ELECTRICITY
Current- rate of flow of charge
Current = charge/time, I=Q/t, unit = ampere (A); 1A = 1C/s
Measured by an ammeter (connected in series)

Electromotive Force, e.m.f. of a cell -energy dissipated or work done by a source in driving a charge round a complete circuit
unit; volt (V); 1 V =1 J/C

Potential Difference, p.d across a device- electrical energy changed into other forms of energy per unit charge
Potential difference = Energy/Charge, V = E/Q, unit; volt (V); 1V = 1J/C, measured by voltmeter(connected in parallel)

Resistance- ability to oppose flow of electricity (electrons), unit = Ω ohm (W)
good conductors-low resistance bad conductors-high resistance

Precaution-to avoid damage to ammeter by overload, rheostat should set to max. resistance before circuit closed
Vary current through resistor by using rheostat.
-record current & voltage > find average resistance (R= V/I)
-plot graph V against I, gradient = resistace

Ohm's Law: the current passing through a wire at constant temperature is proportional to the potential difference between its end

Limitations of Ohms Law- all physical conditions must remain constant
Factors affecting resistance:
-Length ∝ resistance
-Cross-sectional Area ∝ 1/resistance
-Type of wire : copper-good conductor, silver-best conductor of electricity, nichrome-highest resistance
-Temperature : pure metals; T ^, R ^ semi-conductors; T ^, R decreases

Resistor Network
combined resistance of resistors in series = sum of resistors R=R₁+R₂...
combined resistance of resistors in parallel is given by 1/R=1/R₁ + 1/R₂ ...

I-V Graphs (gradient ∝ 1/resistance)
Ohmic conductors (obey Ohms law)

Non-ohmic conductors (don't obey Ohms Law)

₁torch bulb: R ^ as current ^ temp. of filament
₂thermistor: R decreases, rapidly as temp. ^
₃diode: current passes when p.d. applied in 1 direction but almost zero when applied in opp. direction

Unit of Electricity
Energy consumed = power x time = kW x hour = kWh, [1 unit = 1kWh]

	SERIES CIRCUIT	PARALLEL CIRCUIT
CURRENT	constant at every pt in circuit	= sum of current at separate branches
P.D.	p.d. across whole circuit = sum of p.d. in circuit	p.d. across whole circuit = p.d. across each brance
RESISTANCE	R = R1 + R2 + R3	1/R = 1/R1 + 1/R2 + 1/R3

Short Circuit
Due to low resistance > gives high current

Effects of Electricity-Heating + lighting, -Magnetic, -Chemical

Electrical Energy
change in electrical energy = charge x potential difference, E = QV
-energy is released when a current flows through a circuit, amt depends on p.d. + charge, as E (J) = QV = VIt

Power
power = rate of using energy/ rate at which electrical enrgy is being transfered (watts, W, 1 W = 1 J/s)
P = E/t = VIt/t = VI (1 W = 1 J being transferred in one second)
Rate of production of heat : P = I²R = V²/R

Properties of heating elements-made from wires which have:
1high resitivity, 2high m.p., 3not easily oxidised (and become brittle when current makes it red-hot)

Electrical lighting: lights- connected in parallel
1.1 fails, rest not affected, 2.bulbs can be switched on + off individually, 3.w/ same power supply, brighter than in series

Live wire- brown- carries current from power station- changes between -ve & +ve voltage
Switches in live wire to prevent appliance from becoming live > touch > electric shock
Neutral wire- blue- carries current to power station- stays close to zero (earth potential)
Earth wire- yellow- green- carries current away from metal casing, if it becomes live, protects user, has v. low resistance

Double insulation-plastic (insulator) casing used, no earth wire required, casing cant become live

Fuse

-safety device: protects appliance- made of tinned copper wire- weakest point in circuit- melts + breaks circuit when current exceeds certain value
-should have a rating slightly more than max current taken by appliance
-connected to live wire to break circuit and prevent appliance damage
-mains- switched off when fused changed

Dangers of electricity
-damage insulation: live wire exposed > high voltage > electric shock + electric fires
-cable overheating: due to large current > short circuit (live wire touches neutral wire > large current + heat > electric fire) caused by too many plugs in socket
-e: water- low resistance > high current

Electromagnetic induction
when a conductor cuts magnetic field lines > current induced on conductor

Flemings right-hand rule; dynamo rule: used to find direction of induced current

induced current increases with increasing
-speed of motion, -strength of magnet, # of turns per unit length

Faradays Law: emf induced in a conductor is directly to rate at which conductor cuts through the magnetic field lines

Lenzs Law: direction of induced current is such that it opposes the change producing it

I: anti-clockwise > "N-pole" > repel magnet


I: clockwise > "S-pole" > attract magnet

Output emf is increased by increasing
-speed of rotation, -# of turns per unit length, -magnetic field (winding coil on soft iron core / using stronger magnet)

Mutual induction: when the current in a coil is switched on and off or changed an emf is induced in a neighbouring coil
(an example of electromagnetic induction)

current in A; on: magnetic field set up a cut by coil B
current in A: off: magnetic field dies away and cuts coil B
current steady: no change in magnetic field > no current induced
a.c. used to induce a.c. (current always changing > magnetic flux linking the coil changes)

Simple A.C Generator (mechanical energy > electrical energy)

consists of
-rectangular coil between poles of C-shaped magnet
-2 slip rings joined to ends of coil (rotate w/ coil)
coil rotated > emf induced > current induced(direction: right-hand rule)

Transformers

Vp/Vs = Np/Ns
Step-up transformer: Ns > Np : Vs > Vp
Step-down transformer: Ns < Np : Vs < Vp
100% efficient transformer : IpVp = IsVs

Energy loss due to:
Eddy current-current induced on core due to changes in magnetic field > heating effect, laminating core > R high > I low
Resistance in windings-heating effect
Leakage on field lines-not all magnetic field lines pass through secondary coil

Uses of Transformers
To get high voltage from low voltage and vice-versa, for electrical appliances- radio, TV,
Resistance welding- objects welded: connect to secondary coil of step-down transformer, high current > high temp
Induction furnace- Ns = 1 > very high current > melt solder, quickly boil water
National Grid System: power stations: electricity generated at ~11kV- ~25kV > step-up > 132kV > sub-station: step-down > distributed

Electricity carried at very high voltage/tension > I low > less heat loss by heating effect, good insulation needed
V low > high heat loss > fast wasting of insulation

Magnetic Effect of Current
Magnetic field of a wire w/ current flowing into the paper (closer together nearer the wire)

-magnetic line near wire stronger > closer together
Increase strength of flat coil:-increase current & # of turns
Magnetic field pattern of a solenoid
-uniform + stronger inside solenoid > parallel lines close together
Increase strength of solenoid; increase current & # of turns per unit length, use soft-iron core (concentrate field lines)

Uses of electromagnets

ELECTRIC BELL
switch closed > electromagnet magnetised > attracts soft iron armature > hammer strikes gong > circuit broken > electromagnet loses magnetism > springy metal strip restores contact w/ contact screw > circuit complete > repeats as long as switch closed

REED SWITCH
a pair on soft iron / nickel-iron alloy strips (reeds) in glass tube, small gap between reeds, movement of reeds > make / break contact [due to (electro)magnet] (inert gas protects reeds from contamination and oxidation)

MAGNETIC RELAY
current in wire >armature attracted > opens/closes output circuit
-uses v. small current (safe, can activate high current/voltage circuit)

REED RELAY
reed switch in solenoid, current in coil > reeds touch > close circuit
better than magnetic relay as-₁high speed, ₂low input power, ₃contacts protected
Flemings Left-hand rule; motor rule: find direction of force when a current-carrying conductor is in a magnetic field

Loudspeaker

-permanent magnet used in a moving coil loudspeaker has a central cylindrical pole and a surrounding ring pole > strong magnetic field in gaps between poles
-a.c. current > force produced > moves coil forwards and backwards through a short distance
-paper cone attached to coil vibrates >air molecules in front of cone vibrates >sound waves (elec vibration >vibration of air molecules)

D.C. Motor (elec > kinetic energy)

-commutators replace slip rings of a.c. generator
commutator reverses current in loop > loop rotates in one direction
-loop: horizontal- a couple makes the loop rotate 90°
-loop: vertical- no force, momentum carries loop through vertical position
Increase turning effect on loop- increase # of turns, increase current, place soft-iron core in magnetic field

Electricity: Part 2

Thermal Physics
Waves & Sound
Radioactivity
Measurements in Physics
Forces
Energy
Pressure
Light
Magnetism
Electrostatic Charging

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