introduction to electricity & magnetism - department of physics
TRANSCRIPT
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Dr Lisa Jardine-Wright
Cavendish Laboratory
Introduction to Electricity & Magnetism
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Examples of uses of electricity…
Christmas lights
Human bodyElectronic devices
Cars
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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.
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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.
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Electric Charges
• Charge arises because of a transfer of electrons.
• This charge, measured in units called Coulombs (C), is given by
• To charge an object means to transfer electrons from one object to another. They are not created or destroyed, just moved!
C1.6x10electron an on Charge 19−=
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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)
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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)
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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)
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Electrical Symbols & Units
Lamp Resistor
Cell Switch
Voltage = Volts (V)
Resistance = Ohms (Ω)
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Voltage, Current & Resistance
V +-
I R
V = I x RRV
=I
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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.
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Solids:Insulators -vs- Conductors
• Atomic structure of a solid: A lattice
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Solids:Insulators -vs- Conductors
Electrons in the lattice
INSULATOR CONDUCTOR
Bound to atoms Free to move
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Electric Circuits
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Connecting in Series
V+ -
R1
Total R = R1+R2+R3
I = I1 = I2 = I3
I = V/R = V/(R1+R2+R3 )
R2 R3I1
I
I2 I3
I
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Connecting in Parallel
321 R1
R1
R1
R1
++=
V R1 R2 R3
I+-
Total R:
I1I2 I3
I
I = I1 + I2 + I3 RV
=
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Series -vs- Parallel
+ -
1 Ω 2 Ω 3 Ω
I6V
6V
I
+ -
1 Ω
2 Ω
3 Ω
I1 I2 I3
I3
I2
I1
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V
I
+-
I1
I2Sa
Introducing a Switch
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Predict the Action of The Switches Sa - Sd
V +-
Sa Sd
Sc Sb
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Put Lamps 1-5 in Order of Brightness
V +-
2
3
4
5
1
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Put Lamps 1-5 in Order of Brightness
V +-
2
3
4
5
1
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Making Electricity
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How Do Cells Work?
Electrodes (uncharged) made with different metals
Electrolyte: ionic solution
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How Do Cells Work ?
electrode negatively charged
positive ions that pass into solution
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How Do Cells Work?
AI ≠ 0
Ions
Electrons
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The Orange Cell
A I ???
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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.
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The Dynamo
• A dynamo converts kinetic energy into electrical energy through electromagnetic induction.
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Magnetic Field Around a Wire
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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."
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Inducing a Current in a Coil
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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
BANfV =
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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.
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Electricity Summary
• Relation between voltage, current and resistance
• Resistors in series
• Resistors in parallel
V = I x R
Total R = R1+R2+R3
321 R1
R1
R1
R1
++=
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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
BANfV =