Transcript Physics Section 17.3 Apply the properties of electric current

Physics Section 17.3
Apply the properties of electric current
Electric current is the rate at which charge flows through a
conductor. The charges can be either electrons or positive
ions.
I=Q
t
I = current(ampere A)
Q = charge (C)
t = time (s)
An ampere is equivalent to a coulomb of charge
passing through a cross-sectional area in one second.
1A = 1C/1s
http://videos.howstuffworks.com/hsw/18447-electricity-and-magnetism-electric-currentvideo.htm
example
The current passing through a light bulb is .75 A.
How long would it take a total charge of 1.5 C to pass
through the bulb?
example
The current in a circuit is 80.0 μA. How long does it
take 20.0 x 1016 electrons to pass a given point?
A potential difference (electric field) causes charges to
flow.
Conventional current is
defined as the direction
that positive charges
flow. Conventional
current flows opposite
the direction of electrons
in a conductor.
Drift velocity is the average velocity of an electron in
a conductor. It is only a few millimeters per second.
The velocity that the electric field is established in a
conductor is nearly the speed of light. (300,000,000
m/s)
Resistance is the opposition to electric current by a
material or a device.
R=V
I
R = resistance (ohm Ω)
V = potential difference (V)
I = current (ampere)
One ohm equals one volt per ampere.
Ohm’s Law: V = IR
For a constant resistance, current is directly
proportional to voltage. For a constant
voltage, current varies inversely as the
resistance.
http://phet.colorado.edu/en/simulation/battery-resistor-circuit
Materials with a constant resistance over a wide
range of potential differences are ohmic materials.
Materials whose resistance varies are nonohmic.
A diode is a semi-conducting nonohmic device. It has
a low resistance for current in one direction and a
high resistance for current in the opposite direction.
The resistance of a conductor varies directly as its
length and temperature and inversely as its crosssectional area. The structure of the atoms in a
material also affect the resistance.
Resistors are used to control the amount of current in a
conductor.
A rheostat is a device that acts as a variable resistor. It
can be used to vary the amount of current in a circuit.
The human body has resistance close to 500,000 ohms.
example
A light bulb has a resistance of 15 Ω. Find the
current in the circuit when it is connected to a
potential difference of 24 V.
example
The current in a radio is .50 A. Find the potential
difference across the circuit if the resistance of the
radio is 250 Ω.
Assignment
• Page 616
• problems 1 - 7