All High School Physics Resources
Example Questions
Example Question #1 : Power
How many 75 W light bulbs connected to 120V can be used without blowing a 15A fuse?
32
20
18
12
24
24
Known
Power = Current x Voltage
There are in the circuit. We can then divide this number by to determine how many lightbulbs can go into the circuit.
lightbulbs
Example Question #1 : Power
A person accidentally leaves a car with the lights on. If each of the two headlights uses 40 W and each of the two tail lights 6 W, for a total of 92 W, how long will a fresh 12V battery last if it is rated at 75Ah? Assume the full 12V appears across each bulb.
14.7 hr
6.3hr
9.7hr
7.6hr
9.7hr
First, we need to determine how much current is used by the system.
If the battery is rated at 75Ah we can divide this number by the current needed by these devices to determine how long the battery would run.
Example Question #1 : Power
You buy a 75W light bulb in Europe, where electricity is delivered at 240V. If you use the bulb in the United States at 120V (assume its resistance does not change) how bright will it be relative to 75W 120V bulbs?
The bulb is brightest in Europe
The bulb is brightest in the United States
The bulb has the same brightness in both locations
The bulb is brightest in Europe
First, let us determine the amount of current in the bulb while it is in Europe.
We can use this information to determine the resistance of the bulb.
Now let us determine the current going through the bulb when it is in the United States assuming that the resistance is constant.
When we compare these two current values we can see that the bulb has a higher current in Europe and a lower current in the United States. This means that the bulb will be dimmer.