Introduction to Electricity & Magnetism - Department of Physics

Dr Lisa Jardine-Wright

Cavendish Laboratory

Introduction to Electricity & Magnetism

Examples of uses of electricity…

Christmas lights

Human bodyElectronic devices

Cars

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.

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.

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−=

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)

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)

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)

Electrical Symbols & Units

Lamp Resistor

Cell Switch

Voltage = Volts (V)

Resistance = Ohms (Ω)

Voltage, Current & Resistance

V +-

I R

V = I x RRV

=I

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.

Solids:Insulators -vs- Conductors

• Atomic structure of a solid: A lattice

Solids:Insulators -vs- Conductors

Electrons in the lattice

INSULATOR CONDUCTOR

Bound to atoms Free to move

Electric Circuits

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

Connecting in Parallel

321 R1

R1

R1

R1

++=

V R1 R2 R3

I+-

Total R:

I1I2 I3

I

I = I1 + I2 + I3 RV

=

Series -vs- Parallel

+ -

1 Ω 2 Ω 3 Ω

I6V

6V

I

+ -

1 Ω

2 Ω

3 Ω

I1 I2 I3

I3

I2

I1

V

I

+-

I1

I2Sa

Introducing a Switch

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Predict the Action of The Switches Sa - Sd

V +-

Sa Sd

Sc Sb

Put Lamps 1-5 in Order of Brightness

V +-

2

3

4

5

1

Put Lamps 1-5 in Order of Brightness

V +-

2

3

4

5

1

Making Electricity

How Do Cells Work?

Electrodes (uncharged) made with different metals

Electrolyte: ionic solution

How Do Cells Work ?

electrode negatively charged

positive ions that pass into solution

How Do Cells Work?

AI ≠ 0

Ions

Electrons

The Orange Cell

A I ???

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.

The Dynamo

• A dynamo converts kinetic energy into electrical energy through electromagnetic induction.

Magnetic Field Around a Wire

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."

Inducing a Current in a Coil

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 =

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.

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

++=

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 =