Understanding Ohm's Law and Electrical Resistance

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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
 
free electron
 
(actually
positive ions)
path
 
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Resistance Across a Circuit
Resistance Across a Circuit
 
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Resistance in Wires
Resistance in Wires
 
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 …
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
.
 
What is the relationship between resistance and
voltage drop?
Introducing ….
 
V is  potential difference across a resistor
I  is  current
R is resistance
Examples
Examples
 
If a 3 volt flashlight bulb has a resistance of 9 ohms, how
If a 3 volt flashlight bulb has a resistance of 9 ohms, how
much current will it draw?
much current will it draw?
 
 I = V / R =  3 V / 9 
 I = V / R =  3 V / 9 
  = 
  = 
1/3  Amps
1/3  Amps
 
If a light bulb draws 2 A of current when connected to a
If a light bulb draws 2 A of current when connected to a
120 volt circuit, what is the resistance of the light bulb?
120 volt circuit, what is the resistance of the light bulb?
 
 R = V / I = 120 V / 2 A = 
 R = V / I = 120 V / 2 A = 
60 
60 
Check for Understanding
Check for Understanding
Which of the following would cause the current in a circuit to
Which of the following would cause the current in a circuit to
decrease the most?
decrease the most?
1)
Increased voltage and increased resistance
Increased voltage and increased resistance
2)
Increased voltage and decreased resistance
Increased voltage and decreased resistance
3)
Decreased voltage and decreased resistance
Decreased voltage and decreased resistance
4)
Decreased voltage and increased resistance
Decreased voltage and increased resistance
Check for Understanding
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 …
Just for Fun …
Effects of electric current on the BODY- electric shock
Effects of electric current on the BODY- electric shock
human body resistance varies:
human body resistance varies:
100 ohms if soaked with salt water;
100 ohms if soaked with salt water;
moist skin - 1000 ohms;
moist skin - 1000 ohms;
normal dry skin – 100 000 ohms,
normal dry skin – 100 000 ohms,
extra dry skin – 500 000 ohms
extra dry skin – 500 000 ohms
.
.
 
What would be the current in your body if you touch the
What would be the current in your body if you touch the
terminals of a 12-V battery with dry hands?
terminals of a 12-V battery with dry hands?
   I = V/R = 12 V/100 000 
   I = V/R = 12 V/100 000 
 = 0.000 12 A    quite harmless
 = 0.000 12 A    quite harmless
 
But if your hands are moist and you touch 12 V battery,
But if your hands are moist and you touch 12 V battery,
how much current would you draw?
how much current would you draw?
   I = V/R = 12 V/1000 
   I = V/R = 12 V/1000 
 = 0.012 A
 = 0.012 A
                                        a dangerous amount of current.
                                        a dangerous amount of current.
Electric Power
Electric Power
 
Electric power is the rate at which 
Electric power is the rate at which 
electric energy is
electric energy is
converted into another form such as mechanical
converted into another form such as mechanical
energy, heat, or light.
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.
What is power?
 
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:
 
Unit: Js
-1
 = W
 
How much current is drawn by a 60 Watt light bulb
How much current is drawn by a 60 Watt light bulb
connected to a 120 V power line?
connected to a 120 V power line?
 
 
 
What is the resistance of the bulb?
What is the resistance of the bulb?
 
Electric Power – We Do
Electric Power – We Do
How much current is drawn by a 60 Watt light bulb
How much current is drawn by a 60 Watt light bulb
connected to a 120 V power line?
connected to a 120 V power line?
                   
                   
P = 60 W = I V = I x 120
P = 60 W = I V = I x 120
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What is the resistance of the bulb?
What is the resistance of the bulb?
     
     
I = V/R        R = V/I = 120 V/0.5 A
I = V/R        R = V/I = 120 V/0.5 A
   
   
        R = 240 
        R = 240 
 
Electric Power – We Do
Electric Power – We 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
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).
 
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Electric Power – You Do
Electric Power – You Do
 
Paying for electricity
Paying for electricity
 
You pay for electricity by the Kilowatt-hour (kWh).
You pay for electricity by the Kilowatt-hour (kWh).
What is a kWh?  Simply another unit for energy.
What is a kWh?  Simply another unit for energy.
 
 
Paying for electricity
Paying for electricity
 
You pay for electricity by the Kilowatt-hour (kWh).
You pay for electricity by the Kilowatt-hour (kWh).
What is a kWh?  Simply another unit for energy.
What is a kWh?  Simply another unit for energy.
 
 
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Kilowatt-hour (kWh) = 1000 W x 3600 s
 
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1 kWh = 3.6 x 10
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Paying for electricity
Paying for electricity
You pay for electricity by the Kilowatt-hour (kWh).
You pay for electricity by the Kilowatt-hour (kWh).
What is a kWh?  Simply another unit for energy.
What is a kWh?  Simply another unit for energy.
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Kilowatt-hour (kWh) = 1000 W x 3600 s
Kilowatt-hour (kWh) = 1000 W x 3600 s
1 kWh = 3.6 x 10
1 kWh = 3.6 x 10
6
6
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1W x 1s = 1J
1W x 1s = 1J
 
At a rate of 14 cents per kWh, how much does it cost to keep a 100 W light bulb
At a rate of 14 cents per kWh, how much does it cost to keep a 100 W light bulb
on for one day?
on for one day?
energy (kWh) = power (kW) x time (h)
energy (kWh) = power (kW) x time (h)
 
energy (kWh) = 0.1 kW x 24 h = 2.4 kWh
energy (kWh) = 0.1 kW x 24 h = 2.4 kWh
     cost / day = 2.4 kWh x 14 cents/kWh = 
     cost / day = 2.4 kWh x 14 cents/kWh = 
33.6 
33.6 
¢
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Paying for electricity – You do
Paying for electricity – You do
A 9V battery costs $1.25.  If placed in a cd player that draws a 300
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?
mA current, it will last for 32 hours.  What is the price per kWhr?
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Explore the principles of Ohm's Law and electrical resistance, understanding how resistance affects electron flow in circuits, the relationship between voltage drop and resistance, and practical examples of applying Ohm's Law in circuit calculations. Discover the impact of resistance in wires and how resistors influence circuit behavior. Test your knowledge with a check for understanding on factors influencing current flow in circuits.


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  1. Ohms Law

  2. 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. path 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 ( )

  3. 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).

  4. WIRE_VACUUM_SEAL 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

  5. What is the relationship between resistance and voltage drop? Introducing . Ohm s Law V = I R V is potential difference across a resistor I is current R is resistance

  6. 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

  7. 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

  8. 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.

  9. Just for Fun Effects of electric current on the BODY- electric shock Current (A) can be felt Effect 0.001 painful 0.005 involuntary muscle contractions (spasms) 0.010 loss of muscle control 0.015 if through the heart, serious disruption; probably fatal if current lasts for more than 1 second 0.070

  10. 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.

  11. Electric Power What is power? Power = ???? ???? = ?????? ? ???? 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: P = VI

  12. 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?

  13. 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

  14. 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?

  15. 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

  16. Paying for electricity You pay for electricity by the Kilowatt-hour (kWh). What is a kWh? Simply another unit for energy.

  17. 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

  18. 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.

  19. 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? 9?(0.3?)(32 ????) 1000? = 0 .086 kWhr ?? $1.25/0.086 kWh = $14 /kWhr

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