Microscopic view of current
Current:
Current and voltage measurements
Unit ampere: 1 (A) =
Ideal voltmeter:
Ideal ammeter (current meter):
Fig. 17-2c, p.570
Microscopic view
of current
I
= nqvd
A
Fig. 17-4, p.570
Drift velocities, large or small?
E in a metal?
is the metal charged?
Fig. 17-5a, p.572
How can we change resistance?
What do we observe?
If the wires are good conductors:
If we add another battery:
Water “circuit” analogy
Ohm’s law
Resistance
Temperature and resistance
Fig. 17-6a, p.573
1 A battery is
connected in
three ways.
Which way(s)
light(s) up the
bulb (deliver(s)
power)?
A. 1
B. 2
C. 3
D. 1,2
E. 1,3
Fig. 17-6b, p.573
Fig. 17-6c, p.573
How much energy does each electron lose in the
resistor?
A high-voltage transmission line carries 1000A at 700
kV above ground voltage for a distance of 100 km. If
the resistance in the wire is 1 Ω/km (each way)
How much energy per second is lost by all of
them?
What is the voltage drop by the time it has traveled 100
miles?
Power
How much power is delivered to the load (a city)?
P = I ∆V = I
2
( ∆V )
R=
2
R
How much power is lost in heating the transmission
line?
Note: Pbattery = I ∆Vbattery
Energy consumed
If the transmission line were at 100V above ground,
what would the transmission loss be for the same
power plant output?
1 kWh = 3.60 × 106 J
P2. One resistor is 4 ohms. A second resistor is made of the
same material, but is twice as long and has four times the
conducting area, so it has a resistance of ___________ ohms
A. 1
B. 2
C. 4
D. 8
E. 16
A light bulb is connected across a power source with voltage
difference of 100 V. The bulb consumes 200 W of power.
P3. The current is _____ A
A. 2
B. 50
C. 10
D. 200
E. 500
P4. The resistance of the wire is _____ ohms
A. 2
B. 50
C. 10
D. 200
E. 500
P7. Charge flows through a light bulb because of a
battery. Suppose a wire which has a tiny R is
connected across the bulb as shown, which has a large
R.
When the wire is connected,
A. essentially all the charge continues to flow through
the bulb.
B. half the charge flows through the wire, the other half
continues through the bulb.
C. essentially all the charge flows through the wire.
Hint: think about ohm’s law
Two resistors are connected to a battery. The power
consumed in each battery is shown.
P5. The battery with the
greatest resistance is
A. top
B. bottom
C. same
P6. The battery with the
greatest current is
A. top
B. bottom
C. same