Electric Fields A charged object experiences a force inside an electric field

Electric Fields The field does work on charged objects Charges accelerate W = Q x V

Potential Difference Work done to move 1 coulomb of charge 1V = 1 J C -1

Internal Resistance Load, R Internal resistance, r Current, I E.m.f. ( E ) E = I.R + I.r

Internal Resistance Resistance of power supply itself work is done to push charges through power supply hence ‘Lost Volts’ E.M.F. Maximum energy to push unit charge around circuit Terminal potential difference, work to push unit charge through external circuit ( load )

Internal Resistance E = V t.p.d. + V lost V t.p.d. = E - V lost V t.p.d = E - I x r V t.p.d I E.M.F open circuit p.d. NO lost volts - slope = r Short circuit current E = V lost

Wheatstone Bridge R 3 R1R1 R2R2 R3R3 R4R4

Wheatstone Bridge At balance point V rr For out of balance bridge

Alternating Current Peak Voltage r.m.s. voltage

Alternating Current Resistance is independent of frequency

Capacitors Dielectric 0 V+ 5V Charge builds up on plates ( does not flow through dielectric ) 1 F = 1 C V -1

Capacitors Work is done charging up Capacitor Energy ( charge) is stored in Capacitor E e = 0.5.Q.V Q V Area under graph = Energy Stored Slope = Capacitance

Capacitors Capacitors block d.c. yet pass a.c Capacitors supply time delays Capacitors used to smooth a.c. Capacitors used as microphones

Capacitors I directly proportional to frequency I f

Op Amps Voltage Amplifier Ideal Op Amp a) Input current = 0 A b) p.d. across inverting and non inverting pin = 0V

Op Amps : Inverting Mode V in V out R in RfRf

Op Aps : Inverting Mode Saturation occurs at c.a. 85 % of p.supply voltage V out V in

Op Amps : Summing Amp 2 inputs can be added together

Op Amps :Differential Mode Difference between two inputs is amplified V2V2 V1V1 V out RfRf R in R3R3 R4R4

Op Amps :Differential Mode Can be connected to a wheatstone bridge Used in ECG to subtract the 50 Hz mains hum from the heart signal

Op Amps : Output Output current is of micro amp order To drive a speaker or motor a power amp must be used I.e. an NPN transistor or a MOSFET transistor This increases the current

Op Amps : Output NPN is switched on by output from op amp

MOSFET is switched by output from op amp