6.1 Simple circuits. Electric charge

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1 6.1 Simple circuits Electricity is one of many forms of energy. Electrical energy powers your MP4 player, laptop computer, hairdryer, iphone and electric toothbrush. It starts the car and it lights up the streets and your home at night. What makes electrical energy so useful is that it is easily transformed into other forms of energy such as heat, light and sound. Although spectacular, the sparks and lightning bolts of static electricity aren't very practical. Current electricity is the form of electricity that you use every day and is the form that you get from batteries and from power points. To use current electricity, you need an electric circuit. Electric charge Everything is made of atoms, which are themselves made of protons, neutrons and electrons. As Figure shows, protons and neutrons are located in a small and dense core called the nucleus. Around the nucleus spin tiny electrons. Protons and electrons are electrically charged. Protons carry a positive charge (+) and electrons carry a negative charge (-). Neutrons carry no charge and are said to be neutral.

2 Overall, the atom is neutral, because the numbers of protons and electrons are always equal. Their opposite charges balance each other out and so the atom has no overall charge. Figure 6.1.1Atoms are neutral but contain charged particles. Charges form when there are unequal numbers of protons and electrons. However, electrons sometimes get knocked, rubbed or pulled off an atom. This leaves the atom with more protons than electrons, giving the atom an overall positive charge. This 'charged atom' is known scientifically as an ion. In this case, it's a positive ion. The electrons that have been removed then attach themselves to another atom nearby. The atom to which they attach themselves will then have more electrons than protons and so will have a negative charge. A negative ion is formed. Static electricity Static electricity is the build-up of electric charge on a surface. This build-up of charge most commonly occurs because the surface has been rubbed against another surface. Electrons have been rubbed off one surface (charging it positive) and have transferred to the other surface (charging it negative). Static charge usually leaks away after some time into its surroundings, including the air around it. This returns the materials to their original neutral state. However, if the build-up of charge continues, the electrons may jump across a gap from the negatively charged surface back to the positively charged surface. As they jump back, the electrons release all their energy in one go. This converts the energy into the heat, light, sound and motion (kinetic energy) that you observe as a spark or lightning bolt. This is what is happening in Figure

3 Figure 6.1.2A lightning bolt happens when static charge builds up within the atmosphere and then jumps to Earth, a building or another cloud. SciFile Strike me lucky! A bolt of lightning:! is about 5 cm wide! has a temperature of around C (hotter than the surface of the Sun)! is the main source on Earth of plasma, the fourth state of matter! could power a 25 watt light globe for a year! SciFile Bright spark! Electrons from a 240 V power point have insufficient energy to jump the gap in a switch. However, extreme voltages cause air in the gap to break down, allowing a spark to jump across it! A 1 cm gap requires about 3000 V for it to 'spark'. Current electricity The electricity you get from a battery or a power point is not static electricity. It is made up of electrons moving along a wire, like those in Figure This movement of charge is called an electric current. These moving electrons have energy that is transformed into other forms of energy as the electrons pass through things like light globes (transforming electrical energy into light), heating elements (into heat) and motors (into movement). If the electrons don't move, no energy is received by the light globe, heater or motor and so nothing happens. It remains 'off'.

4 Figure 6.1.3Electrical appliances work because electrons flow through their circuits. The energy the electrons carry is then transformed into heat, light or motion. Simple electric circuits Electrons need a path to travel around so that they can deliver their energy. This path is called anelectric circuit. As Figure shows, an electric circuit needs:! an energy source, such as a battery or a generator like the dynamo on a bike. This supplies the electrons in the wire with the energy they require to get them moving around the circuit! an energy user, such as a light globe, heating element or motor. These devices convert the energy that electrons are delivering to them! wires to connect everything, making the circuit complete.

5 Figure 6.1.4A circuit needs an energy source, an energy user and wires to connect them all. The circuit usually has a switch too. Any break in an electric circuit stops the flow of electrons and stops them from delivering their energy. Most electric circuits have switches that deliberately break the circuit, turning it on and off. Circuit components The different parts of a circuit are known as its components. Each component is given a different symbol. This makes diagrams of circuits easier to construct and easier to understand. Some of these components are shown in Figure

6 Figure 6.1.5These symbols are used to show the different components that are connected up to make a circuit. Circuit diagrams A circuit diagram is a simplified and shorthand version of a real circuit. It shows how all the components in the circuit are connected. A torch is an example of a very simple circuit. Its energy source is a battery and its globe transforms electrical energy into light. The circuit diagram for the torch is shown in Figure The battery supplies the electrons with energy. As the electrons flow through the globe, they lose almost all their energy, which is transformed into light energy and some heat energy. The electrons then travel back to the battery, where their energy is replenished.

7 Figure 6.1.6A torch has a battery, a globe, wires and a switch. Remembering 1 State whether the charges of the following are positive, negative or neutral. a an atom b a proton c a neutron d an electron 2 Recall charges by copying the following statements and completing them. To complete each statement, insert the sign = or (is greater than) or (is less than). a In an atom, the number of protons the number of electrons. b In a positive ion, the number of protons the number of electrons. c In a negative ion, the number of protons the number of electrons. 3 List the forms of energy released when a spark jumps across a gap. 4 Name the charged particles that carry an electric current through a circuit. 5 List what an electric circuit needs. 6 List three examples of each of the following components of an electrical circuit. a an energy supplier b components that use electrical energy 7 Recall the symbols for the following electric components by sketching them. a globe b battery c single cell d switch

8 Understanding 8 Define the following terms. a nucleus d neutral c component 9 Calculate whether the particles in Figure would be neutral, positively charged or negatively charged. Figure Applying 10 Identify whether the following are displays of static electricity or current electricity in action. a the spinning motor of a hairdryer b lightning c a spark felt when you touch a doorknob after walking across carpet d the TV when it is on