help with formulas

[rudeboy500]

hi can someone help I have the 301 exam coming up in a few months. I have been trying to some maths samples, the problem is when there is a question i get confused on what formula i should use, is there a way that i can apply the right formula to the question they are asking.
Here are some examples:
1 The resistance of a 230v 3 kW immersion heater element will be?

2 The force on a conductor 750mm long carrying a current of 6A placed at right angles
in a field of 1.2 Tesla will be?

3 A capacitor of 100BF is connected in parallel with a 20@ pure resistor. The group is
connected to a 200v 50Hz supply. Calculate the power dissipated in the combination?

4 A Capacitor of Capacitive Reactance 50, an inductor of 80 Inductive Reactanceand a Resistor of 40 are connected in series to a 200v 50 Hz supply. The impedance​

of the circuit will be?5 A four-pole 50Hz three-phase induction motor is connected to a 400v supply. If the
motor has 4% slip the rotor speed will be?

I dont want the answers,
I want to understand how I can tackle these questions, therefore which formula goes with which question and why you used that formula and why!

hope you can help

Thanks
Rudeboy500

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[TedM]

You're asking in the wrong place - this section of the forum is for PV.

But anyway here's a start:

1. Ohm's Law and Joule's Law
These consists of 4 parts - voltage, current, resistance and power and combine them in related ways.

If you have a question that gives you 2 of those and asks for a third then you can use the formula in different ways to get the answer.

Ohm's Law: Voltage (E) = Current (I) * Resistance (R)
Joule's Law: Power (watts) = Voltage (E) * Current (I)

So in this question you know voltage (230) and power (3000) - make sure you get the units right.From rearranging the formulae you can derive the other 2 - but you are only being asked for resistance.To get resistance you need to know current which you can calculate from the second formula.

3000 = 230 * x
x = 3000/230
x = 13.04 amps

Now you can use the first formula to find the resistance.

230 = 13.04 * x
x = 230/13.04
x = 17.63Ohms

So the answer to Q1 is 17.63 Ohms.

In a correctly functioning 3kW immersion heater then that is the reading you should expect to see if you apply a resistance test meter across the terminals - so if the reading was wildly different from that then you would know that something was wrong with the heating element itself and it would probably need to be replaced.

So bring the formula back into the real world and think of a scenario where you would want to apply the knowledge to a practical situation and you will have more chance of remembering them all.

If you are attending a decent college then your lecturers should be able and willing to explain all that to you.

 

[Bluesknight]

There are two very good books filled with examples: Elecrical Istallation Calculations Basic and Electrical Installation Calculations Advanced.

By A J Watkins and C Kitcher

There are not expensive and will give you plenty of practice.

Good luck with your exams

 

[spikester]

For question one it is easier to use the power formula P=V²/R
So this transposes as R=V²/P and this gives you R=17.63Ω

Just try and get loads of past papers and run through them then you'll get use to the formulas you require.

Good luck with the exams