Current Electricity. 3 rd Years

Transcription

1 Current Electricity 3 rd Years

2 Comparing: Flow of electricity to flow of water.

3 Electric Current An electric current is a flow of electric charge. An electric current is caused by the flow of electrons through a conductor such as metals. Conductors are substances which allow electric current to flow through them freely. Insulators are substances which do not allow electric current to flow through them. i.e rubber. Electricity flowing through a conductor is a form of energy. We use energy conversions every day when we heat and light our house, cook with electrical cookers, use electrical appliances such as TVs, stereos etc.

4 Test if something is a conductor of insulator. A closed circuit is needed for a current to flow. If the terminals of a battery are attached to each end of a conductor the positive terminal will attract free electrons and the negative terminal will replace the electrons. This results in constant flow of electrons or charge in the conductor. This is shown by the bulb lighting. If the terminals of a battery are attached to each end of a insulator, no current flows and the bulb will not light.

5 Simple Electric Circuit

6 Simple Electric Circuits.

7 The battery supplies the force to move the electrons through the wire. In a metal conductor such as wire it is electrons that are carrying the charge. One pole of the battery is negative because a chemical reaction at this pole has left a lot of excess electrons here. The other pole of the battery is positive because the chemical reaction has stripped the pole of lots of electrons, leaving it with a positive charge. Because of the two charges on the poles, the electrons at the negative pole and those in the wire are attracted to the positive pole and therefore move towards it. We now have electrical current. The electrons have energy when they move and can do work.

8 Electric Current Electric current (symbol: I) is the flow of electric charge. It is measured using an AMMETER. The unit of measurement for current is AMPS (symbol: A).

9 Potential Difference For an electric current to flow, a difference in charge is needed between the ends of the circuit. This charge difference is known as POTENTIAL DIFFERENCE. Without this current cannot flow. The battery is the source of energy that provides this potential difference. Electricity will flow in a circuit if it is a closed circuit, has an energy source and has potential difference

10 Voltage Potential difference is also known as VOLTAGE (symbol: V). It is measured using a VOLTMETER Its unit of measurement is VOLTS (symbol:v).

11 Resistance With an electric current, some materials slow down the flow of electrons passing through them. Such materials offer a RESISTANCE to the flow of electrons. The thin filament of a bulb (made of wire which is not a good conductor of electricity) offers resistance to the electrons, it slows them down. More energy is required to force the electrons through. Electrical energy is converted to heat. The bulb gets hot and glows. The bulb is acting as the RESISTOR. Other examples: heating coil of an electric fire or kettle

12 Resistance Resistors are often used in electrical circuits to produce heat or light or to reduce the current flowing in the circuit. Resistors whose resistance can be changed are called VARIABLE RESISTORS or RHEOSTAT. E.g. controlling volume of a radio, dimmer light switch. Increasing resistance means less current will flow. Decreasing resistance means more current will flow.

13 Resistance Resistance (symbol: R) is the ability a substance has to resist the flow of electricity in a circuit. It is measured in Ohms (symbol: Ω)

14 Calculations current, voltage and resistance

15 Relationship between Voltage, Current and Resistance The larger the voltage, the larger the current that can flow. The larger the resistance, the smaller the current in the circuit. Memory tip for equation: VERY IMPORTANT RULE V I x R

16 Example 1: A battery provides a potential difference of 9V across a metallic conductor of resistance 5Ω. Calculate the current flowing. Use the triangle and get I = V R I = 9V / 5Ω I = 1.8A

17 Example 2 What voltage is needed to drive a current of 1.5A through a resistor of resistance 8Ω? Use triangle again V = I x R V = 1.5A x 8Ω V = 12V

18 Example 3 A bulb in a circuit is connected to an ammeter that reads 0.8A. A voltmeter across the bulb reads 5V. What is the resistance of the bulb? Use the triangle again: R = V / I R = 5V / 0.8A R = 6.25Ω

19 Electrical Symbols

20 Units of Electricity Volt (V) measures potential difference (strength of battery / power box) Ampere (A) measures current (amount of charge flowing) Ohms (Ώ) measures resistance

21 Ammeter to measure current.

22 Voltmeter measure potential difference

23 To measure resistance (ohms)

24 Relationship between current, potential difference and resistance

25 Relationship between Current and potential difference

26 Calculations current, potential difference and resistance

27 Series Circuits Bulbs connected in series are connected one after another. When current flows the bulbs light up because their filaments resist the flow of electricity and therefore heat up. They convert electrical energy to heat and light energy. The more resistance (bulbs) in the circuit, the less current flows and the dimmer each bulb will be. If one bulb is disconnected though, the circuit is broken. Christmas lights are sometimes arranged in series. It means current through them is very low, so bulbs in lights will not get too hot. However if one blows, all lights go off.

28 Series circuit

29 Bulbs in Series When 2 bulbs are in series in a circuit then the same current flows through each bulb. But they do shine as brightly as one bulb on its own.

30 Parallel Circuits Bulbs connected in parallel are connected side by side. Each bulb is connected independently of the others. The total resistance in the circuit is much less than in the series circuit, so the current in the circuit is higher than in the series circuit. The bulbs are therefore brighter. Also if one bulb is disconnected, the others continue to light as each is connected independently to the battery. Most lights arranged in parallel, as they can be individually switched on/off without affecting other lights in the circuit.

31 Parallel circuit.

32 Effects of an Electric current. Electrical energy is converted to: 1. Heat energy 2. Magnetic energy 3. Chemical energy

33 Heating Effect Current has difficulty passing through an object with high resistance this causes heat to be given off. Applications: electric kettle, immersion, electric fires, ovens, irons. Fuses if fault occurs fuse wire gets too hot and melts switching off current.

34 Magnetic Effect Compass needle moves in direction of current when placed near electric wire Electromagnet e.g. nail with wire wrapped around it is magnet when current is flowing. Electromagnets used in electric motors, electric bells, loudspeakers, tape recorders, computers, scrap yards.

35 Chemical Effect Electrolysis of water separating into hydrogen and oxygen Electroplating cheap metal covered by a layer of another metal to prevent it from corrosion, used in making cutlery, kettles, car bumpers.