phy B5 current and circuits

electromotive force

• the energy converted into electrical energy from other forms per unit charge in driving a charge round a complete circuit

• OR the electrical power supplied by the source per unit current

potential difference

• work done per unit charge on moving a positive charge between two points along the path of the current → electrical energy converted into other forms per unit charge passing across two points

• conventional current flows from higher to lower potential (opp for electron flow)

current

• direct current: flow of charge carriers

  • when battery connected across ends of conductor, delocalised electrons drift towards positive terminal of battery → produces direct current

  • electron flow is opposite to direction of conventional current

• conventional current (I): rate of flow of positively charged particles

  • on circuit diagrams, conventional current is indicated

  • I = Δq/Δt = Ne/Δt

resistance

• ratio of potential difference across the conductor to the current flowing through it

• due to frequent collisions of drifting electrons with vibrating ions in the lattice

  • when accelerated by potential difference (electric field), free electrons gain KE, but some energy transferred to ions when collide → electrons slow down, limits current flow

• to determine: place ammeter in series to measure current through, voltmeter in parallel to measure pd across

ohm’s law

• current through a metallic conductor is directly proportional to the pd across it, IF temperature and other physical conditions are constant

  • to keep temp constant, immerse in constant temperature water bath

• ohmic conductors have constant V/I ratio

• heating effect

  • ohmic @ small current, low temp → R constant, linear constant gradient

  • non-ohmic @ large current, high temp: lattice ions vibrate more vigorously, collide more frequently with electrons → R increase, V > I

electric power

• work done per unit time to move electric charge across potential difference

internal resistance

• all sources of emf have internal resistance → some energy used to overcome

• ε = I(R+r) = V_R + V_r

• terminal pd = ε – Ir = IR = V_R

  • electrical energy converted to other forms in one full circuit

  • V_R < ε → lost voltage is pd across r (accounts for loss of efficiency)

  • treat internal resistor as another resistor in series (constant I)

  • V_R = ε only if ideal cell (r=0) OR circuit is open (I=0)

I-V characteristics of electrical components

• ohmic resistor: R constant → gradient constant

• filament lamp: I increase, temp increase, R increase → V/I increase so I/V gradient decrease

• thermistor: I increase, temp increase, R decrease → V/I decrease so I/V gradient increase

• diode: forward biased → exponential graph

  • negative I, R infinite → I/V = 0

  • positive I, R = 0, I/V = infinite