Positive Į indicates resistivity increases
with temperature
Negative Į indicates resistivity
decreases with temperatureȡ 2 = ȡ 1 [1 + Į(T 2 íT 1 )]
R 2 = R 1 [1 + Į(T 2 íT 1 )]
ȡ = resistivity
Į = temperature coeff. of resistivity
T = temperature
R = resistance
The hotplate contains a
nichrome wire with a resistance
of 15 at 25°C. What is its
resistance at 375°C? For this
nichrome alloy, Į = 4.0×10í^4 °Cí^1
at 25°C.
R 2 = R 1 [1 + Į(T 2 íT 1 )]
R 2 = R 1 [1 + (4.0×10í^4 °Cí^1 )(T 2 íT 1 )]
R 2 = 15 [1 + (4.0×10í^4 )(350)]
R 2 = 17
25.7 - Electric power
Power is defined as work per unit time. Power also can be stated as the amount of energy transfer per unit time, and this is often a useful
formulation when considering electric circuits.
Electrical devices are often rated on how much energy they use per second. A 100-watt light bulb requires an average of 100 joules every
second. As it operates, the light bulb’s filament warms up and emits heat. This flow of heat from a resistor is called joule heating.
Power can also be calculated as the product of current flowing through a resistor and the potential difference across it. This is the equation in
the first line in Equation 1. Ohm’s law can be used to then restate this equation in terms of resistance and current, or in terms of potential
difference and resistance. We show these equations in the second line in Equation 1.
In the example problem, we simplify things by using an average or constant value for current and potential difference í likely this burner is
powered with an alternating current where these values vary. Alternating current (AC) is a topic later in the textbook.