SPH3U Grade 11 Physics Electricity and Magnetism Test

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