chapter 11- electricity 11- electricity.pdf · ohm’s law . i = v / r . i = current (amperes)...
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Chapter 11- Electricity
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
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)
What is electricity? • Everything is made of atoms which contain POSITIVE particles called PROTONS and NEGATIVE particles called ELECTRONS.
Proton (+)
Neutron
Electron (-)
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)
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
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
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)
Electric Current
•This can be rearranged as
• or
ItQ =
IQt =
Electric Current • The SI Unit for Electric Current is Ampere (A) • Electric Current is measured using an
Ammeter
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.
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
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
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”.
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
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.
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.
Summary: Electrical Charge
• Charge is measured in
Coulombs • which is given the symbol
C
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.
Summary: What is electricity?
So electricity is… movement of electric charge round a
circuit. We call this electric current.
Summary: Charge, Current & Time
• Electric current is given the symbol
I • Electric current is the movement of
negative charges (electrons) in a circuit
Summary: Charge, Current & Time
• Electric Current is the amount of charge flowing per second and is given the unit
Amps (A)
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.
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.
Summary: Circuit Diagrams
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
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.
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.
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
How to Measure Voltage?
• The SI unit for voltage is volt (V).
• A voltmeter is an instrument used for measuring voltages.
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.
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.
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 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
measuring voltage
V
This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT
V
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
fill in the missing ammeter readings.
?
?
4A
4A
4A
3A ?
?
1A
?
3A
1A
1A
V
measuring voltage
V V
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 complete the missing current and voltage readings.
V V
6V 4A
A
A
a)
measuring current & voltage
V
V
6V 4A A
A
A
b)
answers
3V 3V
6V
4A 4A 6V
6V
6V 4A 4A
2A
2A
4A
a) b)
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.
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
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.’
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)
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
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
• 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
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
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?
Vs
I3
I2
I1
R3
+ -
R2 R1
V1 V2 V3
One electrical path from negative to positive therefore series.
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.
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.
Vs
I3
I2
I1
R3
+ -
R2 R1
V1 V2 V3
How do you find total resistance in series?
321 RRRRtotal ++=Add each resistance together.
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.
What type of circuit is this?
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1
V1
V2
V3
More than one electrical path – components connected on different branches therefore parallel.
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1
V1
V2
V3
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
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
321
1111RRRRT
++=
IT IT
Vs
I3
I2
I1
R3
+ -
R2
R1 V1
V2
V3
The resistance in parallel?
• 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.
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.
NOTE!! Hazards of electricity +
Safety Devices + Wiring of an Electric Plug
= Self Study