Electrical Circuits 1 of 31 Boardworks Ltd 2011

Transcription

1 Electrical Circuits 1 of 31 Boardworks Ltd 2011

2 2 of 31 Boardworks Ltd 2011

3 What is an electric circuit? 3 of 31 Boardworks Ltd 2011 An electric circuit is a path for electric charge to flow along. Like all electrical devices, this portable MP3 player contains electric circuits to enable it to play music. What is needed for an electric circuit to work? source of energy a component a complete loop no short circuits

4 How can we model electric circuits? 4 of 31 Boardworks Ltd 2011 A model can help us to understand how current works in an electric circuit. In this model, the moped riders represent the flow of charge and the pizzas represent the electrical energy carried around the circuit. What do the pizza shop and the house of party-goers represent?

5 What are conductors and insulators? 5 of 31 Boardworks Ltd 2011 In some materials, electric charges are free to flow, but in others, they are not. This means the current cannot flow. Conductors are materials that allow the flow of electric charge. Metals are good conductors. Insulators are materials that do not allow the flow of electric charge. Plastics are good insulators.

6 How do metals conduct electricity? 6 of 31 Boardworks Ltd 2011 It is the delocalized electrons involved in metallic bonding that allow metals to conduct electricity. delocalized electrons The delocalized electrons are free to flow through the metal and so carry a current. Insulating materials do not contain free electrons and so current is unable to flow. Ionic solutions are also able to conduct electricity because they have mobile charge-carrying particles.

7 Does it conduct? 7 of 31 Boardworks Ltd 2011

8 Why do cables have plastic coatings? 8 of 31 Boardworks Ltd 2011 Electricity and electric wires can be very dangerous. Insulators can be used to make wires safer. Metals, usually copper, are used to make electrical wires because they conduct current. To prevent electric shocks, the metal wires are coated with a thick layer of plastic. How does this increase safety? Plastic is an insulator and does not conduct current. It is also important to insulate wires because if any part of the circuit is earthed, the charge will not flow.

9 Conductors and insulators 9 of 31 Boardworks Ltd 2011

10 10 of 31 Boardworks Ltd 2011

11 Using circuit diagrams 11 of 31 Boardworks Ltd 2011 Which competitor is finding it easier to build a circuit?

12 Why use circuit diagrams? 12 of 31 Boardworks Ltd 2011 Electronic circuits are usually very complex. It is important to have a clear way of showing how different components are connected together. A standard set of symbols is used to represent different devices and to draw schematic diagrams of circuits, which show how the components are connected.

13 How are wires shown in circuit diagrams? 13 of 31 Boardworks Ltd 2011 Wires or leads are used to carry the current around a circuit between the components. They are usually made from thin copper wires twisted together and covered with a plastic sleeve. circuit symbol for a wire Circuit diagrams can be complex and it is important to be able to distinguish between wires that are joined together and those that cross without being joined. two wires that are connected two wires that cross (i.e. not connected)

14 What is the source of energy in a circuit? 14 of 31 Boardworks Ltd 2011 Mains electricity is often used to power electric circuits. When mains electricity is unsuitable, cells or batteries can be used as the source of electrical energy. This remote control uses batteries. Circuit symbol for a cell Circuit symbol for a battery V

15 Circuit components 15 of 31 Boardworks Ltd 2011

16 Circuit symbols 16 of 31 Boardworks Ltd 2011

17 What is a short circuit? 17 of 31 Boardworks Ltd 2011 Current will always pass along the path of least resistance. The resistance of the wires in a circuit is low compared to the resistance of components, such as bulbs. If current can flow along a path without passing through the components this part of the circuit is called a short circuit. The current can only pass through the bulb. These circuits both contain a short circuit, so the bulb will not be lit up.

18 Which circuit diagram? 18 of 31 Boardworks Ltd 2011

19 19 of 31 Boardworks Ltd 2011

20 What is current? 20 of 31 Boardworks Ltd 2011 Current is a measure of the rate of flow of electric charge in a circuit. Electric charge is measured in coulombs. Current is measured in amperes (A) using an ammeter. It is calculated using the equation: current = charge time or I = Q t A current of 1 A is 1 coulomb of charge flowing every second. So, a current of 5 A is 5 coulombs of charge flowing every second. A An ammeter must be connected in series as it measures the current flowing through it.

21 What is potential difference? 21 of 31 Boardworks Ltd 2011 Potential difference is the work done (or energy transferred) between two points in a circuit, per coulomb of charge. Potential difference is also called voltage. Voltage is measured in volts (V) using a voltmeter. voltage = work done charge or V = W Q A 1 V cell gives 1 joule (J) of energy to each coulomb of charge. So, a 12 V battery gives 12 J of energy to each coulomb of charge. The voltage of a battery is the push it gives the charges in the circuit. V A voltmeter must be connected in parallel as it measures the potential difference across the component.

22 Water model of a circuit 22 of 31 Boardworks Ltd 2011

23 Are they cells or batteries? 23 of 31 Boardworks Ltd 2011 A cell is a source of electricity. A chemical reaction takes place inside the cell, which produces a potential difference (voltage) across the cell. People often refer to cells as batteries, however they are not the same and it is important to use the terms correctly! A battery consists of two or more cells that are joined together. The potential difference across a battery is the sum of the potential differences across the cells.

24 24 of 31 Boardworks Ltd 2011 How can battery voltage be increased? Using more cells in a battery increases the voltage. V 1.5V If several cells are connected in series, the total voltage across the battery can be found by adding the voltages of the cells. So, if 2 cells with a voltage of 1.5V are connected together, the voltage across the battery is 3V. When three cells of 1.5V are connected, what is the voltage across the battery? V V 3V 4.5V

25 Current and voltage true or false? 25 of 31 Boardworks Ltd 2011

26 What is conventional current? 26 of 31 Boardworks Ltd 2011 Before the discovery of the electron, scientists assumed that current was due to positively-charged particles moving from the positive terminal around a circuit to the negative terminal. This way of representing the direction of current is called conventional current. It is now know that charge is carried by electrons, flowing from the negative terminal to the positive terminal. This is called electron flow. Today, both conventional current and electron flow can be used to represent the direction of current.

27 Representing current direction 27 of 31 Boardworks Ltd 2011

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29 Glossary 29 of 31 Boardworks Ltd 2011

30 Anagrams 30 of 31 Boardworks Ltd 2011

31 Multiple-choice quiz 31 of 31 Boardworks Ltd 2011