# ELECTRICITY

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STATIC ELECTRICITY
Electric Charge
Static electricity describes the situation when electric
charges remain stationary. This occurs best with insulators.
Electric charge can be either positive or negative.
LIKE CHARGES
ATTRACT.
REPEL,
UNLIKE
CHARGES
Examples of Electro-Static Electricity
When a charged object is brought close to an uncharged one
the two objects attract each other
Production Electro-Static Electricity
When certain insulating materials are rubbed against each
other they become electrically charged. Electrons are
transferred from one material into the other.
•
•
The material that gains electrons become
negatively charged.
The material that loses electrons is left with an
equal positive charge.
NB: Only electrons move.
Polythene and ebonite gain extra electrons when they are
rubbed and become negatively charged. Cellulose acetate,
perspex and glass have electrons removed when they are
rubbed and become negatively charged.
TESTING FOR A CHARGE
Repulsion between two objects is a reliable test that they are
both charged. The electroscope uses repulsion to indicate
that material is charged.
The symbol for charge is Q. Charge is measured in coulomb
(C)
Attraction and repulsion force
Two bodies that carry different types of charge attract.
When a charged object is brought close to the metal cap,
•
•
Two bodies that carry the same type of charge repel.
Electrons are either repelled from the metal cap to
the gold leaf and metal stem or
Electrons are attracted to the metal cap from the
metal stem and gold leaf.
Charged rod brought near metal plate of electroscope
•
•
When the metal plate and the gold leaf have similar
charges, they will repel each other. When the object
is removed, the leaf falls fully.
If the deflection reduces or leaf collapses, the
charge will be unlike that of the electroscope
When the leaf is raised and the metal cap touched with a
finger or earthed, the electrons will either move from metal
THE LAW OF CHARGES:
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cap to ground or ground to metal cap depending on the
charge on the metal cap.
LIGHTNING
Lightning is an electric discharge in the form of spark or
flash. Before a lightning discharge, charge will accumulate
in clouds as a result of friction between clouds and air
molecules. The bottom of clouds will be negatively charged.
The negative charges in the clouds will induce positive
charges on the ground. At a high sufficient potential
difference between ground and cloud there is discharging
causing a very large spark to jump between them.
DISCHARGING
This releases a tremendous amount of energy in the
lightning flash, most of which is used to heat up the
atmosphere, producing light and give sound as rumblings of
thunder. There is so much energy dissipated that the air is
superheated and explode, we hear this as Thunder.
Electric charge in a charged object can be neutralised or
reduced when the object comes into contact with an object
which has an opposite charge or neutral body or a
conductor. Discharging can also be obtained by earthling the
charged objects.
The Van de Graaff generator
Lightning Conductor: used to protect buildings from
The Van de Graff generator produces a large and
continuous supply of electric charge. In this
machine a rubber belt rubs against a plastic roller
and becomes charged. The charge is carried on the
moving belt up to the metal dome, where it is
collected. A large quantity of charge therefore
builds up on the dome.
• Woollen threads attached to the dome will
repel each other strongly after the
generator has been running for a while.
• When a metal sphere connected to Earth
with lead, is brought near the metal dome,
electric sparks are produced. This occurs
as charges from the dome pass through the
air to sphere and then to the earth. This
collapses.
being stuck by lightning. It is made out of a thick
copper/aluminium rod which connects spikes (sharp metal
points) above the building to a metal plate buried deeply on
the ground. When there is lightning, the charge will be
conducted safely down to the ground without damaging the
building.
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ELECTRIC FIELD
Electric field due to charged plates
The region of space where an electric charge experiences an
electric force due to other charges.
The direction of the field denoted by arrows is the direction
in which a charge placed in the field would move.
Electric field for individual charges
•
Positive charge
Hazards of static electricity
The main danger of static electricity is in situations where a
spark can cause a fire or an explosion.
Fuel pipe problems: When oil or petrol is pumped along
conductor pipes a static charge can build up on the pipe
which could result in a spark. This could cause an explosion.
To avoid this pipe should be earthed.
•
Negative charge
Uses of static electricity:
•
•
Paint spraying
Smoke precipitator
CHARGING
CONDUCTOR
(SEPARATION OF CHARGES)
Electric field between charges
•
a.
BY
INDUCTION
P and Q in contact (Neutral)
Unlike charges
Insulator
b.
•
Like charges
Negatively chargerd rod is brought near sphere P
(without toching P), electrons from P are repelled to
Q, each sphere becomes chareged.
Negative rod
c.
NB: Point C is called the Neutral Point. No electric force is
experienced by a charge placed at this point
P and Q are separated in the presence of the
negatively charged rod
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1.
2.
3.
Demonstrate how P can be charged negatively and
Q positively
Demonstrate how P can be charged negatively
If you hold a copper rod in your hand and rub it
with wool or fur, it will not attract a small piece of
paper. Explain why?
CONDUCTORS AND INSULATORS
d.
After separating spheres (charges), the charged rod
is removed then P becomes positively charged and
Q negatively charged
CHARGING CONDUCTOR BY EARTHING
Electrical conductors are materials in which has a free
electron that are not bound to atoms and can move relatively
freely through the material. Materials such as copper,
aluminium, and silver are good electrical conductors. When
such materials are charged in some small region, the charge
readily distributes itself over the entire surface of the
material.
Electrical insulators are materials in which all electrons are
bound to atoms and cannot move freely through the
material. Materials such as glass, rubber, and wood fall into
the category of electrical insulators. When such materials
are charged by rubbing, only the area rubbed becomes
charged, and the charged particles are unable to move to
other regions of the material.
Obtaining a Positive Charge
Negative rod brought
near sphere P electrons
repelled to the far end
of sphere
Metal sphere
uncharged
ELECTRIC CURRENT
Electric current is defined as the flow of charge per unit time
across a conductor. Current is measured in Amperes (A)
using an instrument called Ammeter. An ammeter is
connected in series with other electrical components.
Negative rod
ELECTRIC CURRENT FLOW
Electrons flow to earth by
touching when the
charged rod is still held
closer to the sphere
Earth is removed first
then the charged rod.
Sphere P will be
positively charged.
Electric current flows from the POSITIVE terminal to a
NEGATIVE terminal.
πͺππππππ(π°) =
Earth
π°=
Exercise
ππππππ (πΈ)
ππππ (π)
πΈ
π
Where: Q is charge in coulombs(C)
t is time in seconds(s)
I is current in Amperes (A)
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Question 1
Calculate the charge passing through a device when a
current of 500mA flows for 3 minutes. Q= 90C
Question 2
Calculate the current flowing when a charge of 240C flows
through a device in 80s. I=3A
A circuit diagram uses a standard set of symbols to show
how electrical components are connected together.
ELECTROMOTIVE FORCE (emf)
Electromotive force is the maximum voltage a cell or battery
can hold, which is the electrical energy that drives charges.
Question 1
Calculate the voltage of a battery if it supplies 300 joules of
energy to 50C of charge. V=60V
Question 2
A coil of copper is connected across the terminals of a
battery of emf 6.8V.
a. what is the total energy transformed when 30C of charge
passes around a complete circuit
b. if this charge circulates in 120s what current is flowing.
ELECTRICAL RESISTANCE
Resistance is the opposition to the flow of electrical current
in a circuit.
A resistor is a device that has a particular resistance
The non-electrical energy converted to the electrical energy
when one coulomb of positive charge passes through the cell
is also called emf. It is measured in Volts (V).
πππππππ(π½) =
ππππππππππ ππππππ (π¬)
πͺπππππ (πΈ)
π¬
π½=
πΈ
The SI unit for resistance is Ohms, symbol omega (Ω)
Ohm’s law
Where: E is energy in joules (J)
Q is charge in coulombs (C)
V is pd/emf in Volts (V)
1 volt is the same as 1 joule per coulomb
Potential Difference
The amount of work done in moving a charge between
two terminals (positive and negative terminals) divided
by the size of the charge. P.D is also known as voltage.
Ohm’s law state that “the current is directly proportional
to the voltage at a constant temperature.”
That is: V α I
Therefore: V = I x Constant
Ohm’s law equations are given as
•
voltage(V) = Current (I) = x Resistance (R)
•
Resistance (R) =
•
πΆπ’πππππ‘(πΌ) =
Voltage (V)
Current (I)
Voltage (V)
Resistance (R)
Voltage is measured in volts (V) and is measured by a
voltmeter (connected in parallel).
Example
If a cell has 1 Volt, it delivers 1 Joule of energy to each
coulomb of charge (J/C).
Calculate the resistance of a lamp if a voltage of 12V causes
a current of 3A to flow through the lamp.
πππππππ(π½) =
ππππππππππ ππππππ (π¬)
πͺπππππ (πΈ)
π¬
π½=
πΈ
Where: E is energy in joules (J)
Q is charge in coulombs (C)
V is pd/emf in Volts (V)
resistance = voltage
current
= 12V / 3A
resistance = 4 ohms (4Ω)
Question 1
Calculate the resistance of a heater if a voltage of 230V
causes a current of 200mA to flow through the heater.
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Question 2
Calculate the voltage across a resistance of 40Ω when a
current of 5A is flowing.
Question 3
Calculate the current flowing through a wire of resistance of
8Ω when a voltage of 12V is connected to the wire.
EXPERIMENT TO VERIFY OHM’ S LAW
The circuit below can be used to obtain a voltage-current
graph of a resistor.
•
•
•
The graph is a straight line through the origin.
The wire or resistor obeys Ohm’s law
Doubling voltage doubles current
VOLTAGE-CURRENT-CHARACTERISTICS
GRAPHS FOR NON-Ohmic CONDUCOTORS
1. Filament lamp
V/I graph bends over as V and I increases, that is resistance
(V/I) of a filament lamp increases as the current increases
makes the filament hotter (temperature increases). The lamp
does not obey Ohm’s law
2. Diode
The variable resistor is used to apply a range of voltages
across the resistor. Every time a set of voltage and current
value are recorded. Recorded values will be used to plot a
graph of Voltage-Current graph. Typical graph should be a
straight line from origin.
V/I shows that current passes when the p.d is applied in one
direction but is zero when is acting in the opposite direction.
Diode has very small resistance when connected one way
around but a very large resistance when the p.d is reversed.
Typical results:
Table 1
The diode has a very high resistance in the reverse direction.
Question 1
3. Thermistor
Use values on table 1 to plot a graph of V/I and calculate the
The resistance of a thermistor decreases as the temperature
increases. V/I graph bends upwards
VOLTAGE-CURRENT-CHARACTERISTICS
GRAPHS FOR METALLIC (Ohmic) CONDUCOTORS
A typical Voltage -Current graph of a wire or a fixed resistor
at a constant temperature is as shown below
Note:
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Resistivity
Circuit symbols
Besides temperature, the resistance (R) of a conductor also
depends on the following
a. Length (L)
b. Cross – Sectional Area (A)
c. Type of material
Therefore resistance can also be given as
π³
πΉ=π
π¨
Where L is length in meter (m)
A is Cross sectional area in meter squares (m&sup2;)
π is resistivity in ohms-meter (Ωm)
R is resistance in ohms (Ω)
Consider the following wire of the
1. Same length but different cross sectional area
Wire S
Wire T
Which wire has higher resistance and why?
2. Same cross sectional area but different length
Wire g
Wire w
Which wire has higher resistance and why?
Question 1
A metal wire of length 100cm and cross sectional area 0.2
mm&sup2; has a resistance of 8.0 Ω. What is the resistance of a
wire of the same metal of length 50cm and cross sectional
area of 0.40mm&sup2;? R = 2.0 ohms
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SERIES CIRCUITS
π = π½π = π½π
In a series circuit all of the components can be controlled by
using just one switch. Electrical components are connected
in one line.
Current, Resistance AND voltage in series
Current at any point is the same in a series circuit.
Each component shares the voltage of the power supply
and so adding more bulbs in series will cause each bulb to
become dimmer
Currents in parallel circuits
The total current through the whole circuit is the sum of the
currents through the separate components.
That is:
π = ππ + ππ
Resistance in Parallel
For resistors in series, the total/equivalent resistance is given
by the sum of resistors in series. That is:
For 2 resistors in series
πΉ = πΉπ + πΉπ
If there are more than two resistors
πΉ = πΉπ + πΉπ + πΉπ
Total resistance for two resistors in parallel is given as
R=
For more than 3 resistors
π
πΉ
P.d across the circuit equal the sum of each p.d across
components.
π½ = π½π + π½π
PARALLEL CIRCUITS
In a parallel circuit all of the components can be individually
controlled by using separate switches. If one light bulb
blows the other bulbs will still carry on working. Bulbs
connected in parallel will light brighter.
Current, Resistance AND voltage in parallel circuits.
The voltmeter reading for component X will be the same as
the voltmeter reading for component Y.
That is: Voltage is the same.
π1 π2
π1 + π2
=
π
πΉπ
+
π
πΉπ
+
π
πΉπ
Exercise
What are the advantages of connecting two lamps in parallel
rather than in series to a power supply? (House wiring)
EXAMPLES
1.
Calculate the currents measured by ammeters A1,
A2 and A3 in the circuit below.
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2.
Fig. 1.2 shows a circuit.
Fig. 1.2
(i) In the space below, draw the circuit using circuit
symbols. [1]
measure the potential difference across the cell. [1]
(iii) When the switch is pressed so that the contacts join,
which of the lamps light up? [1]
(iv) When there is a current in the circuit, ammeter 1 reads
0.5 A.
What current does ammeter 2 read? [1]
(v) One lamp “blows”, so that its filament breaks.
What happens in the circuit? [1]
1. A LIVE wire: with BROWN insulation, the wire always
carries voltage, touching it will be fatal. Fuse and switch are
connected to it.
2. A NEUTRAL wire: with BLUE insulation, the wire is
always at 0V, provide return path for voltage.
3. An EARTH wire: with YELLOW-GREEN striped
insulation. Connected directly to the ground to provide a
path for faulty voltage.
All wires are all surrounded by an outer layer made of
rubber or flexible plastic called Insulation.
The three pin plug
EXERCISE
1.
Label the 3 pin plug below
MAINS USES OF ELECTRICITY
HEATING EFFECT OF CURRENT
Heat is generated when an electric current passes through a
resistor. The heating effect can be used for lighting (bulbs),
cooking, ironing, hating water etc.
2.
What is wrong with this plug’s wiring?
Application of heating effect include: electric stoves,
fuses, lamps/bulbs, ovens, kittles element, iron element
MAGNETIC EFFECT OF CURRENT
Electrical conductor carrying current generate magnetic field
around it, produced magnetic effect is used to interact with
other magnetic field or magnetic material to produce
mechanical movements.
Applications of magnetic effect of current: relays, electric
bells, circuit breakers, DC or AC motors
ELECTRICAL CABLE AND THREE PIN PLUG
Electrical cable consists of:
_________________________________________
_______________________________ 3
Note: The appliance connected with this plug would
probably still work but it would be very dangerous to use!
SHOCKET
The EARTH wire, NEUTRAL wire and LIVE wire
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SAFETY DEVICE
1. Earth: This is a safety feature.
The earth wire is connected to the metal casing of a device.
The other end of this wire is connected to a metal rod or
pipe that goes into the ground next to a building.
The action of the EARTH wire
Fuse Rating
Fuses are only supplied with a limited number of ratings.
Namely 3A, 5A and 13A etc.
Appliances with metal cases such as a tumble dryer are
usually earthed by having the EARTH wire connected to
their metal case.
3. CIRCUIT BREAKERS
A circuit breaker is an electromagnetic device that breaks a
circuit when the current goes above a certain value.
Normally current flows to and fro between the LIVE and
NEUTRAL wires through the heater of the dryer. The metal
case is at zero volts and is safe to touch. If the LIVE wire
became loose inside the dryer it might touch the metal case.
A simple circuit breaker
The metal case would now be dangerous to touch and could
give a fatal electric shock.
However, the EARTH wire provides a low resistance path to
the ground. A large current now flows through the fuse and
causes it to melt. The dryer’s metal casing is now isolated
from the LIVE connection and is safe to touch.
Appliances that have plastic cases, for example hairdryers,
do not need the earth wire connection.
2. Fuses
A fuse is a length of wire designed to melt and so breaking a
circuit when the current passing through it goes above a
certain level.
β
Terminals A and B through the contact and the
electromagnet.
When the current in a circuit increases, the strength of the
electromagnet will also increase. This will pull the soft iron
armature towards the electromagnet.
As a result, spring 1 pulls apart the contact and
disconnecting the circuit immediately, and stopping current
flow. The reset button can be pushed to bring the contact
back to its original position to reconnect the circuit
Comparison of fuses and circuit breakers
The thicker the fuse wire the greater is the current required
to cause it to melt (or fuse).
•
•
•
Both can prevent fire by limiting the current
flowing through a cable or appliance.
Fuses are simple and are cheap to replace.
Circuit breakers act more quickly than fuses and
can be reset.
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4. Double insulation
Many electrical appliances have casings made from an
insulator such as plastic rather than metal. The electrical
parts of the device cannot therefore be touched by the user.
The appliance is said to have double insulation. Such
appliances will only have two-wire cables as they do not
need the EARTH wire.
Question 1
Calculate the power of a light bulb that uses 1800 joules of
electrical energy in 90 seconds.
THE DANGERS OF MAINS ELECTRICITY
1. Fill the missing information
DANGER
HAZARDS
Damaged
insulation
Overheating of
cables
Damp
conditions
sockets
PREVENTION
ELECTRICAL POWER (P)
The electrical power, P of a device is equal to the rate at
which it transforms energy from electrical to some other
form (such as heat).
Electrical power = energy transferred &divide; time
Electrical power is measured in watts (W)
Energy in joules (J)
Time in seconds (s)
also:
1 kilowatt (kW) = 1 000 watts
1 megawatt (MW) = 1 000 000 watts
Electrical power ratings
These are always shown on an electrical device along with
voltage and frequency requirements.
electrical power = electrical energy
time
= 1800 J
90 s
electrical power = 20 watts
Question 2
Calculate the energy used in joules by a heater of power
3kW in 1 hour. E= 10800000J or 10.8MJ
Electrical
power,
and voltage V
P
electric
current,
I
Question 1
Calculate the power of a 230V television that draws a
current of 2.5A. P=575W
Question 2
Calculate the current drawn by a kettle of power 2kW when
connected to the mains 230V power supply. I=8.7A
FUSE RATINGS
The equation:
current = electrical power
voltage
is used to find the fuse rating of a device.
The correct fuse rating is that next above the normal current
required by an appliance.
Example:
A 5A fuse should be used with a device that needs a current
of 3.5A.
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cost in pence = kilowatt-hours x cost per kWh
Fuses of 3A, 5A and 13A are available. What fuse should be
used with a 60W, 230V lamp?
I=P&divide;V
= 60W &divide; 230V
= 0.26A
Fuse to be used = 3A
ELECTRICAL ENERGY E
EXAMPLES
1.
Calculate the energy used in joules by a 12V car starter
motor when drawing a current of 80A for 3 seconds.
E=2880J
2. Calculate the energy used in joules by a hairdryer of
power 1kW in 1 hour. E=3.6MJ
Paying for electricity
An electricity meter is used to measure the usage of
electrical energy.
The meter measures in kilowatt-hours (kWh)
A kilowatt-hour is the electrical energy used by a device of
power one kilowatt in one hour.
CALCULATING COST OF ELECTRICITY
1 . Calculate kilowatt-hours used from:
kilowatt-hours = kilowatts x hours
2 . Calculate cost using:
EXAMPLE
1.
Electricity currently costs about 12t per kWh
Calculate the cost of using an electric heater of power 2kW
for 5 hours if each kWh costs 12t.
kilowatt-hours = kilowatts x hours
= 2kW x 5 hours
= 10 kWh
= 10 units
cost in thebe = kilowatt-hours x cost per kWh
= 10 kWh x 12t
= 120t
cost of using the heater =P1.20
2. Calculate the cost of using a mobile phone charger
power 10W for 6 hours if each kWh costs 12t.
3. Calculate the cost of using THE FOLLOWING
DEVICES for 6 hours if each kWh costs 12t
a) desk-top computer – 300 W
b) hairdryer – 2 kW
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QUESTIONS
1. Fig. 1.1 shows a 12 V battery connected to a number of
resistors.
(ii) using the voltmeter on the range 0 to 30 V is unsuitable.
[2]
(b) (i) Calculate the current in the 12 Ω resistor. State the
formula that you use.
(ii) Calculate the p.d. between A and B in Fig. 3.1.
4 Fig. 4.1 shows a mains extension lead. The six sockets
allow several electrical appliances
to be connected to the mains supply through one cable.
Fig. 1.1
(a) Calculate the current in the 8β¦ resistor.2]
(b) Calculate, for the resistors connected in the circuit, the
combined resistance of
(i) the two 5β¦ resistors,[1]
(ii) the two 4β¦ resistors.[2]
(c) The total current in the two 4β¦ resistors is 6 A.[ 2]
(d) What will be the reading on a voltmeter connected
across
(i) the two 4β¦ resistors,
(ii) one 5β¦ resistor? [2]
(e) The 8β¦ resistor is made from a length of resistance wire
of uniform cross-sectional area.
State the effect on the resistance of the wire of using
(i) the same length of the same material with a greater crosssectional area,
(ii) a smaller length of the same material with the same
cross-sectional area.[2]
b)
2. The lamps in a house are connected in parallel to the
mains supply.
(a) On Fig. 2.1, draw three lamps and their switches
connected to the mains supply.
(b) Each lamp is labelled 240 V, 30W. Calculate the current
in one lamp when it is operating correctly. [2]
(c) State the current from the mains supply when the three
lamps are switched on.
3..Fig. 3.1 shows a circuit in which a voltmeter is placed
across a resistor.
Fig. 3.1
The potential difference across the 12 Ω resistor is 4.0 V.
The voltmeter has three different ranges: 0 to 3.0 V, 0 to 6.0
V and 0 to 30 V. The best range
for use in this circuit is 0 to 6.0 V.
(a) Explain why
(i) using the voltmeter on the range 0 to 3.0 V is unsuitable,
Fig. 4.1
(a) The cable connects the sockets to the mains supply.
The cable contains three wires: live, neutral and earth. State
the colour and what is meant by
(i) live, [1]
(ii) neutral,
(iii) earth. [1]
(b) Six powerful lamps are plugged into the sockets and
switched on, one by one.
(i) State what happens in the cable as the lamps are switched
on, one by one. [1]
(ii) Describe why it can be dangerous when a fuse of the
wrong value is used in the plug. [1]
(c) Explain why your hands should be dry when you put a
plug into a socket.[1]
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