* is the opposition to charge moving through a source of electricity
8
* is the opposition to charge moving through a source of electric * causes electrical energy to be dissipated inside the source as moves through it E = W Q Producing heat ! 5.3 Emf and internal resistance Internal resistance Electromotive force of a source * is the potential (chemical) energy transferred to electrical e one coulomb of charge passes through it
description
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. - PowerPoint PPT Presentation
Transcript of * is the opposition to charge moving through a source of electricity
* 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
E = W Q
Producing heat !
5.3 Emf and internal resistance
Internal resistance
Electromotive force of a source
* is the potential (chemical) energy transferred to electrical energy when one coulomb of charge passes through it
5.3 Emf and internal resistance
r
E
V
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.
r
ER
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= +
E V v+=
IR IrE +=
I ( R + r)E =
5.3 Emf and internal resistance
r
E
V
r
ER
I
EMF Pd across R Pd across r= +
E V v+=
IR IrE +=
3
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.
= 2.5 + 0.5 r
0.5 = 0.5 r
r = 1.0 ohm
“terminal pd” “lost volts”
Power supplied by cell = I E = I(IR +Ir) = I2 R + I2r
5.3 Emf and internal resistance
Power
Maximum power delivered to R
* occurs when the load resistance R matches the internal resistance r
Useful power delivered to R
I2 R = E2 R (R+r)2 I = E
(R+r)
(R+r)I = E
Power delivered
to R
Load resistor Rr
* adjust the variable resistor for different values of current and record the terminal pd across the cell
5.3 Emf and internal resistance
Measurement of emf and internal resistance r
R limits the current to safe levels !
r
Terminal Pd /V
Circuit current / A
Emf = 1.5V
1
2
0.5 1.0
5.3 Emf and internal resistance
Measurement of emf and internal resistance r
Terminal Pd /V
Circuit current / A
Emf = 1.5V
1
2
0.5 1.0
EMF Pd across R Pd across r= +
E V v+=
IR IrE +=
E IrIR -=
V E= rI-
Y c= mX-
(1.2 , 1)(0, 1.5)
m = gradient
m = Y
m = -0.5 = 0.42 1.2
Int resistance r =
Compare with straight line equation:
