Electricity

Electrical Current (definition, units)

• The rate of flow of charge.

• Measured in ampères(A)

• Normally a flow of electrons in metals or a flow of ions in electrolytes

Electrical Current (equation)

I = ΔQ/Δt = charge transfered (coloumbs) / time (seconds)

Charge of an electron

-1.6×10^-19 (=-e)

Elementry charge

1.6×10¹⁹ C

Kirchhoff’s 1^st law

At any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point, electrical charge is conserved

Conventional current

The ‘flow of positive charge’ - it is in the opposite direction to the movement of the electrons in the circuit.

Mean drift velocity

The average velocity of the charge carriers due to the applied electric field. It has to be an average because they’re often moving randomly in all directions.

Equation for drift velocity

I = Anev

Current = Cross-sectional area × number density × elementry charge × mean drift velocity

Number density

Number of charge carriers per metre³

Potential difference

The energy transferred from electrical energy to other forms (heat, light, etc.) per unit charge

Volt

• The energy transferred per unit charge.

• Unit of p.d. and e.m.f, .

• 1 V is the p.d. across a component when 1 J of energy is transferred per 1 C passing through the component

Electromotive force (e.m.f)

The energy transferred from chemical energy (or other forms like light, heat, movement etc.) to electrical energy per unit charge

Kirchhoff’s 2^nd law

In a closed loop of an electrical circuit, the sum of the e.m.f.s is equal the sum of the p.d.s

Equation for resistors in series

R_T = R₁ + R₂ + …

Equation for resistors in parallel

1/R_T = 1/R₁ + 1/R₂ + …

Ohm’s law

The potential difference across a conductor is directly proportional to the current in the component as long as its temperaure remains constant

Equations linking resistance and resisivity (symbol & word)

R=ρL/A

Resistance(Ω) = resistivity(Ωm) × length of wire(m) / cross-sectional area(m²)

(open) switch circuit symbol

(closed) switch circuit symbol

Cell circuit symbol

Battery circuit symbol

Diode circuit symbol

Resistor circuit symbol

Variable resistor circuit symbol

Lamp circuit symbol

Fuse circuit symbol

Voltmetre circuit symbol

Ammetre circuit symbol

Thermistor circuit symbol

Light dependant resistor (LDR) circuit symbol

Light emitting diode (LED) circuit symbol

Capacitor circuit symbol

IV characteristics of resistors

• The potential difference across the resistor is directly proportional to the current in the resistor (V∝I). As a result:

  • A resistor obeys Ohm's law, and so can be described as an ohmic conductor

  • the resistance of the resistor is constant.

• The resistor behaves in the same way regardless of the polarity.

IV characteristics of filament lamps

• The potential difference across a filament lamp is not directly proportional to the current through the resistor. In other words:

  • a filament lamp does not obey Ohm's law, and so can be described as a non-ohmic component

  • the resistance of the filament lamp is not constant.

• The filament lamp behaves in the same way regardless of the polarity.

• Resistance of the filament lamp increases as the p.d. across it increases

IV characteristics of diodes

• The potential difference across a diode (or LED) is not directly proportional to the current through it. This means:

  • a diode does not obey Ohm's law, and so can be described as a non-Ohmic component

  • the resistance of the diode is not constant.

• The diode's behaviour depends on the polarity.

• The resistance of the diode is very high - infinite for practical purposes - up until the threshold voltage (e.g at A). At the threshold p.d. (at B) the resistance gradually starts to drop. Above this value, the resistance drops rapidly (e.g. at C) and the diode has very little resistance.

IV characteristics of thermistors

• The p.d. across the thermistor is not directly proportional to the current through the thermistor. As such:

  • it is a non-ohmic component

  • resistance is not constant

• The thermistor behaves in the same way regardless of the polarity.

• Resistance of the thermistor decreases as temperature increases.

  • This is beacause as the current increases the temperature increases. This leads to an increase in number density and so a drop in resistance. This can be confirmed by comparing R = V/I at various points on the graph

Potential divider

Electrical circuit that uses resistors to deliver only a proportion of the voltage from a battery to a component in order to produce a specific output

Potential divider equation

V_out = V_in × R₂ / (R₁ + R₂)

Where R₁ + R₂ = R_T

Similaraty and difference between e.m.f. and p.d.

• Both are measured in volts/ defined as energy transferred per unit charge

• Charges are losing energy for ___ and gaining energy for ___

Definition of the kilowatt hour

1 ___ is the energy transferred by a 1kW device in a period of 1 hour

How an electron gun produces a beam of high speed electrons

• Electrons are emitted from the hot wire/filament at the rear of the electron gun through thermionic emission

• There is a large p.d. between the filament and an anode.

• Electrons are accelerated towards the anode.

• They pass through a hole/gap in the anode.

Equation relating work done on charged particles and their gain in kinetic energy

eV = ½mv²

work done on electron = gain in KE