# 122 SC 130 Physical Science Laboratory 12 Circuits, conductors, and Ohm's Law

## Questions

What is a closed, open, and short circuit? What materials conduct electricity? How does electrical conduction relate to heat conduction? Is there a linear relationship between the current and voltage for an electrical circuit?

Due to equipment limitations, the following parts do not have to be done in any particular order.

## Terminology

Electrical conductor: An electrical conductor allows electricity to flow through the material.

Electrical insulator: An electrical insulator does not allow electricity to flow through the material.

Cell: A single "battery" is call a "cell." Two or more cells connected together are called a "battery." Each "dry" cell produces 1.5 volts. The symbol for a cell in an electrical diagram is:

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.

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.

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.

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.

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. 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

In part III of today's laboratory the resistance will be obtained from the slope of a current versus voltage graph.

Schematic diagram: A schematic diagram is a symbolic representation of a circuit using abstract symbols for the power source and loads.

Closed circuit: A closed circuit is one in which electricity flows. In a circuit with a light bulb, the circuit is a closed when the light bulb lights.

Short circuit: A short circuit is one in which electricity flows but not through a load. In a circuit with a light bulb, the circuit is a short circuit when the wire gets hot and the bulb does not light. The current is flowing from the cell directly back into the cell without passing through the load.

Open circuit: An open circuit is one in which electricity does not flow. In a circuit with a light bulb, the circuit is open when the light bulb does not light and the wire does not get hot. No current flows in an open circuit.

## Laboratory experiments

### Part I: A cell, a wire, and a bulb [i]

Using only a single 1.5 volt cell, a single wire, and a flashlight bulb, make the bulb light up. The materials are on the north table.

In electrical physics a closed circuit is one in which electric current flows. When the bulb lights, the circuit is a closed circuit. The circuit may also be a closed circuit without lighting the bulb. When this happens the wire will get hot. If the wire gets hot, then you have created a short circuit.

A circuit in which no electricity flows is an open circuit. In an open circuit the bulb does not light and the wire stays cool. Draw [i] three sketches. Draw a sketch of a closed circuit that lights the bulb. Draw a sketch of a short circuit. Draw a sketch of an open circuit. Sketch what you actually see with your eyes, drawing the battery, bulb, and wire. This sketch can be glued or taped into your final laboratory report.

### Part II: Conductors [d][t]

Using the two-cell battery and the bulb, test the materials provided on the middle table for conductivity. Note whether the brightness of the bulb varies for any of the materials. Make a table [d][t] that indicates whether the material is a conductor or a non-conductor of electricity. Include in the same table whether that material was a conductor of heat or a non-conductor of heat in laboratory five.

### Part III: Ohm's law [d][t]

Ohm's law indicates that the voltage V is equal to the current i times the resistance R for a circuit. Three circuits are provided. All three circuits include a two-cell battery, a switch to open and close the circuit, a voltmeter that measures the voltage, and an ammeter that measures the current in amperes. The only difference in the three circuits is the number of resistances. Each bulb is a resistance. Each bulb is wired in series, so the resistances add directly. The circuits with one and two bulbs are shown in the schematic diagrams below.

Using the ammeter, measure the current i in amperes. Using the voltmeter, measure the voltage V in volts. Note how the meters are connected to the circuit. The electricity in the circuit flows through the ammeter. The ammeter is measuring a rate of flow of electricity. The voltmeter is arranged "across" a single light bulb. The voltmeter is measuring an electrical "pressure difference" across a single light bulb.

For each of the three circuits measure the current i and the voltage V and record the data in a table [d] [t]. Include in your table the value of the voltage and current when the circuit is "open" (the switch is not closed).

Note also the brightness of the bulb for each set-up and whether the bulb is hot, warm, or cool.

As you record the current and voltage for each of the three set-ups, calculate the resistance for each by dividing the voltage by the current. Does the resistance have a relationship with the brightness and how hot the bulb is?

## Graph [g]

Make an xy scattergraph [g] of the data in part three. Use the current i as the x variable and the voltage V as the y variable.

## Analysis [a]

The slope of the linear regression should be the resistance R for the light bulb. When the current is zero, the voltage should be zero. The y-intercept should be zero. To force a spreadsheet to make the y-intercept zero we use the LINEST function. Calculate the slope using the following function in Excel:

`=LINEST(voltage V data (y),current i data (x),0)`

In OpenOffice.org Calc the function is:

`=LINEST(voltage V data (y);current i data (x);0)`

The slope R is an estimate of the mean resistance R in Ohms Ω for one bulb across the range of currents tested.

## Conclusion [c]

Wrap up with a summary of your findings.