Transcript SPH3U - K-Moncrief

SPH3U: Electricity
Electric Potential Difference
Electric Potential
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When we turn on a light switch, we can say we
are “using electricity,” but what does this
mean? What does the light bulb take from the
current? To answer these questions, let’s first
look at a few analogies.
Electric Potential

First, imagine a ball held some
distance above the surface of
Earth (but still within the earth’s
gravitational field).
 Work must have been done on
the ball to lift it this high against
the force of gravity.
 The ball’s gravitational potential
energy is increased.
 If the ball is let go, gravity will
make it move back to the Earth,
converting gravitational potential
energy into kinetic energy as the
ball falls.
Electric Potential


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Now, imagine a small positive charge
held at rest a certain distance away
from a negatively charged sphere.
Work must be done on the small
positive charge to overcome the
electric force that exists because of
the negative sphere.
In this case, the small positive
charge has an increase in electric
potential energy as a result. If let
go, the positive charge will move
back toward the negative sphere, in
much the same way that the ball
moved back toward Earth. When it
does this it will lose electric potential
energy.
Electric Potential

In a circuit, when electrons leave the battery
(or source) they have a lot of electric potential
energy.
Electric Potential

As current passes through the circuit, and hits
the light bulb, it experiences opposition to the
flow, resulting in a loss of electric potential
energy. This opposition happens because the
electrons collide with the atoms of the light
bulb as they move through the light bulb.
Electric Potential

As current passes through the circuit, and hits
the light bulb, it experiences opposition to the
flow, resulting in a loss of electric potential
energy. This opposition happens because the
electrons collide with the atoms of the light
bulb as they move through the light bulb.
Electric Potential

Since the electric current loses energy it also
loses electric potential, resulting in an electric
potential difference (V) between two points,
A and B. This can be represented as
W
V
Q
where W is the amount of work that must be
done to move a small positive charge, Q, from
point A to point B.
Electric Potential in a Circuit
Electric Potential Difference

The Electric Potential Difference (V) is defined
as: the change in electric potential when a
charge is moved between two points in an
electric field.

The SI unit for electric potential difference is
the volt (V). 1 Volt is the electric potential
difference between two points if it takes 1 J of
work per coulomb to move a positive charge
from one point to the other. 1 V = 1 J/C
Electric Potential Difference
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Because of the units in which it is measured,
electric potential difference is often referred to
as “voltage.”
Electric Potential Difference

A 12-V car battery is a battery that does 12 J
of work on each coulomb of charge that flows
through it.
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Electric potential difference between two
points in a circuit is measured with a device
called a voltmeter. To measure electric
potential difference, a voltmeter is connected
across the source of electricity and the bulb in
the circuit. This type of connection is called a
parallel connection.
Connecting a Voltmeter
Electric Potential


The electrical energy lost or work done by a charge,
Q, going through a potential difference, V, can be
written
ΔE = QV
Since it is often easier to measure the current and the
time during which it lasts, we can use the equation
Q = IΔt
and, substituting in the first equation, we get an
expression for the electrical energy lost by a current, I,
through a potential difference, V, for a time interval, Δt.
ΔE = VIΔt
Example Problem 1
A 12-V car battery supplies 1.0 × 103 C of charge to
the starting motor. How much energy is used to start
the car?
Example Problem 1
A 12-V car battery supplies 1.0 × 103 C of charge to
the starting motor. How much energy is used to start
the car?
Example Problem 2
If a current of 10.0 A takes 300 s to boil a kettle of
water requiring 3.6 × 105 J of energy, what is the
potential difference (voltage) across the kettle?
Example Problem 2
If a current of 10.0 A takes
300 s to boil a kettle of water
requiring 3.6 × 105 J of energy,
what is the potential difference
(voltage) across the kettle?
Electricity and Safety

Electricity is dangerous because of its effect
on the life support organs of the human body.
The primary factors determining the effect of
electricity on the body are the amount and
path of the current passing through the body.
Electricity and Safety

Currents of less than 0.02 amp may produce
sensations ranging from tingling to sharp pain
(like the current generated by our little handheld generator). A more serious effect occurs
if the current causes muscles to contract. A
person touching a live wire with their
outstretched hand may literally not be able to
let go of the wire due to the current's effect on
the muscles. Currents from 0.03 to 0.07 amp
will begin to impair the ability of the person to
breathe.
Electricity and Safety
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The most dangerous range of currents is from
0.1 to about 0.2 amp. Currents in this range
can cause death by initiating fibrillation
(uncontrollable twitching) of the heart, which
stops the regular flow of blood to the rest of
the body. Currents much larger than 0.1 amp
do not result in fibrillation - instead they stop
the heart completely. If the duration of the
current is short, the heart will usually start to
beat by itself after the current is removed.
Electricity and Safety
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Because of the equation V = IR, we know that
Voltage and Resistance determine the current.
Therefore, high voltages and low resistance
will produce a high current, which is
dangerous.
Electricity and Safety
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As examples, an AA battery provides voltage
of 1.5 volts, a car battery 12 volts and an
electrical outlet 110 volts. The resistance
(which is measured in ohms) of the human
body can range from one hundred to one
million ohms. Wet skin has a much lower
resistance to current than dry skin.
Electricity and Safety

This is why electrical appliances warn against
use while in the shower or bath; although the
voltage of the appliance may not be sufficient
to send large currents through a dry body, the
same voltage may result in a very dangerous
current in a wet body. If the resistance of a
body is as low as 100 ohms, a voltage as
small as 20 volts can lead to fatal currents.
Homework
Read Section 11.3 pages 510-513
 Add to your notes from your readings.
 Answer:
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Page 513 # 1-5