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## Electric Fields

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**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**Internal resistance, r E.m.f. ( E ) Current, I E = I.R + I.r Load, 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 E.M.F open circuit p.d. NO lost volts - slope = r I Short circuit current E = V lost**Wheatstone Bridge**R2 R1 R3 R3 R4**Wheatstone Bridge**• At balance point For out of balance bridge V r**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 • Ee = 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**Rf Rin Vin Vout**Op Aps : Inverting Mode**• Saturation occurs at c.a. 85 % of p.supply voltage Vout Vin**Op Amps : Summing Amp**2 inputs can be added together**Op Amps :Differential Mode**• Difference between two inputs is amplified Rf R4 Rin Vout R3 V1 V2**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