Electricity on the Move

We will learn… What an electric circuit is Learn and understand the meaning of an electric circuit Know how an electrical device receives energy from a source of electricity Efficiency of electrical appliances Learn about hydroelectric power Sustainability Issues

The Basics To make “something” (refrigerator, light, computer, radio controlled car, sewing machine….) turn on we need: An appropriate source of electricity Metal wires insulated with plastic A switch

Definitions: Cell – Source of electrical energy Battery – Combination of cells Resistor – Any one of a number of different components called loads which resist movement of charge (electrons) through a circuit

Definitions Loads – convert electrical energy to other forms of energy E.g. motors convert electrical energy to motion Radios convert electrical energy to sound Lamps convert electrical energy to light Current – Movement of electrons through a circuit

Electrical Circuit Definition: An electrical circuit is a network that has a closed loop, giving a return path for the current A cell or battery provides energy to push negative charges (electrons) through conductors in a circuit. Again this movement of electrons is called current A battery has a negative terminal and a positive terminal

Electrical Circuit The conductors in a simple circuit will be the wires Electrons flow through the wires from high to low energy in response to an electric field established in the wires and appliance (load). Recall that like charges repel like charges. That is precisely what is happening in the circuit, negative electrons are being repelled from the “negative terminal” of the source and subsequently attracted to the positive terminal

Electrical Circuit The source uses energy from some other means to keep the electrons flowing WITHIN the source from positive to negative – chemical or mechanical energy. This completes the loop.

Assignment: Lesson 20 – Circuit Electricity

Voltage In a circuit, movement of electrons is continuous and controlled. An excess of electrons accumulates at one terminal of the battery giving it a negative charge. At the same time, electrons withdraw from the other terminal, making it positive. In the battery energy from chemical reactions separates the charges.

Voltage At the negative terminal, the electrons are highly energized, giving them the potential to do some work on something else, such as lighting a bulb, or heating an element. The electrical energy stored in a battery is called the electrical potential energy. The electrons cannot do any work until the battery is hooked up to a load, and the circuit is closed. All forms of energy are measured in joules (j)

Voltage In an electrical circuit, we describe the potential energy possessed by each coloumb of charge, rather than the total energy of all the charges. The term potential difference means the difference in potential energy per coloumb of charge, at one point in the circuit, compared to the potential energy per coulomb of charge at another point in the circuit. Units for potential energy per unit of charge are joules per coulomb (J/C). We give a special name to this combination of units; the volt.

E.g. If one coulomb of charge has one more joule of potential energy at one point in a circuit compared to another point in the circuit, the potential difference between those two points is one volt.

Calculating Voltage

Calculating Voltage: In a battery, 45J of chemical energy are converted into electrical energy by separating positive and negative charges. This energy places 15C of negative energy at the negative terminal, leaving a deficit at the positive terminal. What is the potential difference between the terminals of the battery?

Electrical Current We have already defined Current or electrical current as the movement of electrons through a circuit. Today we are going to further refine this definition and solve some related problems.

Flow of Current A cell or battery provides enough energy to push negative charges (electrons) through conductors in a circuit Movement of electrons is called the current You have heard this word before, think about the current in a river or stream The current of a river or stream is measured by counting the number of liters of water that pass a certain point every minute

Flow of Current Electrical current is measured by the amount of charge that passes a conducting wire per second. Just as water is measured in liters, charge has a unit of measure too. Charge, denoted by the symbol Q, is measured in coulombs (C) Current, denoted by the symbol I, is measured in Amperes (A) An instrument called an ammeter can be used to measure electric current.

Charge (Coulombs) Current (Amperes) Time (seconds)

Resistance and Current Marbles in a tube can represent electrons being pushed through a circuit. If you hold the tube level, the marbles will leave both ends of the tube. To have all the marbles flow out of one end, you must lift one end so that the two ends of the tube are at different heights or “potentials” The higher you lift one end of the tube, the greater the number of marbles flowing out of the tube.

Two different tubes filled with identical marbles tipped the same amount do not have to have the same current. A tube with a large diameter and a tube with a smaller diameter held at the same angle: What do you notice?

The number of marbles leaving the larger tube is greater than that of the smaller tube. They have different “currents” of marbles. The smaller tube does not allow the marbles to flow as freely as the larger tube. In other words, the smaller tube has more resistance.

Electrical Resistance is the ratio of the voltage to the current. Any electrical component that has electrical resistance slows down the current and transforms electrical energy into other forms of energy. A resistor is an electrical component that has a specific resistance. Resistors can be used to control current or potential difference in a circuit to provide the correct voltage and current to other components of the circuit.

It takes less effort to slide a heavy box across a smooth polished floor compared to pushing the same box across a rough floor. The rough floor provides resistance to the motion of the box. Batteries try to “push” electrons through a circuit. When the charge encounters resistance, some of the electrical energy stored in the electrons is transformed into other forms of energy, such as heat.

Series Circuits Series Circuit – A circuit that has only one path for current to travel. Electrons have only one path to travel. If the switch is opened, all electrons are blocked and the current stops

Series Circuits In an electric circuit, the charge that leaves a 12V battery “loses” all 12V. These losses occur on loads such as light bulbs or resistors, which transform the electrical energy into other forms of energy. Each load in the series circuit loses a portion of the total voltage supplied to the electrons by the battery.

Since there is only one path for the electrons to travel in the series circuit, the current in each part of a series circuit is equal.

Assignment: Lesson 21 What is a Series Circuit