3/29/2015
Phys 208 - Recitation Currents - © J. Hedberg 2015
Electric Currents - Phys 208
1. Currents
1.1 A very simple circuit
Below is a very simple circuit consisting of just a battery and a metal wire.
a
a. Locate along the wire the approximate places were V = 3V, 2V, 1V, 0V.
b. Which direction would the electric field point at points a, b, and c? (up,
down, left, or right?). a and b are located within the wire, and c is in the
middle of the battery.
3V
+
c
0V
c. Describe how the potential changes as you move along the wire, from the
positive terminal of the batter to the negative terminal.
–
b
d. Which direction would an electron move in the circuit? CW or CCW?
e. Which direction is the conventional current moving?
f. Describe the kinematics of the electron as it moves.
g. How does the electron's electric potential energy change as it moves through this circuit?
h. What is the potential energy of the electron turned into as it moves around the circuit?
i. Make a plot of potential as a function of location around this simple circuit.
1.2 Two batteries now
Now, instead of 1 battery that generates 3 voltage potential difference, we have two 1.5 volt
batteries connected in different ways.
+
+
+
+
+
1.5V
1.5V
1.5V
–
–
–
1.5V
+
–
–
1.5V
1.5V
–
–
a. Make a plot of the potential as a function of position along this three circuits.
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3/29/2015
Phys 208 - Recitation Currents - © J. Hedberg 2015
b. Also, indicate the direction of the electron flow and the conventional current direction.
1.3 Odd shaped conductor
Here is a conductor made of two piece with different diameters,
through section A can be called .
and
. The current passing
a. In section B, is the current greater, less than, or equal to the current in section A?
b. Compare also, the current densities between the two parts.
c. Which side has the greater electric fields strength?
d. Which side has the greater electron drift speed?
1.4 Let's make some changes
If you double the current in a wire, what happens to the a) current density, b) the charge-carrier
density, c) the mean time between collisions, d) the drift speed of the electrons
1.5 Resistance and Temperatures
What do you think would happen to the resistance of a piece of wire it were heated up? Explain.
1.6 Two diameters
Two wires are made of the same metal, have the same length, but have different diameters (wire 1
is smaller than wire 2). If they are each exposed to the same electric fields strengths, how will the:
a. current densities compare?
b. currents compare?
c. electron drift speeds compare?
1.7 Two types of metal, one wire.
Here is a metal wire made up of two different types of metal. The left section has a conductivity
given by , and the right has a conductivity given by
which is greater than .
a. If a current
side?
is flowing on the left side, what will the current
be on the right
b. How will the current densities compare?
c. How will the electric fields in the wires compare?
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