# Calculate Joules Used by 100W Light Bulb in 2 Hours

• DB
In summary, the conversation is about finding the amount of energy in Joules used by a 100 W light bulb in 2 hours. The formula W=J/s is used to solve the problem, with the power (P) and time (T) being represented by variables. The correct answer is 720000 Joules, but the person is seeking help as their teacher believes they may have used the wrong method. The speaker argues that their approach is better than their teacher's approach.
DB
How much energy in Joules is used by a 100 W light bulb in 2 hours?

$$W=\frac{J}{s}$$

$$100 W=\frac{x_{j}}{7200_s}$$

$$720000=Joules$$

My teacher said I probably got the right answer but I did the wrong work. I think I did so too. Can some1 help me out?

Thanks

DB said:
How much energy in Joules is used by a 100 W light bulb in 2 hours?

$$W=\frac{J}{s}$$

$$100 W=\frac{x_{j}}{7200_s}$$

$$720000=Joules$$

My teacher said I probably got the right answer but I did the wrong work. I think I did so too. Can some1 help me out?

Thanks
Your answer is "correct". However, it's better to work problems using equations with variables representing the various quantities.
In words:
{Power} = {Energy}/{Time}
Now let {P = Power (in J/sec)}, {E = Energy (in J)}, and {T = Time (in sec)}, so that:
P = E/T
::: ⇒ E = P*T

P = (100 Watts) = (100 J/sec)
T = (2 hours) = (7200 sec)
E = (100 J/sec)*(7200 sec) = (720000 Joules)

~~

Last edited:
DB said:
How much energy in Joules is used by a 100 W light bulb in 2 hours?

$$W=\frac{J}{s}$$

$$100 W=\frac{x_{j}}{7200_s}$$

$$720000=Joules$$

My teacher said I probably got the right answer but I did the wrong work. I think I did so too. Can some1 help me out?

I would argue that your approach is better than the teacher's approach; especially when using a Texas Instruments calculator. TI calculators have an equation solver, so you don't actually need to manipulate the formula. The less you screw with an equation, the less likely you are to make silly algebra errors.

Last edited:
Check out my method of writing formulas

http://myfiles.dyndns.org:8080/math/shawn_equation_style.jpg

Notice how nothing is moved around from the original formula. The value I'm trying to find isn't even isolated.
You're in a physics class, so do physics. Math is for the losers in the math class

Last edited by a moderator:

## 1. How do you calculate the energy used by a 100W light bulb in 2 hours?

The energy used by a 100W light bulb in 2 hours can be calculated by multiplying the power (100W) by the time (2 hours) in seconds. This gives us an answer of 200 joules (J).

## 2. What is the formula for calculating energy usage?

The formula for calculating energy usage is: Energy (J) = Power (W) x Time (s).

## 3. Can I use any unit of time to calculate energy usage?

Yes, as long as you convert the time into seconds before plugging it into the formula. For example, 2 hours would be converted to 7200 seconds (2 hours x 60 minutes/hour x 60 seconds/minute).

## 4. What if my light bulb has a different wattage?

If your light bulb has a different wattage, you can simply plug in the new power value into the formula. For example, if your light bulb is 60W and was used for 2 hours, the energy usage would be 120 joules (60W x 2 hours x 60 minutes/hour x 60 seconds/minute = 120J).

## 5. Is there a way to reduce the energy usage of my light bulb?

Yes, you can reduce the energy usage of your light bulb by using a lower wattage bulb or by turning off the light when it is not needed. You can also switch to energy-efficient LED bulbs, which use significantly less energy compared to traditional incandescent bulbs.

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