The Control of Electricity in Circuits

Electricity and Electric Circuits Electrical energy is transferred by the movement of electric charge Electric current – the term used to describe the movement, or flow, of electric charges from one place to another Electric circuit – a controlled path through which electric current passes

The Parts of an Electric Circuit A simple circuit has four basic parts. Power Source – Where the electric current comes from. Electric Load – Something that uses and transforms the electricity into a new form (ie: toaster, stove, etc)

Control Device – something to control the flow (ie: on/off) such as a switch. Connector – a conducting wire that provides a path for electric current to flow to each part of the circuit.

Smartboard Circuit

Textbook Pg. 301 - #’s 1, 2, 4 and 5

Ways To Describe Circuits When describing whether a circuit is operating or not we describe it as being open or closed. Closed circuit – when a circuit is operating, and current is flowing. The switch is on (the circuit has no “open” parts to stop the flow). Open circuit - when a circuit is not operating, and the current is not flowing. The switch is off (the circuit has an “open” part and stops the flow).

Electric Potential (Voltage) Electric potential - the maximum possible electrical energy that any source possesses. Electric potential is commonly referred to as voltage and can be thought of as pressure through a hose >>> the higher the voltage the higher the “pressure”. The unit used to measure electric potential (ie: voltage) is the volt (V). Table 1 on page 303 lists some sources of electric potential, with typical voltage values.

Electric Current An electric current is made up of moving electric charges Electric current (I) is a measure of the rate at which electric charges move past a given point in a circuit. Ampere (A) is the SI unit used to measure electric current. Table 1 on page 314 lists the electric current required to operate some electrical loads.

Electrical Resistance Resistance (R) is the ability to impede (slow/stop) the flow of electrons in conductors. The Ohm is the SI unit for resistance and is shown with the “Ω” symbol. Resistors are an electrical device that is designed to slow/stop the flow of electrons in conductors. This loss of electrical potential is often referred to as potential difference (difference in voltage). The coils on a stove are resistors designed to stop the flow of electrons and by this process heat is created.

It is easier to understand 1) Electric Potential, 2) Current and 3) Resistance when thinking of water in a closed system. Water in a closed system

Ohm’s Law This law states that the difference in electrical potential (voltage) between two points in a circuit is directly related to the electric current (amps) flowing through the conductor. The resistance created by the resistor is what causes this difference (drop in voltage).

Ohm’s Law Continued This law can be expressed in the form of an equation that shows how the voltage will drop when related to the Current (I) and Resistance (R). Potential Difference (voltage drop) = Electric Current x Electrical Resistance V = I x R It is important to remember the “V” is not voltage of the power source but the difference between this voltage and the voltage after the resistor.

Remember: Potential difference (voltage drop) is measured in volts (V) Electric current (I) is measured in amperes (A) Resistance (R) is measured in ohms (Ω)

Problem 1 What is the voltage drop across the tungsten filament in a 100 W light bulb? The resistance of the filament is 144 Ω and a current of 0.833 A is flowing through it.

Answer 1 V = I x R I = 0.833 A R = 144 Ω V = (0.833 A)(144 Ω) V = 120 V

Problem 2 An electric toaster is connected to a 120 V outlet in the kitchen. If the heating element in the toaster has a resistance of 14 Ω, calculate the current flowing through it.

Answer 2 V = I x R V = 120 V R = 14 Ω I = ? I = V/R I = 120 V / 14 Ω I = 8.6 A

Problem 3 The current required to operate an electric can opener is 1.5 A. What is its resistance if the supply voltage is 120 V?

Answer 3 V = I x R V = 120 V I = 1.5 A R = ? R = V / I R = 120 V / 1.5 A R = 80 Ω

Problem 4 The measured voltage at point #1 in the diagram below was 120 V. After passing the resistor shown the voltage was measured at 80V. If the amperage was measured at 0.833 A what was the resistance?

Answer 4 V = I x R V = 120 V – 80V = 40V I = 0.833 A R = ? R = V / I R = 40V / 0.833 A R = 48 Ω

More Practice Questions 1, 5 – 7 on page 319

Assignment on Ohm’s law Due _____________?

Full class on an assignment using only V=IR for entire period to make sure they’ve got it.

Cells in Series and Parallel There are two basic kinds of circuits: series circuit and parallel circuit. When dry cells (basic batteries) are not connected in series they can produce only about 2 V. Connecting cells in series allows you to obtain much higher voltages.

Resistors in Series Resistance = R1 + R2 + R3… An electrical circuit in which more than one resistor is wired to one another in a single source. The negative terminal of one cell is connected to the positive terminal of another. Every time we connect a new cell (source) we increase the overall voltage.

- The voltage (electric potential) is twice as high and therefore the energy is high enough to run 2 motors at once.

Resistors in Parallel 1/Resistance = 1/R1 +1/R2 + 1/R3 (maybe don’t do this to avoid confusion but still explain diagram with parallel and effects on Voltage (None) and Current (will be higher) An electric circuit in which more than one cell (source) is connected to the electrical load by its own separate path or circuit. The positive terminals connect together and the two negative terminals connect together. Every time we do this we double the electric charge (the electric charge motor will operate for twice as long) as each cell drains slower.

- The voltage (electric potential) will not be any higher as the cells are not “stacked” but the drain on the cells will be half as much so the motor will run twice as long.

Need questions and practice on creating schematic diagrams of series and parallel circuits with varying resistance, voltage and current. Maybe exploration with multimeters to measure and calculate…