1. It should be as low as possible. It should be as high as possible.
To maximize the percentage of the power from the emf of a battery that is delivered to a device, what should the internal resistance of the battery be?
It should be as low as possible.
It should be as high as possible.
The percentage does not depend on the internal resistance.
Remarks for instructors: Answer (1). Power is delivered to the internal resistance of a battery, so decreasing the internal resistance decreases this “lost” power and increase the percentage of the power delivered to the device.

2. The reading does not change.
With the switch in the circuit of Figure 28.4a closed, there is no current in R2 because the current has an alternate zero-resistance path through the switch. There is current in R1, and this current is measured with the ammeter (a device for measuring current) at the bottom of the circuit. If the switch is opened (Fig. 28.4b), there is current in R2. What happens to the reading on the ammeter when the switch is opened?
Remarks for instructors: Answer (2). When the switch is opened, resistors R1 and R2 are in series, so the total circuit resistance is larger than when the switch was closed. As a result, the current decreases.
Figure 28.4
The reading goes up.
The reading goes down.
The reading does not change.

3. The reading does not change.
With the switch in the circuit of Figure 28.8a open, there is no current in R2. There is current in R1, however, and it is measured with the ammeter at the right side of the circuit. If the switch is closed (Fig. 28.8b), there is current in R2. What happens to the reading on the ammeter when the switch is closed?
Figure 28.8
Remarks for instructors: Answer (1). When the switch is closed, resistors R1 and R2 are in parallel, so the total circuit resistance is smaller than when the switch was open. As a result, the current increases.
The reading increases.
The reading decreases.
The reading does not change.

4. It increases It decreases It remains the same
In Active Figure 28.3, a third resistor is added in series with the first two. What happens to the current in the battery?
Figure 28.3
Remarks for instructors: Answer (2). Adding another series resistor increases the total resistance of the circuit and therefore reduces the current in the circuit.
It increases
It decreases
It remains the same

5. It increases It decreases It remains the same
In Active Figure 28.3, a third resistor is added in series with the first two. What happens to the terminal voltage of the battery?
Figure 28.3
Remarks for instructors: Answer (1). The potential difference across the battery terminals increases because the reduced current results in a smaller voltage decrease across the internal resistance.
It increases
It decreases
It remains the same

6. In Active Figure 28.5, a third resistor is added in parallel with the first two. What happens to the current in the battery?
Remarks for instructors: Answer (1). If a third resistor were connected in parallel, the total resistance of the circuit would decrease and the current in the battery would increase.
Figure 28.5
It increases
It decreases
It remains the same

7. In Active Figure 28.5, a third resistor is added in parallel with the first two. What happens to the terminal voltage of the battery?
Remarks for instructors: Answer (2). The potential difference across the terminals would decrease because the increased current results in a greater voltage drop across the internal resistance.
Figure 28.5
It increases
It decreases
It remains the same

8. impossible to determine
Consider the circuit in Figure and assume the battery has no internal resistance. Just after the switch is closed, what is the current in the battery?
Figure 28.18 .
impossible to determine
Remarks for instructors: Answer (3). Just after the switch is closed, there is no charge on the capacitor. Current exists in both branches of the circuit as the capacitor begins to charge, so the right half of the circuit is equivalent to two resistances R in parallel for an equivalent resistance of ½R.

9. impossible to determine
Consider the circuit in Figure and assume the battery has no internal resistance. After a very long time, what is the current in the battery?
Figure 28.18
Mr W Hint: Try continuing this from the previous problem (switch closed)
AND then try it with the switch open (as shown) .
impossible to determine
Remarks for instructors: Answer (4). After a long time, the capacitor is fully charged and the current in the right-hand branch drops to zero. Now, current exists only in a resistance R across the battery.