FORM 5 2.1
CHAPTER 2:ELECTRICITY
[ ……../ 25 x 100 = ….……% ]
Electric fields and charge flow
A student student is able to : describe an electric field. • sketch electric field lines showing the direction of the field. • state the relationship between electric charge and electric current. • define an electric current •
1.
An electric field exists in the space surrounding a _____________ _____________ body. body.
2.
Draw Draw the the pat patte tern rns s of of ele elect ctri ric c fiel field d lin lines es of the the cha charg rges es belo below: w: (a)
(b)
(c)
(d)
3.
Like charges ___________ ___________ each each other , other , unlike charges ______ charges __________ ____ each each other.
4.
An electric ric curre rrent is defined as ____________ __________________ ______________ ______________ ____________ ______
Electric current, I =
5.
Ch arg e, Q Time, t
,
(Q measured in ______ in ___________, _____, C) C)
Electric cu curre rrent is is me measured wi with an an _____________ ______________ _ in in SI unit _____________
1
Exercise 1.
A charge of 30 coulombs flows through a bulb in1 minute. What is the current flowing in the bulb?
1000 C of charges to flow through the lamp?
4.
2.
A fuse wire ire will ill bu burn if a curre rrent of 13A flows through it for 100ms. Determine (a) the quantity of charge that flows through the fuse wire.
A car battery can supply a curren rent of 4A for 5s. What is the amount of charge stored in the battery?
(b) the number of electrons that flow through the fuse wire if the charge on 1 electron is 1.6x10-19C.
3.
A current of of 2. 2.0A flflows th through a lamp. How long does it take for
5.
Draw Draw the the sym symbo bols ls of thes these e ele elect ctri rica call ite items ms.. Cells
ammeter voltmeter
Switch : open : closed
connecting wire resistor
bulb
2
rheostat 2.2
The relationship between electric current and potential difference [ ………/30 x 100 = …………. % ] A student student is able to to : • • • • • •
1.
define potential difference. describe the relationship between current and potential difference. state Ohm’s Law define resistance explain factors that affect resistance solve problems involving potential difference, current and resistance.
The ____________ _________________V _____V,, between two points in a circuit is defined as the ____________ _______________ ___ W W, in moving _______________ of charge from one point to the other point. It is measured with a ______ a ____________ ________ __ in in SI unit ______ unit _________ ___ V
2.
Sket Sketch ch a grap graph h to show show the the rel relat atio ions nshi hip p betw betwee een n pote potent ntia iall diff differ eren ence ce , V and current, I for a resistor that obeys Ohm’s law.
3.
State the Ohm’s law. Ohm’s law state that the ______ the __________ ____ that that passes through an ohmic conductor is ____________ ___________________ ______________ __________ ___ to the ____________ ____________________ ________ applied applied across it if the ______ the ____________ _____________ _______ and and other physical conditions are constant. V
By Ohm’s law, V I
4.
∝
I ,
= constant c onstant
[ known as the resistance ]
The ____________ ___________________ _______ of of a conductor is defined as the ratio of the potential difference, V, acro across ss the the cond conduc ucto torr to the the curr curren ent, t, I, flowi flowing ng throu through gh the conductor.
3
R= 5.
V I
In the table below, write down the relevant hypothesis for each of the expe experi rime ment nts s to inve invest stig igat ate e the the fact factor ors s affe affect ctin ing g the the resi resist stan ance ce,, R of the the conductor.
Factors e h t f o h t g n e L , r o t c u l a n o i t c e s s s o r c e h T e l a i r e t a m f o e p y t e h T r f o e r u t a r e p m e t e h T r o
Diagram
Hypothesis The resistance,R is _______________________ to the length of the conductor
l
6.
The resistance,R is _______________________ to the cross-sectional area of the conductor
Differ Different ent cond conduct uctors ors with with the same physical cond condit itio ions ns have have diff differ eren ent t resistance
The higher the temperature of the conductor, the higher the _________________
What What are are the the fact factor ors s aff affec ecti ting ng the the res resis ista tanc nce e of of a cond conduc ucto tor? r? a)
____________ _________ _______________ _________ ______
b)
____________ _________ _______________ _________ ______
c)
____________ _________ _______________ _________ ______
d)
____________ _________ _______________ _________ ______
4
Graph
Exercise In the the diag diagra rams ms belo below w, det dete ermin rmine e the the valu values es for for V, I and R .
1.
V=?
9V
(a)
(b)
1.5A
A
6Ω
6Ω
I=____
R=____
R=____
V=____ I =
V= = =
(c)
A
I=?
(d)
12V
240V
3mA
A
I=?
2kΩ
R=?
I= ___
R=____
V= ___
V=_____
R=
I=
5
A
2.3
Series and parallel circuits
[……../ 33 x 100 = …………..% ]
A students students is able to to : •
•
• • •
1.
identify series and parallel circuits. compare the current and potential difference of series circuits and parallel circuits. determine the effective resistance of resistors connected in series. determine the effective resistance of resistors connected in parallel. solve problems involving current, potential difference and resistance in series circuit, parallel circuits and their combinations.
Match the circuits below
Parallel circuit
Series circuit
2. Series circuit
Parallel circuit
1.
I = I 1
=
I 2
=
I 3
=
2.
V = V 1
+
V 2
+
V 3
R1 + R2
+
R3
3.
R
=
.....
1.
I = I 1 + I 2
+
....
2.
V = V 1
+
....
1
3.
4. Has only ____path for the current to flow. 5. The current is the ______ the ______ throughout throughout the circuit. 6. The component with the largest resistance has the ______ the ____________- potential difference across it. 7. ___ 7. ___ current current flows when the switch is
R
=
1 =
R1
+
V 2
=
I 3
V 3
1 +
R2
+
=
1 +
.....
R3
.....
+ ....
4. Has ______ Has __________ ____ path path for the current to flow 5. The current is different at __________ __________ points. 6. The potential different across all the components is the ______ the _________ ___ 7. Current stops only in the branch that is
6
open.
3.
open. Current __________ __________ to to flow in other branches. Determine the total effective resistance between point X and Y .
Answer Answer
Answer
7
Answer
8
Answer Answer
Answer Answer
Answer Answer
Finally
9
4. In the circuits below, determine the reading of the ammeter marked by ‘?’
(a)
(b) ____ (b) ____
(c) _____
(d) _________ _________
(e) ______ (e) _______ _ (f) _______ (g) ______ (g) _________ ___ (h) ______ (h) _________ ___
5. In the circuits below, determine the reading of the voltmeter marked by ‘?’
10
(a) ______ (a) ________ __
(d) ______ (d) _____________ _______
(b) __________
(c) ______ (c) _________ ___
(e) ______ (e) _______ _
(f) ______ (f) ______
5.
A 8Ω 8Ω resis resistor tor and and a 12Ω 12Ω resi resisto storr are are conne connecte cted d in in ser series ies.. The The curr curren entt in in the the 8Ω resistor is 1.5A and the potential difference across it is 12V. What is the current and the potential difference across the 12Ω resistance?
6.
When When two two iden identic tical al resi resisto stors rs are are con conne necte cted d in in para paralle llel, l, the the effec effectiv tive e resis resistan tance ce is 3Ω. What is the effective resistance if the two resistors are connected in series?
2.4
Electromotive force and internal resistance
11
[ ……/ 15 X 100% = ………..]
A student student is able to : Define electromotive force (e.m.f.). Explain internal resistance. • Determine e.m.f. and internal resistance. • Solve problems involving e.m.f. and internal resistance •
•
1.
What is meant by the electromotive force (e.m.f.)?
The electromotive force (e.m.f.) is defined as the ___________ ___________ by by a source in driving ______ driving ____________ ___________ _____ of of charge through the a complete circuit.
What is meant by the internal resistance (r) of a cell? The internal resistance, r, is the ________________________ a cell
3.
Calc Calcul ulat ate e the the amou amount nt of e.m. e.m.ff in in cir circu cuit it belo below w.
Answer
Answer
E=
E=
Answer
Answer
E=
E=
4.
12
The equation for the graph above is given by : V = -r I + E
V= potential difference r= internal resistance E= e.m.f. of cell I = Current
From the graph obtain the value of e.m.f, E and the internal resistance, r.
E= r=
5.
A dry dry cel celll has has an an elec electro tromot motive ive force force,, e.m. e.m.f. f. of of 3.0 3.0 V and and inter interna nall resis resistan tance, ce, r of of 0.5 Ω is connected in series with a resistor, R Ω. If a current of 0.4 A flows through the circuit, calculate (a) the potential difference across the resistor R
(b) resistance R
13
6.
A dry dry cel celll with with an intern internal al res resist istan ance ce of of 2.0Ω 2.0Ω is con conne necte cted d to to a bulb. bulb. The pote potenti ntial al difference across the bulb is 2.4V when the current flow is 0.3A. Determine the e.m.f of the dry cell.
7.
A ce cell of e.m e.m.f .f 12V an and in interna rnal re resis sistan tance r Ω is connected to a bulb with resistance 2Ω . The current in the circuit is 5A. What is the internal resistance of the cell?
2.5
Electrical energy and power
[……/ 17 x 100 = …………..% ]
14
A student student is able to to : • • •
Define electrical energy Define electric power Solve problems involving electrical energy and power
1.
_____________ ___________________ _________ ___ ,, E, is released when electric charges flow through any two points in an electric circuit.
2.
Elec Electr tric ical al ener energy gy,, E, E, is is mea measu sure red d in in __________ __________ . One ________ of ________ of electrical energy is released when ______ when ____________ __________ ____ of of electric charge flows through a potential difference of _____________ ________________ ___ Electrical energy, E = V Q
V, potential difference Q, number of charges
E = _________
[ substitute Q= It ]
= __________ [ substitute V= IR]
3.
_______________ is defined as the rate of energy dissipated or transferred. SI unit for power is ___________. Hence, Power =
timetaken
= VI
4.
Efficiency = =
energyoutput energyinput power ........... power ..........
x 100%
x 100%
Exercise
15
1.
The curre current nt in a toy toy car car is 7.0A 7.0A and and the volta voltage ge of the batte battery ry is 3.0V. 3.0V. Find Find the the power delivered to the motor and energy dissipated in the motor in 6.5 minutes of operation.
2.
A 40W 40W fluore fluoresce scent nt lamp lamp tran transfe sferr 60% 60% of the elect electric rical al energ energy y to to ligh lightt ene energy rgy.. How much light energy does it emit in two minutes.
3.
An electr electric ic kettl kettle e with with a pow power er of of 2kW 2kW is used used for for 10 minute minutes, s, three three times times a day day.. If the cost of electricity is 25 cent per unit, what is the cost of operating the kettle for 30 days?
7.
A mot motor or whic which h is conn connec ecte ted d to a 120 120V V DC volt voltag age e sou sourc rce e pro produ duce ces s 320W 320W of mechanical power. The current from the voltage source is 3.0A. (a) What is the input power of the motor?
(b) What is the efficiency of the motor?
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