an electric heater uses 3/5 watts of power each hour. how many watts of power will the heater use in 1/2 hour?

Answer 1

3/10th of course. 3/5 divided by two.

However I suspect you might want to re-read the original question as heaters typically take many THOUSAND times this amount of power - as in 2000 - 3000 watts not 3/5 ths of a watt!

Mark


edit: doug, I do follow the difference between watt hours and watts. However to say that question was extremely misleading in the extreme is understating it. Heh, if he had been talking about an indcutive load and given me the PF I could have told him the difference in KW and KVA - or if preferred KWH KVAH

Trying to make some sens out of "3/5 watts of power each hour" I figured, perhaps wrongly - as meaning 3/5 WH. In which case it will only consume 3/10 WH in 30 mins.

Really not quite sure why I even bothered adding this now :(

Answer 2

First get a sound understanding of units. Power (watts) is energy/second, so "uses 3/5 watts of power each hour" isn't a sound statement. You can say "a 3/5 watt heater runs for an hour". In fact your power company probably charges for energy in units of kilowatt hours (amount of energy consumed in an hour by a device that uses 1000 watts)
Another less significant error is that 3/5 watt is an extremely small rating for a heater; more likely it was 3/5 kilo watt.

Similarly, your question, "how many watts of power will the heater use in 1/2 hour" is also flawed.

Anyway, the energy consumed in a half hour would be half the energy consumed in an hour, if the power is equal in both cases and is the same throughout the duration

Answer 3

The heater uses power at the rate of 3/5 watt no matter how long it's on. A watt is a measure of how rapidly energy is used and is equal to 1 J/s. So in 1800 seconds (1/2 hour) it will use
(3/5)*1800 = 1080 J.

Doug
EDIT: Neither you -nor- Mark T seem to know the difference between energy and power.