Physics help please!!?

Question

1) Using power = current X voltage, find the current drawn by a 1200 w hair dryer conected to 120 V. Answer in unitd of A.

2) A typical television dissipates 265 W when plugged into a 115 V outlet. Find the resistance of the television. Answer in units of ohms

Answer 1

1. For this one, you don't really need physics help; just a bit of elementary algebra.

If power = current * voltage, then dividing both sides by voltage will give

power / voltage = current.

If the voltage is in volts and the power in watts, the current will come out in amperes and will not need conversion. In this example,

1200 W / 120 V = 10 A

2. This one is a bit harder because you have to use two equations to get what you want and you must get the equation relating resistance from somewhere other than the problem itself:

Power (P) = Voltage (V) * Current (I)
Voltage (V) = Current (I) * Resistance (R)

By dividing both sides by I, we can get the equation

V / I = R

Unfortunately, we don't know what I is, but we have another equation from which we can get it. We have already done that derivation in part 1, where we found that

I = P / V

Rather than use I in the resistance equation, we can use the expression P / V instead, to get

V / (P / V) = R

Simplifying the left side gives us the new equation

V^2 / P = R

R will come out in ohms if V is in volts and P in watts, so no unit conversions will be necessary. Plugging in our values to the equation,

R = (115 V)^2 / 265 W
= 49.9 ohm

A couple of practical problems with example 2, by the way. The day has long passed when a typical television set dissipated as much as 265 W. Today's television sets are more likely to dissipate one third to one fourth that much power.

The other problem is modeling a television set as a resistance. A TV set's electrical load might have been reasonably modeled as a resistor in the days of vacuum tubes when most of the power was taken up to drive the tubes' heaters, but semiconductors have no such power requirement and the load presented by the power supply is likely to vary by quite a bit from a simple resistor. However, very new electronic devices may have power supplies deliberately designed to look a lot like resistors to the power supply specifically because resistors cause less power loss in the transmission lines than other kinds of loads for the power consumed.

Answer 2

1. Power=v*i
1200 Watts =120 volts * i amps
i=10 amps

2. Power = v*i and v=i*r so Power=v^2/r
265 W=(115 V)^2/ R
R=115^2/265 = 49.906 Ohms

Answer 3

P = IV so 1200 W = 120 V* I ---> I=1200/120 A = 10 A

P = V^2/R so R = V^2/P = (115)^2/256 = 51.67 Ohms

Answer 4

1...Volts x Amps = Watts.....Watts ÷ Volts = Amps
= 1200 ÷ 120 = 10 Amps.
2...265 ÷ 115 = 2.3 Amps.
R = Volts ÷ Amps = 115 ÷ 2.3 = 50 Ω