G9 Unit 6 Electricity

Unit 6:
Electricity

Learning Contents
• Electrical Quantity (Electric Current, Voltage and
Electrical Resistance)
• Relationship between Electric Potential Difference,
Electric Circuits and Connections
• Calculations of Electrical Energy and Electricity Cost
• Safety Electrical Appliances Wisely and Safely

What is Electricity?
Definition:
Electricity is a form of energy that can be easily changed to other
forms.

Where does Electricity come from?
Mainly 2 sources:
1) Power Stations
- Supply a lot of electricity
- Used in many electrical appliances
2) Electric Cells (batteries)
- Supply a little electricity
- Portable
- Safe

Kinds of Electricity
Current
• The flow of electric
charges carried through a
material such as wire or
other conductors
• Examples
– Power lines
Static
• An electric charge that
collects or builds up on
the surface of an object
• Examples
– Lightning

Conductors and Insulators
Insulators
• Materials that do
not allow electric
charges to flow
freely through them
• Examples:
– Wood
– Glass
– Rubber
Conductors
• Any material that
allows electric
charges to move
through it
• Examples:
– Metals (esp. copper)
– Your body
– Electric wires

How does an Electrical Appliance Work?
• To make an electrical appliance work,
electricity must flow through it.
• The flow of electricity is called an
electric current.
• The path along which the electric
current moves is called the electric
circuit.

What is an Electric Current?
Definition:
An electric current as the
rate of flow of charges
through conductor.

Electric Charges
• Electric charges are made up of positive charges (protons)
and negative charges (electrons).
• When these charges flow in a circuit, a current is produced.

• The electrons are pushed
away from negative
terminal and attracted
towards the positive
terminal.
How does electricity flow?

Electrical Quantity
Electric Current, Voltage and Resistance

Definition:
An electric current is
the rate of flow of
electric charges in a
circuit.
electric cell
filament
flow of electrons
connecting wire

How to Measure Current?
• The SI unit for electric current is
ampere (A).
• Different electrical components
and appliances require different
sizes of current to turn them on.

Instrument to Measure Current
• An ammeter is an instrument used for
measuring electric current.

Direct Current (DC)
• An electric
current that
flows in one
direction.

Alternating Current (AC)
• The direction of
current flowing in
a circuit constantly
reverse and forth
back.

Electrical Potential Difference or Voltage
• An electric cell gives energy to the electrons and pushes them round a
circuit.
• Voltage is the electrical energy supply needed to move each unit of
charge from one point to another or a potential difference of two
points.
• Different voltages are supplied by different cells and batteries.
1.5 V Dry Cell
9 V Dry Cell

How to Measure Voltage?
• The SI unit for voltage is volt (V).
• A voltmeter is an instrument used for measuring
voltages.

Resistance
• When an electric current flows through a circuit,
there will be some resistance that opposes it.
(similar to friction).

Resistance
• Good conductors of electricity have LOW
RESISTANCE. (Eg. Metal objects)
– Electricity is able to flow through them very easily
• Poor conductors of electricity have HIGH
RESISTANCE. (Eg. Wood, cloth)
– Electricity is not able to flow through them easily

Resistance
• The SI unit for
resistance is ohm ()
• Different electrical
components have
different resistance.
• Rheostat is used to
measure the resistance.

2
4

2
4

Relationship between
Electric Potential
Difference, Electric Current
and Electrical Resistance
• Ohm’s Law

Ohm’s Law
• The current that flows through a conductor is
directly proportional to the voltage across the
ends of the conductor, provided that the
temperature and other physical conditions are
unchanged.
• The relationship between voltage, current and
resistance can be written as : V=IR

• https://phet.colorado.edu/si
ms/html/ohmslaw/latest/ohms-law_en.html

Chart

Formula for Ohm’s Law
• V (E) = I x R
• I = V
R
• R = V
I

Sample Problem 1:
Find the current through a 12Ω resistive circuit when 24
volts is applied.
Given:
R = 12 Ω
V = 24 V
I = ?
Solution:
I = V R
I = 24
12
I = 2 A The current is 2 A.

Find the resistance of a circuit that draws 0.006
amperes with 12 volts applied.
Given:
R = ?
V = 12 V
I = 0.006
Solution:
R = V I
R =
12
0.006
I = 2, 000Ω
Sample Problem 2
The current is 2,000Ω.

Find the applied voltage of a circuit that draws 0.2
amperes through a 4800-ohm resistance.
Given:
R = 4, 800 Ω
V = ?
I = 0.2 A
Solution:
V = I R
V = ( 0.2)(4, 800)
V = 960 V
Sample Problem 3
The applied voltage of a circuit is 960V.

Sample Problem 4
What voltage or potential difference is used by an electrical
appliance that draws 0.4 amps of current and has a resistance of
3 ohms?
Given:
R = 3 Ω
V = ?
I = 0.4 A
Solution:
V = I R
V = ( 0.4)(3)
V = 1.2V
The voltage or potential difference is used by an electrical
appliance is 1.2 V.

Sample Problem 5
A light bulb uses 240 volts of electricity and draws a current
of 2 amps. What is its resistance?
Given:
R = ?
V = 240 V
I = 2 A
Solution:
R = V I
R = 240
2
I = 120Ω
The resistance of the
lightbulb is 120Ω.

Seatwork: Calculate the problem below. Show your solution.
1. What current flows through a hair dryer plugged into a 110 Volt
circuit if it has a resistance of 25 ohms?
2. A 12 Volt car battery pushes charge through the headlight
circuit resistance of 10 ohms. How much current is passing
through the circuit?
3. An electric heater works by passing a current of 100 A though
a coiled metal wire, making it red hot. If the resistance of the
wire is 1.1 ohms, what voltage must be applied to it?
4. A subwoofer needs a household voltage of 110 V to push a
current of 5.5 A through its coil (circuit). What is the resistance
of the subwoofer?
5. An electric toaster is connected to a 120-V outlet in the
kitchen. If the heating element in the toaster has a resistance
of 14Ω, calculate the current flowing through it.

Electrical Circuits
and Connections

• Simple circuit – are connected
with wires, a battery, a switch
and a bulb.
• When the switch is closed, the
circuit forms a complete
circuits around which a current
flows.
Complete Circuits

Open and Closed Circuit
Closed Circuit
• An electric circuit that
electric current flows in
a circuit completely.
• Open Circuit
• An electric circuit that
electric current cannot
pass through.

Circuit Diagram

Components Found in an Electric Circuit
• A cell is a single unit which
converts chemical energy to
electrical energy to deliver a
voltage while a battery is
composed of a number of cells in
series to get increased voltage.

• Fuse is an electrical device
that can interrupt the flow
of electrical current when it is
overloaded.
• A power supply is a device
that supplies electric power
to an electrical load.

• A switch is an electrical component
that can break an electrical circuit,
interrupting the current or
diverting it from one conductor to
another
• Two-way switch can be used in
pairs so that either can switch a
light on or off.

• Ammeter- instrument used to
measure electric current
• Voltmeter – measure the voltage
• Galvanometer – used to detect
the flow of charges or electric
current.

• Bulb
• Variable resistor (rheostat) rheostat:
resistor for regulating current
• Fixed resistor is a resistor having a
definite resistance value that cannot
be adjusted.

Series and Parallel
Circuits

Workbook p. 160

How are the brightness of light bulbs in a series circuit and
a parallel circuit similar or different?

Series Circuit
•Series Circuit – a circuit that contains only one
current path

Characteristics of a Series Circuit
•If one bulb is spoilt or removed, the circuit becomes
incomplete and no current is allowed to flow through
the circuit. The other bulb will not light up.
•When more bulbs are connected in series, the bulb will
become dimmer.

Voltage drop
•As each device in series
uses power, the power
carried by the current is
reduced.
•As a result, the voltage is
lower after each device
that uses power.

Rules for Calculating Series Circuit Values
1. The current is the same at any point in the
circuit. I total
= I 1
=I 2
=I 3........
2. The applied voltage is equal to the sum of the
voltage drops across all the resistors. V total
=V 1
+
V 2
+V 3
....

Sample Problem 1
A series circuit contains a 12-V
battery and three bulbs with
resistances of1Ω, 2 Ω, and 3 Ω.
What is the current in the
circuit?

1. Looking for:
• …current (amps)
2. Given
Voltage = 12V; resistances = 1Ω, 2 Ω, 3 Ω.
3. Formula:
• R t
= R 1
+R 2
+R 3
• Ohm’s Law I = V ÷ R
4. Solution
• R t
= 6 Ω
• I = 12 V ÷ 6 Ω = 2 amps

Determine the (a) total
resistance of the entire circuit,
(b) the current through each
resistor, and (c) the voltage drop
across each resistor.
Sample Problem 2
4 Ω
36 V
6 Ω
I = I 1 = I 2 = I 3
R t = R 1 + R 2 + R 3
V = V 1 + V 2 + V 3
8 Ω
b. I = I 1 = I 2 = I 3
a. R t = R 1 + R 2 + R 3
R t = 4 Ω + 6 Ω + 8 Ω
R t = 18 Ω
I = V
R t
I = 36 V
18 Ω
I = 2 A

V 1 = I R 1
c. I = 2 A
V = I R
V 1 = (2A)(4 Ω)
V 1 = 8 V
4 Ω
V = V 1 + V 2 + V 3
V 2 = I R 2
V =8V +12V+ 16V
36 V
6 Ω
V 2 = (2A)(6 Ω)
V = 36 V
V 2 = 12 V
8 Ω
V 3 = I R 3
V 3 = (2A)(8 Ω)
V 3 = 16 V

The circuit shown contains a 9-
volt battery, a 1-ohm bulb,
and a 2-ohm bulb.
Calculate the circuit’s total
resistance and current.
Then find each bulb’s voltage
drop.
Sample Problem 3

1. Looking for:
• …total resistance; voltage drop each bulb
2. Given
• …Voltage = 9V; resistances = 1Ω, 2 Ω.
3. Formula:
• R t
= R 1
+R 2
+R 3
• Ohm’s Law I = V ÷ R
4. Solution- part 1
• R t
= 3 Ω
• I = 9 V ÷ 3 Ω = 3 amps

4. Solution- part 2
– Use resistance to find current
I = 9 V ÷ 3 Ω = 3 amps
• Solution- part 3
– Rearrange Ohm’s law to solve for voltage
– Use current to find each voltage drop
V = I x R
V 1 = (3 A) x (1 Ω) = 3 volts
V 2 = (3 A) x (2 Ω) = 6 volts (3 + 6 ) = 9 V

• Determined the total current in the circuit.
Sample Problem 4:
• Given:
V = 10V
R1 = 20 Ω
R23 = 19Ω
R4 = 20Ω
I total = ?

• Determined the total current in the circuit.
Sample Problem 4:
• Solution:
R total = R1 + R23 + R4
= 20Ω +19Ω+20Ω
= 59Ω
I total = V÷R
= 10 ÷ 59
= 0.17 A
The total current in the
circuit is 0.17A

Sample Problem 5:
• Determined the total current in the circuit.
Given:
V = 45V
R1 = 5Ω
R2 = 10Ω
R3 = 7.5 Ω
I total = ?

Sample Problem 5:
• Determined the total current in the circuit.
Solution:
R t = R1 + R2 +R3
=5Ω + 10Ω + 7.5 Ω
= 22.5Ω
I total = V ÷ R
= 45 ÷ 22.5
= 2A
The total current in the
circuit is 2A.

Place values in the tables.
V A R
R1
R2
R3
Total
2 V
4 V
6 V
0.002 A 1000 Ω
0.002 A 2000 Ω
0.002 A 3000 Ω
12 V 0.002 A 6000 Ω

Parallel Circuits
•In parallel circuits the current can take more than one
path.

Characteristic of Parallel Circuits
• The number of paths for the flow of current is
increased.
• If one bulb is spoilt, or removed from the circuit, the
other bulb will not be affected and will continue to
light up.
• When more bulbs are connected in parallel, the bulb
will have the same brightness.

Parallel Circuit Calculation Rules
•The voltage across any branch is equal to the source
voltage.
• V total = V 1 = V 2 ………
•The total current is equal to the sum of the branch
currents.
• I total = I 1 + I 2 …

1
R t
= 1 R 1
+ 1 R 2
1
R t
=
1
+ 1
20Ω 30Ω
1
= 3+2 = 5 R t
60 60
R t = 60
5
R t = 12 Ω

1
R t
= 1 R 1
+ 1 R 2
+ 1 R 3
1
R t
=
1
+ 1
+ 1
10Ω 20Ω 40Ω
1
= 4+2+1
R t
40
= 7 40
R t = 40
7
R t = 5.71 Ω

Solving for Parallel Resistance
• Calculate using reciprocal
formula:
1
R t
= 1 R 1
+ 1 R 2
+ 1 R 3
1
= 1 + 1 + 1
R t
2Ω 3Ω 5Ω
R1=2 R2=3 R3=5
1
= 15+10+6
R t
30
R t = 30
31
= 31
30
R t = 0.97 Ω or 1 Ω

Sample Problem 1
•Find the total current.
•I1 = 10V / 10 Ohm = 1 A
I2 = 10V / 20 Ohm = .5 A
•I 1 + I 2 = I t
1A + .5A = 1.5A
•The total current is 1.5 A.

Sample Problem 2
All of the electrical outlets in Jonah’s
living room are on one parallel
circuit.
The circuit breaker cuts off the
current if it exceeds 15 amps.
Will the breaker trip if he uses a light
(240 Ω), stereo (150 Ω), and an air
conditioner (10 Ω)? Assuming that
the voltage is 120 V.

Solving Problem 2:
1. Looking for:
– whether current exceeds 15 amps
2. Given:
– ……resistances = 240 Ω; 150 Ω; 10 Ω
– ……. Voltage = 120V
3. Relationships:
– Assume voltage for each branch = 120 V
– Ohm’s Law I = V ÷ R
– I total = I 1 +I 2 +I 3
4. Solution:
– I light = 120 V ÷ 240 Ω = 0.5 amps
– I stereo = 120 V ÷ 150 Ω = 0.8 amps
– I a/c = 120 V ÷ 10 Ω = 12 amps
0.5 A
0.8 A
+12.0 A
13.3 A
Breaker will not trip

Sample Problem 3
• In the diagram below, find
the total current, I.
• Find the total current.
• I 1 = 12V ÷ 2Ω = 6 A
• I 2 = 12 V ÷ 3Ω = 4A
• I 3 = 12V ÷ 6Ω = 2A
• I total= I 1 + 1 2 +1 3
• = 6A + 4A + 2A
• =12 A
• The total current is
2A.

Calculations of
Electrical Energy and
Electricity Cost

Electrical power is the rate at which an
electrical appliance uses electrical energy.
All appliances have a power rating.

• Power is measured in watts (W).
• 1000 watts = 1 kilowatt (kW).
• 1 watt of power means that 1 joule of energy is used every second.
Appliances that need to create heat, such as
washing machines, cookers, hair dryers and
kettles, usually use the most power.
TVs, radios and computers usually
use the least amount of power.

Calculating the units of electricity
electrical energy = Power x time
E is the energy transferred in kilowatt-hours, kWh
P is the power in kilowatts, kW
t is the time in hours, h.
The amount of electrical energy (i.e. the amount of
electricity) used by an appliance depends on its power
and how long the electricity is used for.

Sample Problem 1:
• Samson spends an hour each day drying his hair with an electric hair
dryer with a power rating of 1.5 kW. How much is the electrical
energy consumed by Samson?
Given:
P = 1.5 kW
t = 1 hr
E = ?
Solution:
E = P x t
= ( 1.5 kW)(1hr)
= 1.5kWh
The electrical energy consumed by Samson is

Sample Problem 2:
Calculate the electrical energy consumed by switching on a refrigerator
(rated as 220V, 500 W) for 24 hours in kWh.
Given:
P = 500W
= 0.5 kW
t = 24 hr
E = ?
kW
Solution:
E = P x t
= ( 0.5 kW)(24hr)
= 12kWh
The electrical energy consumed by switching on a refrigerator for
20 hours is12kWh.

Sample Problem 3:
• A consumer uses a 6 kW immersion heater, a 4 kW electric stove and
three 100 watt lamps for 10 hours. How many units (kWh) of
electrical energy have been converted.
Given:
P = 6kW; 4Kw; 100W
t = 10 hr
E = ?
Solution:
E = P x t
= (6kW+4kW + 0.1kW)(10hr)
=(10.1kW)(10hr)
= 101kWh
The electrical energy that have been converted in 101kWh.

Conclusion:
• Electrical power is the rate at which an electrical
appliance uses electrical energy.
• Calculating Electrical Power is P = IV.
• Electrical energy can be calculated by Energy = Power x
Time

Calculation of Electricity Cost in Thailand
• Electricity pricing in the residential sector of Thailand
– Base Tariff or Cost of Electrical Energy Consumption
– Fuel Adjustment Charge (Ft)
– Tax (7% VAT).

Calculating the cost of electrical energy used
• The cost of electrical energy used is based on the
number of kilowatt-hour (kWh) used.
• Cost of electrical energy consumption = total domestic
units × tariff rate
A list of price charge
per domestic unit
Electrical energy
used

Sample Problem 1:
A household which have a 98 units in a month.
How much is the electricity bill cost?
Electricity Rate Price per unit kWh Amount
First 15 units 2.3488
Next 10 units 2.9882
Next 10 units 3.2405
Next 65 units 3.6237
15
10
10
63
35.2320
29.8820
32.4050
228.2931
Total Electricity Bill
325.8121 bht

Sample Problem 2:
A household which have a 100 units in a month.
How much is the electricity bill cost?
Electricity Rate Price per unit kWh Amount
First 15 units 2.3488
Next 10 units 2.9882
Next 10 units 3.2405
Next 65 units 3.6237
15
10
10
65
35.2320
29.8820
32.4050
235.5405
Total Electricity Bill
333.0595 bht

Importance of Safety
Precaution in the Use of
Electricity

Using Electrical Appliances Wisely and Safely

Three possible dangers in electricity
• Insulation can be damaged by overheating of cable dues to
excessive current flowing through the cable or wire.

Three possible dangers in electricity
• Exposed live wire can cause severe electric shock if the user
accidentally touches it

Three possible dangers in electricity
• Short-circuit of overloading can cause large current to flow in the
conducting wire.

Safety features at home to prevent
• Fuse – a safety device and every electrical appliance
should have a fuse with the correct rating to prevent it
from overloading.

• Earth wire - a safety wire
and connects the metal
case of the appliance to
the earth.

• Double insulation – is a safety feature in an electrical
appliance that can substitute for an earth wire.

• Circuit breakers
– Miniature Circuit Breaker (MCB)
– Earth Leakage Circuit Breaker (ELCB)

• will break the circuit when
there is a short-circuit or when
overload current passes
through a circuit . The MCB can
be switched back on to get the
electric supply again.
Miniature Circuit Breaker (MCB)

Earth Leakage Circuit Breaker (ELCB)
• Can detect small leakage
current from the live
wire to the earth wire.
The ELCB will switch off
all the circuits in the
house in a vey short
time once the leakage is
detected.

Unit Test