7.2 ohms law

X

Y

A BBulb

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.

7.2

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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,E Charge, Q Charge,Q

SI unit is Volt (V) = Joule per coulomb


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

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1.Name the device used to measure electrical current.

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 an electrical 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 negative terminal of the dry cell?

1. Name the device used to measure potential difference.

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

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

3. How is an voltmeter connected in an electrical 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 to find 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 the readings. 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 ohmic conductor 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 is directly proportional to the potential difference across it if the temperature and 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 Law ancal

cond

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iFrom Ohm’s Law, V I or

= 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 ratio of the potential difference (V) across the conductor to the current (I) flowing through it.

Ohm’s equation = V = IR V = potential difference I = electric current

R = resistance


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

Factors Diagram Hypothesis Graph

Len

gth

of

the

con

du

cto

r, l

The longer the length of the conductor, the __________ its resistanceResistance is __________ proportional to the length of a conductor

Th

e cr

oss

-se

ctio

nal

are

a o

f th

e c

on

du

cto

r, A

The larger the cross - sectional area, the ____________ the its resistanceResistance is __________ proportional to the cross-sectional area of a conductor

Th

e ty

pe

of

the

mat

eria

l of

the

co

nd

uct

or

Different conductors with the same physical conditions have ________ resistance

Th

e te

mp

erat

ure

o

f th

e co

nd

uct

or The higher the temperature

of a conductor, the _________ the resistance

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

Resistance of a conductor, and cross-

Hence, resistance of a conductor, or

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.

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 atoms that exist 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 a computer.

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 the electrical energy to turn into heat , so some electrical energy is lost along the way if we are trying to transmit electricity from one place to another through conductor.

Advantage of ResistanceIt is resistance that allows us to use electricity for heat and light. The heat is generated from electric heaters or the light that we get from light bulbs is due to the resistance of the wire. In a light bulb, the current flowing through a resistance filament causes it to become hot and then glow.

A superconductor is a material whose resistance becomes zero when its temperature drops to a certain value called the critical temperature.

This enables superconductors to maintain a current with no applied voltage 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 to another.

(d) 2 volt is two joules of work done to move 2 coulomb of charge from 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 heat 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 passed through it and 25 J of electrical energy is converted to light and heat energy. What is the potential 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 to transform electrical energy to light and heat energy in 50 s?

5. 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?

V = IR

6. The graph shows the result of an experiment to determine the resistance of a wire. The resistance of the wire is

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3 A A

20 V

Bulb

V/V

I/A0 5

1.2


TUTORIAL 7.2

1. Which graphs shows the relationship between potential difference, V, and current, I, for a resistor that obeys Ohm’s law? (2003)

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

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

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

Y is high

B. the resistance of the cable across X and Y is very high

C. the body of the bird has a low resistance

D. the current flowing through its body is very small

4. Diagram below is a graph shows the relationship between the potential difference and the current of four different conductors, P, Q, R and S. (2006)

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

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

Calculate the electrical energy released by 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 not influence the resistance of a wire? (2007)A. Length of wireB. Material of the wireC. Cross-sectional area of the wire

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D. Hardness of the wire7. Figure shows the circuit used to

investigate the relationship between potential difference, V and current, I for a piece of constantan wire. The graph of V against I from the experiment is as shown in the figure below.

(a) What quantities are kept constant in this experiment?

_____________________________

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

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

____________________________increases, hence the gradient

incriii) a shorter constantan wire is used

____________________________

8. Diagram below shows an electric circuit which is used to investigate the relationship between electric and potential difference across a conductor XY.

(a) What is the function of the voltmeter?

(b) Underline the correct answer in the bracket to complete the sentence below.

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 which is kept constant in the experiment.

…………………………………………..

9. Figure 6 shows the graph of current against potential difference across three conductors P, Q and R.

(a) Among the three conductors, which one or ones obey Ohm’s law? Give reason.

_____________________________

_____________________________

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(b) State Ohm’s law.

_____________________________

_____________________________

(c) The resistance R of a conductor is given as:

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

3. Figure 4.1(a) shows a filament light bulb labeled “240 V, 100 W”. Figure 4.1(b) shows a graph of potential difference against current for two materials, P and Q. One of the materials will be chosen as filament in a bulb.

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

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7.2 ohms law