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p. 458-464
11.3 Ohm’s LawRelating Current, Voltage, and Resistance
German physicist Georg Ohm (1787-1854) found relationship between potential difference & current.
He kept potential difference & current CONSTANT but varied the length of the conductor (changes resistance).
Ohm’s Experiment Observations: as the length of the
conductor increased, the current decreased.
Recall: increasing length of wire increases resistance (if resistance increases current decreases provided that voltage stays the same)
Findings: For certain materials, when you plot
a graph of the voltage versus the current, you get a straight – line relationship.
The slope of the straight line represents the resistance of the material.
The steeper the slope of the straight line, the greater the resistance
Relationships… The relationship written: R = resistance in ohms V = voltage OR potential difference
in volts (v) I = current in amperes (A) This Relationship is called Ohm’s Law which states: “as the potential difference across a load increases so does current”
Ohm’s relationship to calculate resistance of a load in a circuit…
Ohm’s equation rearranged to solve for current:
Ohm’s relationship rearranged to calculate voltage across a load…
Current through Loads in Series If you have a circuit with one load the total
Resistance of the circuit will be different than if you have two or more loads connected in series
Electrons have only one path to follow and with two or more loads they have more bumps to deal with along the way, and from this the current flowing through a circuit with two or more loads will be less than the current flowing through a circuit with one load.
A) Electrons leaving the battery will have two possible paths to follow and since each of those paths have the same load the current splits in two
What do you think will happen in figure 5 (b)?
The electrons will have three possible paths to follow and the current will split in three.
As a
result:
The total resistance in a parallel circuit will be less than if the loads were connected in series since the electrons that flow through one branch go through only one load
Since the electrons flow through one branch and go through one load than ALL the electric potential energy that the electron receives from the battery will be converted into light and heat
From this, the voltage drop across each parallel load will be the SAME as the voltage drop across the battery
Again, the brightness of each lamp in a series circuit decreases as you connect more lamps, let’s examine electric potential energy of the circuit to explain this. RECALL BELOW:
The battery converts chemical energy into electric potential energy.
This potential energy gets transferred to all the electrons that leave the battery and creates the current flow in the circuit
As electrons move through the circuit the electric potential energy gets converted to kinetic energy
Circuit having one load: When electron passes through the load the electric
potential energy that the electron received from battery gets converted into light and heat.
Once the electron completes one loop, it returns to the battery only to be given more electric potential energy which allows the electron to continue moving through the loop of the circuit over and over again
Since voltage is related to electric potential energy the voltage drop across the one load will be the SAME as the voltage across the battery
Circuits having two identical loads: An electron leaving the battery will have the same
amount of electric potential energy as if there were one load
Reason: the battery can only supply so much potential energy.
For electron to go through each lamp only half of the electric potential energy gets converted into light and heat.
Since only half gets converted though each lamp, the voltage drop across each lamp is half the voltage drop across the battery
The more the identical lamps connected in series means less of an electrons electric potential energy gets converted to heat and light.
The voltage drop across each load decrease
Vsource___________
An accidental low resistance connection between two points in a circuit, causing excess current (recall: decrease resistance-increase current)
Electrical device will not work and could be dangerous. Wire becomes hot and could start a fire (supplement p. 462)
If there is too much current flowing through a circuit, it will become overloaded.
This can be dangerous because it can cause the wires to heat up and start a fire.
A fuse prevents too much current from flowing through a circuit.
If the current is too high, the wire inside it melts.
This opens the circuit so no current can flow.
Once a fuse has melted, it needs to be replaced.
A fuse box holds all the fuses for all the circuits in a house or apartment.
Like a fuse, a circuit breaker stops electric current from flowing in an overloaded circuit.
It works like a switch.
If the circuit is overloaded, the circuit breaker trips, opening the circuit.
After the problem is fixed, the circuit breaker can be reset to close the circuit.
Unlike a fuse, a circuit breaker can be used over and over again.
A circuit panel has a circuit breaker for each circuit in a house or building.
3 different kinds of connections
1. Hot or “live” wires – carry current to the appliance- red or black
2. Neutral wire - carries current away from the load
- white
3. Ground wire - carries unsafe current to the ground
- green or bare copper wire
Connects to the ground.
It gets rid of electric current by letting it flow to the Earth.
It is a safety feature that ensures unsafe current flows to the ground rather than through you.
Appliances used near water are a danger because water conducts electricity.
Parts of your home that are close to water often have a GFI outlet.
A sensor measures the current flowing in the hot wire and the neutral wire.
If there is a difference in current, the circuit breaker opens the circuit.
If you push the “test” button, the power to the outlet is cut.
The “reset” button restores the power.
Homework: Read 11.3 and supplement your notes Complete p. 460-461 ALL practice
problems (#1-3 x 3boxes) p. 467 #1,3,7-10,12
Chapter 11 review p. 470-471 #1-4,6,8-10,13
videoshttp://www.youtube.com/watch?v=5laTkjINHrg ELECTRIC CURRENThttp://www.youtube.com/watch?v=YGvu9iqjJq4&feature=related RESISTANCEhttp://www.youtube.com/watch?v=F1p3fgbDnkY&feature=related VOLTAGE
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