Magnetic Field Pattern in SOLENOID

Do u know how to draw a magnetic field pattern in a solenoid???
Watch this! Its part of the online tuition discussion about solenoid..

SPM Question:Electromagnet - Paper 1

SPM F.A.Q: National Grid Network

If you study based on SPM checklist subtopic 3.5 that i listed in previous post, u'll be having less problem answering SPM physics' questions. TAk percaya??

Lets analyse the following SPM year 2005:
Question 12:
(a) Most of our electrical energy comes from hydroelectric power stations and thermal power stations. These power stations are connected by cables to transmit electricity to users in industries, offices, schools and houses. This system is called the national grid network.

(i) State the transformations of energy in a hydroelectric power station. [1 mark]
(Refer checklist 2)


(ii) Explain briefly the importance of the national grid network system in distributing electrical energy to the users. [4 marks]
(refer checklist 5)

Answers:
i. Gravitational Potential Energy -->Kinetic Energy--> Electrical Energy
(to understand about energy transformation, u need to appreciate principle of conservation of energy)

ii. ( u can choose any 4 of these advantages)

• It enables continuous supply of electricity to the end user.
  
• If one of the station break down, the other network can still operate.
• Overall cost of the transmission network can be reduced
• Energy loss is minimised due to use of high voltage transmission
• General maintenance can be done without any interruption of power suply in other area.

Lets analyse the following question:
Figure 12.1 shows how a model of an electric transmission system can be set up in a laboratory. The model consists of a power station that generates 12 V of alternating current (a.c) that transmits the electrical energy to the users using transmission wires and transformers P, Q and R. If the resistance of the transmission wires between transformers P and Q in the model at Figure 12.1 is 30ohm , calculate:

(i) the current that flows in the transmission wire,
***Why don't u try this on your own :)

(ii) the power loss due to the heating effect of the currect in the transmission wire.
(refer checklist no. 4 for a clue)

[5 marks]

Answer:
i. 0.8A
ii. 19.2 W

So my advice is, make sure u cover all the aspect that being asked in those checklist.

SPM Questions: Transformer

In previous post,I gave u some SPM checklist instead of detailed notes about transformer. The reason i did that is to let u to do some work & revise what had been asked from the checklist.
Another reason bcoz i hate to spoon feed my student...

Usually..SPM questions are designed from those checklist. You can find these checklist in your text books under the column called learning outcome. If u have covered & understood everything that being asked in the outcome,i'm sure u can grab at least a 'B' for physics. However to get an A.. u need to do lots of extra exercises for P1,P2 & P3.
I know some of u hate to refer yr txt book..but trust me, those outcomes are your guidance..

Here i want to disucss some SPM passed year questions about transformer.
In my previous checklist, i asked u to state & relate the equation of ideal transformer.If u really understand the equation for ideal transformer, u shoudnt having prblem answering these questions. However your undrstanding on electricity must be strong enough..
Lets check'em out..

1.
SPM 2004




The important keyword in this questions is 'ideal transformer'.So in the xm i strongly advice u to underline this keyword. For ideal transformer, the output power is equal to input power.Since the bulb is having the same rating voltage & power, so it will produce same brightness. Answer: B







2.
(b) Figure 4.2 shows a circuit consisting of a transformer, an ammeter and two light bulbs. The ammeter reading is 0.5 A and both bulbs light up with normal brightness.












(i) What is the output voltage of the transformer?
(ii) Calculate the efficiency of the transformer?

i) Answer=12V
Explaination:
The bulbs are arranged in parallel. So the voltage across the bulbs are the same i.e 12V. The voltage across the bulb represents the output voltage of the transformer.
Basically this questions wanna test your basic understanding about electric circuit.

(ii) Calculate the efficiency of the transformer.
Efficiency = (Pout / Pin) x 100

Total Pout=2(24) =48W
Pin = VI =240 ( 0.5)= 120W

Eff= (48) / 120) } x 100
= 40%

Basically this questions wanna test your basic understanding about electric circuit. Please make sure u understand the parallel & series circuit calculation..

SPM Questions: Motors & Generators

As i mentioned before, to understand application of each physics subtopic is vital. Usually in SPM they'll ask u about this in section B & C , paper 2.
For example, let us analyse an SPM question taken from year 2006 about motors.
But as usual, those who created the question loved to confuse the students.

Take a look at the diagram below.
Lets discuss the 1st part..the question asked them
i. explain how AC motor can rotate

If we analyse the diagram, they gave a labeled diagram for an AC motor. If u notice, we hardly find notes on reference book or textbook about AC motor. (try & find it)But we have been given in details about the structure of AC generators. If the student tend to simply memorize, they might had been described the AC generator working principle instead of AC motors. I would advice in this case for u to underline the word motor as to make u alert of things that being asked.

Basically to answer the 1st part, u just simply describe details of the turning effect in dc motor and mention what happen if the current changed in direction from the AC supply. U can draw diagram to help u describing the principle.

Now on the 2ndpart:
The question asked the student to modify the device into DC motor & make it rotate faster.
Again.. i strongly advice u underline these words, "DC motor & rotate faster". So in your answer you must give points on both cases.
-First, u need to explain how u are going to convert it into DC motor
..here comes the importance of revising the application of each subtopic. If u never read about motors, u are having difficulties answering this question.
Moreover in this case they combine some concept in Electronics to change the AC to DC.

-Then, give points on how to make it rotate faster.... I'm not gonna give u the answer for this, coz u can find it in your reference book or cerdik's passed year's book. So apa tunggu lagi, bukak lah buku..pegi baca ;)

What i'm trying to emphasize here is, please dont take for granted of any applications in each subtopic. In this case, u really need to identify the differences between AC-DC motors &
AC-DC generators.
But dont simply memorize them, understand how they work.
Who knows, it might come out in your trial..








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SPM Questions:Electromagnet - 3.2

3.2 Electromagnetism Paper 1(DOWNLOAD)

SPM Questions: Electromagnet 3.1

SPM Questions on electromagnet: 2004 -05

1 The diagram shows an arrangement of apparatus to study the pattern of magnetic fields which are formed when current flows through conductors:

Answer: B

To answer this one, 1st determine the direction of current entering the cardboard for both conductors.

For each case, apply right hand thumb rules to determine the direction of magnetic field.Look at diagram below:

2. The diagram shows an electromagnet.

Which factor does not influence the strength of the electromagnet ?

A Material used for the wire

B Material used for insulator of the wire

C Number of turns of wire

D Current flowing in the wire..

Answer:B, coz insulator wont affect the magnetic field strength

This question asked for a wrong answer..so beware, always underline important phrase like, does not.

The other answers will influence the strength of electromagnet;

- A: different wire materials used would give dfferent resistance which then affect amount of current flows.

strength of magnetic field depends on current flows.

- C: No. of turns wire...bigger number of turn produce stronger magnetic field

- D:amount of current...higher current give stronger magnetic field