7/29/2019 7.2 ohms law 09.doc

1/12

X

Y

A

Bulb

Ideas of Potential Difference

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 is due to the difference in the gravitational potential energy.

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.

1 Volt = 1 joule per coulomb. The potential difference across two points in a circuit is 1 Volt if 1 Joule of work is done in

moving 1 Coulomb of charge from one point to the other.

7.2

62

The potential difference, V between two points in a circuit is defined as the work

done when 1 C of charge moves between two points in an electric field.

Potential difference, V = Work done, W or V = Energy,ECharge, Q Charge,Q

Q

E

Q

WV == SI unit is Volt (V) = Joule per coulomb

7/29/2019 7.2 ohms law 09.doc

2/12

Device and symbol

ammeterCells

voltmeter Switch

connecting wireConstantan wire //eureka wire

resistance

bulb

rheostat

Measuring Current and Potential Difference/Voltage

Measurement of electricity Measurement of potential difference/voltage

(a) Electrical circuit (a) Electrical circuit

(b) Circuit diagram (b) Circuit diagram

63

7/29/2019 7.2 ohms law 09.doc

3/12

1. Name the device used to measure electricalcurrent.

2. (a) What is the SI unit for current?

(b) What is the symbol for the unit ofcurrent?

3. How is an ammeter connected in anelectrical circuit?

4. The positive terminal of an ammeter is

connected to which terminal of the dry cell?

5. What will happen if the positive terminal ofthe ammeter is connected to the negativeterminal of the dry cell?

1. Name the device used to measurepotential difference.

2.(a) What is the SI unit for potentialdifference?

(b) What is the symbol for the unit ofpotential difference?

3. How is an voltmeter connected in anelectrical circuit?

4. The positive terminal of a voltmeter is

connected to which terminal of the drycell?

Experiment: To investigate the relationship between current and potential difference for anohmic 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?

Do Experiment 2.4 in Practical Book page 35 and make a PEKA report.

64

7/29/2019 7.2 ohms law 09.doc

4/12

Referring to the figure (a) and (b) complete the following table to plan & conduct an experiment tofind the relationship between current and potential difference.

Aim:

Hypothesis:

Variables

Procedure

Control MV

Measure RV

RepeatTabulateAnalyze

Manipulated variable:

Responding variable:

Controlled variables:

Apparatus: Rheostat, constantan wire, switch, connecting wire, batteries,ammeter, voltmeter

Turn on the switch and adjust the rheostat until the ammeter reads the current, I= 0.2 A.

Read the value of the potential difference, V, from the voltmeter. Record thereadings.Repeat the experiment for I = 0.3 A, 0.4 A, 0.5 A, 0.6 ATabulate the data.Plot a graph of V against I.

Repeat the experiment by replacing the constantan wire, which is ohmicconductor with an electric bulb which is a non-ohmic conductor.

Constantan wire Light bulbI/ A V / V I/ A V / V

65

7/29/2019 7.2 ohms law 09.doc

5/12

Ohms law states that the electric current, I flowing through a conductor isdirectly proportional to the potential difference across it if the temperatureand other physical conditions are constant.

Discussion1. Sketch the graph V vs I and describe the shape of each graph.

ohmic conductor constantant wire non ohmic conductor light bulb

2. What is the relationship between V and I for the ohmnic conductor?

_________________________________________________________________________3. Find the gradient of the graph for the ohmic conductor.

4. What is the physical quantity represented by the gradient of the graph?

_________________________________________________________________________

5. Why are the two graphs in the experiment different from each other?

_________________________________________________________________________

_________________________________________________________________________

6. What is the function of the rheostat in the circuit?_________________________________________________________________________

7. What precautions must you take in order to obtain more accurate results?

________________________________________________________________________________

__________________________________________________________________

Describe the relationship between current and potential difference

The greater the potential difference or voltage, the greater the current flow. When the potential difference between two points in a circuit increases, the current flowing

through it increases.

When the potential difference (V) between the points decreases, the current (I) decreases. The potential difference is directly proportional to the current flowing through it.

State Ohms Lawancal

cond

66

7/29/2019 7.2 ohms law 09.doc

6/12

I

V=

iFrom Ohms Law, V I or

I

V= constant = gradient

The graph of V against Iis straight line through the origin.

The gradient of the graph is constant.

Define Resistance, R

The unit of resistance is _________________

An _____________ ____________ is one which obeys Ohms law, while a conductor which

does not obey Ohms law is known as a _____________ conductor

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. When the value of the resistance, R is large, the current, I flowing in the conductor is

(small / large)

4. What are the factors affecting the resistance of a conductor?

a) .

b) .

c) .

d) .

67

The resistance (R) of a conductor is defined as the ratioof the potential difference (V) across the conductor to thecurrent (I) flowing through it.

Ohms equation = V = IR V = potential differenceI = electric currentR = resistance

7/29/2019 7.2 ohms law 09.doc

7/12

5. Write down the relevant hypothesis for the factors affecting the resistance in the tablebelow.

Factors Diagram Hypothesis Graph

Lengthof

the

conducto

r,l

The longer the length of theconductor, the __________its resistanceResistance is __________proportional to the length ofa conductor

Thecross-sectional

areaofthe

A

The larger the cross -sectional area, the

____________ the itsresistanceResistance is __________proportional to the cross-sectional area of a conductor

Thetypeofthe

materialofthe

ctor

Different conductors with thesame physical conditionshave ________ resistance

Thetemp

eratureof

theconductor

The higher the temperatureof a conductor, the

_________ the resistance

6. From the table above, the following can be stated:

Resistance of a conductor, lR andA

R1

cross-

Hence, resistance of a conductor,A

lR or

A

lR =

where = resistivity of the substance

Do Experiment 2.5 in Practical Book page 37. Make a PEKA report.

68

7/29/2019 7.2 ohms law 09.doc

8/12

The resistance of a metal increases with temperature The resistance of a semiconductor decreases with temperature.

Applications of Superconductor

Magnetic-levitation is an application where superconductors perform extremely well.Transport vehicles such as trains can be made to float on strong superconducting magnets,virtually eliminating friction between the train and its tracks.

Magnetic resonance imaging (MRI) is to determine what is going on inside the human body.By exposing the body to a strong superconductor-derived magnetic field, hydrogen atomsthat exist in the bodys water and fat molecules are forced to accept energy from the magneticfield. They then release this energy at a frequency that can be detected and displayedgraphically by a computer.

Electric cable made of superconductors will increase the efficiency of electrical powertransmission as the loss of energy in the form of heat is greatly reduced.

Disadvantage of ResistanceResistance causes some of theelectrical energy to turn into heat ,so some electrical energy is lostalong the way if we are trying totransmit electricity from one placeto another through conductor.

Advantage of ResistanceIt is resistance that allows us to useelectricity for heat and light. The heat isgenerated from electric heaters or the lightthat we get from light bulbs is due to the

resistance of the wire. In a light bulb, thecurrent flowing through a resistancefilament causes it to become hot and thenglow.

A superconductor is a material whoseresistance becomes zero when itstemperature drops to a certain valuecalled the critical temperature.

This enables superconductors tomaintain a current with no appliedvoltage at that temperature.

Able to sustain large currentsSmaller power loss during transmissionLess heat energy is wastedSmall-sized motors and generators can be used.

69

7/29/2019 7.2 ohms law 09.doc

9/12

Exercise 1 : Solve problems involving potential difference, current and resistance

1. Tick () the correct answers

True False

(a) Unit of potential difference is J C

-1

(b) J C-1 volt, V

(c)The potential difference between two points is 1 volt if 1 joule ofwork is required to move a charge of 1 coulomb from one pointto another.

(d)2 volt is two joules of work done to move 2 coulomb of chargefrom one to another in an electric field.

(e) Potential difference Voltage

2. If a charge of 5.0 C flows through a wire and the amount of electrical energy converted intoheat is 2.5 J. Calculate the potential differences across the ends of the wire.

3. A light bulb is switched on for a period of time. In that period of time, 5 C of charges passedthrough it and 25 J of electrical energy is converted to light and heat energy. What is thepotential difference across the bulb?

4. When the potential difference across a bulb is 20 V,the current flow is 3 A. How much work done totransform electrical energy to light and heat energy in50 s?

5. What is the value of the resistor in the figure, if thedry cells supply 2.0 V and the ammeter reading is 0.5 A?

V = IR

6. The graph shows the result of an experiment todetermine the resistance of a wire. The resistance of thewire is

TUTORIAL 7.2

70

3 A

A

20

V

Bulb

V/V

I/A0 5

1.2

7/29/2019 7.2 ohms law 09.doc

10/12

1. Which graphs shows the relationshipbetween potential difference, V, andcurrent, I, for a resistor that obeys Ohmslaw? (2003)

2. Which circuit can be used to determinethe resistance of a bulb? (2003)

3. The diagram shows a bird perched on ahigh voltage cable.

The bird does note experience an electricshock because . (2005)A. the potential difference across X and

Y is highB. the resistance of the cable across X

and Y is very high

C. the body of the bird has a lowresistance

D. the current flowing through its bodyis very small

4. Diagram below is a graph shows the

relationship between the potentialdifference and the current of fourdifferent conductors, P, Q, R and S.(2006)

Which conductor has the highestresistance?A. P C. RB. Q D. S

5. Diagram below shows an electric circuit.The reading of the ammeter is 0.2 A andthe reading of the voltmeter is 2.8 V.

Calculate the electrical energy releasedby the bulb in 2 minutes. (2006)A. 0.56 JB. 1.12 JC. 28.00 JD. 67.20 JE. 1680.00 J

6. Which of the following factors does notinfluence the resistance of a wire? (2007)A. Length of wireB. Material of the wireC. Cross-sectional area of the wireD. Hardness of the wire

71

7/29/2019 7.2 ohms law 09.doc

11/12

7. Figure shows the circuit used toinvestigate the relationship betweenpotential difference, V and current, I for apiece of constantan wire. The graph of Vagainst I from the experiment is as shown inthe figure below.

(a) What quantities are kept constantin this experiment?

_____________________________

(b) State the changes in the gradient ofthe graph, ifi) the constantan wire is heated

____________________________

ii) a constantan wire of a smallercross-sectional area is used

____________________________increases, hence the gradient

incriii) a shorter constantan wire is used

____________________________

8. Diagram below shows an electric circuitwhich is used to investigate therelationship between electric andpotential difference across a conductorXY.

(a) What is the function of the voltmeter?

(b) Underline the correct answer inthe bracket to complete the sentencebelow.

When the electric current increases,(i) the potential difference (increases,decreases, remains unchanged)

(ii) the resistance will (increase,decrease, remain unchanged)

(c) State one physical quantity whichis kept constant in the experiment.

..

9. Figure 6 shows the graph of currentagainst potential difference across three

conductors P, Q and R.

(a) Among the three conductors, whichone or ones obey Ohms law? Givereason.

_____________________________

72

7/29/2019 7.2 ohms law 09.doc

12/12

_____________________________

(b) State Ohms law.

_____________________________

_____________________________

(c) The resistance R of a conductor isgiven as:

I

VR =

What is the resistance of X when acurrent of 0.4 A flows through it?

3. Figure 4.1(a) shows a filament light bulblabeled 240 V, 100 W. Figure 4.1(b)shows a graph of potential differenceagainst current for two materials, P andQ. One of the materials will be chosen asfilament in a bulb.

Based on the graph in Figure 4.1(b),compare the resistance of materials Pand Q. Choose the more suitablematerial to be used as filament bulb.Explain your choice.

73