 # Electric Current. Answer Me!!!  Why are electric wires made from metal?

## Presentation on theme: "Electric Current. Answer Me!!!  Why are electric wires made from metal?"— Presentation transcript:

Electric Current

Conductivity  Metals are good conductors of electricity. This is because metals have “free” electrons that can easily flow through a metal.  Nonmetals are poor conductors of electricity, because electrons cannot move easily through them.

Periodic Table of Elements

Electric Current  Electric Current is the number of charges that pass through a point in one second.  Current is determined by the following equation:  I = current  q = number of charge  t = time  If 1 Coulomb of charge passes through a point in 1 second, a current of 1 Ampere (A) is said to exist.

Practice Problem 1  Two coulombs of charge pass a point in four seconds. How much current (I) flows through the conductor?

Practice Problem 2  If 10 C of charge are transferred through an electric circuit in 5.0 seconds, what is the current in the circuit?

Practice Problem 3  An operating lamp draws a current of 0.50 A. What is the amount of charge passing through the lamp in 10 seconds?

Practice Problem 4  In a lightning flash, a charge of 10 C was transferred from the base of a cloud to the ground. The current was 1.0 x 10 4 A. Find the amount of time it took for the flash to travel.

Final Thought…  The current traveling from the cathode to the screen in a television picture tube is 5.0 x 10 -5 A. How many electrons strike the screen in 5 seconds?

Answer Me!!!  What are 3 examples of good conductors of electricity?

Potential Difference  Potential difference pushes charges and causes them to flow.  A battery can be used to create a potential difference.  Potential difference is measured in volts (V) by a voltmeter.

Current Flow  There are two ways to describe current Conventional Current: Positive charge flows to negative. (Think electric field lines) Electron flow: Electrons flow from negative to positive.

Resistance  Resistance is the opposition to the flow of charges.  The resistance of an object depends on characteristics of that object and the environment.  Resistance is measured in Ohms ()

Ohm’s Law  Although resistance is a property of the material current is passing through, potential difference and current are related to resistance by the following equation:  I = current measured in amperes  R = resistance measured in ohms  V = potential difference measured in volts

Practice Problem 6  In a simple electric circuit, a 110 V electric heater draws 2.0 A of current. What is the resistance of the heater?

Practice Problem 7  How much current flows through a 12 flashlight bulb running at 3 V?

Practice Problem 8  What is the potential difference across a 2 resistor that draws 2 C of charge per second?

Practice Problem 9  A metallic conductor obeys Ohm’s Law. Draw a graph that represents the relationship between potential difference (V) across the conductor and the resulting current (I) through the conductor. V I

Answer Me!!!  How could you change the resistance of a wire without changing the material it is made from?

Resistivity  Resistivity ()is a measure of a material’s ability to resist the flow of electrical current.  Resistivity is measured in m and common materials are found on your Reference Tables

What determines resistance?  Resistance (R) of a conductor is affected by its length (L), cross-sectional area (A), and resistivity ()

Temperature  As the temperature of a material (most materials) is increased, the resistivity of that material will increase.

Practice Problem 10  A copper wire is connected across a constant voltage source. The current flowing in the wire can be increased by increasing the wire’s Cross-sectional Area Length Resistance Temperature

Practice Problem 11  The diagram below shows a circuit in which a copper wire connects points A and B. What can be done to decrease the electrical resistance between A and B?.A.A.B.B

Practice Problem 12  A copper wire is part of a complete circuit through which current flows. Draw a graph that represents the relationship between the wire’s length and its resistance. Resistance Length

Practice Problem 13  Several pieces of copper wire, all having the same length but different diameters, are kept at room temperature. Draw a graph that represents the relationship between resistance and cross-sectional area. Resistance Area

Practice Problem 14  A 1.0 m length of nichrome wire with a cross-sectional area of 7.85 x 10 -7 m 2 is connected to a 1.5 V battery. Calculate the resistance of the wire.