What is current? cgrahamphysics.com Current

Transcription

1 What is current? cgrahamphysics.com 2016 Current 9/4/2016

2 Do you know your circuit? Without talking to another or sharing any ideas draw a complete electrical circuit with symbols including... - two bulbs, cell, and a switch to turn it on and off. Now draw another one with the same components but make it so that one bulb can be turned off and the other is on all the time.

3 im Describe what is needed to measure current. Explain how the resistance of a circuit affects the current. Key Words Current 전류 mmeter 전류계 mps () 암페어 ( 전류측정단위 ) Resistance 전기저항

4 Have you ever wondered? How do we measure electricity?

5 Can you solve the puzzle? farmer is traveling home with his fox, chicken and some corn. He has to cross a river, and the only way to cross is in a small boat which can hold the farmer and just one of the three items he has with him. The problem is, he has to be very careful about what he chooses to leave behind at any time. If the fox and chicken are left alone, the fox will eat the chicken. If the chicken and the corn are left alone, the chicken will eat the corn. To solve the puzzle, you must show how the farmer can get himself and his three items across the 9/4/2016 river without losing any cgrahamphysics.com of them. 2016

6 Solution The man and the chicken cross the river, (the fox and corn are safe together), he leaves the chicken on the other side and goes back across. The man then takes the fox across the river, and since he can't leave the fox and chicken together, he brings the chicken back. gain, since the chicken and corn can't be left together, he leaves the chicken and he takes the corn across and leaves it with the fox. He then returns to pick up the chicken and heads across the river one last time.

7 Or is this the solution?

8 Can we use this example and represent it as an electrical circuit? Design a simple electrical circuit that follows the puzzle. You have need a power pack set on 6 V, a bulb, some connecting wire, and four switches: 3 single-pole and 1 double-pole Each switch represents one of the items: the farmer, the cat, the chicken and the corn (you have to figure out which need to be SP switches and which one needs to be a DP switch). The switches are mounted on a small panel, in a horizontal row representing the river, Label each switch Farmer, fox, Chicken, Corn. The circuit is to be designed so that if either of the problematic pairs (fox-chicken, or chicken-corn) are left alone on the same side of the river, the light bulb lights up, indicating an incorrect solution (you can add a 6 V buzzer, too, if you like). Since the boat can hold only two items, players can use only two switches per "move".

9 Current and Energy Electricity is caused by Electrons moving round a circuit: electrons electrons electrons electrons electrons electrons electrons electrons

10 Current and Energy There are millions of electrons moving round the circuit!! They pick up energy at the cell and give it to the light bulb. They keep on going round, picking up and dropping off energy

11 model for electricity The flow of electrons (or skiers) is called the current. The faster the flow of electrons, the bigger the current

12 Ski Slope Model... Skiers are lifted to top of mountain by ski lift which is like a cell or battery Skiers represent the electrons moving in wires current flowing Speed of skiers is the amount of current flowing i.e. 1 Skiing pathways are like wires Height of mountain is the voltage of the cell or potential difference Skiers convert energy as they ski down mountain like a bulb Skiers can take different paths to show resistance

13 model for current The electrons are like a stream of tiny lorries, all carrying energy and dropping it off at the bulb.

14 nother model for current

15 Trucks Model... Trucks are like charges moving Traffic lights stop them mount of current is speed of trucks Trucks deliver goods to customers (energy) Trucks pick up energy at their supply depot (energy) Traffic jams slow them down (resistance)

16 Measuring current electrons electrons electrons electrons electrons electrons electrons electrons electrons We can measure the current using an ammeter, it measures the number of electrons passing a point per second. It measures 9/4/2016 current in cgrahamphysics.com mps (), 2016 e.g. 3.

17 model for current In our model the mmeter is like a bridge, where we count the number of lorries going under each second.

18 What we know so far The number of electrons passing a point each second is called the current. It is measured in mps (), using an ammeter connected in series. The symbol for an ammeter is:

19 n ammeter is placed in a circuit like this:

20 What is the current at different points in a series circuit? 1. Copy the circuit diagram into your book Write a prediction about where the current might be biggest (1, 2 or 3) and explain it using a model of your choice. I predict that the current will be...because using the... model...

21 Current in a series circuit Set up the circuit and record the current at each point. (You will have to move the ammeter each time). 5 minutes + - You will need: 1 battery pack 1 ammeter 2 bulbs 4 wires

22 What did you find? Current is the same at all points in a series circuit. The current is not used up! Was your prediction correct? How can we use a model to explain that? 0.5

23 Measuring Current Flow... chemical cell or battery of cells provides a flow of electricity. This flow registers on a meter called an ammeter which counts electrons they flow through the wires. Can you predict the two other ammeter readings?

24 Measuring Current Flow... In a series circuit the current is the same everywhere as electrons push around the circuit like water down a pipe. It all travels at the same speed. Can you predict the other ammeter readings?

25 What happens to the current as you add more bulbs to a series circuit? Choose 1, 2 or 3 Write a prediction about which circuit will have the largest current and explain it using a model of your choice. I predict that the current will be largest for? bulbs because using the... model...

26 Recording your results Copy the table into your book. Number of bulbs 1 Current / 2 3

27 + - What happens? Set up the following circuits and record the current in your table. 5 minutes Do you notice anything else? You will need: You should have found that 1 more bulb doubling the number of bulbs 1 more wire halves the current.

28 + - What did you find? Can we use a model to explain this?

29 What obstacles affect the flow of current? How do different components affect the flow of current?

30 What affects the size of the current? Current can be changed by increasing or decreasing the voltage of the circuit. Components in a circuit that reduce the size of the current adding resistance to the circuit. ll wires and components resist current. Resistance is a measure of how hard it is for electrons to move in an electrical circuit.

31 Resistance Resistance is the opposition to the flow of electricity. When you add more bulbs to a circuit the current decreases and the bulbs get dimmer. Bulbs have a high resistance which makes it hard for the current to flow. The more bulbs you add, the harder it is for the current to flow so the lower the current. If you double the resistance the current will halve.

32 Summary Circuit 1 Circuit 2 V V R V Delete the wrong answer: Increasing the number of cells increases/decreases the current that flows in the circuit. The current/voltage depends on the current/voltage. R V

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34 Series circuits key ideas 1. In a series circuit the current is the same in all parts of the circuit. 2. The supply voltage is shared between the components in a series circuit. (The sum of the voltage across each component is the same as the total supply voltage.) 3. The current depends on the voltage in any circuit. 4. Resistance opposes the current flow.

35 Questions 1 What does an ammeter measure? the size of the current 2 Draw the symbol for an ammeter. c) What current will ammeter 3 show? 3 Look at the circuit. a) What current will ammeter 1 show? 0.1 (or 0.1 amps) b) What current will ammeter 2 show? 0.1 (or 0.1 amps)

36 Multiple-choice quiz

37 Key words Current - the amount of electrons that move past a point in a given time period mmeter - this measures the current. It is placed in series with the circuit mps () the unit in which current is measured Resistance opposes the electron flow. It is a measure of how hard the electrons have to be pushed to make it through the circuit