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* is the opposition to charge moving through a source of electricity

5.3 Emf and internal resistance. Internal resistance. * is the opposition to charge moving through a source of electricity * causes electrical energy to be dissipated inside the source as charge moves through it. Producing heat !. Electromotive force of a source.

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* is the opposition to charge moving through a source of electricity

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  1. 5.3 Emf and internal resistance Internal resistance * is the opposition to charge moving through a source of electricity * causes electrical energy to be dissipated inside the source as charge moves through it Producing heat ! Electromotive force of a source * is the potential (chemical) energy transferred to electrical energy when one coulomb of charge passes through it E = W Q

  2. 5.3 Emf and internal resistance E E V r r A high resistance voltmeter measures terminal Pd. Open circuit Pd of a cell equals the EMF in volts, because the current through the cell is negligible. Energy supplied per coulomb by cell Energy changed per coulomb by external circuit Energy wasted per coulomb by internal resistance = + I EMF = Pd across R + Pd across r R E = V + v = IR + Ir E = I ( R + r) E

  3. 5.3 Emf and internal resistance E E V r r Q. Find the internal resistance of a cell if its emf is 3V and the pd across an external resistance is 2.5 volts when 0.5 A flows. EMF = Pd across R + Pd across r E = V + v “terminal pd” “lost volts” I = IR + Ir E = + 0.5 r 2.5 3 R = 0.5 0.5 r r = 1.0 ohm

  4. 5.3 Emf and internal resistance Power Power supplied by cell = I E = I(IR +Ir) = I2 R + I2r Useful power delivered to R Maximum power delivered to R Power delivered to R * occurs when the load resistance R matches the internal resistance r (R+r)I = E I2 R = E2 R (R+r)2 I = E (R+r) r Load resistor R

  5. 5.3 Emf and internal resistance Measurement of emf and internal resistance r * adjust the variable resistor for different values of current and record the terminal pd across the cell r R limits the current to safe levels ! Terminal Pd /V 2 Emf = 1.5V 1 Circuit current / A 0.5 1.0

  6. 5.3 Emf and internal resistance Measurement of emf and internal resistance r EMF = Pd across R + Pd across r E = V + v = IR + Ir E - Ir = E IR Terminal Pd /V rI - = E V Compare with straight line equation: - 2 = c mX Y Emf = 1.5V m = gradient (0, 1.5) m = Y m = -0.5 = 0.42 1.2 (1.2 , 1) 1 Int resistance r = Circuit current / A 0.5 1.0

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