Current, Charge and Potential Difference
- Current = rate of flow of charge. Q= It, where Q = charge in Coulombs, t = time. (1c = 1A s^-1)
- Measured using ammeter in series.
- I dependant on drift velocity; I = nevA (n=no. carriers, e=charge per electron (1.6*10^-19) v=drift velocity, and A is cross sctional area of the wire) all directly proportional
- Different materials have different no.s of charge cariers. Metal have lots-- free electrons in outer shells, Semiconductors have less. Insulators have as few as possible.
- In liquids and gases, carriers are ions. Ionic crystals (NaCl) don't conduct if solid, only molten.
- Gases are insulators until p.d. is high enough to rip electrons from atoms, making a spark.
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Electrical Energy and Power
- Power is the rate of energy transfer in W (Js^-1). Calculated by P = VI, or P = E/t
- Because V = how much energy per C, I = how many C per s, so P = how much energy per s
- Using V = IR, sub to get P = V^2 / R and also P = I^2 * R
- E= ItV (power * time gives energy) because power can be calculated in several ways, find that first then multiply by time to give energy.
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Resistance and Resistivity
- As p.d. flows across a component, current flows. Its size depends on resistance of component-- how hard for current to flow. R = V/I, measured in Ohms. 1 Ohm = 1V makes 1A flow.
- Resistance depends on length, area of wire (thicker = less), and resistivity (varies per material)
- Resistivity= resistance per m length wire with area of 1m^2. measred in ohm metres.
- p = RA / I resistivities of conductors are v. small.Check area is in m^2
- For ohmic conductor, R = constant. Obey Ohm's law: while temp. = constant, p.d directly proportional to current.
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- Potential divider = circuit with resistors in series. The potential difference is divided according to the ratio of the resistors, so you can get a specific potential difference by connecting between the resistors. Vout = R1 / (R1 + R2) * Vs.
- This circuit is usually used for callibrating voltmeters, which have a very high resistance.
- But, if something with v. low resistance goes across R1, the resistors are effectively in parallel. So, total R is less than R1, and the calculation wont work.
- Add LDR (high R in dark) or NCT thermistor (high R when cold) as resistor, and you get a Vout that depends on the light/temp. With a transistor, that makes a switch.
- In a potentiometer, a variable resistor replaces R1 and R2, so Vout can be precisely altered to fit needs by adjusting R1 and R2 relative to each other. Goes from 0V to Vs (source)
- Used for volume controls.
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E.m.f. and Internal Resistance
- Resistance = collisions between electrons and atoms, resulting in electrons losing energy.
- In batteries, chemical energy makes electrons move, but they collide with atoms in battery.
- Therefore, batteries have resistance and warm up when used.
- How much electrical energy produced per coulomb called electromotive force. Despite very helpfully not actually being a force. Measured in V.
- Terminal p.d. = energy transferred when 1 C of charge flows through load (total) resistance (R).
- If no internal resistance (r), terminal p.d. would = e.m.f. Energy wasted = lost volts (v)
- e per C supplied = e per C used by components + e per C wasted in internal resistance
- e.m.f. = V + v (terminal p.d. + lost volts) or I(R + r) (current*load resistance + internal resistance) or V = e.m.f. - v or e.m.f. = V + Ir (terminal p.d. + Current * internal resistance)
- ost power supplies need low r, e.g. car battery. Usually <1 Ohm. HT and EHT supplies need high r, because produce high I and it's safer then if short-circuits.
- Easiest way to measure e.m.f is to put high-resistance voltmeter across its terminals. As a (small) current flows, some volts must be lost. Value measured is less than e.m.f.
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- Graphs that show how resistance varies as p.d. increases across a component.
- if Current is on the y-axis, 1/gradient gives the resistance: shallower graph=more resistance.
- Curve shows resistance is changing. For a metallic conductor, I/V characteristic is a straight line, temp. is constant, so current = directly proportional to p.d. Therefore, they're ohmic.
- For a filament lamp, characteristic is a curve through the origin that gets shallower as p.d. increases.Increasing current heats wires, causing more KE in particles and more collisions with the electrons, so they lose energy. R increases linearly with temp in most metallic conductors.
- Semiconductors worse than metals, (fewer carriers)
- The resistance of a thermistor is dependent on temp.In NTC thermistors, R down as temp up.
- R of LDRs decreases with light intensity-- provides energy, releasing electrons, greater no.
- Semiconductors are used in these sensors; fewer carriers: see changes to environment in form of increased no. charge carriers due to external energy releasing more.
- Diodes (LEDs) only allow current to flow in one direction (direction = forward bias).
- Most diodes need threshold p.d. of 0.6v before conduction.
- In reverse bias R is v. high, and current that flows is really tiny.
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Domestic Energy and Fuses
- Units of electricity charged for = kWh (Energy = Power* Time) 1kWh=3.6million J
- Cost of electricity = no. of units * price per unit. Duh. Always make sure units are converted.
- Fuses are tiny wires betweeen live terminal and the appliance. In power surge, high current melts fuse, breaking cirsuit. Prevents shocks and fire.
- Have fuse rating to make safe-- should be the next value up from current used in the circuit.
- The earth pin in the plug connectc to the appliance's case via earth wire. If fault develops so that live wire touches case, current flows safely out through earth wire. Simples. The surge from this melts the fuse.
- To find the right fuse rating, look at the power rating on the appliance, and the voltage rating.
- I = P/V
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Conservation of Energy and Charge
- As charge flows, it isn't used up: whatever charge flows into a junction will flow out again.
- Same goes for current as it's the rate of flow of charge, which doesn't change.
- Kirchhoff's first law is as follows: total current entering junction = total current leaving it.
- Energy also conserved. Surprisingly. Energy transferred to a charge =e.m.f., from a charge =p.d.
- In a closed circuit, they must be equal: total e.m.f. around series circuit = sum of p.d.s across components (e.m.f. = sum (IR)) <-- = Kirchhoff's second law.
- Because of dear old Kirchhoff's laws, we know that in a series circuit Current is the same throughout, e.m.f. split (E = V1 + V2 + V3 etc), and
- load resistance = R1 + R2 + R3.
- For parallel circuits, current splits at each junction, so I = I1 + I2 + I3. p.d. is same across all components. For resistance, 1/R total = 1/R1 + 1/R2 + 1/R3 etc)
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