Physci text 13 Electricity

13 Electricity

Voltage, Current, Power, and Resistance

If one thinks of standing under a waterfall, there are two factors that contribute to the force of the water. The first is how far the water falls - how high is the waterfall. The second is how much water is falling - the amount of water. The higher the waterfall, the greater the force on a person standing underneath. The greater the amount of water, the greater the force on a person standing underneath. The height of a waterfall is called "the head" and the amount of water flowing is called the "current." Similarly, electricity flowing in a circuit is characterized by two factors: voltage and current. Similar to head, the voltage is a measure of the electromotive "pressure" in the circuit. The amount of electricity flowing is called the "current." These are just "analogies" and they are not exactly comparable, but the factors for a stream can help you think about electricity.

Voltage: The voltage is the amount of electrical "pressure" in a circuit. Voltage is measured in volts using a voltmeter. The letter for voltage is a capital letter V. This can be confusing because the variable for Voltage is capital letter V and the units for voltage is also a capital letter V.

Current: The current is the amount of electricity flowing in a circuit per unit time. Current is measured in amperes (amps) using an ammeter. The letter for current is the lower case letter i.

Power: Power is the amount of electrical energy consumed per second. Power can be calculated from the relationship P = iV where P is the power in Watts, i is the current in amperes, and V is the voltage in Volts.

Resistance: The resistance refers to the resistance to the flow of electricity in a circuit. Resistance is defined by the drop in voltage as a current flows through a load. A load can be a light bulb or any other electrical appliance. Loads include coffee pots, fans, rice cookers, ovens, refrigerators, cell phone speakers, microwave ovens, televisions - anything powered by electricity. The letter for resistance is a capital letter R. Resistance is calculated by using Ohms law: V = iR. The units for resistance, technically volts per ampere, are called Ohms and use a capital Greek letter omega: Ω

The symbol for resistance in a circuit diagram is a repeated "w" shape.

For students who might later work for a local power utility they will be introduced to a graphical variation of the equations above. 

Electrical company workers learn that the vertical line is a multiplication line. The horizontal line can be a division line or equals sign. All of the relationships can be derived graphically from these two diagrams. P = iV while i = P ÷V.

A coffee maker uses 900 Watts according to the plate on the bottom of the coffee maker. The coffee maker is a standard 120 volt appliance. Wall outlets supply 110 to 120 volts. Using Ohms law and the power relationship above we can calculate the resistance and the current. To calculate the resistance we would need the current. The power relationship above can give us the current. P = iV or i = P ÷ V. 900 ÷ 120 = 7.5 amps. Using the current i = 7.5 amps, the resistance R = V ÷ i = 120/7.5 = 16 Ω

A wall outlet is typically designed to handle no more than 18 amps. One can add the amperages for appliances and if the total exceeds 18 amps, the circuit is overloaded and can overheat. In theory a circuit breaker should trip (open and disconnect the circuit) if the amperage exceeds 20 amps on a single phase household circuit. If the breaker does not trip, then there is a risk of fire.

Cash power is used across Micronesia. The units on the cash power meter are kilowatt hours: one hour of 1000 Watts of power. Or two hours of 500 Watts. Or ten hours of 100 Watts. Power is money. What uses the most power in a house? A slides presentation includes photos of many appliances with their electrical ratings. 

As of mid-2024 Pohnpei Utility Corporation supplied roughly 7.5 megawatts of power to the island. Chuuk Power and Utility Company supplied roughly 2 megawatts for Weno. Kosrae Utility Authority may be less than a megawatt - the villages are certainly less than a megawatt but freezers at the Okat operation pull a large and not known amount of power. KUA has apparently asked the company to work on providing their own power via generators. 

Laboratory 13: Ohm's law

Ohm's law indicates that the voltage V is equal to the current i times the resistance R for a circuit. 

In a schematic diagram a battery or batteries is marked with an E for "source of Electromotive force."

R refers to Resistance. When working in Desmos be aware that the "Statistics" r and R² are not resistance, those are statistical measures of how well the data fits to the regression equation.

In this laboratory the voltage is measured across the resistance, also known as the load, while the current is measured "inside" the circuit. 

In laboratory thirteen an Ohm's law kit is used to determine the resistance in Ohms of a resistor. 

The kits include a multimeter. The number of batteries on the circuit is changed to produce different currents and voltages. The current versus the voltage yields the resistance as the slope. 

The ammeter on the left is measuring 14.5 milliamps inside the circuit while the voltmeter on the left is measuring 1.43 volts across the resistor for a measured resistance of 98.6 Ω. Resistor is rated at 100 Ω ±5%

If there is enough equipment, groups use pairs of multimeters, one for the current, one for the voltage. Otherwise a single multimeter is used and wiring is rerigged for current measurements and voltage measurements. In way, the latter arrangement forces the students to think more about how each is being measured. 

The kits have 100 Ohm and 330 Ohm resistors. The reports follow the usual format of an introduction, data table with caption, labelled graph with trendline, analysis via Desmos, and a discussion of results.