal numbers of positive and negative
charges (electrons) >> neutral*When dust particles fall from
shoes onto the carpet, the electrons of the dust particles are
displaced towards the carpet. The positive charges are displaced
away from the carpet.*The force of attraction between the negative
electrons on the dust particles and the positive charges on the
carpet causes the dust particles to stick to the carpet.
4. Earthing of a vehicle
*Vehicles (car, lorry tanker, aircraft) will become charged due
to friction with the air.*When a person touches the outside of the
vehicle, the highly charged vehicle will discharge through him and
the person will receive a brief electric shock.*If the vehicle is a
petroleum tanker, the sparks produced during discharge may start a
fire.*To avoid this, a long metal chain which touches the ground is
usually tied to the back of the vehicle.*Thus, the electric charge
produced by friction can flow from the vehicle to the earth through
the metal chain.Applications of electrostatic charges1.
Electrostatic crop sprayer
*A strongly charged metal wire charges the tiny droplets of
pesticide as they leave the nozzle.*The droplets of pesticides have
the same charge, so they repel each other and spread out evenly to
cover a wide area.*When a droplet is near a leaf, it induces an
opposite charge on the leaf and is attracted to it.
2. Electrostatic paint sprayer
*The droplets of paint coming out from a nozzle are charged
positively as the nozzle is connected to a positive potential.*As
the droplets of paint have same charge, they repel each other to
form a wide fine spray.*The body is earthed during spraying and
thus the droplets of paint are attracted to the metal body, giving
a very even coating.*save cost, even spraying, reach inaccessible
parts
3. Electrostatic air filter (to clean air in room or
factory)
*When the fan on top spins, dirty air is sucked from below
through a filter netting. Bigger dirt particles are filtered off by
the netting.*The dirty air then moves up through two sets of metal
grids. Grid A is charged positive and Grid B is charged
negative.*Dust particles are charged by the grids. Negatively
charged particles are attracted to grid A and positively charged
dust particles are attracted to grid B.*The foul smell of the air
is removed through absorption by charcoal placed on top. Thus what
flows out of the filter is fresh clean air.
4. Electrostatic precipitator (remove ash and dust from the
waste gases in power stations or factory chimneys before being
discharged to the atmosphere)(1) Smoke particles pick up a negative
charge.(2) Smoke particles are attracted to the collecting
plates.(3) Collecting plates are knocked to remove the smoke
particles
5. Photostating machine (Photocopier)*The selenium-coated drum
is charged positively as it rotates past a charged rod.
*Drum becomes evenly charged.
*Print image are focused on the drum. Parts which receive light
lose all their charges. Dark areas on the drum surface retain their
charges.
*Fine powdered ink (toner) which is negatively charged is
attracted to the charged areas of the drum.
*The toner is transferred to the paper and the images are
printed on the paper.
*The toner is heated to melt the powdered ink and fix the images
permanently on the paper.
2.1.3 Electric current, I
1. Electric current, I = rate of flow of charge
2. SI unit of current, I =1 A is the steady current that flows
through a wire when a charge of 1 coulomb flows through the wire in
1 second.Hence, 1 A = 1 C s-1
3. The amount of charge, Q, flowing through any conductor is
given by :
4. When free electrons move through the wire from B to A, the
electric current, I, flows from A to B.
ExerciseA fuse wire will blow if the current flowing through it
exceeds 5.0 A for a time of 80 ms.a) What is the quantity of
electric charge that has passed through the fuse wire in 80 ms?b)
How many electrons have passed through the fuse wire in that time,
given that the charge on 1 electron is 1.6 10-19 C?2.1.4 The
electric field
1. Electric field = a region surrounding a charged body where
electrostatic force can be experienced.
2. An electric field can be represented by:
3. Arrangement used to observe electric field patterns:
4. Patterns of electric field using different types of
electrode:Electric field patterns
Effect of an electric field1. A ping-pong ball coated with a
conducting material
*When a high voltage is applied across the two plates, the ball
is observed to be stationary.*This is because opposite charges are
induced on the surface of the ball, so that the ball is attracted
by the positive and negative charge plates with equal force, F.*If
the ping-pong ball is slightly displaced to one side, the ball will
bounce back between the two plates continuously.
2. Spreading of a candle flame
*When the high d.c. voltage connected to plates P and Q is
switched on, it is observed that:a) the candle flame spreads out in
two opposite directionsb) the spread of the flame towards the
negative (P) plate is biggerExplanationa) Heat energy from the
candle flame produces ionization of the air molecules. Positive and
negative ions are formed around the flame.b) Positive ions are
heavier, and move slower. Movement of the positive ions towards the
negative plate P causes a bigger spread of the flame.c) Electrons
are lighter and move at a higher speed. When they move towards the
positive plate Q, they cause a smaller spread of the flame.
3. Lightning
*When a negatively charged thundercloud passes above a tall
building, positive charges are induced on the roof.*A strong
electric field between the cloud and the roof produces a strong
force of attraction between the opposite charges.*Electrons will
suddenly accelerate from the cloud to the roof and this causes the
building to be struck by lightning.*A lightning conductor with
sharp spikes is fixed on the highest part of the roof to reduce the
risk of lightning strikes in two ways:a) The lightning conductor
carries some induced charges to the ground and also cancels out
some of the charges on the cloud. This reduces the chances of
lightning striking.b) If lightning does strike, the lightning
conductor provides an easy path for the electrons to pass to the
ground without damaging the building.2.2 Relationship between
electric current and potential difference2.2.1 Potential difference
(Voltage)
Electric potential at X (+ve terminal) ( ) Electric potential at
Y (-ve terminal)>> Electric current flows from X to Y,
passing through an electric bulb, because of the electric potential
difference across the battery.
1. The potential difference between two points in a circuit
causes an electric charge to move between the two points.
2. An electron will move from a point of low potential to a
point of high potential.Hence, an electric current I will flow from
a point of low potential to a point of high potential.
3. Potential difference, V, between two points: work done (or
energy produced) when 1C of charge moves between the two points in
an electric field.
Or
4. The SI unit of potential difference ( or voltage ) is volt
(V).1 Volt is the potential difference between two points if 1 J of
energy is produced when 1 C of charge flows between the two
points.
Exercise
If 2 1019 electrons pass a point on a wire in 5 seconds, what is
the magnitude of the current that flows through the wire?
The energy released between points A and B is 60 J when a charge
of 20 C flows from A to B.a) What is the potential difference
between A and B?b) If the time taken for the 20 C to flow between A
and B is 40s, what is the value of the current flowing through the
bulb?
Chapter 2 : Electricity
2.2.2 13
2.2.3 Ohms Law
Ohms law states that the current, I, flowing through a metal
conductor is directly proportional to the potential difference, V,
across the conductor (if the temperature and other physical
conditions remain constant)
1. Graph for a conductor that obeys Ohms law ( )
2. Resistance, R = ratio of the potential difference (or
voltage), V, across the conductor to the current, I, flowing
through it.
3. 1 is the resistance of a conductor when a potential
difference of 1 volt applied across it produces a current of 1A
through it.
ExerciseA 9 V battery connected across a resistance wire
produces a steady current of 0.3 A through the wire. If the
internal resistance of the battery can be neglected, what is the
resistance of the wire?
5.0 V is applied across a 2.0 resistor. What current will flow
through the resistor?
2.2.4 Factors that affect resistance1. Four factors that affect
resistance of a conductor:
Resistance
Length of conductor(wire or filament)
Cross-sectional area of conductor(diameter or number of
wires)(Highway)
Temperature of the conductoratoms in metallic latticevibrate
stronger > block the movement of electrons >flow slower
Type of materialSilverCopperConstantanNichrome
A resistance wire of resistance 3 has length 0.2 m and
cross-sectional area 0.02 mm2.What is the resistance of another
wire of the same material, but with length 0.80 m and cross
sectional area 0.04 mm2?
2.2.5 Superconductors
1. Resistance of a metal increases with temperature.Resistance
of a semiconductor decreases with temperature.
2. A superconductor = a material whose resistance become ZERO
when its temperature drops to a certain value called the critical
temperature, Tc.E.g. mercury becomes a superconductor when its
temperature reaches 4.2 K.
3. Other examples of superconductors:MaterialCritical
temperature, Tc (K)
Zinc0.88
Aluminium1.19
Mercury4.15
Lead7.18
YBa2Cu3O790
Ti-Ba-Ca-Cu-O125
4. When a small permanent magnet is moved above the surface of a
superconductor, a current is induced inside the superconductor.5.
As the resistance of the superconductor is zero, the induced
current will continue to flow for years, even after the permanent
magnet is removed.
6. As the magnetic field produced by a superconductor is
opposite in direction to that of the permanent magnet, the
permanent magnet will be acted by an opposing magnetic force. Thus,
it will float above the surface of the superconductor.
Advantages of superconductors*At critical temperature, a very
large current can flow through the superconductor without
overheating, as its resistance is zero.*The magnetic field produced
by a superconductor is many times more powerful than that produced
by any permanent magnet.
Uses of superconductorsa) As superconducting coils (in
electromagnets)*produce very powerful magnetic fields*used in
scanning devices in hospitals (CT-Scan and MRI)
b) As high temperature superconducting cables (in power
transmissions)*Efficient! Low cost! Low power loss!c) Make strong
superconducting electromagnets (in MAGLEV train magnetic levitation
train) *the train floats about 1 cm above the tracks (due to strong
magnetic repulsion between the base of the train and the railway
tracks)*the train can slide at high speed (500km/h) without
friction
2.3 Series and parallel circuits1. Electrical circuit = complete
path along which electric current can flow2. A circuit made of
electrical components2.3.1 Resistors in a series circuit
Series circuit = circuit components are connected end to end
consecutively to provide a single path for current flow through all
the components
*All components in series circuit have the __________ current*If
one component fails to allow current to flow through, the whole
circuit breaks off.
1. Current, I = 2. Potential difference, V = = = 3. Resistance,
R =
2.3.2 Resistors in a parallel circuit
Parallel circuit = components are placed side by side and their
corresponding ends are joined together
*Failure of one component does not affect the other
components.1. Potential difference, V = = 2. Current, I = =
= 3. Since,
Thus,
Advantages of parallel cicuits1. If one of the electrical
appliances does not function, the other appliances can still be in
use.2. Additional appliances can be connected in parallel with the
existing appliance without the need for more voltage.3.
Therfore,most of the household applian ces are connected in
parallel.
Effective resistance (Combined resistance)Effective resistor = a
single resistor that replaces 2 or more resistors in seires or
parallel= has the same effect on the circuit= allow same amount of
current to flow in the circuitCombined circuitCombined circuit = a
combination of resistors connected in series and in parallel
Calculate the effective resistance
2
3 5 3 2
5
2 4 2 Calculate the effective resistance
3
6 4
2.4 Analysing electromotive force and internal
resistanceInternal resistance1. A circuit is a closed loop through
which current can continuously flow.a) _____________ = path taken
by the current outside the cell b) _____________ = path taken by
the current within the cell
2. When current flows in a circuit (internal or external),
resistance is produced.a) For external circuit = ______________b)
For internal circuit = ______________ or source resistance
3. What is internal resistnace or source resistance?The
resistance within the cell or battery itself , (due to its
electrolyte or electrodes).(A cell consists of electrodes in a
chemical electrolyte.When the cell is connected in a circuit, the
current flowing in the electrolyte through the electrodes
experience internal resistance.)
To show the exsistence of internal resistance1. Turn on a torch
for 20 minutes. The dry cells in the torch becomes hot (due to
resistance)2. Hot (resistance) happens when current flows in
resistor.3. Curent flows through the cell and external circuit.4.
Therefore, resistors in the external circuit become hot
too.Electromotive force (e.m.f.)1. A cell = an electrical source
which uses chemical reactions to produce a current (changes
chemical energy into electrical energy)
2. A battery = combination of two or more cells connected in
series
3. _________________________________________________ has to be
in a circuit to make a current flow through the circuit.
4. Electromotive force, E, of a dry cell = total electrical
energy given to one coulomb of charge flowing through the CELL
If a dry cell has E = 1.5 V, the dry cell will provide 1.5 J of
electrical energy to every 1 C of charge passing through the
cell.This 1.5 J of energy is transferred to all parts of the
circuit.
Relationship between e.m.f. & terminal potential
difference
Electromotive force*Work done by a source in driving 1 coulomb
of charge around a COMPLETE CIRCUIT (external and internal
circuit)*can be measured by voltmeter connected to the terminals of
a cell on open circuit
Terminal potential difference , Vt*Work done by a source in
driving 1 coulomb of charge through the EXTERNAL RESISTOR*can be
measured by voltmeter connected to the terminals of a cell on
closed circuit (when the cell sends current through the external
resistor)
Since Vt < E ; E Vt = Vd
Lost Volts, Vd*Work done by a source in driving 1 coulomb of
charge through the CELL*OR, potential difference required to drive
the current through the internal resistance*cant be measured
directly
# Work done = Energy# E.m.f = energy supplied by a cell per
coulomb of charge to the whole circuit
1. 2. 2.1 2.2 2.3 2.4 2.5 Analysing electrical energy and
powerElectrical power
Power,P = amount of energy transferred in one second
1 watt = power of an electrical appliance which can produce a
totoal energy of 1 J in 1 sec
Cost of using electrical energy1. Unit for cost of using
electrical energy = kilowatt-hour (kWh)2. 1 kWh = total energy
consumed by an electrical appliance of power 1 kW in 1 hour.1 kWh
=
3. If a television set of power 700 W is switched on for 6 hours
a day, then the total electrical energy used in a day is
Energy = Power x Time
Exercise1. The lamp of a motorcycle is labelled 12 V, 15 Wa)
Explain the meaning of 12 V, 15 Wb) What is the value of the
current flowing through the lamp when it is connected to a 12 V
supply?How much is the resistance of the filament of the lamp?
2. The usage of electrical appliances in Alis household in one
day is as shown in the table below:ApplianceNumber of
unitsPowerTime duration used
Lamp8100 W12 h
Television1700 W 8 h
Kettle12 kW1 h
Fridge1400 W12 h
Fan370 W10 h
a) Determine the total electrical energy (in kWh) used in Alis
hosue in 1 day.b) Calculate the cost of electrical energy usage in
Alis house in one week if the cost per unit is as follows:First 100
units: 22 cent per unitEvery additional unit: 26 cent per unit
Efficiency of electrical energy1. Efficiency =
2. If power output = power in put, then the electrical appliance
has 100% efficiency.
3. For most electrical appliances, efficiency < 100% due to
loss of useful input power as heat.
a) Filament lamp and fluorescent lamp Function is to produce
light Both types produce heat as side products Fraction of
electrical energy that is converted to heat = wastage Fluorescent
lamp produce less heat, higher efficiency Fluorescent lamp - 4 to 6
times more efficient Efficiency of lamp =
A tungsten-filament lamp and a fluorescent lamp, each with a
power rating of 40 W, produce 8 W and 38 W of light energy
respectively. What is the efficiency of each lamp?
b) Hairdryer
Consists of fan and heating coils made from nichrome To increase
efficiency, users can choose cool air, warm air or very hot air by
switching the hairdryer
c) Comparing the efficiency of electric kettle and imersion
heater
Both appliances consist of wires made from nichrome Efficiency
of electric kettle is higher*has cover that prevents heat loss to
surroundings Efficiency of immersion heater is lower*has long
connecting wire that has resistance (waste)
Importance of using energy efficiently Depletion of major fuel
supplies (natural gas, coal, oil) Only renewable energy (nuclear,
hydroelectric, wind) can be used to generate energy constantly
Ways to improve efficiency of energy usage1. Better insulation
or ventilation of buildings Reduce heat loss during cold weather
Less fuels needed to keep interior temperature at comfortable
level2. Combined heat and power Waste heat produced in the colling
water can be channeled back to household building
3. Better use of electricity Use fossil fuels instead of
electricity to heat homes and offices during cold weather(avoid
loss of energy) Use electricity to run machinery, computer,
domestic appliances and so on.
4. Change of lifestyle Use less electrical energy Use public
transport Switch on fans and air-conditioners when neccessary
Switch off unused electrical appliances Carry out maintenance
service on electrical appliances from time to time (fridge,
air-conditioners, water heaters) Clean air filters of
air-conditioners every month Clean the cooling fins at the back of
fridge every two months Wipe the surfaces of filament bulbs and
fluorescent lamps from time to time