Download Unit 4 - Section 13.3 2011 Charge and Current

Grade 9 Academic Science – Unit 4 Physics (Electricity)
Electric Current
Section 13.3 Pages 556-557
Electric Charge
The unit of electric charge is the coulomb.
Electric Current (I)
 A measure of the RATE of electron flow past a given point in a circuit
 Measured in amperes (A)
Ordinary matter is made up of atoms that have positively charged nuclei and negatively charged
electrons surrounding them. As noted, the rate of flow of electric charge is called electric current
and it is measured in amperes.
NOTE: In introducing one of the fundamental properties of matter, it is important to realize that
simplified sketches and constructs are used to introduce concepts. There is much more
to the story. NO significance should be attached to the circles representing the proton
and electron. The circles do not imply size or shape.
Electrically, the most important opening idea, is that electrons and protons have a
property called "charge" which is the same size but opposite in polarity (i.e., proton is +
and electron is -). The proton has 1836X the mass of the electron but exactly the same
size charge. The idea of charge suggests the proton and electron will strongly attract
each other. As well, like charges (two protons or two electrons) strongly repel each
other. "Like charges repel and unlike charges attract".
For any electrical device to work, there must be a FLOW of electrons.
For an analogy of current flow to water moving
over a waterfall, read Page 556
Current is measured in units called 'amperes'. The number of amperes in a wire relates to how
many electrons pass a cross section of
the wire per second. With a higher the
current (higher amperes), the number of
electrons that pass a cross section of
the wire per second is increased or
greater (see illustration).
NOTE: The unit of measurement of
current is AMPERES (symbol A). It is
named after André-Marie Ampère the
founder of a device to measure current.
The symbol of current is I
Measuring Current
An Ammeter is device used to measure electric current.
As shown, an ammeter is connected in
SERIES with the load. This setup ensures that
all the electrons that flow through the load also
flow through the ammeter.
Current involves the flow of a VERY HUGE
NUMBER of electrons. If an ammeter
indicates a current of 1 A, it means 6.2 X 1018
electrons are passing through the ammeter
each second.
Current flows from a higher
potential (…more negative) to a
low potential (…few electrons).
NOTE: The direction of the
current will change if the
negative and positive poles of
the source exchange.
Safety with Electric Currents
Large currents can damage equipment and cause fires
A fuse or circuit breaker in your home ensures the proper amount of current reaches your
electrical equipment. The fuse or breaker is connected in series with the electrical circuits of your
house. If too much current is demanded, the current also flows through the fuse or breaker. The
fuse breaks or the breaker trips. It is like a switch….the current is turned off.
Electrical Shock and the Human Body
Your brain coordinates movement and reactions via electrical signals sent through your nervous
system.
Small electrical shocks can be dangerous to your health
 A current of 0.001 A can give a tingling sensation
 A current of 0.05 to 0.15 A causes muscle contraction or convulsions. This level is called
the “LET GO” threshold. Beyond this value, you CANNOT let go of the object giving the
shock.
 A current of 1.0 to 4.3 A causes your heart to stop beating. NOTE: a wall outlet delivers
a current of 15 A.
HOMEWORK
Page 557, Questions 1-4
Grade 9 Academic Science – Unit 4 – Physics – Electricity
Notes on Current
Electric Charge
The unit of electric charge is the coulomb.
Ordinary matter is made up of atoms that have positively charged nuclei and negatively charged
electrons surrounding them.
The rate of flow of electric charge is called electric current. Current is measured in amperes.
NOTE: In introducing one of the fundamental properties of matter, it is important to realize that
simplified sketches and constructs are used to introduce concepts. There is much more
to the story. NO significance should be attached to the circles representing the proton
and electron. The circles do not imply size or shape.
Electrically, the most important opening idea, is that electrons and protons have a
property called "charge" which is the same size but opposite in polarity (i.e., proton is +
and electron is -). The proton has 1836X the mass of the electron but exactly the same
size charge. The idea of charge suggests the proton and electron will strongly attract
each other. As well, like charges (two protons or two electrons) strongly repel each
other. "Like charges repel and unlike charges attract".
Although it is electrons that are the mobile charge carriers and responsible for electric current in
conductors, it is convention to take the direction of electric current as if the positive charges which
are moving. In common applications such as determining the direction of force on a current
carrying wire, treating current as positive charge motion or negative charge motion gives identical
results. The conventional current direction is the direction from high voltage to low voltage -- high
energy to low energy. In this way, the current flow is similar to the flow of water from high
pressure to low (see diagram).
Current – Note Summary
Electric Charge
 The unit of electric charge is the coulomb.

Ordinary matter is made up of atoms that have positively charged nuclei and negatively
charged electrons surrounding them.

NO significance should be attached to the circles representing the proton and
electron. The circles do not imply size or shape.

Electrons and protons have a property called "charge" which is the same size but
opposite in polarity (i.e., proton is + and electron is -). The proton has 1836X the
mass of the electron BUT exactly the same size charge. The idea of charge
suggests the proton and electron will strongly attract each other. Like charges
(two protons or two electrons) strongly repel each other. "Like charges repel and
unlike charges attract".

The rate of flow of electric charge is called electric current. Current is measured in
amperes.

The conventional current direction is the direction from high voltage to low voltage -- high
energy to low energy.