Ohm’s Law –Pg 681

Electric Current, Voltage and Resistance

Electric Current

Is the flow of charge.

In solid conductors, electrons carry the charge through the circuit.

Electric current is measured in AMPERES (A).

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.

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.

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).

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

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.

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

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

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.

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.

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 !!!

Do Now

Turn to Pg 696 and answer Q 5, 10, 11, 12, 13, 14, 15, 16, 17, 34, 35