Goals:

APES lab: Electrical heating

Goals:

1. Learn how to use the VOM meter (yellow) to measure voltage, current and resistance
2. Learn how to measure Voltage, Current, Power and energy using a kill-a-Watt meter
3. Learn how to calculate power from Volts x Amps
4. Learn how to calculate power from current ^2 x resistance
5. Learn how to calculate energy used from power x time
6. Learn how calculate the electrical energy needed to heat water

VOM meter

This is a VOM (Volt-Ohm-Milliammeter) meter:

1. Measure your human resistance using the Ω setting, record this.
2. Measure the resistance of several humans, record this.
3. Measure the resistance of your hot water heater with the plug out, record.
4. Measure the voltage of several batteries, record these
5. Using the AC voltage setting (~) measure the voltage at the outlet, record.
6. You may have access to a meter with current, save this step for later, but identify the setting and how you must change the leads
7. Why does your resistance fluctuate?
8. How is this used as a lie detector?

Kill-a-Watt meter:

This is a Kill-a-Watt meter:

Note the buttons in order, left to right:

Voltage, Current, Power, frequency, energy

1. Plug the unit into the wall, note the voltage. How is this different from what you got in part one? Record.
2. Plug in a water heater device (tea maker) and measure current with the unit on and off. Record.
3. Measure the power, record.
4. There are two others you may try: VA and PF. Record these for later.
5. The last purple button measures kWh, but it works on a start-stop basis. More on this later.

Calculating power

1. While your water is heating, measure current and voltage. Calculate power from voltage x current
2. Using your value for resistance for your water heater, calculate power using current ^2 x resistance. Record and compare with your answer in #1 above

Calculating energy:

1. Fill your water heater with 1000 ml of cool water, measure the temperature.
2. Plug your heater into the Kill-a-Watt meter, start your timer and the kWh button.
3. Time how long it takes to bring the water to a boil
4. Calculate energy by power x time, using time in hours and power in kW
5. If electricity costs $0.40/kWh, how much did this cost?

Heating water:

1. Use the calculator or online to convert kWh into calories, record.
2. Using the formula below, calculate calories:

Q = mc∆t

Q = heat in calories (note: small c, Calories are food calories = 1 kcal)

m = mass in grams (same as ml for water)

c = specific heat of water (ability to hold heat)

∆t = change in temperature (degrees C)

1. Are these two values for heat the same?
2. Why or why not?
3. 4.18 joules = 1 calorie, so how many joules did you put into the water?
4. If 1 Watt = 1 Joule/second, use your time to compare electrical power with heat power.

Lab question:

Your 4500 W home hot water heater has a capacity of 50 gallons or 200 liters.

Water comes in at 20°C and is heated to 70°C

1. How many grams is this?
2. How many calories will this take?
3. How long will this take?
4. How much will it cost at $0.40/kWh?
5. How would solar hot water panels change this?