What is the strategy?
- Use the equation for power as a function of current and voltage to find the current through the hair dryer.
- Use Ohm’s law to find the hair dryer’s resistance.
Physics principles and equations
Power in an electric circuit
P = IǻV
Ohm’s law
ǻV = IR
Step-by-step solution
We use the power equation to find the current through the hair dryer.
Now we can use the current to find the resistance of the hair dryer.
Step Reason
1. P = IǻV power equation
2. solve for I
3. I = (1100 W)/(120 V) = 9.2 A evaluate
Step Reason
4. ǻV = IR Ohm’s law
5. solve for R
6. evaluate
25.11 - Sample problem: power transmission
Variables
What is the strategy?
- Find the current in the wire using the equation for power in terms of the potential difference and the current.
- Calculate the power dissipated by the power lines using the equation for power dissipated by a resistor.
A power plant provides 450 MW of
power through transmission lines at a
potential difference of 500 kV. What
current flows through the lines?
For a city 300 km away from the
power plant the total resistance in the
transmission lines is 60 ohms. What
fraction of the generated power is
dissipated as “line loss”?
If the generated power is transmitted
at a potential difference of 175 kV
instead, what fraction is dissipated?
generated power P = 450×10 (^6) W
transmission line potential difference ǻV
resistance R = 60
dissipated power PD