Electric Current
Answer Me!!!

Why are electric wires made from
metal?
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:
q
I
t




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 104 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
(W)
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:
V
R
I



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
12W flashlight bulb running at 3 V?
Practice Problem 8

What is the potential difference
across a 2W 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 (r)is a measure of a
material’s ability to resist the flow of
electrical current.
Resistivity is measured in Wm 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 (r)
R
rL
A
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.
.A
.B
What can be done to decrease the electrical
resistance between A and 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 m2 is connected to a 1.5 V
battery. Calculate the resistance of
the wire.