Chapter 2 - Electricity - Lesson 16-17

8/7/2019 Chapter 2 - Electricity - Lesson 16-17

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LESSON 16 - Electric FieldBy the end of this lesson, you should be able to: Describe an electric field. Sketch electric field lines showing thedirection of the field.

Describe the effect of an electric charge on acharge. Solve problems involving electric charge andcurrent,

When the plastic comb and the plastic ruler are rubbed, they

are charged. The charged plastic comb and the charged plastic

ruler can attract the bits of paper and the fine stream of water

even though they are not in contact.

However, nothing happens to the bits of paper or the fine

stream of water when the charged plastic comb and thecharged plastic ruler is far.


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This charged object has an electric field around itself. Any

other charged objects which come into the field will

experience a force-

The electric field is strongest near the charged object, hence, theforce is strongest there. The field gets weaker as you move

further away from the charged object.

The electric field pattern between two parallel charged plates.

The direction of the electric field is from the positive electrode to

the negative electrode. Field lines do not cross each other The

field lines between two plates are parallel and equally spaced,

except near the edges. Thus, the electric field is uniform betweentwo parallel plates except at the edges.


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When a higher potential difference is applied, the number of field

lines increases. The closer the lines, the stronger the electric

field within that region.

The electric field direction is always directed away from the

positive charge and towards the negative charge.

The density of lines indicates the strength of the field. More lines

indicate stronger electric field which is due to a larger quantity of

charge.


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Observe the electric field line pattern when different

electrodes are used:

A straight electrode and a point electrode.

Two different charges on point electrodes.

Two same charges on point electrodes.


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When the ball touches the positive plate, it loses some negative

charges and is immediately repelled from the positive plate and

attracted to the negative plate.

On touching the negative plate, the ball collects negative charges

and is repelled back towards the positive plate.

The effect of an electric field on a candle flame.


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The heat of the flame ionises the air molecules to become

positive and negative charges.

When a voltage is applied between the two plates, positive

charges are attracted to the negative plate; while the

negative charges are attracted to the positive plate.

The positive charges are heavier than the negative charges.As the positive charges move towards the negative plate,

they drag along most of the flame,

This causes the flame to disperse more to the negative

plate.

ANALYSING THE RELATIONSHIP BETWEEN ELECTIC CURRENT AND POTENTIALDIFFERENCELESSON 17 - Potential Difference

By the end of this lesson/ you should be able to: Define potential difference. Determine the potential difference in an electric circuit.. Relate potential difference with the output energy.

Negatively charged clouds induced positive charges to build up on the


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ground. Hence, an electric field is formed between the clouds and the

ground.

The movement of these charges is due to the potential difference

between

the clouds and the ground. So, what is potential difference?

What is the force that opposes the motion as the book islifted? Are you doing any work when lifting up the book?

When you lift a book from a table, you are moving the book

against the gravitational force. In other words, you are doing

work against the gravitational force to lift the book up.

Name the energy obtained by the book when it is at position

A and position B. Compare the height and the energy at both

positions- Which position has more energy?


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When an object is at a certain height from the ground, it has

gravitational potential energy (GPE).

The gravitational potential energy (GPEB) at B is greater than

the gravitational potential energy (GPEA) at A because

position B is higher than A.

When moving the book from position A to position B/ you are

doing work against the gravitational force.

And the work done from position A to position B is the

difference in their gravitational potential energy.

Work done, WAB= difference in potential energy

=GPEB-GPEA

=mghB mghA

Hence, there is a difference in potential energy between

positions A and B and this difference equals to the work done

in moving the object against the gravitational force.


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A positive charge is moved towards the positive plate in an

electric field. What is the force opposing the positive charge

as it moves towards the positive plate? Why?

The opposing force is the repulsive force as like charges repel.

Hence when the positive charge moves from point A to point

B, if will experience greater repulsive force.

Between point A and point B, at which point do you think the

charge has to have greater electrical potential energy (EPE)?

Why?

The charge has greater electrical potential energy (EPE) when it is

closer to point B than at point A because it has to overcome

greater repulsive force at point B.

EPEB>EPEA

Is any work done when the charge moves from point A to

point B?

Work is defined as force multiplied by displacement in the

direction of force.


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Hence, the positive charge is doing work because force is

needed to oppose the repulsive force when moving from

point A to point B.

Work done on moving a positive charge between point A andpoint B equals to the difference in the electrical potential energy;

WAB = EPEB- EPEA

The difference in the electrical potential energy is also known

as potential difference.

When a positive charge moves from the lower potential

energy (A) to higher potential energy (B), it is doing positive

work or gain in electrical potential energy,EPE.


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But what happens when the positive charge moves from point

B to point A? Will it gain or lose potential energy?

Wh

en a charge moves fromh

igh

er potential (B) to lowerpotential (A), the charge loses potential energy or potential

drops.

As energy is conserved, this energy is released into other

forms of energy such as heat energy or light energy.

Whathappens when a negative charge moves towards the

positive charge? Will ifgain orlose potential energy?

A negative charge would have lowerEPE at points ofhigher

potential compared to the lower potential due to its

attraction with a positive charge.


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Hence, the negative charge oran electron losesEPE as it

flows from lower potential to a higher potential.

That is why, in a circuit, current, I, which is regarded as the rate

offlow ofpositive charges/ moves from thehigherpotential

(positive terminal) to lower potential (negative terminal).

But electrons move in the opposite direction that is from lower

potential (negative terminal) to higher potential (positive

terminal).

In both cases, the electric current (positive charges) or

electrons (negative charges) will only flow in an electric

circuit when there is a potential difference in the circuit.


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Thus, potential difference is defined as work done in moving a

unit of charge between 2 points in a circuit.

The work done is equivalent to the electrical energy to move

the charge between the 2 points.

Potential difference between 2 points is also referred as the

voltage difference or simply voltage and is symbolised by V.

The potential difference, can be summarised as:

W= work done, E= energy released ,Q = quantity of charge

The unit for potential difference, I, is joules per coulomb

(JC-1) which is also known as volts (V).

1 joule per coulomb =1 volt

What are the sources of potential difference in an electric circuit?

One of the easiest ways to supply a potential difference between

two points is by using a battery. There are many kinds ofbatteries-

The main purpose of batteries is to supply electrical energy.

A simple battery has two terminals (metal posts) that provide a


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means for connecting wires.Do you know which the positive and negative terminals are?

The quantity commonly called'the voltage of a battery is actually

the potential difference between its two terminals.

The potential difference between the two terminals can be

determined by using a voltmeter.

The voltmeter is connected parallel to resistors, bulbs or any other

electrical components.


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What can you say about the brightness of the lamps? Are they thesame? Do you think they have the same output energy?

What can you say about the brightness of the lamps? Are they the

same? Do you think they have the same output energy?

Lamp A is brighter than lamp B. Thus, lamp A releases more energy.

Note the ammeter readings. What can you say about the amount

of current that flows in lamp A and lamp B?

The same amount of current flows into lamp A and lamp B even


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though they are of different brightness.

Note the voltmeter readings. Is the potential difference across lamp A

the same as the potential difference across lamp B?

The potential difference is higher across the lamp that emits brighter

lights or that releases more energy.

Thus, these comply with the equation :

potential difference, Vis directly proportional to the output energy,

E.

The output energy represents the work done to move the charge

between two points.

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Chapter 2 - Electricity - Lesson 16-17