Transcript 4. Electricity in Circuits

Lesson.4: Electricity in Circuits
Objectives
Recognise that a complete loop is required
for a circuit to work.
Explain the behaviour of simple circuits in
terms of the flow of electric charge.
DEFINE CHARGE, P.D AND CURRENT
Understand the effects of resistance in a
circuit.
Be able to calculate resistance, p.d and
current with an equation.
Most of this lesson is at a higher level than the
exam requires.
What is an electric current?
A flow of charged particles. In metals these
particles are electrons which have a
negative charge.
Current Flow
When we talk about current flowing we say that
current flows out of the positive terminal of the cell
and back into the negative terminal.
Conventional Current
What do we call this flow?
Electron Flow
However, the electrons flow from the negative side of
the battery to the positive side. These are the particles
which are actually moving through the conductor.
Conventional current and Electron flow
When
electrons
discovered,
do were
electrons
have?
What
will
a positive
and
a negative
What charge
do
two
negative
charges
do physicists worked out
what
was
really
charge
donear
to each
other?
if placed
tohappening.
each
other? They
Negative
will repel each other.
Negatively charged electrons are repelled out of the
negative terminal of the cell. They
travel round
the
Theythen
will attract
each other.
circuit and are attracted back to the positive terminal.
Conventional
current
electrons
Conventional
current
We use an ammeter to measure the current in a
circuit. We connect it in series as shown below.
Question
Can you name the other components in the
circuit diagram?
12V
A
As electrons are too small, and there are so many
of them, the ammeter can’t count how many
electrons pass by in the circuit.
It counts the number of groups of electrons which
pass by. These groups are called Coulombs (C).
1Coulomb = 6 million, million, million electrons!
12V
A
Question
What would happen to the reading on an
ammeter if it continually counted the number
of coulombs (groups of electrons) passing by?
12V
A
Question
What must the ammeter measure then?
12V
A
Question
What must the ammeter measure then?
The number of Coulombs passing each second.
The number of Coulombs per second.
12V
A
The number of Coulombs per second.
If ammeters measure in Amps or Amperes (A).
1 Amp is a flow of 1 Coulomb per second
1A = 1C/s
and
2A = 2C/s
12V
A
etc…
Measuring Current
Circuit .1
Set up the circuit shown
below. Move the ammeter
into the three positions
marked on the diagram
recording a current in the
table each time.
Position
Current (A)
A1
A2
A3
12V
A1
A3
A2
Circuit .2
Set up the circuit shown
below. Move the ammeter
into the three positions
marked on the diagram
recording a current in the
table each time.
Position
A1
A2
A3
12V
A1
A2
X
A3
X
Current (A)
Circuit .3
Set up the circuit shown
below. Measure the
current at each of the
power pack voltages
shown in the table.
Voltage (V)
3V - 12V
A
3
6
9
12
Current (A)
Now, check the results with your teacher. If
you do not need to repeat any measurements,
pack the equipment away and please answer the
questions below.
1. Does the current in a series circuit change
anywhere in the circuit?
2. What happens to the current when it meets a
junction where three wires join?
3. How does the current change when the power
pack voltage is increased?
Conclusions
 The current in a series circuit is constant.
Charge is NEVER used up!
 When a current reaches a junction it splits up.
 If the power supply or battery voltage is
increased, the current increases.
This is because each coulomb receives more
energy and so can travel faster through the
circuit.
What is a Voltage?
Also known as an E.M.F and P.D in different
circumstances.
It’s the energy transferred to or from a coulomb of
charge. When charge carries the energy, it is
known as electrical energy.
It’s measured using a Voltmeter connected in
parallel in a circuit.
12V
X
v
X
X
It’s measured using a Voltmeter connected in
parallel in a circuit.
Question
What reading would the voltmeter in the
diagram show?
12V
X
v
X
X
A voltmeter looks at the energy each coulomb
carries at the first connection and the second
connection.
The reading it shows in volts is equal to the
energy per coulomb at 1 minus the energy per
coulomb at 2.
12V
1
X
v
2
X
X
A volt is the amount of energy transferred to or
from each coulomb.
So 1V = 1J/C and 2V = 2J/C
etc…
12V
1
X
v
2
X
X
Question
In the diagram below, is the voltmeter
measuring energy ‘taken’ from each coulomb
or ‘given’ to each coulomb?
12V
1
X
v
2
X
X
The voltage reading below gives the amount of
energy ‘taken from each coulomb and transferred
to light and heat by the bulb.
When energy is ‘taken’ from a coulomb we call
the reading a potential difference or p.d.
12V
1
X
v
2
X
X
Question
In the diagram below, is the voltmeter
measuring energy ‘taken’ from each coulomb
or ‘given’ to each coulomb?
12V
v
X
X
X
The voltage reading below gives the amount of
energy ‘given’ to each coulomb as electrical
energy.
When energy is ‘given’ to a coulomb we call
the reading an Electromotive force or E.M.F
12V
v
X
X
X
Measuring Voltage [P.D]
Circuit .1
Set up the circuit shown
below. Move the
voltmeter into each of the
four positions and record
the p.d (only use one
voltmeter).
Position
P.D (V)
V1
V2
V3
V4
12V
v1
X
X
X
v2
v3
v4
Circuit .2
Set up the circuit shown
below. Move the
voltmeter into each of the
three positions and record
the p.d (only use one
voltmeter).
Position
V1
V2
V3
12V
v2
v1
X
v3
X
P.D (V)
Now, check the results with your teacher. If
you do not need to repeat any measurements,
pack the equipment away and please answer the
questions below.
1. What relationship can you see between
voltage V1 and V2, V3 and V4 in circuit 1?
2. What relationship can you see between
voltage V1 and V2 and V3 in circuit 2?
Conclusions
 The voltage each coulomb has must be
transferred to other forms in one journey
around a circuit.
 Components connected in series share the
voltage between them.
 Each path of a parallel circuit receives an
equal voltage.
End Lesson?
Starter Questions
1. Does the current in a series circuit change
anywhere in the circuit?
2. What happens to the current when it meets a
junction where three wires join?
3. How does the current change when the power
pack voltage is increased?
4. What is the relationship between V1, V2, V3
and V4?
12V
v1
X
X
X
v2
v3
v4
5. What is the relationship between V1, V2 and
V3?
12V
v2
v1
X
v3
X
Lesson.4: Electricity in Circuits
Objectives
Recognise that a complete loop is required
for a circuit to work.
Explain the behaviour of simple circuits in
terms of the flow of electric charge.
DEFINE CHARGE, P.D AND CURRENT
Understand the effects of resistance in a
circuit.
Be able to calculate resistance, p.d and
current with an equation.
How will the current in the circuit below change
if…?
12V
A
X
How will the current in the circuit below change if
I add another bulb?
12V
A
X
X
What quantity increased to cause the current to
decrease?
12V
A
X
X
X
Resistance
Is a measure of how hard it is for a current to flow
in a circuit.
Definition;
where
R= V
I
R = resistance ()
V = potential difference (V)
I = current (A)
 is pronounced as Ohms
R= V
I
From this equation it can be seen that;
for a given resistor, current increases as pd
increases and vice versa;
for a fixed pd, current decreases as resistance
increases and vice versa.
Normal resistors have
fixed values, (unless
they are heated by
large currents!).
Variable resistors like
the one in the picture
which is also shown in
the circuit diagram
below do as their name
suggests. They can be
varied between a
minimum and
maximum value,
depending on how they
are made.
This basic circuit can
be used to collect
results to calculate
the resistance of the
component.
Altering the
variable resistor
gives several
readings of p.d and
current which can be
used with the
equation to calculate
the resistance.