COVERED UNDER ELECTRIC CHARGE BY TRIPLE GROUPS ONLY
Electric charge
Electric charge can be either
positive or negative.
In an atom an electron has a
negative charge that is of the
same size as the positive
charge of a proton. Neutrons
have no electric charge.
As an atom has the same
number of electrons as protons
it is uncharged.
Electric circuits
An electric current will only flow if there is a
complete, unbroken electric circuit, that
contains a power supply.
A circuit diagram uses a standard set of symbols to
show how electrical components are connected
together.
Circuit symbols
cell
a cell is required to push
electrons around a circuit
battery a battery consists of two or
more cells
wire
wires should always been drawn
as straight lines
wire junction
switch a switch enables the current in
a circuit to be turned on or off
indicator
often a light bulb – this is used to
show whether or not a circuit is on
light bulb
old symbol – the indicator symbol
is now used
A
ammeter
measures electric current in
amperes (A)
V
voltmeter
measures voltage in volts (V)
fixed
resistor
a resistor is used to limit
the current in a circuit
variable
resistor
thermistor
a device whose resistance
decreases with temperature
light dependent
resistor (LDR)
a device whose
resistance decreases
with brightness
diode
a diode only allows current to flow in
one direction (indicated by the arrow)
light emitting
diode (LED)
fuse
a diode that emits light when
it allows the flow of electric
current
a fuse is designed to melt and so break
an electric circuit when too much
electric current flows
heater a device used to convert
electrical energy to heat
Electric current
An electric current is the rate of flow of
electric charge.
An electric current of one ampere (A)
flows when a charge of one coulomb (C)
passes a point in an electric circuit in one
second
In metallic conductors (e.g. copper wire)
electrons carry negative charge from the
negative side of a power supply, around a
circuit and back into the positive side.
The arrow shows the
direction of electron flow.
In a series circuit
Current is the same at any point in the circuit
2.5 A
2.5 A
Do Now
2.5 A
2.5 A
Can you copy
this please?
Next draw a
parallel circuit
with two
branches
Today’s lesson
• Looking at current in different circuits
• Recall and use: Q = It
• Understand that voltage is the number of
joules per coulomb transferred
• Looking at voltage in different circuits
In a parallel circuit
The current splits (total current stays the
same)
2.5 A
2.5 A
1.25 A
1.25 A
In a parallel circuit
The current splits (total current stays the
same)
2.5 A
2.5 A
1.25 A
☺
1.25 A
Can you copy
this please?
Milliamps - conversions
1 A = 1000 mA
1 mA = ?
Milliamps!
1 A = 1000 mA
1 mA = 0.001 A
COVERED UNDER ELECTRIC CHARGE
Electrical conductors and insulators
An electrical conductor
is a material through
which electric current
flows easily.
All metals are
conductors.
Electrical insulators
have a very high
resistance to the flow of
electric current.
Complete the table below:
copper
rubber
steel
mercury
paper
plastic
diamond
graphite
conductor
1
insulator
2
conductor
3
conductor
insulator
4
insulator
5
insulator
6
conductor
Charge-current equation
electric charge = current x time
Q=Ixt
also: I = Q ∕ t
and: t = Q ∕ I
charge
current
time
Question 1
Calculate the charge passing through a
device when a current of 500mA flows for 3
minutes.
Q=Ixt
= 500 mA x 3 minutes
= 0.5A x 180s
charge = 90C
Question 2
Calculate the current flowing when a charge of
240C flows through a device in 80s.
I=Q÷t
current = 240 C
80s
current = 3A
Now complete the questions on the sheet.
Do Now - Please
copy and complete
Complete:
Q
I
60 C
2A
13 A
960 C
3C
t
5s
4 minutes
50 mA
Learning today
• Understand that voltage is the number of
joules per coulomb transferred
• Looking at voltage in different circuits
• The definitions of emf and pd
• The advantages of connecting bulbs in
parallel.
Answers
Complete:
Q
I
t
60 C
2A
30 s
65 C
13 A
5s
960 C
4A
4 minutes
3C
50 mA
60 s
Voltage
A battery gives electrical charge energy.
The voltage of a battery is equal to the energy in
joules provided when a charge of one coulomb
passes through the battery.
voltage = energy ÷ charge
1 volt is the same as 1 joule per coulomb
Voltage(emf)
☺
☺
☺
V
☺
☺
☺
I’m checking the
difference in
energy (per
coulomb) between
the 2 red arrows
☺
☺
☺
☺
☺
1 Volt = 1 Joule per coulomb
Voltage (p.d.)
☺
☺
I’m checking the
difference in
energy (per
coulomb) before
and after the lamp
☺
☺
☺
☺
V
☺
☺
☺
☺
☺
1 Volt = 1 Joule per coulomb
Voltage – copy
Voltage V is the amount of energy
transferred (changed) per coulomb of
charge.
Unit – volts V
It is measured with a voltmeter in parallel
1 volt is the same as 1 joule per coulomb
The unit is named after the scientist Volta famous
for making the first batteries
Choose appropriate words to fill in the gaps below:
charge
Electric current is the rate of flow of electric _______.
coulombs
Electric charge is measured in _________.
energy . The amount of energy
A battery provides electrical _______
coulomb of electric charge passing is equal to
provided per _________
voltage
the ________
of the battery.
joules
mains
The _______
supply gives 230 ________
to every coulomb of
charge.
WORD SELECTION:
coulombs charge mains joules
coulomb voltage
energy
EMF and PD
• Electromotive force • Potential difference
is please copy the
is
definitions on page
179
EMF and PD
both have the volt as a unit
• The electromotive • The potential
force (emf) of a
difference (PD)
cell or a power
across a
supply is the work
component is the
done per unit of
work done per unit
charge by the cell
of charge in driving
in driving charge
charge through the
around a complete
component.
circuit (including
the cell itself
In a series circuit
The sum of the voltages across the lamps
equals the voltage across the cells
9V
3V
3V
3V
In a series circuit
The sum of the voltages across the lamps
equals the voltage across the cells
9V
☺
3V
3V
3V
Can you copy
this please?
In a parallel circuit
In a simple parallel circuit, voltage across
each lamp equals the voltage across the
cells
5V
5V
5V
In a parallel circuit
In a simple parallel circuit, voltage across
each lamp equals the voltage across the
cells
5V
5V
☺
5V
Can you copy
this please?
Do Now
Ammeter &
Voltmeter
Learning today
• The advantages of connecting bulbs in
parallel.
• Resistance
Electric current flow – conventional
current
Electric current flows
from the POSITIVE
terminal of a power
supply around a circuit to
the NEGATIVE terminal.
The longer thinner line of
the symbol for a cell is
the positive terminal.
In the circuit above the diode is
aligned so that it allows current
to flow through the radio.
Series circuits
Circuit components are said to be connected in
series if the same electric current passes through
each of them in turn.
3A
3A
3A
3A
3A
3A
3A
The cell and the two lamps are in series with each other and
so the same electric current passes through all of them.
Parallel circuits
The voltage across each component connected in
parallel is the same.
The voltmeter reading for
component X, V1 will be the
same as the voltmeter
reading for component Y, V2.
In a series circuit all
of the components
can be controlled by
using just one
switch.
Each component
shares the voltage of
the power supply
and so adding more
bulbs in series will
cause each bulb to
become dimmer.
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.
Exercise
What are the advantages of connecting two lamps
in parallel rather than in series to a power supply?
When connected in parallel:
1. the lamps are brighter than when connected in series
2. the lamps can be controlled individually with switches
3. one lamp will continue working even if the other does not
Currents in parallel circuits
The total current
through the whole
circuit is the sum of
the currents through
the separate
components.
5A
5A
3A
2A
3A
2A
Calculate the currents measured by ammeters A1,
A2 and A3 in the circuit below.
6A
A3
A1
A2
A1 = 2A
A2 = 4A
2A
A3 = 6A
Question
Draw a circuit diagram
for the torch shown
below.
Choose appropriate words to fill in the gaps below:
connected together in series they will
When components are _________
current flowing through each of them.
all have the same _________
When components are connected in parallel to each other
voltage
they will each have the same _________.
parallel
Lamps are usually connected in __________
to each other as
switches
this allows them to be controlled individually by _________
blows the others can still continue to
and if one lamp _______
operate.
WORD SELECTION:
blows current
switches
parallel connected voltage
Online Simulations
Fifty-Fifty Game on Conductors & Insulators - by
KT - Microsoft WORD
Signal Circuit - PhET - Why do the lights turn on
in a room as soon as you flip a switch? Flip the
switch and electrons slowly creep along a wire.
The light turns on when the signal reaches it.
Charge flow with resistors in series and parallel NTNU
Circuit Construction DC Only - PhET - An
electronics kit in your computer! Build circuits
with resistors, light bulbs, batteries, and
switches. Take measurements with the realistic
ammeter and voltmeter. View the circuit as a
schematic diagram, or switch to a life-like view.
Simple parallel circuit with motor and lamps Freezeway.com
Simple parallel circuit with motor and lamps and
a short circuiting switch - Freezeway.com
Switch quiz circuit - Freezeway.com
Two way switches with a flight of stairs Freezeway.com
Bulb circuit diagram quiz - Freezeway.com
Hidden Pairs Game Circuit Pairs Quiz basic
circuit symbols with this pairs game - by eChalk
Hidden Pairs Game on Circuit Symbols - by KT Microsoft WORD
Electric Current Quizes - by KT - Microsoft
WORD
BBC KS3 Bitesize Revision:
Circuit symbols
Series and parallel circuits
Measuring current and voltage
Current in series circuits
BBC AQA GCSE Bitesize Revision:
Circuit symbols & diagrams
Series & parallel connection
Current & potential difference
Cells and circuits
Series circuits
Parallel circuits
Today’s lesson
• Understand the term power
• Recall and use: P =VI
• Recall and use E = VIt
Power
The amount of energy used by a device
per second, measured in Watts (Joules
per second)
A
V
Power = voltage x current
P = VI
Power
The amount of energy used by a device
per second, measured in Watts (Joules
per second)
A
V
☺
Can you copy
this please?
Power = voltage x current
P = VI
Power of a lamp
Measure the power of the lamp at it’s operating voltage.
What is the electrical energy being turned into?
A
V
Power = voltage x current
P = VI
Example
• A 200 W television is plugged into the
110V mains. What is the current in the
television?
P
V I
X
Example
• A 200 W television is plugged into the
110V mains. What is the current in the
television?
• I = P/V = 200/110 = 1.8A
P
V I
X
Example
• A kettle uses 240V and 8A. What is its
power?
P
V I
X
Example
• A kettle uses 240V and 8A. What is its
power?
• P = VI = 240x8 = 1920W (=1.9kW)
P
V I
X
Remember
Power is the amount of energy used by a
device per second, measured in Watts
(Joules per second)
A
V
Power = voltage x current
P = VI
Total energy
So the total energy transformed by a lamp
is the power (J/s) times the time the lamp
is on for in seconds,
E = VIt
E = energy transformed (J)
V = Voltage (also called p.d.)
I = current (A)
t = time (s)
Example
• A kettle uses 240V and 8A. What is its
power?
• P = VI = 240x8 = 1920W (=1.9kW)
• How much energy does the kettle use in 5
minutes?
P
V I
X
Example
• A kettle uses 240V and 8A. What is its
power?
• P = VI = 240x8 = 1920W (=1.9kW)
• How much energy does the kettle use in 5
minutes?
• E = VIt = 240x8x300 = 576000 J
P
V I
X
Simple!
Let’s try some questions!