ohm’s law –pg 681 electric current, voltage and resistance
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Ohm’s Law –Pg 681
Electric Current, Voltage and Resistance
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Electric Current
Is the flow of charge.
In solid conductors, electrons carry the charge through the circuit.
Electric current is measured in AMPERES (A).
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Net charge of a wire
The net charge of a wire is ZERO.
Number of electrons in the wire equal the number of protons in the atomic nuclei.
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Voltage Sources
Something that provides a potential difference is called a voltage source. Ex: Batteries n generators.
Provides an “electric pressure” for the electrons to move between the terminals.
In U.S, plugs have a “pressure” of 120 V.
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Electric Resistance Current depends on the resistance the
conductor offers to the flow of charge – Electric Resistance.
Resistance depends on the conductivity of the material (how well it conducts), its thickness and its length.
Thick wires have less resistance and longer wires have more resistance.
Electric resistance is measured in OHMS (Ω).
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Ohm’s Law
Gives the relationship between voltage, current and resistance.
States that The current in a circuit is directly
proportionate to the voltage across the circuit and inversely proportionate to the resistance of the circuit.
Current (I)= Voltage (V) Resistance (R) I = V/R
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Current in a circuit
1. If the voltage is doubled, while the resistance is the same, the current _________.
2. If the voltage is the same, while the resistance is doubled, the current _________.
So to decrease current, we have to ______ resistance.
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Problems on Ohm’s Law
Calculate the current in a toaster that has a heating element of 14 ohms when connected to a 120-V outlet.
Given: V= 120V; R = 14 Ohms U: I=? Eq: I = V/R Subst: I = 120/14
= 8.6 A
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How much current moves through your fingers of resistance 1200Ω, if you touch them to the terminals of a 6V battery?
Given: R= 1200Ω; V= 6V Unknown: I=? Eq: I = V/R Subst: I = 6/1200 = 0.005 A
Problems on Ohm’s Law
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Causes of Electric Shock
More current flows through you if your body has less resistance.
Your body has less resistance if you are wet, so you are more likely to get a severe shock.
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High-Voltage Wires
Birds do not receive a shock as they are at the same potential as the wire.
If they touch the ground (or another wire) while touching the wire, they will get a shock.
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Rescuing shock victims
First clear them of the electric power supply wires with a wooden stick (any non conductor) so that you don’t get electrocuted yourself !!!
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Do Now
Turn to Pg 696 and answer Q 5, 10, 11, 12, 13, 14, 15, 16, 17, 34, 35