PHYSICS D.C. CIRCUITS!

D.C. CIRCUITS!

Introduction:

Hey again everyone! Hope you all are good. Today we are going to study about D.C. Circuits, D.C. is the abbreviate for direct current. In this tutorial, we’ll study how the arrangement of an electric circuit will affect the electrical properties like charge, current, e.m.f., p.d. and resistance. Ready?

Series Circuit:

The diagram shows a series circuit. All bulbs and ammeters are connected in series. Do you note that all ammeters are recording the same value? Yeah so here we are: current at any point in a series circuit is the same, no matter how much load you put in. In both bulbs above, same amount of current is flowing.

Take a look at the above circuit, it has the same battery and same bulb (only one this time though) and the ammeter is still giving the same reading. Hence, load doesn’t affect the current value in a series circuit. Easy? Let’s move on!

Potential Difference across a series circuit:

Now in the above diagram, two resistors are connected in series. We know that current will remain same at each point, only the voltage will change. T o find the e.m.f. of the battery, simply add up all the voltages in the series circuit, that is:

V (e.m.f. of battery) = V1 + V2

So in scientific terms, we can say that in a series circuit, the sum of potential difference across each component is equal to the potential difference across the whole circuit (e.m.f.).

Resistance in a series circuit:

Coming to the resistance in a series circuit, consider the same circuit.

The resistance of each resistor is R1 and R2. We learned the formula V = IR in the tutorial on current electricity. So

according to this, p.d. across R1 is V1 is V1 = I R1 and that across R2 is V2 = I R2.

We know that

V = V1 + V2

V = IR1 + IR2 = I (R1+ R2)

Therefore V / I = R1 + R2 .

From the above derivation we can see that the combined resistance R is the sum of all the resistors in the circuit. Hence in a series circuit, the combined resistance is the sum of all the resistances.

Q1. Determine the current in the ammeters A1 and A2 .

Hint: The current in a series circuit is same and remember ho w the tot al resistan ce is ca lculated in serie s.

Parallel Circuits:

The most simple explanation can be: in a parallel circuit, there is more than one path through which electric current

can flow. And as we learned that in a series circuit, current all around the circuit remains the same. In parallel its

different. Take a look at the circuit below:

We can see from the circuit diagram that each lamp has its own branch. The three lamps are in parallel, so this is a parallel circuit. There are junctions between each branch which are represented by the black dots.

If we set this circuit up and connect ammeters as shown, we find that: A1 = A2 + A3 + A4

In other words, the currents add up. An important rule for all parallel circuits is: the currents in each branch add up to the total current.

More scientifically, in a parallel circuit, the sum of individual currents in each of the parallel branches is equal to the main current flowing into and out of the parallel branch.

P.d. across a parallel circuit:

P.d. in a parallel circuit is more like current in series: it remains same!

In the circuit above, the p.d. shown on each voltmeter is the same as the battery voltage.

Resistance in a parallel circuit:

The diagram shows exactly how combined resistance of resistors in parallel can be found.

Q2. Calculate the total resistance in the following circuit where

R1 = 2 O,R2=4O andR3=6O.

There are two advantages of connecting bulbs in parallel. Firstly, they glow more brightly in parallel because with the same e.m.f. of the battery, each bulb in series gets less voltage than the power e.m.f. while in parallel, all bulbs will get the same e.m.f. as we just learnt. More voltage means more current according to V = IR and hence, brighter the

bulb! Secondly, in parallel, if one bulb goes out, the other continues to glow normally. While in series, when one bulb goes out, the circuit breaks and then obviously, other bulbs won’t light up.

Differences between series and parallel (Summary):

Okay let’s just quickly revise the differenc es between series and parallel circuits.

●In series, current remains same at every point while voltage varies. In parallel, current varies while voltage remains the same at every point.

●To find the total resistance in series just simply add up the resistances, and for parallel apply the formula R (total) = (1 / R1) + (1 / R2) + (1 / R3) …

●Now in series, the sum of all p.d.s is the e.m.f. T ry to remember what we learned about p.d. in the tutorial on current electricity.

●Here’s a new point: to find the P.d. of any load (a resistor for instance) for which you know the resistance, the total resistance of the circuit and the total voltage, apply the formula

P.d. = ( R / Rtot al) x Vto tal.

Potential Divider:

A potential divider is not as complicated as its name sounds. It is simply a circuit with resistors arranged in series. We know that the potential difference across each resistor in a series arrangement changes according to the resistance of the resistor.Hence, we can divide a main voltage into two voltages.

Take a look at a simple potential divider circuit above. Vin is the e.m.f. supplied by the cell which has been divided

into two potential differences across each resistor R1 and R2. The Vout across R1 is then used to drive another part of

circuit.

We can find the current through the resistors R1 and R2 by using:

I = ( V / R1 + R2 )

Hence the potential difference Vout across R2 is given by:

Vout = IR2 = ( V / R1 + R2 ) x R1

Now find the Vout in the following diagram:

Here’s how you’ll solve it:

Answers:

Q1. 0.5 A

Q2. 1.1 O

PHYSICS CURRENT ELECTRICITY

CURRENT ELECTRICITY!

Introduction:

Welcome back everyone! Hope you are all good. T oday we’ll learn about current electricity. We have studied in the tutorial on static electricity that insulators and conductors become charged when electrons are added or removed from them. We know that these electrons are stationary, but what will happen if these electrons are provided with a conducting path? The electrons will start to flow, and moving electrons produce electric current.

Electric Current:

As we just saw, elec tric current is produced when electrons flow. T hese electrons always flow from a negatively charged to a positively charged end. T his is the electron flow.

Now coming to the convectional current, in the previous days it was assumed that current flows from positive to negative end and it is widely held today. T his is called the convectional current flow. The diagram below makes it clearer.

Electric current (I) is a measure of the rate of flow of electric charge (Q) through a given cross section of a conductor. Such that:

I = Q / t

The SI unit of current is ampere (A). An ammeter is used to measure current.

Now before moving on, you should all be aware of how a simple circuit looks like.

In the circuit shown above, it consists of:

1.a source of electromotive force that drives electric current (e.g. battery)

2.a load on which moving charges can do a useful job (e.g. a bulb)

3.conductors to connect the components together (e.g. copper wire)

4.switch to open or close a circuit.

Task: Goo gle the sym bols used in circuit diagram for diff erent apparatuse s.

Electromotive Force and Potential Difference:

Electromotive force (e.m.f) is the energy required to move a unit positive charge from one end of the circuit to another. Such that:

E = W / Q

where E is the e.m.f. of the power supply, W is the amount of electrical energy converted from electrical to non- electrical forms (work done) and Q is the amount of charge. T he SI unit for e.m.f. is Joule per Coulomb or volt (V). Remember that e.m.f. is the movement of charge through the entire circuit.

The diagram above shows the voltage calculated of the cell, and as cell is providing voltage to the entire circuit, it is hence e.m.f.

Now, potential difference (p.d.) is the amount of electrical energy consumed to move a unit positive charge from one point to another in an electrical circuit. Such that:

V = W / Q

where V is the p.d., W is the electrical energy converted to other forms and Q is the amount of charge. T he SI unit for this is the same as that for e.m.f. and that is volt (V).

The diagram shows how p.d. can be calculated across the bulb (between two points).

Resistance:

Resistance, as the name suggests, is the measure of how difficult it is for an electric current to pass through a material, copper wire let’s say. So it is basically the restriction (resistance) of a material to the free moving electrons in the material. If you compare it with the friction in moving objects, it’s quite correct.

Now in more scientific terms, resistance R of a component is the ratio of the potential difference across it to the current I flowing through it, such that:

R = V / I

where R is resistance, V is the p.d. across the component (note that across a component, that is, between two points, it’s p.d. and not e.m.f.) and I is the current flowing through it.

The SI unit of resistance is ohm (O).

Resistance is measured using a conductor called resistor. Resistors are of two types: fixed and variable (rheostats). Now you can tell exactly from the name what these are, right? Fixed resistors have a fixed value while variable resistors can vary the resistance and are used in circuits to vary current.

Ohm’s Law:

Ohm’s Law states that the current passing through a metallic conductor is directly proportional to the p.d. across the ends, provided the physical conditions (such as temperature) are constant.

Such that:

IaV

where I is current and V is p.d.

and this drives us to the formula which we have already learned, i.e. V / I = constant = R

From this, we can make another conclusion that resistance of a metallic conductor remains constant under steady physical conditions, and such conductors which obey Ohm’s law are called ohmic conductors. For ohmic conductors, I-V graph has a constant gradient (i.e. inverse of resistance), as shown below:

Not all conductors obey Ohm’s Law, such conductors are non-ohmic conductors. T he resistance of such conductors can vary, but how do we differentiate? The I-V graphs of different conductors can help us differentiate.

For example, for a filament lamp, when the p.d. across the lamp increases, the current does not increase proportionally. T he graph below makes it clearer:

The deviation of I-V graph from straight line is due to increase in the resistance of the filament with temperature. T he graph is straight line in initial stage because the increase in resistance of the filament with the temperature due to small current is not appreciable. As the current is further increased, the resistance of the filament continues to

increase due to rise in temperature (Though the gradient is decreasing, how can we say that the resistance is increasing? That’s because slope is the inverse of gradient in this case). How is the temperature rising? It’s rising because as the bulb remains on for a long time, more energy is dissipated to heat energy.

Task: Goo gle other non-ohm ic conductors and f ind out ho w the ir resistance varies in an I-V graph.

Resistivity:

Apart from temperature, there are other factors as well on which R depends. As for temperature, the higher the temperature of metallic wire, the larger the resistance.

The resistance depends on

1.the length l of the wire,

2.the cross-sectional area A or thickness of the wire, and

3.the type of material.

To memorize how these factors affect the resistivity of the conductor, memorize the following formula: R = p (l / A)

where R is the resistance, p (a constant) is the resistivity, l is the length and A is the cross-sectional area of the wire. This shows that Raland that Ra1/ A.

So now me have made it quite easy: as R is directly proportional to length, the longer the length of the wire, the greater is its resistance, and as R is inversely proportional to cross-sectional area of wire, the greater is its cross sectional area, the lower is its resistance.

Now for the type of material, every material has a different resistance. For example, the resistance of silver is 1.6 x 10-8 Owhilethatofgraphiteis3000x10-8 O.

IGCSE CHEMISTRY PAPER 6 revision

1) What is the purpose of ice or cold water? To cool down the gas so that it condenses and turns into a liquid.

2) when the gas collecting tube is upside down, give a property of this gas. It is less dense than air.

3) Why is a pencil used in drawing thenorigin line in chromatography? If pen was used, it ill dissolve giving colours and so, the experiment won't be accurate.

4) When using ethanol, give a better apparatus arrangement, and why? Cover apparatus wth a lid, because ethanol is volatile.

5) In the tests and observation tables, when in the first row, they tell you that upon heating condensation occured, there will be a question asking what does this show about the solid? It is hydrated.

6) when copper is used in the test, and the answer is four marks, the best answer is: light blue precipitate (ppt) which is soluble in excess to form dark blue solution.

7) Why is this experiment done in a fume cupboard? It releases harmful gases that are poisonous. It is toxic.

8) Which result appears to be inaccurate? It is the point not appearing on the drawn graph, you read it's x-axis and write it with a reason indicating that it doesn't occur in the graph.

9) Why should the solid be crushed? It increases surface area for a faster rate of reaction.

10) Why is the experiment made in a well-ventilated room? To prevent the burning of the substance.

11) explain the term (decant). Filter/Pour the liquid leaving the solid alone.

12) Why is concentrated sulphuric acid not used to dry ammonia? Because it will react with the base ammonia, which is neutralization reaction.

13) Why should samples be taken from different parts of the field? To get more accurate results.

14) Suggest why it is important to know the pH of the soil. To see which is the best place for growing, and wht kind of base to use for neutralizing it.

15) What is necessary for rusting? Water (humidity) and oxygen (air).

16) Suggest why in an experiment for rusting the water level increases. Oxygen is used up, and water is used to take its place.

17) For electrolysis, state the observations. The bulb will light - A metal is formed on the cathode - Fizz of gases produced.

18) Suggest a suitable material for electrodes. Graphite - Carbon - Steel.

19) when copper oxide is reacted with hydrogen, what is the colour change? Black to brown, because copper oxide is reduced to copper.

20) How can you distinguish between water and ethanol? Use cobalt chloride paper, it turns from pink to blue with water, but there will be no change with ethanol.

21) how can you distinguish between sulpharic acid and aqueous sodium sulphate? There are three tests, you can use a metal carbonate in which carbon dioxide will be produced with sulpharic acid but there'd will be no change with sodium sulphate. You can add a metal, in which hydrogen is produced with sulpharic acid, but no change in sodium sulphate. Finally, you could use an indicator like litmus paper, it will change to red with sulpharic acid, but there will be no change with sodium sulphate..

22) how can you distinguish between hydrochloric acid and nitric acid? Add silver nitrate, in which white ppt will be formed with hydrochloric acid, but there will be no reaction with nitric acid.

23) What is the purpose of the mineral wool? To absorb and hold the liquid.

24) When there is a delivery tube involved in a question, what precaution should be taken in the experiment when the heat is removed? Remove the delivery tube from water to prevent suck-back.

25) In rate of reactions, always include the word "collisions between particles".

26) How can you distinguish between alkanes and alkane? Use bromine water, in which the alkene will decolourise it to colorless, but nothing happens with an alkane,

27) how can you distinguish between chlorine and sodium chloride? Add litmus paper, it will bleach with chlorine, but nothing happens to it with sodium chloride.

28) How can you distinguish between copper sulphate and copper carbonate? Acidify with hydrochloric acid, and add barium chloride, there will be white ppt with sulphate, but no white ppt with carbonate. OR just add hydrochloric acid, in which nothing happens with sulphate, but a fizz or effervescence of carbon dioxide will occur with carbonate.

29) When a measuring cylinder is used, and they ask for a change in apparatus to get more reliable results, you should say that a biuret can be used instead as it is more accurate.

30) volume of reagent used decreases if it is more concentrated.

31) in an experiment observation of pH value, and they ask what type of acid/base is used, your answer should be weather weak or strong. A strong acid lies between pH values of 0 and 2, and a weak one lies between 3 and 6. 7 is neutral. A weak base lies between 8 and 11, while a strong one lies between 12 and 14.

32) A concentrated acid is an acid that contains a large number of H+, hydrogen ions. Vice versa with dilute acid.

33) A concentrated base is a base that contains a large number of OH-' hydroxide ions. Vice versa with dilute base.

34) a strong acid is one that ionizes completely giving H+ in solutions. Vice versa with a weak one..

35) A strong base is one that ionizes completely giving OH- in solutions. Vice versa with a weak one..

36) Concentrated: is a solution that contains a large number of solute or little amount of water is involved.

37) How can you make crystals? 1)heat till point of crystallization. 2) leave to cool gradually. 3) filter, dry and collect the crystals!

38) How can you detect the point of crystallization? Place a stirring rod in the solution and see the formation of the first crystals on it.

39) When you crush, you use a pestle and mortar.

40) Grass is ground with ethanol rather than water because clorophyll is more soluble in ethanol.

41) Colour of rusty iron fillings is brown (orange and red are I think accepted)

42) If pure oxygen was used instead of air, rusting will be faster.

43) you can speed up the drying process by using a fan or by increasing temperature or by using a hair-drier if you have one ) , NOT a catalyst.

44) The action of a lie big condenser is to change steam to water.

45) to check for the purity for a collected solvent, test it's melting or boiling point.

46) The chromatogram needs to be sprayed with locating agent is amino acids are investigated because they are colorless.

47) If water contained salt, this will have no effect on rusting, however if a bigger substance is being rusted, it will be slower.

48) Hydrated copper sulphate will turn from blue to white upon heating.

49) saturated: no mo solute can be dissolved in a solvent AT A CONSTANT TEMPERATURE..

50) An excess amount of reactant is used to make sure all the other reactant will be used.

51) Sometimes, crystals are dried using filter paper instead of heating to prevent the complete loss of water from crystals, and to prevent crystals from breaking.

52) how could you know which reactant is in excess? At the end of the reaction, the excess reactant will be visible.

53) excess means more than what is needed.

54) when lead bromide is used, you can use a fume cupboard or use goggles,lab coat,gloves,,, because it is toxic.

55) to separate two different solutions with different boiling point, use fractional distillation.

56) Physical test of water: heat, it will boil at 100 degrees Celsius, or heat ice and it will melt at 0 degrees Celsius.

57) chemical test of water: Add blue cobalt chloride paper, it turns pink OR add anhydrous copper sulphate, it turns blue.

58) fire will be produced if alcohol is touched with lighted splint, therefore a water bath should be used when heating it. Lagging or cloth can be used to control temperature for accurate results.

59) unreacted reactant is called excess.

60) As reactants are heated, the particles gain energy, move faster, and their kinetic energy increases therefore there will be more collisions and rate increases.