ELECTRICITY CALCULATIONS
06 AUGUST 2013
Lesson Description
In this lesson, we:

Work through several electric circuit calculations
Key Concepts
Important Equations
I = current (A)
Q = charge (C)
Δt = time (s)
V = potential difference (V)
W = work / energy (J)
R = resistance (Ω)
Resistors in series
Resistors in parallel
P = power measured in watt (W)
Questions
Question 1
(Adapted from Grade 11 Exemplar 2007)
The simplified circuit diagram represented alongside shows part of
the electric wiring of a model doll’s house. Bulb B and a fan F are
connected to a 12 V car battery. The internal resistance of the car
battery cannot be ignored.
a.) The bulb is rated 12 V; 15 W. When switch S1 is closed, at
maximum voltage, the bulb allows a maximum current of 1,25 A
to pass through it. Calculate the resistance of the bulb filament.
b.) With switches S1 and S2 closed, the bulb glows a little dimmer.
Fully explain why the bulb glows dimmer.
Question 2
In the circuit represented, each cell has an emf of
1,6V and an internal resistance of 0,25. Two
resistors of 10 and 30 are connected in parallel
and the ammeter is connected such that it
measures the circuit current.
Switch S is open.
What is the reading on the following?
a.)
b.)
Voltmeter V
Ammeter A
Switch S is now closed.
Calculate the following:
c.)
d.)
e.)
The effective resistance of the combination of resistors
The reading on ammeter A
The reading on voltmeter V
Question 3
Peter sets up the following circuit. He uses a battery with an emf of 12 V. The internal
resistance of the battery is so small that he can ignore its effects. His circuit contains four
resistors, an ammeter and a switch. The diagram shows the values of the resistors. He then
closes the switch, S.
a.)
b.)
c.)
Calculate the total resistance of the circuit.
Calculate the reading on the ammeter.
He then connects a voltmeter across various points in the circuit. Calculate the reading he
gets on the voltmeter when he connects it across the following points.
i.
P and Q
ii.
W and X
Question 4
(Adapted from IEB Exemplar 2012)
Consider the following basic circuit found in a motor car. A battery with internal resistance r and emf
of 12 V is connected in series to a bulb, resistor R, Hooter H and a starter motor M, each with a
respective resistance of 1 Ω, 16 Ω, 8 Ω and 35 Ω. When the current is flowing in the circuit the
potential difference across the battery drops to 10,8 V.
a.)
b.)
c.)
Calculate the current flowing in the circuit
Calculate the resistance r
Calculate the power of resistor R.
Links



Series circuit calculations: http://www.youtube.com/watch?v=zgONtlnph3c
Parallel circuit calculations: http://www.youtube.com/watch?v=7NMnWn5guko
Series and parallel circuits http://www.youtube.com/watch?v=x2EuYqj_0Uk