Ohm’s Law

Review of last Class  Electrical Resistance  Resistance of some basic household appliances  Typical Lamp cord ~ << 1 ohm  Typical Lightbulb ~ 100 ohms  Iron ~ 15 to 20 ohms  Electric toaster ~ 15 to 20 ohms  Hairdryer ~ 15 to 20 ohms

Review of last Class

Ohm’s Law  Is the relationship between voltage, current, and resistance.  Ohm’s Law states  That the current in a circuit is directly proportional to the voltage impressed across the circuit, and inversely proportional to the resistance of the circuit  Current = Voltage / Resistance -> I = V/R

Ohm’s Law  For a circuit of constant resistance, current and voltage are proportional.  What does this mean?  You will get twice the current through a circuit for twice the voltage across the circuit.  I.E. – the greater the voltage, the greater the current.  What happens if the resistance is doubled? (Remember that the relationship between current and resistance is inversely proportional)  If the resistance is doubled for a circuit, the current will be half of what it would be otherwise.  I.E. – the greater the resistance, the less the current.

Ohm’s Law  Example Problems  Consider a potential difference of 1 volt impressed across a circuit that has a resistance of 1 ohm. What is the current produced in this circuit?  I = V/R  I = 1Volts / 1 Ohm  I = 1 V/Ω or 1 Ampere  Consider a voltage of 12 volts impressed across the same circuit. What will the measure of the current?  I = V/R  I = 12V / 1Ω  I = 12 V/Ω or 12 A

Ohm’s Law  Interactive Class Question:  How much current is drawn by a lamp that has a resistance of 100 Ohms when a voltage of 50 volts is impressed across it?  Answer:  I = V/R  I = 50 V/ 100 Ω  I = ½ A

Ohm’s Law and Electric Shock  What causes electrical shock in the human body?  Current or Voltage?  The damaging effects of electric shock are the result of current passing through the body.  Knowing this what do we see from Ohm’s Law?  Current depends on the voltage applied and also on the electrical resistance of the human body.

Ohm’s Law and Electrical Shock  What is the resistance of the human body?  Electrical resistance of the human body depends on condition and range  If you are soaked with salt water ~ 100 ohms  If your skin is very dry ~ 500,000 ohms  Perspective:  If you touch the ends (terminals) of a 12 V battery with your fingers you would normally not feel anything.  Your body usually provides a resistance of 100,000 ohms  If you touch the terminals of a 24 V battery with your fingers you would barely feel a tingle

Ohm’s Law and Electrical Shock Current (Amperes)Effect 1 mACan be felt 5 mAPainful 10 mAInvoluntary muscle contractions 15 mALoss of muscle control 70 mAIf through the heart, serious disruption can occur, probably fatal if the current lasts for more than 1 second.

High Voltage Wires  Why can these birds perch on these wires that carry such high voltage?

High Voltage Wires  Answer:  Every part of the birds body is at the same high potential, therefore it does not feel any effects.  Same goes for the person in the picture to the right.

High Voltage Lines  Suppose you fall off a bridge and are able to grab onto a near by High Voltage Power Line as you are falling. You grab onto the line with both hands. What happens to you??

High Voltage Lines  Why do you not get shocked?  Because there is not a potential difference.  As long as you touch nothing else, you stay at the high potential difference as the wires  However, if you reach over and grab onto another wire of different potential with one of your hands you will get shocked – ZAP!

Ground Wires  Mild shocks occur when surfaces of appliances are at an electrical potential different from that of the surfaces of other nearby objects.  If you touch surfaces of different potentials, you get shocked. Why?  Because you are a pathway for the current to travel  To prevent this problem, ground wires are used.  Ground wires connect the outside of the object to a third prong of a three wire electric plug. This third prong is then tied to a grounding rod through the circuitry of the building.