Electrostatics, circuits, semiconductors, induction, transformers, safety ยท 76 terms
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The study of the properties of charged particles at rest.
Occurs when a body loses electrons (deficit of electrons).
Rubbing a glass rod with silk: electrons move to silk, leaving glass positively charged.
Occurs when a body gains electrons (surplus of electrons).
A region of space around a charged body where a positive test charge would experience a force other than gravity.
Like an invisible force-bubble around a charge โ anything else charged within it feels a push or pull.
A line that shows the direction of force a positive test charge would experience โ lines go from + to โ.
Field lines never cross. Closer together = stronger field.
The force per unit positive charge at a point in an electric field.
E = F/q
The force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
Double the distance between charges โ force drops to a quarter.
F = kqโqโ/rยฒ. k = 9ร10โน N mยฒ/Cยฒ
The work done per unit positive charge in moving a charge from one point to another.
Like height difference in a gravitational field โ charge "falls" from high potential to low potential.
V = W/q. 1 volt = 1 joule per coulomb.
1 volt is the potential difference between two points when 1 joule of work is done moving 1 coulomb of charge between them.
The potential difference between the terminals of a source when no current is being drawn from it โ the maximum voltage a source can provide.
A battery's EMF is its "open circuit" voltage โ once you draw current, it drops slightly due to internal resistance.
EMF is not truly a force โ it's a voltage. Distinguish from terminal voltage.
A device capable of storing electric charge (and electrical energy) in the electric field between two conductors separated by an insulator.
A capacitor is like a short-term rechargeable energy store โ charges up fast, releases fast.
The ratio of the charge stored to the potential difference across the capacitor.
C = Q/V. 1 farad = 1 coulomb per volt.
The rate of flow of electric charge.
Like water flow in a pipe โ more charge flowing per second = higher current.
I = Q/t. 1 ampere = 1 coulomb per second.
1 amp is the constant current which, when flowing through two infinitely long parallel conductors placed 1 m apart in a vacuum, produces a force of 2ร10โปโท N per metre between them.
This is the SI definition. Know it for the exam.
The SI unit of electric charge. 1 coulomb is the charge that passes a point when 1 amp flows for 1 second.
Q = It
A material through which electric charge flows easily โ has many free electrons.
Metals are conductors. Copper is used in wires because it has many free electrons.
A material through which electric charge cannot flow โ few or no free electrons.
Plastic, rubber, and glass are insulators โ used to coat wires for safety.
The opposition to the flow of electric current in a conductor โ the ratio of voltage to current.
A narrow pipe opposes water flow; a high-resistance wire opposes current flow.
R = V/I (Ohm's Law)
1 ohm is the resistance of a conductor when a current of 1 A flows through it under a potential difference of 1 V.
At constant temperature, the current through a conductor is directly proportional to the potential difference across it.
V = IR. Must state "at constant temperature."
The intrinsic resistance of a material โ a property of the material itself, independent of shape or size.
Copper has very low resistivity (good conductor); rubber has very high resistivity (insulator).
R = ฯl/A
The rate of heat produced in a conductor is proportional to the square of the current flowing through it.
Doubling the current quadruples the heat generated.
P = IยฒR. Heat = IยฒRt
A circuit with no junctions โ all components connected in a single loop. Same current through all components.
Like a single-lane road โ all cars must pass through every junction.
R_total = Rโ + Rโ + โฆ Voltage divides.
A circuit with junctions where current splits between branches.
Like multiple lanes on a motorway โ more paths for current to take.
1/R_total = 1/Rโ + 1/Rโ + โฆ Current divides, voltage same across each branch.
A circuit structure using two resistors to divide a voltage between two sub-circuits.
V_out = V_in ร Rโ/(Rโ+Rโ)
A circuit arrangement used to find the value of an unknown resistance by balancing ratios.
Like a see-saw of resistances โ when it's balanced, the unknown can be calculated.
P/Q = R/S when balanced (galvanometer reads zero).
A material with resistivity between that of a conductor and insulator.
Silicon: not a good conductor, not a good insulator โ but its conductivity can be precisely controlled.
Conduction in a pure semiconductor due to electron-hole pairs created by thermal energy.
Increased conduction in a semiconductor due to the deliberate addition of impurities (doping).
The controlled addition of impurities to a semiconductor to increase its conductivity.
Adding a tiny amount of phosphorus to silicon creates extra free electrons โ suddenly it conducts much better.
Silicon doped with boron (3 outer electrons), creating extra "holes" (positive charge carriers). Net charge is still zero.
Majority carriers: holes. Minority: electrons.
Silicon doped with phosphorus (5 outer electrons), creating extra free electrons. Net charge is still zero.
Majority carriers: electrons. Minority: holes.
When P and N type semiconductors are joined, a one-way conductor is formed โ current flows only from P to N (forward bias).
Like a one-way valve for electricity.
Depletion layer forms at junction. Forward bias: P to positive terminal. Reverse bias: no current.
The region at a P-N junction depleted of majority charge carriers due to recombination, creating a built-in electric field.
Positive terminal connected to P-type, negative to N-type โ depletion layer narrows and current flows.
Positive terminal connected to N-type, negative to P-type โ depletion layer widens and (virtually) no current flows.
The conversion of alternating current (AC) to direct current (DC) using a diode or bridge rectifier.
Phone chargers rectify AC from the wall to DC for the battery.
A P-N junction diode that emits light when in forward bias, as holes and electrons recombine.
LEDs are very efficient lights โ almost no heat wasted compared to incandescent bulbs.
A semiconductor whose resistance decreases as temperature increases.
Used in thermostats and temperature sensors.
A semiconductor whose resistance decreases as light intensity increases.
Used in automatic street lights โ resistance drops in daylight, switching the light off.
A semiconductor device with three terminals (base, collector, emitter) that can act as a switch or amplifier.
A safety device: a thin wire that melts when excessive current flows, breaking the circuit. Single use.
Rated at slightly above the normal operating current of the appliance.
A resettable safety device that breaks a circuit when current exceeds a preset value, using magnetic properties.
Unlike a fuse, an MCB can be reset after tripping โ no replacement needed.
A safety device that compares outgoing and returning current โ if different (current leaking to earth), it trips instantly.
Most sensitive household safety device โ protects against electrocution.
An atom or group of atoms that has lost or gained electrons, giving it a net electric charge.
The decomposition of a compound by passing an electric current through it in its molten or dissolved state.
Current that periodically reverses direction โ mains electricity is AC at 50 Hz in Ireland.
AC can be stepped up/down by a transformer. DC cannot (directly).
Current that flows in one direction only โ batteries produce DC.
An instrument for measuring electric current โ connected in series.
Very low resistance โ must be in series.
An instrument for measuring potential difference โ connected in parallel.
Very high resistance โ must be in parallel.
The amount of energy used when a 1000 W appliance runs for 1 hour. 1 kWh = 3.6 MJ.
The unit on your electricity bill.
Units of electricity = power (kW) ร time (hours)
A region of space where a magnetic force can be experienced โ where an isolated north pole would feel a force.
The space around a magnet where other magnets or iron objects feel a force.
Lines showing the direction and strength of a magnetic field โ from N to S outside a magnet, tangent gives field direction.
Field lines never cross. Closer = stronger field.
A vector quantity equal to the force per unit length per unit current on a conductor perpendicular to the field. B = F/Il.
The unit of magnetic flux density. 1 T means a 1 m conductor carrying 1 A perpendicular to the field experiences a force of 1 N.
The number of magnetic field lines passing through a surface. ฮฆ = BA cosฮธ.
The unit of magnetic flux. 1 Wb = 1 Tยทmยฒ.
If the right hand grips a current-carrying wire with the thumb pointing in the direction of the current, the curled fingers show the direction of the magnetic field around the wire.
For a current-carrying conductor in a magnetic field: thumb = force (motion), index = field direction, middle = current direction. All three perpendicular.
FBI rule: Force, B (field), Current. Used for motors.
A solenoid with a soft iron core โ magnetised when current flows, demagnetised when current stops.
Cranes in scrapyards use electromagnets to pick up and drop metal objects.
The creation of an EMF in a conductor due to a changing magnetic flux through a circuit.
Moving a magnet through a coil generates a voltage โ this is how generators work.
The magnitude of the induced EMF is proportional to the rate of change of magnetic flux through the circuit.
EMF = โฮฮฆ/ฮt. More turns = bigger EMF: EMF = โNฮฮฆ/ฮt
The direction of the induced current is always such as to oppose the change in magnetic flux that caused it.
Pushing a magnet into a coil: the coil acts like a magnet repelling it โ the induced current opposes the incoming change.
Lenz's law is a consequence of conservation of energy.
When a changing current in one coil induces an EMF in a nearby coil due to changing magnetic flux.
The basis of the transformer.
When a changing current in a coil induces a back-EMF in the same coil, opposing the change.
A device that changes the voltage of AC using mutual induction between a primary and secondary coil around an iron core.
Power stations step up voltage to 100,000 V for transmission (less energy lost), then step it down to 230 V for homes.
Vs/Vp = Ns/Np (ideal transformer). Only works with AC.
A device that uses electromagnetic induction to convert mechanical energy to electrical energy.
A wind turbine: wind spins a coil in a magnetic field โ changing flux โ induced EMF โ electricity.
An electromagnetic switch โ a small current in one circuit controls a large current in another via a solenoid.
The conversion of AC to DC using a diode or full-wave bridge rectifier.
A domestic wiring circuit where sockets are connected by a loop of wire โ both ends connect to the distribution box. Reduces resistance/heating.
Used for normal plug sockets.
A circuit where a single cable runs directly from the distribution box to one large appliance (e.g. electric shower).
Connecting all metal pipes, taps, and tanks to earth to prevent electrocution if a fault occurs.
An electronic device made from transistors that performs a Boolean logic operation.
Output is HIGH only when ALL inputs are HIGH.
Truth table: only 1 AND 1 = 1.
Output is HIGH when ANY input is HIGH.
Truth table: 0 OR 0 = 0; all others = 1.
Output is the inverse of the input (HIGH becomes LOW, LOW becomes HIGH).