2.1 CHARGE AND ELECTRIC CURRENT - - Get a Free Blog Here

Physics Module Form 5 Chapter 2- Electricity GCKL 2011

2-1

CHARGE AND ELECTRIC CURRENT

Van de Graaf

1. What is a Van de Graaff generator? Fill in each of the boxes the name of the part shown.

A device that ...................... and ........................................ at high voltage on its dome.

2. You will feel a brief _________ shock when your finger is brought close to the dome of the

generator.

(B) EXPLANATION

i. When the motor of the Van de Graaff generator is switched on, it drives the rubber belt. This

cause the rubber belt to rub against the roller and hence becomes _______ charged. The

charge is then carried by the moving belt up to the metal _______ where it is collected. A

large amount of _________ charge is built up on the dome.

ii. The electric field around the metal dome of the generator can produced a strong force of

___________ between the opposite charges. ___________ will suddenly accelerate from the

finger to the dome of the generator and causes a spark.

2.1

dome

+ + +

+

+

+

+ +

+

+


2-2

iii. When the wire touches the dome, the microammeter needle is deflected. This shows that a

__________ is flowing through the galvanometer.

iv. The electric current is produced by the flow of ____________ from earth through the

galvanometer to the metal dome to neutralize the positive charges on its surface.

v. The metal dome can be safely touched with the finger as all the positive charges on it have

been ________________.

2. What will happen if the charged dome of the

Van de Graaff is connected to the earth via a

microammeter? Explain.

There is a ................of the pointer of the

meter.

This indicates an electric current ..............

The microammeter needle is returned

to its .................................. position when the

Van de Graaf is switched off.

3. Predict what will happen if a discharging metal

sphere to the charged dome.

When the discharging metal sphere is

brought near the charged dome, .................

occurs.

An electric current ....................

4. The flow of electrical charges produces .........................................

+ + +

+ +

+ +

+

+

+ + + +

+


2-3

Electric Current

1. Electric current is defined as the ................................................................................................

2. In symbols, it is given as:

where I = ...............................

Q = ...............................

t = ...............................

(i) The SI unit of charge is (Ampere / Coulomb / Volt)

(ii) The SI unit of time is (minute / second / hour)

(iii) The SI unit of current is (Ampere / Coulomb / Volt) is equivalent to (Cs // C-1

s // Cs-1

)

(iv) By rearranging the above formula, Q = ( It / t

I / I

t )

3. 1 Coulomb (C) = 1 Ampere Second (As)

4. Example :

Charge of 1 electron = ……………………..

Charge of 1 proton = …………………….

5. Total Charge :

I = Q

t


2-4

Electric Field

a. An electric field is a ................in which an ......................... experiences a .........................

b. An electric field can be represented by a number of lines indicate both the ................ and

....................of the field.

c. The principles involved in drawing electric field lines are :

(i) electric field lines always extend from a .............................................. object to a

.........................-charged object to infinity, or from ................. to a ..................-charged object,

(ii) electric field lines never ..................... each other,

(iii) electric field lines are ...................in a ....................... electric field.

EFFECT OF AN ELECTRIC FIELD ON A PING PONG BALL

(a)

(b)

(c)

Observation:

(a) The ball will still remain ..........................

This is because the force exert on the ball by the

............................ plate is .................. to the

force exerted on it by the ........................ plate.

(b) If the ping pong ball is displaced to the right

to touch the ............................... plate, it will

then be charged with ........................... charge

and will be pushed .......................... the

.......................... plate.

(c) When the ping pong ball touches the

........................... plate, it will be charged with

........................... charge and will be pushed

........................... the ............................. plate.

This process repeats again and again, causes the

ping pong ball ............................ to and fro

continuously between the two plates.


2-5

Conclusion

1. Electric field is a ......................................................................................................

2. Like charges .................. each other but opposite charges ........................ each other.

3. Electric field lines are ...................... in an electric field. The direction of the field lines is

from ................... to ..........................

EXERCISE 2.1

1. 5 C of charge flows through a wire in 10 s. What is the current in the wire?

2. A charge of 300 C flow through a bulb in every 2 minutes. What is the electric current in the

bulb?

3. The current in a lamp is 0.2 A. Calculate the amount of electric charge that passes through

the lamp in 1 hour.

4. If a current of 0.8 A flows in a wire, how many electrons pass through the wire in one

minute? (Given: The charge on an electron is 1.6 x 10-19

C)


2-6

An electric current of 200 mA flows through a resistor for 3 seconds, what is the

(a) electric charge

(b) the number of electrons which flow through the resistor?

IDEAS OF POTENTIAL DIFFERENCE

(a)

(b)

Pressure at point P is ................... than the

pressure at point Q

Water will flow from .... to .....when the

valve is opened.

This due to the ............... in the pressure of

water

Gravitational potential energy at X is ........

than the gravitational potential energy at Y.

The apple will fall from ... to ...when the apple

is released.

This due to the ................... in the gravitational

potential energy.

P Q

X

Y

2.2


2-7

(c) Similarly,

Point A is connected to .............terminal

Point B is connected to ..............terminal

Electric potential at A is ......................... than the electric

potential at B.

Electric current flows from A to B, passing the bulb in

the circuit and .........................the bulb.

This is due to the electric ............................... between

the two terminals.

As the charges flow from A to B, work is done when

electrical energy is transformed to ......... and .......energy.

The .................................... between two points in a

circuit is defined as the amount of work done, W when

one coulomb of charge passes from one point to the

other point in an electric field.

The potential difference,V between the two points will

be given by:

where W is work or energy in Joule (J)

Q is charge in Coulomb (C)

A B

Bulb

V = echQuantityof

Work

arg = Q

W


2-8

EXPERIMENT 1: TO INVESTIGATE THE RELATIONSHIP BETWEEN CURRENT

AND POTENTIAL DIFFERENCE FOR AN OHMIC CONDUCTOR.

(a) (b)

Figure (a) and figure (b) show two electrical circuits. Why do the ammeters show different readings?

Why do the bulbs light up with different intensity?

Referring to the figure (a) and (b) complete the following table:

(a) Inference The current flowing through the bulb is influenced by the potential difference

across it. (b) Hypothesis

(c) Aim To determine the relationship between current and potential difference for a

constantan wire. (d) Variables

(i) manipulated variable :

(ii) responding variable :

(iii) fixed variable :

Apparatus / materials :


2-9

Method :

1. Set up the apparatus as shown in the figure.

2. Turn on the switch and adjust the rheostat so that the ammeter reads

the current, I= 0.2 A.

3. Read and record the potential difference, V across the wire.

Tabulation of

data

:

Current,I/A Volt, V/V

0.2 1.0

0.3 1.5

0.4 2.0

0.5 2.5

0.6 3.0

0.7 3.5

Analysis of data : Draw a graph of V against I .


2-10

Discussion

: 1. From the graph plotted.

(a) What is the shape of the V-I graph?

......................................................................................................................I

is a straight line that passes through origin

(b) What is the relationship between V and I?

.......................................................................................................................

.......................................................................................................................

2. The resistance, R, of the constantan wire used in the experiment is equal

to the gradient of the V-I graph. Determine the value of R.

3. What is the function of the rheostat in the circuit?

......................................................................................................................

Conclusion :

rough it increases as long as

Ohm’s Law

(a)

Ohm’s law states

that the electric current, I flowing through a conductor is directly proportional to the

potential difference across the ends of the ohmic conductor,

if temperature and other physical conditions remain constant

(b) By Ohm’s law: V I

I

V= constant


2-11

(c) The constant is known as .................................... of the conductor.

(d) The unit of resistance is …………………………………

Factors Affecting Resistance

1. The resistance of a conductor is a measure of the ability of the conductor to (resist / allow) the

flow of an electric current through it.

2. From the formula V = IR, the current I is (directly / inversely) proportional to the resistance,

R.

3. Write down the relevant hypothesis for the factors affecting the resistance in the table below.

4. From, the following can be stated:

Hence, resistance of a conductor, R

Factors Diagram Hypothesis Graph

Len

gth

of

the

con

du

cto

r, l

Th

e cr

oss

-sec

tio

nal

area

of

the

con

du

cto

r, A

Th

e ty

pe

of

the

mat

eria

l o

f th

e

con

du

cto

r

Th

e te

mp

erat

ure

of

the

con

du

cto

r


2-12

So R or R = where = resistivity of the

substance

5. i) Electric charge, Q = ( It / t

I /

I

t )

ii) Work done, W = (QV / Q

V /

V

Q )

EXERCISE 2.2

1. If a charge of 5.0 C flows through a wire and the amount of electrical energy converted into heat

is 2.5 J. Calculate the potential differences across the ends of the wire.

2. A light bulb is switched on for a period of time. In that period of time, 5 C of charges passed

through it and 25 J of electrical energy is converted to light and heat energy. What is the potential

difference across the bulb?

3. The potential difference of 10 V is used to operate an electric motor. How much work is done in

moving 3 C of electric charge through the motor?


2-13

4. When the potential difference across a bulb is 20 V,

the current flow is 3 A. How much work done to

transform electrical energy to light and heat energy

in 50 s?

5. What is the potential difference across a light bulb of

resistance 5 when the current that passes through it

is 0.5 A?

6. What is the value of the resistor in the figure, if the dry

cells supply 2.0 V and the ammeter reading is 0.5 A?

7. If the bulb in the figure has a resistance of 6 , what is

the reading shown on the ammeter, if the dry cells

supply 3 V?

8. If a current of 0.5 A flows through the resistor of 3

in the figure, calculate the voltage supplied by the dry

cells?

3 A

A 20 V

Bulb


2-14

9. Referring to the diagram on the right, calculate

(a) The current flowing through the resistor.

(b) The amount of electric charge that passes through

the resistor in 30 s

(c) The amount of work done to transform the electric

energy to the heat energy in 30 s.

10. The graph shows the relationship between the potential

difference, V and current, I flowing through two

conductors, X and Y.

a) Calculate the resistance of conductor X.

b) Calculate the resistance of conductor Y.

c) If the cross sectional area of X is 5.0 x 10-6

m2, and the length of X is 1.2 m, calculate its

resistivity.

I/A

V/V

0 0

X

Y

2

8

2

12 V

5 I

I


2-15

SERIES AND PARALLEL CIRCUITS

Current Flow and Potential Difference in Series and Parallel Circuit

SERIES CIRCUIT PARALLEL CIRCUIT

1. Effective Resistance:

R =

2. Current:

3. Potential Difference:

V =

1. Effective Resistance:

R =

2. Current:

3. Potential Difference:

V =

Effective resistance, R

(a)

(b)

2.3


2-16

(c)

(d)

(e)

(f)

(g)

(h)

EXERCISE 2.3

1. The two bulbs in the figure have a resistance of 2 and 3

respectively. If the voltage of the dry cell is 2.5 V, calculate

(a) the effective resistance, R of the circuit

(b) the main current, I in the circuit (c) the potential difference across each bulb.


2-17

2.

There are two resistors in the circuit shown. Resistor R1 has a

resistance of 1. If a 3V voltage causes a current of 0.5A to flow

through the circuit, calculate the resistance of R2.

3.

The electrical current flowing through each branch, I1 and I2, is 5

A. Both bulbs have the same resistance, which is 2. Calculate

the voltage supplied.

4. The voltage supplied to the parallel is 3 V. R1 and R2

have a resistance of 5 and 20. Calculate

(a) the potential difference across each resistor

(b) the effective resistance, R of the circuit

(c) the main current, I in the circuit (d) the current passing through each resistor


2-18

ELECTROMOTIVE FORCE AND

INTERNAL RESISTANCE

Electromotive force

Figure (a) Figure (b)

1. An electrical circuit is set up as shown in figure (a). A high resistance voltmeter is connected

across a dry cell which labeled 1.5 V.

a) Figure (a) is (an open circuit / a closed circuit)

b) There is (current flowing / no current flowing) in the circuit. The bulb (does not light up /

lights up)

c) The voltmeter reading shows the (amount of current flow across the dry cell / potential difference

across the dry cell)

No current flow

R

Voltmeter reading,

e.m.f.

Voltmeter reading,

potential difference, V < e.m.f., E

E , r

Current flowing

2.4


2-19

2. The switch is then closed as shown in figure (b).

a) Figure (b) is (an open circuit / a closed circuit)

b) There is (current flowing / no current flowing) in the circuit. The bulb (does not light up / lights

up)

c) The voltmeter reading is the (potential difference across the dry cell / potential difference across

the bulb / electromotive force).

d) The reading of the voltmeter when the switch is closed is (lower than/ the same as / higher than)

when the switch is open.

e) State the relationship between e.m.f , E , potential difference across the bulb, VR and drop in

potential difference due to internal resistance, Vr.

3.

a) Why is the potential difference across the resistor not the same as the e.m.f. of the battery?

The potential drops as much as V across the internal resistance

b) Determine the value of the internal resistance.

Since E = V + Ir

= + r

r =

Therefore, the value of the internal resistance is


2-20

EXERCISE 2.4

1

A voltmeter connected directly across a battery gives a reading of

1.5 V. The voltmeter reading drops to 1.35 V when a bulb is

connected to the battery and the ammeter reading is 0.3 A. Find the

internal resistance of the battery.

2 A circuit contains a cell of e.m.f 3.0 V and internal resistance, r. If the external resistor has a

value of 10.0 and the potential difference across it is 2.5 V, find the value of the current, I in the

circuit and the internal resistance, r.

3 A simple circuit consisting of a 2 V dry cell with an internal resistance of 0.5. When the

switch is closed, the ammeter reading is 0.4 A.

Calculate

(a) the voltmeter reading in open circuit

(b) the resistance, R (c) the voltmeter reading in closed circuit


2-21

4

Find the voltmeter reading and the resistance, R of the

resistor.

5 A cell of e.m.f., E and internal resistor, r is connected to a rheostat. The ammeter reading, I and

the voltmeter reading, V are recorded for different resistance, R of the rheostat. The graph of V

against I is as shown.

From the graph, determine

a) the electromotive force, e.m.f., E

b) b) the internal resistor, r of the cell

e.m.f.

6

2

2 /A

/ V


2-22

ELECTRICAL ENERGY AND POWER

Electrical Energy

Electrical Energy and Electrical Power

1. Potential difference, V across two points is the energy,E dissipated or transferred by a

coulomb of charge, Q that moves across the two points.

2. Therefore,

3. Hence,

4. Power is defined as the rate of energy dissipated or transferred.

5. Hence,

Electrical Energy, E Electrical Power, P

From the definition of

potential difference, V

Power is the rate of transfer of electrical

energy,

Electrical energy converted, E

; where Q = It

Hence,

; where V = IR

Hence,

; where I = V

R

SI unit :

SI unit :

Electrical energy dissipated, E

Charge, Q

Potential difference, V =

E = VQ

Energy dissipated, E

time, t

Power, P =

2.5


2-23

Power Rating and Energy Consumption of Various Electrical Appliances

1. The amount of electrical energy consumed in a given period of time can be calculated by

Energy consumed = Power rating x Time

E = Pt where energy, E is in Joules

power, P is in watts

time, t is in seconds

COST OF ENERGY

Appliance Quantity Power / W Power / kW Time

Energy

Consumed

(kWh)

Bulb 5 60 8 hours

Refrigerator 1 400 24 hours

Kettle 1 1500 3 hours

Iron 1 1000 2 hours

Total energy consumed, E =

= kWh

Cost = kWh x RM 0.28

= RM


2-24

EXERCISE 2.5

1. How much power dissipated in the bulb?

(a)

(b)

2.

Calculate :

(a) the current, I in the circuit (b) the energy released in R 1 in 10 s.

(b) the electrical energy supplied by the battery in 10 s.

5 V

R = 10

5 V

R = 10

R = 10

R1=2 R2=4 R3=4

V= 15V I


2-25

3. An electric motor is used to lift a load of mass 2 kg to a height 5 m in 2.5 s. If the supply voltage is

12 V and the flow of current in the motor is 5.0 A, calculate

(a) Energy input to the motor

(b) Useful energy output of the motor

(c) Efficiency of the motor


2-26

REINFORCEMENT EXERCISE CHAPTER 2

Part A: Objective Questions

Which of the following diagrams shows

the correct electric field?

2.

Diagram1show a lamp connected to a

resistor and a battery.

Calculate the power used by the light bulb.

A 6 W

B 12 W

C 20 W

D 50 W

3. When the switch is on, the current that

flows in an electronic advertisement board

is 3.0 x 10 -5

A. What is the number of

electrons flowing in the advertisement

board when it is switched on for 2 hours ?

[ Charge of an electron = 1.6 x 10 -19

C ]

A 3.84 x 1011

B 1.67 x 1014

C 1.35 x 1018

4. A current of 5 A flows through an

electric heater when it is connected to

the 240 V main supply. How much

heat is released after 2 minutes?

A 1 200 J

B 2 400 J

C 14 400 J

D 144 000 J

5. An electric bulb is labeled “240V,

60W”. How much energy is used by

the bulb in one minute if the bulb is

connected to a 240V power supply?

A 60 J

B 360 J

C 600 J

D 3600 J

6. The diagram shows a cell of negligible

internal resistance connected to two

resistors

What is the value of current, I?

A 0.45 A

B 0.40 A

C 0.25 A

Diagram 1

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