E.m.f & Internal Resistance (OCR A Level Physics): Flashcards
Exam code: H556
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Define electromotive force (e.m.f.).
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Cards in this collection (33)
Define electromotive force (e.m.f.).
The amount of chemical energy converted to electrical energy per coulomb of charge when charge passes through a power supply.
How can e.m.f. be measured experimentally?
By connecting a high-resistance voltmeter in parallel with the cell in an open circuit, so no current flows.
Define internal resistance.
The resistance between the terminals of a power supply that causes electrical energy to be dissipated within the supply itself, which is why a cell warms up over time.
A cell can be modelled as a source of e.m.f. with an .......... connected in series.
A cell can be modelled as a source of e.m.f. with an internal resistance connected in series.
Define terminal potential difference.
The potential difference across the terminals of a cell, given by V = IR.
Define lost volts.
The voltage lost within a cell due to its internal resistance: v = ε – V = Ir.
True or False?
If a cell has internal resistance, the terminal p.d. is equal to the e.m.f.
False.
The terminal p.d. is always lower than the e.m.f. because some voltage is lost across the internal resistance (lost volts).
E.m.f. can be defined as the .......... voltage available to the circuit.
E.m.f. can be defined as the total, or maximum, voltage available to the circuit.
Write the equation linking e.m.f. (ε), current (I), the load resistance (R) and the internal resistance (r).
ε = I(R + r)
How are the load resistor and the internal resistance arranged when a cell is connected in a circuit?
They are treated as being connected in series.
A battery of e.m.f. 7.3 V and internal resistance 0.3 Ω is connected in series with a 9.5 Ω resistor. Calculate the current in the circuit.
I = ε ÷ (R + r) = 7.3 ÷ (9.5 + 0.3) = 7.3 ÷ 9.8 = 0.75 A
For the same battery (e.m.f. 7.3 V, internal resistance 0.3 Ω, current 0.75 A), calculate the lost volts.
v = Ir = 0.75 × 0.3 = 0.22 V
True or False?
The current supplied by a cell depends only on the load resistance, not the cell's internal resistance.
False.
Since ε = I(R + r), the current also depends on the internal resistance, r.
In the required practical to determine internal resistance, state the independent and dependent variables.
Independent: resistance, R
Dependent: voltage, V, and current, I
What are the resolutions of the voltmeter and ammeter typically used in this experiment?
Voltmeter: 1 mV
Ammeter: 0.1 mA
When a graph of V (y-axis) against I (x-axis) is plotted for this experiment, what do the gradient and y-intercept represent?
Gradient = – r (negative internal resistance)
Y-intercept = ε (e.m.f.)
Why should the switch only be closed for as long as it takes to record each pair of readings?
To prevent the internal resistance of the cell changing (e.g. due to heating) during the experiment, which would introduce a systematic error.
Give precautions that reduce random error in this experiment.
Use fairly new cells; wait for readings to stabilise before recording; take repeat readings (at least three) and calculate a mean.
Comparing V = –rI + ε to y = mx + c, the gradient represents the ...........
Comparing V = –rI + ε to y = mx + c, the gradient represents the negative internal resistance (–r).
True or False?
Using old, run-down batteries makes no difference to the accuracy of this experiment.
False.
The e.m.f. and internal resistance of run-down batteries can vary during the experiment, introducing random error, so fairly new cells should be used.
Define potential difference (p.d.).
The energy transferred per unit charge flowing from one point to another in a circuit, measured in volts (J C-1).
How is potential difference measured in a circuit?
Using a voltmeter connected in parallel with the component.
The p.d. of a power supply connected in series is always .......... between all the components in the circuit.
The p.d. of a power supply connected in series is always shared between all the components in the circuit.
If a bulb has a p.d. of 3 V across it, how much energy is transferred to the bulb per coulomb of charge passing through it?
3 J of energy per coulomb.
What is the key difference between potential difference and e.m.f. in terms of energy transfer?
P.d. describes energy transferred from electrical energy to other forms in a component (energy loss). E.m.f. describes energy transferred from the power supply to the charges (energy gain).
True or False?
E.m.f. and potential difference are measured in different units.
False.
Both are measured in volts, equivalent to joules per coulomb (J C-1).
State the equations relating energy transferred (work done), W, to charge, Q, for (a) e.m.f. and (b) potential difference.
(a) W = εQ
(b) W = VQ
What is 1 volt equivalent to, in terms of energy per unit charge?
1 V = 1 J C-1
Define the electronvolt (eV).
A unit of energy equal to the work done by an electron accelerated through a potential difference of 1 volt.
Write the equation for the kinetic energy gained by an electron accelerated through a potential difference V.
eV = ½mv2
To convert an energy value from electronvolts to joules, .......... by 1.6 × 10-19.
To convert an energy value from electronvolts to joules, multiply by 1.6 × 10-19.
An electric kettle requires 0.4 MJ to boil a cup of water, supplied by a 230 V mains e.m.f. Calculate the charge supplied.
Q = W ÷ ε = (0.4 × 106) ÷ 230 = 1700 C
True or False?
One electronvolt (eV) and one joule (J) are the same size unit of energy.
False.
1 eV = 1.6 × 10-19 J, which is far smaller than 1 J.
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