SUMMARY EXERCISE
1. Explain what electrical quantity each of the two meters measures, and what units are
used for the measurement.
A voltmeter measures the electrical pressure difference, also called voltage or
voltage difference or potential difference, in units of volts. An ammeter
measures the flow rate of charge, also called rate of charge transfer, current or
amperage, in units of amperes.
2. In terms of experimental measurements, how is resistance defined? What units are used
for measuring resistance?
The resistance of a resistor is defined as the result of the following operations:
- connect the resistor to a battery
- use a voltmeter to measure the electrical pressure difference across the
resistor in volts
- use an ammeter to measure the flow rate of charge through the resistor in
amperes
- calculate the ratio of the voltage to the current, which is the resistance in
ohms.
Thus the amount of resistance indicates the amount of electrical pressure
difference needed to cause a unit flow rate of charge through the resistor.
3. A good measuring instrument should have as little interference as possible on the system
being measured. For both an ammeter and a voltmeter, describe:
a) whether the resistance of an ammeter and a voltmeter is high or low
b) how the meter should be connected in a circuit in relation to the circuit elements
c) why each meter needs to have its particular resistance to be a good measuring device
Ammeter – low resistance. Since charge must flow through the meter, we
want as little resistance to flow as possible so as not to change flow rate in the
circuit. The meter must be connected in series with circuit elements.
Voltmeter – high resistance. We want as little flow as possible through the
meter, so as not to affect the electrical pressure difference between the points
in the circuit. The meter must be connected in parallel with a circuit element.
4. Describe how the resistance of the X and Y resistors was affected by the different
voltages you used. Describe how the resistance of the long bulb was affected by the
different voltages you used. Compare the behavior of the two kinds of devices. Does each
type of device obey Ohm’s law?
The resistance of the X and Y resistors was not significantly affected by the
different voltages, and thus they obey Ohm’s law. The resistance of the long
bulb increases with increasing voltage, and thus the bulb does not obey Ohm’s
law.
PASCO scientific
Teachers Resource Guide
T-242
Voltage
Voltage
Voltage
5. For a circuit containing a battery, two bulbs and a capacitor in series, sketch graphs
describing how the voltage across each element and the current through each element will
behave during the charging of the capacitor.
Time
Time
Capacitor
Battery
Light Bulbs
Current
Current
Current
Time
Time
Time
Time
Capacitor
Battery
Light Bulbs
Be sure to hold class discussion around these diagrams.
Capacitor: the voltage increases with time, approaching a maximum value
(equal to the battery) and the current decreases with time, approaching zero.
Battery: the voltage remains constant; current is high the moment the battery
is connected to the circuit, and decreases with time, approaching zero.
Bulbs: the voltage difference across each of two bulbs is approximately half
that across the battery; the current begins high and decreases with time.
6. Consider the equivalent resistance of each of the following combinations of bulbs. List
each in order from the least resistance to the greatest resistance.
Then devise a method and build circuits to check your answers.
PASCO scientific
Teachers Resource Guide
T-243
C < D < A < B Students could construct a circuit with one long bulb in series
with each of these combinations to observe the changing brightness of the
‘test’ bulb. Or connect each of these in series with an ammeter and a battery
to determine the rate of flow; the larger the current, the less the resistance.
R
R
A
R
L
B
R
R
R
R
C
R
L
R
R
D
7. Power is the rate of energy transfer, so which household bulb would be brighter: a 100watt or a 60-watt bulb?
The 100-watt bulb has a greater power rating, so it is transferring energy at a
greater rate; thus it is the brightest.
8. Which bulb (a 100-watt or a 60-watt) has the largest resistance? Use the relationship
P = ∆V • I in your explanation.
Each household bulb is designed to operate in a circuit with the same voltage,
120 volts. So the ∆V is the same for both. The one with the highest power
rating must have the greatest current flowing through it:
P = ∆V • I for the 60-watt bulb
P = ∆V • I
for the 100-watt bulb.
The one with the greatest current through it (100-watt) has the least resistance.
9. Compare a rechargeable battery and a capacitor:
(a) In what ways are they similar?
(b) In what ways are they different?
(a) Similar:
1. Both store energy when charge is pushed in at one terminal and out of the
other.
2. Both behave like a pump that can push charge around the circuit.
(b) Different:
1. A chemical reaction occurs in a battery when it is pushing charge through,
but no reaction occurs in a capacitor.
2. The pressure difference across a capacitor changes dramatically across a
capacitor during charging/discharging, but the change of ∆V across a battery
is very slight unless it is shorted.
PASCO scientific
Teachers Resource Guide
T-244