SPH3U University 11 Physics Electromagnetism Test Study Notes and Outline

Format

10 marks – communication

10 marks – application

40 marks – knowledge Textbook-like questions

Topics

• Electric fields

• Calculating electric properties

• Solving a circuit

• Charging objects

• Types of magnets

• Drawing magnetic fields

• Domain theory

• Key Terms

• Electromagnets right hand rules

• Circuits with solenoids (electric bell)

• Motors

• Electromagnetic induction

• Lenz’ law

• Transformers

• Magnetic storage

• CRT

Electric Fields

– forces are visualized using the field theory

– field of force exists in a region of space when an object placed at any point in the field experiences a force

– forces occur between objects

– defined as: force per unit positive charge

– vector quantity

– ε = Fe / q – electric field is equal to electric force over charge

– measured in Newtons/Coulomb

– field strength gets stronger with shorter distances

– fields can be represented with lines that indicate direction of force

– field lines never cross

– similarly, gravitational field is defined as the gravitational force per unit mass

– field strength = Kq/d2

Calculating Electric Properties

– current is defined as the amount of charge that passes a point each second

– I is the symbol for current

– Current can either be direct of alternating

– Direct current (DC): current flows in only one direction, created by batteries

– Alternating current (AC): current alternates direction, created by generators

– Amperes (A) are the units for current

– Current = charge over time

– I = Q/t

– Resistance depends on material, length of wire, and the cross-section of the wire

– R is the symbol and ohms are the units

– Electrical potential difference is the voltage of the circuit

– Symbol is V and units are volts (V)

– More potential difference increases the amount of current

– V = I x R

Circuits

– Electron flow is opposite to current

– Short circuits occur when there is little to no resistance and high current

– current is the same at every point in a closed loop (in series)

– current splits up at a branch/junction

– the total current of each branch adds up to the current entering the branch

– potential difference adds up in series

– potential difference is equal in parallel

– resistance adds up in series

– Resistors in parallel use this equation: 1/Requivalent = 1/R1 + 1/R2 + etc…

– Voltage law: sum of the increases in electrical potential = sum of decreases in electrical potential

– Current law: total electrical current before a junction = total electrical current out of the junction

Charging Objects

– induction: when the charge of an object changes when a different charged object is brought near

– conduction: when electrons are actually passed on from an object

Types of Magnets

– magnets have poles (N and S)

– opposite poles attract and similar poles repel

– natural magnets are often found on earth in mines

– e.g. lodestone, magnetite

– artificial magnets are made by mining various metals

– they can be very strong and are used in many products

– ferromagnets become magnets when brought close to other magnets

– they become magnetized for a period of tie, and are mostly steels or irons

Domain Theory

– the smallest part of a magnet is called a dipole

– a group of these dipoles is called a domain

– no such thing as a mono-pole – there has to be a N and a S not either or

– in magnets the dipoles all point in one direction

– in ferromagnets are aligned randomly

– when a ferromagnet is magnetized, the dipoles line up and act like a magnet

– magnetic field lines point away from North towards South

Key Terms (Pg 447)

– demagnetization: when aligned dipoles return to random directions, for soft ferromagnetic materials, they demagnetize when removed from the magnetic field

– reverse magnetization: occurs when magnets are placed in strong enough magnetic fields and the poles go in the opposite directions

– breaking a bar magnet: produces new pieces with dipole alignments similar to the original

– magnetic saturation: when the max number of dipoles of an object are aligned

– induced magnetism by earth: iron in earth’s magnetic field will have their dipoles aligned while heated or vibrated

– keepers for bar magnets: bar magnets become demagnetized over time due to the reverse of polarity – by storing in pairs with small pieces of iron (keepers), this can be prevented

Right Hand Rules

– Right hand rule #1: with straight wires, thumb points in direction of current and the curl of the fingers indicates magnetic field

– Right hand rule #2: with solenoids, wrap fingers in the direction of the current and the thumb points to N

– Right hand rule #3: with motors, use an open hand – fingers point in direction of magnetic field, the thumb points in the direction of current (I), and the force is away from the palm

Solenoids

– when wrapping wire, the field goes through the centre of the coiled wire – coiled wire pretty much turns into a bar magnet

– this wire is called solenoid

– solenoids allow for controlled magnets

– strength of the solenoid depends on: current in coil, number of loops in coil, and type of core material

– relative magnetic permeability (K)

– K = magnetic field strength in material / magnetic field strength in a vacuum

Motors

– when a wire or coil carries current it creates a magnetic field

– if the field of the wire is near another magnetic field, the wire or coil can be made to move

Electro-Magnetic Induction

– a coil conducting current creates a magnetic field

– moving a wire through a magnetic field created electrical current

– law of electro-magnetic induction – an electric current is induced in a conductor whenever the magnetic field surrounding the conduction changes

– mutual induction – changing current in one coil produces a current in another coil (through induction) – mutual induction can be demonstrated with Faraday’s iron ring

Lenz’ Law

– current induced in a coil by a magnetic field is in such direction that the magnetic field that the coil creates opposes the changing field that originally induced the current

Transformers

– secondary coil with more windings creates higher voltage but less current

– transformers are modified versions of Faraday’s ring – used to increase/decrease voltage of an AC source

– Equation: Voltage of the secondary coil / voltage of primary coil = # of windings of secondary coil / # of windings of primary coil

– Vs / Vp = Ns / Np

– If you double the number of windings in the secondary coil, the voltage in the coil doubles

– In an ideal transformer, the power in both coils are equal

– Another equation: Vp / Vs = Is / Ip – V is voltage and I is current

– 2 types of transformers: step up and step down

– Step up creates a higher voltage in secondary coil while step down creates a lower voltage in secondary coil

Magnetic Storage and CRT

– refer to textbook readings to understand these two concepts