20.3 Electric Circuits

What we have learned so far… Electric charges Static electricity Coulomb’s Law Electric current Insulators and Conductors Resistance and Voltage Ohm’s Law 7

What we are going to learn… How do we keep track of the flow of electrons? How do series and parallel circuits differ? How do you calculate electric power and electrical energy? What devices make electricity safe to use? 4

Circuit Diagrams Remember that, in order for electricity to power our TVs and computers, electrons have to be moving through them. An electric circuit is a complete path through which charge can flow. Wires in a house form a complex network of circuits, and each wire must allow electrons to flow through it in order to be useful. 3

Circuit Diagrams The idea of electric circuits is very similar to roads. In order for them to be useful, they have to allow cars to travel to different places. Also, dead ends do not allow for a lot of traffic flow. Disconnected wires are like dead ends, they don’t allow electrons to flow past them. 4

Circuit Diagrams If you want to know possible routes to get to your friends house, you would look at a road map (or use some computerized road map that would tell you where to go). Electricians have to know where the electrons can flow. In order to do that, they use something like a road map - a circuit diagram. 3

Circuit Diagrams A circuit diagram uses symbols to represent parts of a circuit, including a source of electrical energy and devices that are run by the electrical energy. It essentially maps out where the electrons can flow and what they might encounter. 2

Circuit Diagrams A circuit diagram includes many symbols: Anything that uses electricity is seen as a resistor. This symbol could represent a light bulb or a computer. A battery is represented by the following symbol. Note that current “flows” from the positive terminal to the negative terminal. 3

Circuit Diagrams Switches are important! (Like light switches) A switch is like a draw bridge. When the bridge is up, no cars can travel. 2

Circuit Diagrams When the switch is up, no electrons can flow, so no electricity. When the switch is down, the circuit is complete and then electrons can flow. This is then called a “closed circuit.” 2

Types of Circuits There are basically two types of circuits: series and parallel. Once again, each of them act like roads for electrons. To understand them, we can think about cars on a road. 3

Series Circuits In a series circuit, charge has only one path through which it can flow. So the electrons only have one road to drive on. If anything happens to just one section of the path, all of the electrons will feel the effects! So, if one section of the path is blocked, all the electrons will be blocked from traveling through the path. 4

Series Circuits And if electrons can’t travel, there is no electric current. When a light bulb burns out, it essentially blocks the path. So, for two light bulbs “in series,” if one of them burns out, then the other one will not work either. 3

Series Circuits If you add more light bulbs to the series circuit, what does that do to the electrons? Adding a light bulb is like adding something difficult for electrons to travel through. 2

Series Circuits The more resistance you add, the more difficult of a time the electrons have to travel through the whole path. As a result of more resistance, the electrons have to go slower (less current), and each bulb shines less brightly. 2

Parallel Circuits Imagine what would happen if circuits in your home were wired in series… If a light bulb burned out, the television would turn off! 2

Parallel Circuits To avoid this problem, circuits in the home are mostly wired in parallel. A parallel circuit is an electric circuit with two or more paths through which charges can flow. 2

Parallel Circuits This is like having two roads to drive on. If one of the paths gets blocked, the electrons can still drive on the other unblocked road. So if one element stops functioning in a parallel circuit, the rest of the elements still can operate. 3

Parallel Circuits Also, the more parallel paths you have, the less resistance electrons feel. More parallel paths mean there are more roads to for the electrons to take, which means an easier time for the electrons to travel. 2

Power and Energy Calculations The purpose of electricity is to power other appliances. Recall that power is the rate of doing work. The rate at which electrical energy is converted to another form of energy is electric power. The SI unit for power is the “watt” (W). 1 W = 1 joule / second We usually measure power in kilowatts (kW) for convenience. 6

Power and Energy Calculations Electric power can be calculated by multiplying voltage by current. Electric Power: P (watts) = I (amps) X V (volts) Every time you turn on a 75-watt light bulb, you use electric power. 4

Calculating Power An electric oven is connected to a 240-volt line, and it uses 34 amps of current. What is the power used by the oven?

Finding Electrical Energy To find the electrical energy used by an appliance, multiply power by time. E = P x t Most electric power companies usually determine the cost of your electric bill using kilowatt-hours as a unit of energy. 3

Electrical Safety Electricity can cause serious damage to your health, so it is important to know how to avoid electric shock. Inspectors check all new houses to make sure electrical wiring is installed safely. 2

Electrical Safety All wires must be able to carry the maximum expected current. The amount of current depends on the number of devices that are in the circuit. The more devices turned on, the more current you get. 2

Electrical Safety If the current exceeds the circuit’s safety limit, the wire may overheat and start a fire. In order to prevent this, we use fuses and circuit breakers. A fuse is a device that is inserted into a circuit. The fuse has a wire that is designed to melt if too much current passes through it. 3

Electrical Safety This is called “blowing a fuse,” and this event breaks the circuit so that electricity doesn’t flow. The fuse has to be replaced before the circuit can be used again. 2

Electrical Safety Circuit breakers are like fuses, and in most houses, they replace the need to have a fuse. A circuit breaker is a switch that opens when current in a circuit is too high. The circuit breaker must be reset before the circuit can be used again. 3

Personal Safety Touching any open wire is dangerous, so wires are usually put inside the walls of the building. Also, we use three-prong plugs to prevent shocks. The third prong connects to ground, so if a short circuit develops, the charges flow through the grounding wire instead of your body. The transfer of excess charge through a conductor to Earth is called grounding. 4

Short Circuits In a short circuit, current finds a short path through the circuit with less resistance than the full path. This means that current will flow excessively, which will result in overheating, fire or explosions. If you are close to an object that is “shorting out,” there is a chance for a very nasty shock. 3

Effect of Current on the Body 1 mA = slight tingling sensation 5 mA = slight shock 5-30 mA = Painful shock; loss of muscular control mA = Extreme pain; severe muscular contractions. Breathing stops A = Nerve damage; heart stops, death is likely. 10 A = Severe burns; heart stops, death is probable.