7.2 ohms law 09

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X

Y

A

Bulb

Sek Men Sains Muzaffar Syah

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.

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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

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

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Device and symbol

ammeter Cells

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

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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 of current?

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 of the ammeter is connected to the negativeterminal of the dry cell?

1. Name the device used to measure potentialdifference.

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

(b) What is the symbol for the unit of potential difference?

3. How is an voltmeter connected in anelectrical circuit?

4. The positive terminal of a voltmeter is

connected to which terminal of the dry cell?

Experiment: 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?

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

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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

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Ohm’s 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 Ohm’s Lawancal

cond

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I

V =


i From Ohm’s Law, V ∝ I or

I

V = constant = gradient

The graph of V against I is 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 Ohm’s law, while a conductor which does

not obey Ohm’s 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) …………………………………………………………….

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

Ohm’s equation = V = IR V = potential differenceI = electric currentR = resistance

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

Factors Diagram Hypothesis Graph

L e n g t h o f

t h e

c o n d u c t o

r , l

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

T h e c r o s s - s e c t i o n a l

a r e a o f t h e

A

The larger the cross -sectional area, the

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

T h e t y p e o f t h e

m a t e r i a l o f t h e

u c t o r

Different conductors with thesame physical conditionshave ________ resistance

T h e t e m p

e r a t u r e o f

t h e c o n d u c t o r

The higher the temperature of a conductor, the _________the resistance

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

Resistance of a conductor, l R ∝ and A

R1

∝ cross-

Hence, resistance of a conductor, A

l R ∝ or

A

l R ρ =

where = resistivity of the substance

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

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• 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. Transportvehicles such as trains can be made to ‘float’ on strong superconducting magnets, virtuallyeliminating friction between the train and its tracks.

•

Magnetic resonance imaging (MRI) is to determine what is going on inside the human body. Byexposing the body to a strong superconductor-derived magnetic field, hydrogen atoms thatexist in the body’s water and fat molecules are forced to accept energy from the magnetic field.They then release this energy at a frequency that can be detected and displayed graphically by acomputer.

• Electric cable made of superconductors will increase the efficiency of electrical power transmission 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 place toanother 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.

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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 of work is required to move a charge of 1 coulomb from one point toanother.

(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 into heatis 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 in 50s?

5. What is the value of the resistor in the figure, if the drycells 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

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1. Which graphs shows the relationshipbetween potential difference, V, andcurrent, I, for a resistor that obeys Ohm’slaw? (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 body isvery small

4. Diagram below is a graph shows the

relationship between the potentialdifference and the current of four differentconductors, 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 released bythe 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

7. Figure shows the circuit used toinvestigate the relationship between potential

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difference, V and current, I for a piece of constantan wire. The graph of V against Ifrom the experiment is as shown in the figurebelow.

(a) What quantities are kept constantin this experiment?

_____________________________

(b) State the changes in the gradient of the graph, if i) the constantan wire is heated

____________________________

ii) a constantan wire of a smaller cross-sectional area is used

____________________________increases, hence the gradient

incr iii) a shorter constantan wire is used

____________________________

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

(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 Ohm’s law? Givereason.

_____________________________

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7.2 ohms law 09