Introduction to Electricity & Magnetism. Dr Lisa Jardine-Wright Cavendish Laboratory

Transcription

1 Introduction to Electricity & Magnetism Dr Lisa Jardine-Wright Cavendish Laboratory

2 Examples of uses of electricity Christmas lights Cars Electronic devices Human body

3 Electricity? Electricity is the presence and motion of charged particles. Electric current is the flow of charged particles around an closed path an electric circuit.

4 Electric Charge There are two types of charge, which are labeled positive and negative. Like charges repel, Unlike charges attract. Charge is never created or destroyed.

5 Electric Charges Charge arises because of a transfer of electrons. This charge, measured in units called Coulombs (C), is given by Charge on an electron = 1.6x10 19 To charge an object means to transfer electrons from one object to another. They are not created or destroyed, just moved! C

6 Electric Forces & Charge If an electrical force moves a charge a certain distance, it does work on that charge. The work done by this force: Work done = charge x potential difference, W = QV Potential difference is the voltage drop across two points. Units of voltage = Volts (V)

7 Electric Current Electric current is the charge flowing through a point per unit time. Current = Charge / Time I = Q / t Unit of current = Ampères (A) Two types of current in everyday life: Direct current (DC) and alternating current (AC)

8 Electrical Resistance Ohm's law states that, in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference across the two points. (providing physical conditions remain constant). Units of resistance = Ohms (Ω)

9 Electrical Symbols & Units Lamp Resistor Cell Switch Voltage = Volts (V) Resistance = Ohms (Ω)

10 Voltage, Current & Resistance I = V R I R V = I x R V + -

11 Electrical Conductivity Good electrical conductors, such as copper, have a low resistance. Poor electrical conductors, such as concrete, have a high resistance. Current is the flow of the outer electrons of atoms through the material. Resistance then results from collisions of electrons with other electrons and with atoms.

12 Solids: Insulators -vs- Conductors Atomic structure of a solid: A lattice

13 Solids: Insulators -vs- Conductors Electrons in the lattice Bound to atoms Free to move INSULATOR CONDUCTOR

14 Electric Circuits

15 Connecting in Series I 1 R 1 I R 2 R 2 I 3 3 I + - V I Total R = R 1 +R 2 +R 3 I = I 1 = I 2 = I 3 I = V/R = V/(R 1 +R 2 +R 3 )

16 Connecting in Parallel I I 1 I 2 I V R 1 R 2 R I 3 Total R: 1 R = 1 R + 1 R R3 I = I 1 + I 2 + I 3 = V R

17 Series -vs- Parallel 1 Ω 2 Ω 3 Ω I 1 I 2 I 3 I 3 I 2 3 Ω 2 Ω I + - I I 1 1 Ω 6V + - 6V

18 Introducing a Switch I V + - Sa I 2 I 1

19 Predict the Action of The Switches S a -S d S a S d V + - S c S b

20 Predict the Action of The Switches S a -S d S a S d V + - S c S b

21 Predict the Action of The Switches S a -S d S a S d V + - S c S b

22 Predict the Action of The Switches S a -S d S a S d V + - S c S b

23 Predict the Action of The Switches S a -S d S a S d V + - S c S b

24 Predict the Action of The Switches S a -S d S a S d V + - S c S b

25 Predict the Action of The Switches S a -S d S a S d V + - S c S b

26 Predict the Action of The Switches S a -S d S a S d V + - S c S b

27 Predict the Action of The Switches S a -S d S a S d V + - S c S b

28 Predict the Action of The Switches S a -S d S a S d V + - S c S b

29 Predict the Action of The Switches S a -S d S a S d V + - S c S b

30 Predict the Action of The Switches S a -S d S a S d V + - S c S b

31 Predict the Action of The Switches S a -S d S a S d V + - S c S b

32 Put Lamps 1-5 in Order of Brightness V

33 Put Lamps 1-5 in Order of Brightness V

34 Making Electricity

35 How Do Cells Work? Electrodes (uncharged) made with different metals Electrolyte: ionic solution

36 How Do Cells Work? electrode negatively charged positive ions that pass into solution

37 How Do Cells Work? Electrons A I 0 Ions

38 The Orange Cell A I???

39 Magnetism Natural magnets have North and South Poles. Like poles repel and opposite poles attract. Magnetic field lines flow from North to South. Natural magnets are made from Iron, Nickel, and Cobalt. Magnetic substances can be induced by magnets to become magnets.

40 The Dynamo A dynamo converts kinetic energy into electrical energy through electromagnetic induction.

41 Magnetic Field Around a Wire

42 Lenz s Law and Induction Lenz's law enables us to determine the direction of the induced current: "The direction of the induced current is such as to oppose the change causing it."

43 Inducing a Current in a Coil

44 Inducing a Current in a Coil Size of the electromotive force (voltage, V ) in a coil depends on: The strength of the magnet, B the cross-sectional area of the coil, A the number of loops in the coil, N And its frequency in or out of the coil, f V = BANf

45 Making an Electromagnet If you wrap a wire around an iron core, such as a nail, and you send electrical current through the wire, the nail will become highly magnetized.

46 Electricity Summary Relation between voltage, current and resistance V = I x R Resistors in series Total R = R 1 +R 2 +R 3 Resistors in parallel 1 R = 1 R + 1 R R3

47 Magnetism Summary A dynamo converts kinetic energy into electrical energy through electromagnetic induction. Lenz s Law - "The direction of the induced current is such as to oppose the change causing it." Size of the electromotive force (voltage, V ) for a magnetically induced current V = BANf