Download File - Physics with Miss OO

Introduction to Autumn Term
Miss O’Donnell
What questions are we looking at
today?
• What is an electric current?
• How can we calculate the charge flow in a
circuit?
• What are charge carriers?
Current flow conditions
• What conditions must be satisfied to make a
current pass round a circuit?
– Circuit must be complete
– There must be a source of potential difference
– What is Ohm’s Law?
• How does this relate to electric current?
• How is the current carried?
• What carries the current?
Show current
through a
conductor using
VPLab9
Explaining
Electricity in
Electricity DC
The convention is
to show the
direction of current
in a circuit is from
positive to
negative
Why is this?
Units and Equations
• Unit of current is:
– Ampere (A)
• Unit of charge is:
– Coulomb (C): this is equal to the charge flow in
one second when the current is 1 ampere
– Symbol for charge is Q
• For a current I, the charge flow ∆Q = Ix∆t
• Why do we use the ∆ sign?
Lesson 2
Current and charge analogies
Actual
Analogy
Electrons flowing in
a circuit.
Balls flowing through
a hollow pipe.
The cell pushes the electrons in the
wire.
An engine pushes the balls in the pipe.
When the electrons go through the
When the balls have to be squeezed
bulb or a resistor, they cause the wires through a thin section of the pipe, they
to heat up.
will make the pipe hot.
When the cell runs out the current
stops flowing.
When the engine runs out of fuel it will
stop pushing the balls. The flow stops.
Measuring the current is counting the
number of Coulombs passing a certain
point in the circuit each second.
Watching the number of balls passing a
certain point of the pipe each second
The current is the same everywhere in
the circuit.
The no. of balls entering the narrow
strip is equal to no. of balls exiting it.
The speed of the balls is constant
throughout.
Actual
Analogy
Electrons flowing in
a circuit.
Balls flowing through
a hollow pipe.
The cell pushes the electrons in the
wire.
An engine pushes the balls in the pipe.
When the electrons go through the
When the balls have to be squeezed
bulb or a resistor, they cause the wires through a thin section of the pipe, they
to heat up.
will make the pipe hot.
When the cell runs out the current
stops flowing.
When the engine runs out of fuel it will
stop pushing the balls. The flow stops.
Measuring the current is counting the
number of Coulombs passing a certain
point in the circuit each second.
Watching the number of balls passing a
certain point of the pipe each second
The current is the same everywhere in
the circuit.
The no. of balls entering the narrow
strip is equal to no. of balls exiting it.
The speed of the balls is constant
throughout.
Actual
Analogy
Electrons flowing in
a circuit.
Balls flowing through
a hollow pipe.
The cell pushes the electrons in the
wire.
An engine pushes the balls in the pipe.
When the electrons go through the
When the balls have to be squeezed
bulb or a resistor, they cause the wires through a thin section of the pipe, they
to heat up.
will make the pipe hot.
When the cell runs out the current
stops flowing.
When the engine runs out of fuel it will
stop pushing the balls. The flow stops.
Measuring the current is counting the
number of Coulombs passing a certain
point in the circuit each second.
Watching the number of balls passing a
certain point of the pipe each second
The current is the same everywhere in
the circuit.
The no. of balls entering the narrow
strip is equal to no. of balls exiting it.
The speed of the balls is constant
throughout.
Actual
Analogy
Electrons flowing in
a circuit.
Balls flowing through
a hollow pipe.
The cell pushes the electrons in the
wire.
An engine pushes the balls in the pipe.
When the electrons go through the
When the balls have to be squeezed
bulb or a resistor, they cause the wires through a thin section of the pipe, they
to heat up.
will make the pipe hot.
When the cell runs out the current
stops flowing.
When the engine runs out of fuel it will
stop pushing the balls. The flow stops.
Measuring the current is counting the
number of Coulombs passing a certain
point in the circuit each second.
Watching the number of balls passing a
certain point of the pipe each second
The current is the same everywhere in
the circuit.
The no. of balls entering the narrow
strip is equal to no. of balls exiting it.
The speed of the balls is constant
throughout.
Actual
Analogy
Electrons flowing in
a circuit.
Balls flowing through
a hollow pipe.
The cell pushes the electrons in the
wire.
An engine pushes the balls in the pipe.
When the electrons go through the
When the balls have to be squeezed
bulb or a resistor, they cause the wires through a thin section of the pipe, they
to heat up.
will make the pipe hot.
When the cell runs out the current
stops flowing.
When the engine runs out of fuel it will
stop pushing the balls. The flow stops.
Measuring the current is counting the
number of Coulombs passing a certain
point in the circuit each second.
Watching the number of balls passing a
certain point of the pipe each second
The current is the same everywhere in
the circuit.
The no. of balls entering the narrow
strip is equal to no. of balls exiting it.
The speed of the balls is constant
throughout.
Actual
Analogy
Electrons flowing in
a circuit.
Balls flowing through
a hollow pipe.
The cell pushes the electrons in the
wire.
An engine pushes the balls in the pipe.
When the electrons go through the
When the balls have to be squeezed
bulb or a resistor, they cause the wires through a thin section of the pipe, they
to heat up.
will make the pipe hot.
When the cell runs out the current
stops flowing.
When the engine runs out of fuel it will
stop pushing the balls. The flow stops.
Measuring the current is counting the
number of Coulombs passing a certain
point in the circuit each second.
Watching the number of balls passing a
certain point of the pipe each second
The current is the same everywhere in
the circuit.
The no. of balls entering the narrow
strip is equal to no. of balls exiting it.
The speed of the balls is constant
throughout.
Adding another cell will increase
the current.
Adding another motor will speed up
the balls in the pipe.
Adding a resistor will cause the
current in the whole circuit to be
reduced. Current is still the same
everywhere. The current in P is
equal to the current in Q.
Adding an obstruction in the pipe will
make the balls go slower. The rate of
flow of the balls at is the same as the
rate at Q.
Adding another cell will increase
the current.
Adding another motor will speed up
the balls in the pipe.
P
Q
Adding a resistor will cause the
current in the whole circuit to be
reduced. Current is still the same
everywhere. The current in P is
equal to the current in Q.
Adding an obstruction in the pipe will
make the balls go slower. The rate of
flow of the balls at is the same as the
rate at Q.
The sum of the currents entering
a point is equal to the sum of the
currents leaving that point.
The number of balls entering any junction
is the same as the number of balls exiting
that junction.
Rearrange the formula and answer the
questions
•
•
•
•
The charge flow for a current of 1A in 10 seconds
The charge flow for a current of 3A in 10 seconds
The charge flow for a current of 5A in 200 seconds
Calculate the average current in a wire through which a charge of
15C passes in 5s
• Calculate the average current in a wire through which a charge of
15C passes in 100s
• ANSWERS:
–
–
–
–
–
10C
30C
1000C
3A
0.15A
What is the charge on an electron?
• Each electron has a charge of 1.6 x 10-19C
• That means that a current of 1A along a wire is
due to 6.25 x 1018 electrons passing along the
wire each second
• Question:
– Calculate the number of electrons passing a point in
the wire in 10 mins when the current is:
• 1.0μA
• 5.0A
– Answer:
• 3.8 x 1015
• 1.9 x 1022
Objectives
• Can we answer these questions?
• A steady current of 0.25A passes through a torch bulb for 6
minutes.
• Calculate the charge which flows through the bulb in this
time.
• In an electron beam experiment, the beam current is 1.2mA.
Calculate a) the charge flowing along the beam each minute
b) the number of electrons that pass along the beam each
minute
Quiz
Current and charge quiz
1. Calculate the charge passing through a
lamp in three minutes when a steady current
of 0.4 A is flowing.
I=
Q
t
Q=It
Q = 0.4 x 3 x 60 = 72 Coulomb
Current and charge quiz
2. Calculate the number of electrons flowing
through a resistor when a current of 2.3A
flows for 5 minutes
Q
I=
t
Q=Ixt
Q = 2.3 x 5 x 60 = 690 Coulomb
no. of electrons = 690 /1.6x10-19 = 4.31x1021
Current and charge quiz
3. What is the current in a circuit if
2.5x1020 electrons pass a given point every 8
seconds
Charge (C) =
no. of electrons x charge of one electrons
Charge (C) =
2.5x1020 x 1.6x10-19 = 40 Coulombs
I=
Q
t
Current = 40/8 =
5 Amps
Current and charge quiz
4. How long does it take for a current of
0.3A to supply a charge of 48C?
Q
I=
t
t = Q/I
t = 48/0.3 = 160 seconds
Current and charge quiz
5. How many electrons pass a point when a
current of 0.4A flows for 900 seconds?
Q
I=
t
Q=Ixt
= 0.4 x 900
= 360 Coulomb
no. of electrons = total charge / charge of one electron
no. of electrons = 360 / 1.6 x 10-19 = 2.25 x 1021
Current and charge quiz
6. A torch bulb passes a current of 120 mA.
How many coulombs of charge flow through the lamp in 1
minute?
Q=Ixt
= 120x10-3 x 60
= 7.2 Coulomb
Current and charge quiz
7. A car battery is rated as 36 A h.
In principle this means it could pass a current of 1 A for
36 h before it runs down. How much charge passes through
the battery if it is completely run down?
Q=Ixt
= 1 x 36 x 60 x 60
= 129600 Coulomb
Current and charge quiz
8. An electron beam in a beam tube carries a
current of 125 A.
(a) What charge is delivered to the screen of
the tube every second?
(b) How many electrons hit the screen each
second?
(a)
Q=Ixt
(b)
no. of electrons = 125 x 10-6 / 1.6x10-19 = 7.8 x 1014
= 125 x 10-6 x 1 = 125 x 10-6 Coulomb
summary
Charge is a s a fundamental property of some types of
particles (e.g. protons and electrons). There is a law of
force: like charges repel, unlike charges attract. These are
the forces that push charges around electric circuits.
An electron has a charge of -1.6x10-19 Coulombs
A proton has a charge of + 1.6x10-19 Coulombs
Charge carriers are any charged particles that can move.
e.g. electrons, protons or ions.
When charge carriers move, they transfer energy from
the source e.g. the cell to the component e.g. the bulb.
One Coulomb is the charge of 6.2x1018 electrons
Current is the rate of flow of charge.
One Amp flows when one Coulomb of charge passes a
certain point in a circuit in one second.
4 – Electric current
Current
What is an
3.1.3
and
electric
current?
charge
How can we
calculate the
charge flow in a
circuit?
What are
charge carriers?
Electrical
46conduction
47
More about
charge carriers
How science
works Rechargeable
batteries
How science
works –
Physics and
the human
genome
project
Key diagram
–
Convention
for current