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TOPIC 5
ELECTRICITY AND MAGNETISM
ELECTRIC FIELDS
CHARGE: can be either +ve or -ve
⇒ like charges repel ⇒ opposite charges attract
ELEMENTARY CHARGE (e ⁻ ) ⇒ the actual charge of each proton / e⁻
1e ⁻ : 1.6x10⁻¹⁹C UNIT: coulombs (C) - scalar ● an object with an equal amount of +ve & -ve charge ⇒ ELECTRICALLY NEUTRAL ● electric charge is always conserved - change can be neither created nor destroyed total charge before = total charge after ● while charges could migrate from one body to another, the total charge remains the same ● charges ↓ than the electronic charge are not observed in nature
EXPERIMENT: if we rub a rubber balloon on a piece of wool, the balloon strips e⁻ from the
wool captures them
FORMATION OF IONS
POSITIVE ION / CATION: if an e⁻ is removed from an atom, the atom has a net +ve charge NEGATIVE ION / ANION: if an e⁻ is added to an atom, the atom has a net -ve ion
CONDUCTORS: have many free e⁻ that act as charge carriers
EG: all metals and graphite
CONDUCTION IN METALS
● the metal atoms in a solid are bound by metallic bonds
CONDUCTION BY FREE ELECTRONS IN A METAL
- when a metal solidifies form a liquid, its atoms form a regular lattice arrangement
- as the bonding happens, e⁻ are donated from the outer shells of the atoms to a common sea of e⁻ that occupies the entire volume of the metal ● the sea of free e⁻ around the +ve ions are responsible for the electrical conduction ● the e⁻ interact with the vibrating ions and transfer their KE to them ⇒ it is this transfer of E from e⁻ to ions that accounts for the phenomenon that we call RESISTANCE
THE ENERGY TRANSFER IN A CONDUCTOR ARISES AS FOLLOWS:
ABSENCE OF AN ELECTRIC FIELD: the free e⁻ are moving and
interacting with the ions in the lattice, but they do so at random + average speeds close to the speed of sound in the material ⇒ nothing in a material makes an e⁻ move in any particular direction
PRESENCE OF AN ELECTRIC FIELD: an electric force will act on the e⁻ with their -ve
charge, since the electric field is the direction in which a +ve charge moves ⇒ the force of the e⁻ will be in opposite direction to the electric field in the metal ***** in the presence of an electric field, the -vely charged e⁻ drift along the conductor
CONDUCTORS IN GASES & LIQUIDS: possible in other materials too
● some gases & liquids contain free ions ● when an electric field is applied to these materials the ions will move ⇒ +ve in the direction of the field & -ve in the opposite way ⇒ when this happens, electric current is observed ● if the electric field is strong enough, it can itself lead to the creation of ions in gas / liquid - electrical breakdown ⇒ a common effect during electrical storms (when lighting moves between a charged cloud and the Earth)
SEMICONDUCTORS: lie between conductors and insulators
EG: metalloids
INSULATORS: do not allow passage of electric charge as do not have many free e⁻
EG: wood / glass
CHARGING BY INDUCTION
OR
ELECTRIC FIELD STRENGTH: electric force per unit charge (acting on q due to the
presence of Q) experienced by a small +ve point / test charge at a given point ● according to Newton’s 3rd Law, object A repels B ⇒ B repels A with the same type of force / same magnitude but different direction
ELECTRIC FIELD LINES: shows the direction of the force on a small +ve test charge ⇒
the same direction as the ELECTRICAL FIELD STRENGTH by a tangent to the field lines ● always from +ve to -ve ● field lines never touch / intercept each other ● magnitude of the field at a point comes from the density of the field lines around that point
RADIAL FIELDS - MONOPLES: single charge
NON UNIFORM ELECTRIC FIELD
UNIFORM ELECTRIC FIELD: field lines are straight / parallel / equally spaced
⇒ the electric field lines curve outwards near the edge of the plates - EDGE EFFECT
COULOMB'S LAW
Ɛ₀: permittivity in vacuum, so when the charges are in different mediums ( eg: water) we
need to use a different value of Ɛ
PERMITTIVITY: how good the space is in transferring the electric field
ELECTRIC CURRENT (I): rate of flow of electric charge carried by charge-carriers
( eg: e⁻ - conduction e⁻) ● e⁻ travel in the opposite direction to the field as they have -ve charge
OCCURRENCE: occurs only in a conductor with the presence of an electric field
- normally, there is movement of charges in both directions, os it is cancelled and no current flows
- when the electric field ‘rises’, the current forms inself instantaneously ● conventional current flows from +ve to -ve ● e⁻ flow from -ve to +ve
❖ power supplied transfer energy to the e⁻ ⇒ as the e⁻ move through the conductors, they collide with the +ve ions in the lattice and transfer the energy gained from the field to the ions ● when there is a potential difference, there has to be an electric field ⇒ when the switch is closed, e⁻ flow around the circuit What happens to an e ⁻ as it goes round the circuit once? ● the e⁻ gain electric potential energy as it moves through the cell ● the e⁻, then, leaves the cell and begins to move through the connecting lead ● It moves through the switch which also gains a little energy from the charge carrier ● after moving through another lead, e⁻ reaches the lamp ⇒ the metal lattice in the filament lamp gains energy and as a result, the ions vibrate at greater speeds and with a greater amplitude ⇒ T°C↑ ⇒ the lamp will be lit PS: leads to ↓ electrical resistivity ⇒ they don’t require much energy transfer to allow the e⁻ through ⇒ the potential difference from one end of the lead to the other is small
EQUIPOTENTIAL LINES: lines in which the potential difference is equal
● perpendicular lines to the electric field lines ● potential difference gradient: the distance between equipotential lines
ELECTROMOTIVE FORCE: the work done per unit charge in moving charge
around a complete circuit ● when a charge flows through the circuit, electrical energy can go into another form ( eg: internal energy) / can be converted from another form ( eg: light in solar cells) ⇒ the term emf will be used when energy is transferred to the e⁻ in eg: a battery X → ELECTRICAL ⇒ the term potential difference will not be used when the energy is transmitted from the electrical form ELECTRICAL → X
ELECTRONVOLT (eV): work done to move 1e⁻ across a potential difference of 1V
● from W = qV:
HEATING EFFECT ON ELECTRIC CURRENTS
● when we apply a potential difference (V) across a wire, the e⁻ inside speed up and eventually collide INELASTICALLY with surrounding atoms (known as lattice atoms) in the wire, causing ↑T°C electric PE ⇒ gain of KE (e⁻) ⇒ loss of KE (inelastic collision) ⇒ thermal energy of the wire
ELECTRIC CIRCUIT: interconnection of electrical components in a closed loop
CELL BATTERY
LAMP SWITCH
A.C. POWER SUPPLY D.C. POWER SUPPLY
AMMETER VOLTMETER
FIXED RESISTOR VARIABLE
RESISTOR
LDR THERMISTOR
DIODE FUSE
GALVANOMETER EARTH
OHMIC COMPONENT: obeys Ohm’s Law & has a constant R
NON-OHMIC COMPONENTS: components that don't obey Ohm’s Law
LAMP / FILAMENT LAMP: ohmic at ↓I / ↑I cause great T°C↑, making R↑
⇒ atoms vibrate more, so e⁻ collide more with atoms, making it harder for energy to pass through
SEMICONDUCTING DIODES: only allow current flow in one direction (forward), so ↑R in
the other direction
THERMISTOR: as T°C↑, R↓ ⇒ the lattice ions vibrate more and impede charge-carriers
movement
RESISTIVITY ( ρ ): determined by the material the object is made of
● the R of an object is proportional to its length (L) & inversely proportional to its cross-sectional area (A)
POWER DISSIPATION: energy is transferred from chemical energy in the battery, to
electrical energy used by circuit components & then, to the surroundings
POTENTIAL DIVIDER: a circuit component that changes the voltage according go each
specific situations
KIRCHHOFF’S CIRCUIT LAWS
JUNCTION / CURRENT LAW: conservation of charge flow per unit time ⇒ the sum of all
currents flowing into a junction must equal to the sum of all currents flowing out
LOOP RULE: conservation of electric potential energy per charge ⇒ for a complete look of
an electric circuit all of the electric potential rises added together, must equal, all of the electric potential drops added together
ELECTRIC CELLS
CELL: in a circuit acts as a source of electrical energy and creates an electric potential
difference at its terminals
BATTERY: made up of 2 cells connected ⇒ chemical energy transformed into
thermal/mechanical energy
PRIMARY CELL (NON-RECHARGEABLE): cells used until they are exhausted and then
discarded
SECONDARY CELL (RECHARGEABLE): possibility of the reversion of chemical
reactions into original form - eg: lead-acid cell ● RECHARGING PROCESS: use by passing current through the circuit in opposite direction to the current during the discharge
DISCHARGING A CELL: the terminal potential difference of a typical practical electric
cell loses its initial value quickly / has a stable & constant value for most of its lifetime, followed by a rapid ↓ to 0 as the cell discharges completely
PERMANENT MAGNETS: a hard-magnetic material that has been permanently
magnetised ⇒ induces its own magnetic field ● more useful when they do not need to be turned off - eg: fridge magnet
ELECTROMAGNETS: a coil of wire wrapped around a magnetically soft core - can be
turned on and off ● have the ability to be turned on/off so they can be used for situations such as moving scrap metal
MAGNETIC FIELDS: a region around a magnet where a force acts on an another magnet /
magnetic material - caused by the presence of magnets or moving charges ⇒ strongest at the ends of a magnet UNIT: Tesla (T)
MAGNETIC FLUX DENSITY: strength of the magnetic field
MAGNETIC EFFECTS OF ELECTRIC CURRENTS
MAGNETIC FIELD PATTERNS
DIRECTION: North to South magnetic fields coming out of the page - like the tip of an arrow magnetic fields going into the page - like the nock of an arrow
RIGHT HAND GRIP RULE
● ↑ the current through the wire, ↑ the strength of the magnetic field ● reversing the direction of the current through the wire reverses the direction of the magnetic field
CURRENT-CARRYING WIRE SOLENOID
ELECTROMAGNETIC INDUCTION
● when a wire moves across a magnetic field, an e.m.f. Is induced in it ⇒ it is a part of a complete circuit, causes a current to flow ● the induced current flows in such a direction that it opposes the change it produces ● the induced e.m.f. can be ↑ by moving the wire more quickly, using a stronger magnetic field / ↑ the length of the wire ● an e.m.f. is also induced if a changing magnetic field links with a conductor EG: a magnet is moved into a coil, the magnetic field through the coil changes and an e.m.f. Is induced in it - the more quickly the magnetic field changes, the ↑ the e.m.f.
MAGNETIC FORCE ON A MOVING CHARGE
● a charge moving in a magnetic field experiences a force ONLY IF there is a perpendicular component of the motion to the magnetic field ( eg: velocity is not parallel to the field) ***** no magnetic force on a moving charge if the charge moves along the field direction ***** flip the direction for -ve charges
MAGNETIC FIELD STRENGTH (B) DEFINITION BY CHARGES: force acting per unit
charge per speed, on a +ve moving charge perpendicular to the magnetic field
FORCES ON A CURRENT-CARRYING WIRE
● a current-carrying wire consists of e⁻ moving in 1 direction ● when a current carrying wire is placed in a magnetic field, it experiences a force
MAGNETIC FIELD STRENGTH (B) DEFINITION BY CURRENT: force acting per unit
current in a wire per unit length, which is perpendicular to the field ❖ magnetic force between 2 current-carrying wires depends on each wire’s direction ⇒ if currents are in the same direction - ATTRACT ⇒ if currents are in opposite directions - REPEL
