Download Unit 4 - Section 13.5 2011 Potential Difference

Grade 9 Academic Science – Unit 4 – Electricity
Potential Difference – Voltage
Section 13.5 Pages 560-561
Basic electricity includes voltage, current and resistance. What are these terms?
Item
Symbol
Unit of Measurements
Symbol
Voltage
V
Volts
V
Current
I
Amps
A
Resistance
R
Ohms
Ω
Potential Difference or Voltage (V) is the difference in electric potential energy per unit charge
measured at two different points. Electric potential energy per unit charge is often referred to as
“electrical potential” BUT this is not correct because electrical potential is also force, potential
drop, etc. Potential Difference is measured in volts.
The illustration shows Potential Difference in two
batteries (“D-sized” and “AA-sized”). Stored
electrical energy in the cell (i.e., battery) moves from
the area of “greater potential” to the area of “lower
potential.” In this case, it is the movement from the
region with more electrons (-) to the area of no
electrons. When the flow of electrons cross through
the resistor, the resistor uses converts some of the
potential electrical energy into kinetic energy (usable
energy such as heat or light) and the resistor works
(e.g., lamp lights, motor turns, clothes warms up).
For a model of Potential Difference using a waterfall
as an analogy, see Page 560.
To continue….
There is a Potential Difference between the two terminals of the battery cell. Electrons leave the
negative terminal (…area of higher Potential Difference) carrying electric potential energy and
move toward the positive terminal (…area of lower Potential Difference).
If there is a resistor or a load (e.g., toaster, washing machine, computer), some of the energy is
used to run or operate the machine. THUS, the electrons reach the positive terminal with LESS
electric potential energy. Once inside the cell, there are chemical reactions to give electrons
more energy and send the “re-energized” electrons out of
the negative terminal again.
To measure voltage in a circuit, a voltmeter is used. A
voltmeter is a device use the measure the potential
difference. As shown, a voltmeter is connected in parallel
with the load or energy source. In this way, the voltage
measures the difference in volts between the input to the
load and the outflow from the load.
Do you understand? Explain the paragraph above in your own words.
HOMEWORK
 Page 561, Questions 2-3
In the simplest terms

CURRENT is FLOW, i.e., the movement of an electric charge in an electrical circuit

VOLTAGE is FORCE, i.e., the energy needed to make the charge flow

RESISTANCE is STOPPING THE FLOW, i.e., something that prevents the charge from flowing
If you think it through, these three things must be related. For example, if I know the force (voltage) moving
the charge (current), I could calculate the amount of resistance needed to completely stop the flow.
DO I GET IT?
In my circuit, I have a toaster with a resistance of 25 Ω. I want to use a toaster, and I know I have a flow or
current of 100 A.

Can I determine the amount of voltage needed to make the toaster work? Yes or No. Explain your
answer

If I can, what does the voltage tell me?

Can you think of a mathematical formula using resistance and current that would allow you to
calculate the voltage
In my house, do I want to stop the flow completely? In other words, do I want to add so many electrical
appliances (e.g., computer, IPOD recharger, hair blower, straightening iron, toaster, television) to a circuit
that the flow of electrical charge completely stops (Yes or No)? Explain your answer.
Electrical Load is a device that converts electrical energy into the needed form of energy such as heat or
light. Using this definition, which of the following items is an electrical load?
Item
Electrical Load
Y or N
Light Bulb
Toaster
Colour television
Car
Water heater
If you answered “No” for any item, explain your answer.
Does it take a lot of electrical current to kill a person? No. It takes only a very small amount. SO…read
electrical safety warnings…they may save your life.
A human body uses electricity to make muscles contract. Nerve cells produce about 0.08 V of electricity to
make this occur. When higher voltages stimulate the muscles, the contractions are stronger.
DO I GET IT?
A human heart does not beat normally when this muscle contracts too much, too strongly or too quickly.
How does a Heart Defibrillator re-establish normal heart rhythm?
MORE INFORMATION

Most of us do NOT feet an electrical current below 0.001 A; yet at 0.002 A, the muscles start to tingle.
To protect human health, the maximum safe current level is 0.005 A.

At a current of 0.016 A, a person cannot let go of the object applying the electrical shock. This is called
the “let-go threshold.”

At 0.050 A, the heart will stop pumping.

Yet, 0.833 A is the current needed to light a 100 W light bulb. That level of current is enough to kill 50
adult humans.