Transcript Slide 1 - Uplift Summit Intl

Electrical resistance (symbol R)
• Electrons flow due to potential difference. If the potential
difference is removed, then electrons stop flowing.
• As electrons move, they bump into other atoms, this slows
them down and impedes their motion.
atoms
(actually
positive ions)
free electron
.
Resistance (R) is a measure of the degree to
which an object impedes the flow of current.
Resistance is measured in Ohms ()
path
Resistance Across a Circuit
How do resistors affect circuits?
The energy of electricity comes from the movement of the
electrons. So, by slowing down the electrons, resistance
causes the charges to lose energy / electric potential.
Most of the loss of potential comes from the circuit elements,
such as lights and motors (often just called resistors) which
are using the kinetic energy of the electrons to do something
(make light, turn a motor, etc.). We call this loss of electrical
potential voltage drop (or potential drop).
Resistance in Wires
Most of the time, wires are designed to have
as little resistance as possible, mainly by
making them short, thick, and out of a good
conductor, such as copper.
So, in most cases, we can completely ignore the
resistance of the wires within our circuits.
But, in some cases, the wire is designed to have
high resistance (and high ‘loss’ of energy to heat),
usually by making the wire very very long …
What is the relationship between resistance and
voltage drop?
Introducing ….
V is potential difference across a resistor
I is current
R is resistance
Examples
• If a 3 volt flashlight bulb has a resistance of 9 ohms, how
much current will it draw?
• I = V / R = 3 V / 9  = 1/3 Amps
• If a light bulb draws 2 A of current when connected to a
120 volt circuit, what is the resistance of the light bulb?
• R = V / I = 120 V / 2 A = 60 
Check for Understanding
Which of the following would cause the current in a circuit to
decrease the most?
1)
2)
3)
4)
Increased voltage and increased resistance
Increased voltage and decreased resistance
Decreased voltage and decreased resistance
Decreased voltage and increased resistance
Check for Understanding
If the resistance of a circuit were tripled, then the
current through the circuit would be ____.
1. one-third as much
2. three times as much
3. unchanged
4. ... nonsense! There would be no way to make
such a prediction.
Just for Fun …
Effects of electric current on the BODY- electric shock
Current (A)
Effect
0.001
can be felt
0.005
painful
0.010
involuntary muscle contractions (spasms)
0.015
0.070
loss of muscle control
if through the heart, serious disruption; probably
fatal if current lasts for more than 1 second
human body resistance varies:
100 ohms if soaked with salt water;
moist skin - 1000 ohms;
normal dry skin – 100 000 ohms,
extra dry skin – 500 000 ohms.
What would be the current in your body if you touch the
terminals of a 12-V battery with dry hands?
I = V/R = 12 V/100 000  = 0.000 12 A
quite harmless
But if your hands are moist and you touch 12 V battery,
how much current would you draw?
I = V/R = 12 V/1000  = 0.012 A
a dangerous amount of current.
Electric Power
What is power?
Power =
𝑊𝑜𝑟𝑘
𝑡𝑖𝑚𝑒
=
𝐸𝑛𝑒𝑟𝑔𝑦 𝑐ℎ𝑎𝑛𝑔𝑒
𝑡𝑖𝑚𝑒
Unit: Js-1 = W
Electric power is the rate at which electric energy is
converted into another form such as mechanical
energy, heat, or light.
What does a 60W light bulb mean?
It converts electrical energy into light/heat energy at a
rate of 60 J per second.
If we play around with the electric potential energy
equation, we can derive another form of the power
equation that is more useful in circuits:
Electric Power – We Do
• How much current is drawn by a 60 Watt light bulb
connected to a 120 V power line?
• What is the resistance of the bulb?
Electric Power – We Do
• How much current is drawn by a 60 Watt light bulb
connected to a 120 V power line?
P = 60 W = I V = I x 120
so I = 0.5 A
• What is the resistance of the bulb?
I = V/R
R = V/I = 120 V/0.5 A
R = 240 
Electric Power – You Do
1. Calculate the resistance and the current of a 1500-Watt
electric hair dryer plugged into a US household outlet
(120 V).
2. The sticker on a compact disc player says that it draws
288 mA of current when powered by a 9 Volt battery. What
is the power (in Watts) of the CD player?
Electric Power – You Do
1. Calculate the resistance and the current of a 1500-Watt
electric hair dryer plugged into a US household outlet
(120 V).
I = P / V = (1500 W) / (120 V) I = 12.5 Amp
R = V / I= (120 V) / (12.5 Amp) R = 9.6
2. The sticker on a compact disc player says that it draws
288 mA of current when powered by a 9 Volt battery. What
is the power (in Watts) of the CD player?
P = I • V = (0.288 A) • (9 V) P = 2.59 W
Paying for electricity
You pay for electricity by the Kilowatt-hour (kWh).
What is a kWh? Simply another unit for energy.
Paying for electricity
You pay for electricity by the Kilowatt-hour (kWh).
What is a kWh? Simply another unit for energy.
Physicists measure energy in joules, but utility companies
customarily charge energy in units of kilowatt-hours (kW h), where :
Kilowatt-hour (kWh) = 1000 W x 3600 s
1 kWh = 3.6 x 106 J
1W x 1s = 1J
Paying for electricity
You pay for electricity by the Kilowatt-hour (kWh).
What is a kWh? Simply another unit for energy.
Physicists measure energy in joules, but utility companies
customarily charge energy in units of kilowatt-hours (kW h), where :
Kilowatt-hour (kWh) = 1000 W x 3600 s
1W x 1s = 1J
1 kWh = 3.6 x 106 J
At a rate of 14 cents per kWh, how much does it cost to keep a 100 W light bulb
on for one day?
• energy (kWh) = power (kW) x time (h)
•
energy (kWh) = 0.1 kW x 24 h = 2.4 kWh
cost / day = 2.4 kWh x 14 cents/kWh = 33.6 ¢
 for one month that amounts to $ 10.1.
Paying for electricity – You do
A 9V battery costs $1.25. If placed in a cd player that draws a 300
mA current, it will last for 32 hours. What is the price per kWhr?
9𝑉(0.3𝐴)(32ℎ𝑜𝑢𝑟𝑠)
𝑊
𝑘𝑊
1000
= 0 .086 kWhr
$1.25/0.086 kWh = $14 /kWhr