Resistance (R) is a measure of the tendency of something to resist the flow of charge. The resistance of a particular resistor is related to the length (L) and cross-sectional area (A) of the particular resistor, and the resistivity (ρ) of the material of which it is made:

R = ρL/A (units: for R units are Ohms or Ω, for L units are m, for A use m² and for ρ use Ω-m)

Power (P) is defined in general as the time (t) rate of change of the energy (E) by

P=ΔE/Δt

For electric circuits then this becomes

P=IV

Heat revisited: When a mass (m) of material (with specific heat capacity c) absorbs heat energy (Q) it changes temperature (T) according to the formula

Q=mcΔT

For water, c is 4186 J/kgC˚

Procedure

Find a piece of nichrome wire and measure its length and diameter. Use these to find the resistance of the wire. (Resistivity of nichrome is 1.0x10^-6).
Coil the wire up so it will fit into a styrofoam cup without touching the sides and the ends of the wire are able to be attached to the leads from the power supply.
Place an empty Styrofoam cup on the scale and set the scale to zero. Add water until it is about two thirds full and note the mass. (What does this represent?)
Hook the power supply leads across the nichrome and place it in the water. DO NOT TURN IT ON YET.
Use a thermometer to measure the initial temperature of the water. Set the power supply to 12V, plug it in and start the timer.
Once the water has increased in temperature by 10 to 15 degrees note the time and unplug the power.
Calculate the current drawn by the wire and the power it consumes from the voltage (12V) and the resistance calculated in step 1.
Calculate the heat absorbed by the water using the heat capacity formula. Use the formula for power as a function of energy change and time to get the power for the heat absorbed by water. Compare this to the result from step 7.