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Chapter 11- Electricity
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Course Content • Definition of Electricity • Circuit Diagrams
– Series and Parallel Circuits
• Calculating total resistances • Measurement of Electricity
– Ammeters and Voltmeters – Ohm’s Law
• Safety Devices and Hazards • Wiring the Plug
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What is Electricity
• An atom is a fundamental unit of matter • made up of
• protons (with a positive charge) • neutrons (neutral – no charge) • electrons (with a negative charge)
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What is electricity? • Everything is made of atoms which contain POSITIVE particles called PROTONS and NEGATIVE particles called ELECTRONS.
Proton (+)
Neutron
Electron (-)
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What is Electricity?
• Electricity – the flow of electrons through a circuit i.e. the flow of electrical charges
• The path along which the electric current moves is called the ELECTRIC CIRCUIT
• The device which causes the flow of electrons through a circuit is called a CELL or POWER SOURCE (Battery)
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What is Electricity? • Electrons move round a circuit from negative
terminal to the positive terminal of the cell, producing an electric current. (note: like charges repel, opposites attract)
• The rate of flow of electric charges is called ELECTRIC CURRENT
connecting wire electric cell
filament
flow of electrons
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Electric Current
• Conventional Current flow – electric current flows from positive terminal to negative terminal
• Alternating Current (AC) – electric current flows in a clockwise direction and then changes direction to flow anti-clockwise
• Direct Current – electric current flows only in one direction
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Electric Current • Electric current is given the symbol, I • Can also be defined as - the amount of charge
flowing per second • Electric Current is measured in Amps (A) • Thus, Current in Amps (A) =
tQI =
Charge transferred in coulombs (C)
time in seconds (s) Current in Amps (A)
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Electric Current
•This can be rearranged as
• or
ItQ =
IQt =
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Electric Current • The SI Unit for Electric Current is Ampere (A) • Electric Current is measured using an
Ammeter
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Ammeter • It must be connected in series in the circuit. • Positive side of ammeter must be connected
nearest to the positive terminal of the battery (electric cell), and vice versa.
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Electric Circuit Electric circuit :- is a continuous and closed path of an electric current. A schematic diagram of an electric circuit comprising of a cell, electric bulb, ammeter and plug key.
A +
-
+ - bulb
cell
ammeter
plug key
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Electric Circuits • Electric circuits are
made up of electrical components
• These components must be joined together without any gap in between to form a closed circuit.
connecting wires
electric cell
circuit board light bulb
Note:
Components refer to the light bulb, wires, battery
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Electric Circuits • Incomplete circuits are called open circuits.
connecting wire is
missing no source of
electrical energy
Both the circuits in the diagram are incomplete, hence they are known as “open circuits”.
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An electric current flows only when there is:
• a source of electrical energy and
• a closed circuit connecting wires
electric cell
circuit board light bulb
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How to draw Circuit Diagrams Component Symbol Component Symbol
An electric cell Battery
Light bulb (lamp) Switch
Switch (open)
Switch (closed)
Connecting wires (not joined)
Connecting wires (joined)
+ +
Symbols are used to represent the various electrical components in circuits.
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Summary: Electrical Charge
• Electric charge is given the symbol
Q • Electrons are the charge carriers that
flow in an electrical circuit from the negative to positive terminals.
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Summary: Electrical Charge
• Charge is measured in
Coulombs • which is given the symbol
C
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Summary: Electrical Charge
• The charge on a proton is
1.6 x 10-19C
which is the same size as the charge on an electron.
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Summary: What is electricity?
So electricity is… movement of electric charge round a
circuit. We call this electric current.
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Summary: Charge, Current & Time
• Electric current is given the symbol
I • Electric current is the movement of
negative charges (electrons) in a circuit
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Summary: Charge, Current & Time
• Electric Current is the amount of charge flowing per second and is given the unit
Amps (A)
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Summary: Charge, Current & Time
•If current is charge flowing per second then
tQI =
time in seconds (s)
Current in Amps (A)
Charge transferred in coulombs (C)
so a current of 1 A is 1 C of charge transferred in 1 s.
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Summary: Circuit Diagrams Component Symbol Component Symbol
An electric cell Battery
Light bulb (lamp) Switch
Switch (open)
Switch (closed)
Connecting wires (not joined)
Connecting wires (joined)
+ +
Symbols are used to represent the various electrical components in circuits.
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Summary: Circuit Diagrams
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Different types of circuit
• There are different ways in which you can connect cells and electrical components (such as lamps) to create a circuit:
series parallel a mixture of both
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Series Circuit • A series circuit connects the components one after
the other
• A single loop is formed
• A break in any part of a series circuit stops the flow of current in the whole circuit.
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Parallel Circuit • A parallel circuit divides into two or more branches.
• The current divides and flows through each parallel branch.
• If a component breaks or is removed, the other components remain on.
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Voltage • An electric cell gives energy to the electrons and pushes
them round a circuit. Voltage is a measure of how much energy the electrons receive.
• Different voltages are supplied by different cells and batteries.
12 V Car Battery
1.5 V Dry Cell
9 V Dry Cell
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How to Measure Voltage?
• The SI unit for voltage is volt (V).
• A voltmeter is an instrument used for measuring voltages.
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Voltmeter
• Voltmeters must be connected in parallel to the circuit.
• The positive side of voltmeter is connected to the positive
terminal of the cell, and vice versa.
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Recall: Ammeter!!! • It must be connected in series in the circuit. • Positive side of ammeter must be connected nearest to the
positive terminal of the battery (electric cell), and vice versa.
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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 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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measuring voltage
V
This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT
V
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measuring current SERIES CIRCUIT
PARALLEL CIRCUIT
• current is the same at all points in the circuit.
2A 2A
2A
• current is shared between the components
2A 2A
1A
1A
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fill in the missing ammeter readings.
?
?
4A
4A
4A
3A ?
?
1A
?
3A
1A
1A
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V
measuring voltage
V V
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 complete the missing current and voltage readings.
V V
6V 4A
A
A
a)
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measuring current & voltage
V
V
6V 4A A
A
A
b)
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answers
3V 3V
6V
4A 4A 6V
6V
6V 4A 4A
2A
2A
4A
a) b)
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The circuit is no longer complete, therefore current can not flow
The voltage decreases because the resistance increases. The current flow in each bulb is the same.
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Resistance
• When an electric current flows through a circuit, there will be some resistance that opposes it. (similar to friction)
• It can be measured by dividing voltage by the current.
R = V I
R = Resistance
V = Voltage
I = Current
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Ohm’s Law
I = V / R I = Current (Amperes) (amps) V = Voltage (Volts) R = Resistance (ohms)
Ohm’s law states that : ‘The current flowing through a conductor is directly proportional to the potential difference between its ends provided its temperature remains constant.’
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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 • The SI unit for resistance is ohm (Ω)
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Example 1 • An electric rice cooker operates at 240 V and
uses a current of 8 A. What is the resistance of the rice cooker?
Voltage (V) = 240 V
Current (I) = 8 A
R = V I
R = 240
8
= 30 Ω
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Resistors in Series
• When resistors are connected
in series, the resistance will
add up and increase.
• R = R1 + R2 + R3… 2 Ω 3 Ω
The total resistance for this circuit is:
2 Ω + 3 Ω = 5 Ω
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• When resistors are connected in parallel, the resistance will decrease
• This is because they will provide alternate routes for the current to flow.
Resistors in Parallel
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Resistors in Parallel
6 Ω
6 Ω
1 R
= 1 R
1 R
1 R
+ + 1 2 3
What is the resistance of the
circuit?
1 R
= 1 6
1 6
+
1 R
= 2 6
1 3
= R = 3Ω The final resistance is
smaller
![Page 56: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/56.jpg)
Summary: Series and Parallel Circuits
Voltage, Current and Resistance Vs
I3
I2
I1
R3
+ -
R2 R1
V1 V2 V3
What type of circuit is this?
![Page 57: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/57.jpg)
Vs
I3
I2
I1
R3
+ -
R2 R1
V1 V2 V3
One electrical path from negative to positive therefore series.
![Page 58: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/58.jpg)
Vs
I3
I2
I1
R3
+ -
R2 R1
V1 V2 V3
What is the relationship between the three currents?
321 III ==The current is the same at each point.
![Page 59: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/59.jpg)
Vs
I3
I2
I1
R3
+ -
R2 R1
V1 V2 V3
What is the relationship between the four voltages?
321 VVVVs ++=They add to equal the supply voltage.
![Page 60: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/60.jpg)
Vs
I3
I2
I1
R3
+ -
R2 R1
V1 V2 V3
How do you find total resistance in series?
321 RRRRtotal ++=Add each resistance together.
![Page 61: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/61.jpg)
Disadvantages of Series Circuits?
• When one component fails the whole circuit fails.
• The current is the same at all points and the
voltage is divided between the bulbs. The more bulbs added the dimmer each one is.
![Page 62: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/62.jpg)
What type of circuit is this?
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1
V1
V2
V3
![Page 63: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/63.jpg)
More than one electrical path – components connected on different branches therefore parallel.
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1
V1
V2
V3
![Page 64: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/64.jpg)
What is the relationship between the four currents?
321 IIIIT ++=
The four currents add to give the total current.
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1 V1
V2
V3
![Page 65: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/65.jpg)
What is the relationship between the four voltages?
321 VVVVS ===
Each voltage is equal to the supply voltage.
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1 V1
V2
V3
![Page 66: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/66.jpg)
321
1111RRRRT
++=
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1 V1
V2
V3
The resistance in parallel?
![Page 67: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/67.jpg)
• If more resistors are connected in parallel the total resistance will always decrease
• This is because there are more
branches through which the electricity can flow.
![Page 68: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/68.jpg)
Advantages of the Parallel Circuit?
• When one bulb fails the rest of the circuit continues to work.
• The more components, the lower the resistance. The total current drawn increases.
• Voltage in each branch is the same as the supply voltage therefore bulbs in parallel will each be as bright as a single bulb.
![Page 69: Chapter 11- Electricity - NUST 11- Electricity.pdf · Chapter 11- Electricity . ... between to form a closed circuit. connecting wires . electric cell . light bulb . ... 2 Ω + 3](https://reader035.vdocuments.us/reader035/viewer/2022081721/5b32bae47f8b9a2c0b8cdc97/html5/thumbnails/69.jpg)
NOTE!! Hazards of electricity +
Safety Devices + Wiring of an Electric Plug
= Self Study