radmaste learner guide · 6. in this activity we do not measure speeds of trolleys. we only measure...

RADMASTE LEARNER

GUIDE

PRACTICAL ACTIVITIES FOR CAPS GRADE 12

PHYSICS

A Learner Guide for CAPS Grade 12 Physical Sciences with Learner Activities for Prescribed (formal assessment), Recommended (non-formal assessment) and selected other Practical Activities for Physics.

PREPARED BY:

RADMASTE Centre, Wits School of Education, 1st Floor Marang Block, 27 St Andrew’s Road, Parktown, 2193;

Private Bag 3, WITS, 2050 Websites: www.radmaste.org.za; www.microsci.org.za

http://www.radmaste.org.za/

http://www.microsci.org.za/

CONTENTS

PHYSICS

PAGE

MECHANICS 3

Activity 12 P1 Conservation of Momentum 4

Activity 12 P2 Determining the Acceleration due to Gravity 6

ELECTRICITY AND MAGNETISM 12

Activity 12 P3 INTERNAL RESISTANCE: Investigating the relationship between Vext and I in the circuit

13

Activity 12 P4 Series – Parallel Networks of Resistors 15

Activity 12 P5 Force on a Current Carrying Wire in a Magnetic Field 21

Activity 12 P6 Build a Simple Electric Motor 22

PPA = PRESCRIBED PRACTICAL ACTIVITY

RPA = RECOMMENDED PRACTICAL ACTIVITY

RPA

PPA

PPA

RPA

PPA

PPA

PAGE Activity 12 P1 Conservation of Momentum 4

Activity 12 P3 Determining the Acceleration due to Gravity 6

MECHANICS

PPA

RPA

© RADMASTE LEARNER GUIDE: PRACTICAL ACTIVITIES FOR CAPS GRADE 12 PHYSICS 4

ACTIVITY 12 P1: CONSERVATION OF MOMENTUM LEARNERS’ INSTRUCTIONS Introduction

An explosion, is the opposite of a collision – a system breaks into two or more parts, like a gun that fires a bullet. During an explosion parts of a system move apart from each other after a brief, but intense interaction. The explosive forces which could be from an expanding spring or from expanding hot gases, are internal forces. If the system is isolated, its total momentum during the explosion will be conserved. It is often the case that the total momentum of the system is zero. For example before a gun shoots a bullet, the system of the gun-bullet has momentum zero since both gun and bullet are initially stationary. The same applies to a system of two spring loaded trolleys that you will meet in the following activity:

pbefore = pafter 0 = pafter 0 = m1v1 + m2v2

You need:

Two trolleys, one of which with compressed spring.

A track with a barrier on either end

Weight blocks

Measuring tape or meter ruler

Suitable pen or pencil to make markings on the track.

Spring balance (optional)

What to do

1. Place the track on a horizontal surface, like a flat desk or on the floor.

2. Place the two trolleys end-to-end on the track, with the spring of the one trolley compressed.

3. Load a block on the other trolley, as shown in the diagram below.

barrier barrier Trolley A

Trolley B

SA SB

PPA


4. Begin by placing the trolleys near the middle of the track. Release the spring and listen for the impacts against the barriers. The trolleys strike the barriers at different times, showing that they had different speeds. Now, by trial and error, find a position on the track from which the trolleys strike the barriers simultaneously – you should hear just a single bump.

5. Assume that each trolley has a mass m. The mass of each weight block is also about the same, i.e.

m. You may verify this by finding the weight of the trolleys and blocks, using for example a spring balance.

6. When this position has been found, measure the distances SA and SB, that the trolleys travel to the barriers.

7. Repeat step 4 with a different number of blocks on each trolley, for example two blocks on B and

none on A, or one block on A and two blocks on B.

8. Complete a table, like the one shown below, for entering your data and measurements:

Trolley A Trolley B

Mass of A SA (cm)

Mass of B SA (cm)

To think about

1. Decide on what makes up your system in this experiment.

2. What was the total momentum of the system before the explosion?

3. What must be the total momentum of the system after the explosion?

4. Are the magnitudes of the momenta of the two trolleys equal after the explosion?

5. Are the speeds of the two trolleys equal after the explosion?

6. In this activity we do not measure speeds of trolleys. We only measure distances between trolleys and barriers. Why is this enough?

7. Perform the necessary calculations with the data gathered in your table, to decide whether the

law of conservation of momentum is observed in this experiment of trolleys. Show your calculations.

8. What are major sources of error in this experiment? How does error affect your results?


LEARNER’S INSTRUCTIONS ACTIVITY P3 Determining the acceleration due to gravity Introduction Objects that move vertically, near the surface of the Earth, either because they have been thrown vertically up or vertically down, move with the same, constant acceleration. These are the free-falling objects. During a free-fall, the acceleration points always downwards and it is due to the gravity of the Earth. This is the acceleration due to gravity with a magnitude of 9,8 m/s2. We give it the special symbol g. So, free-falling objects undergo a uniform accelerated motion with an acceleration of 9,8 m/s2, but, provided that we can ignore air resistance.

The diagram alongside shows a small rock dropped from rest. We assume that the rock is free falling because we can ignore air resistance. This means that its speed increases by 9,8 m/s each passing second.

Aim: In this Activity you will use a ticker timer and ticker tape to measure the acceleration due to gravity of a free-falling object. The object will be a mass-piece of about 50 g, attached at the end of the ticker tape. The ticker timer provides a simple way to take measurements of distance (through the spacing between the dots) and time (through the period of the ticker timer), which are necessary to study the motion of an object. Once we have measurements of distance and time we can find other parameters of the motion, like, as in our case, acceleration. We can do this by either a) using our data and appropriate kinematics equations to calculate other parameters of the motion, or b) we can use our measurements to produce the position-time and velocity-time graphs of the motion. The gradient of the velocity-time graph is the acceleration of the object, as you have learned in grade 10. You need: Ticker timer, ticker tape and carbon disc 12 V power supply or 8 1,5 V cells (D) and suitable cell-holder Two crocodile clip wires, of appropriate length for connections A 50 g mass-piece Sticky tape Measuring tape or metre-ruler Calculator Graph paper for Method B Optional: Retort stand and clamps to clamp the ticker timer on a table

We cannot say, for example, that a flat piece of paper falls freely! Its large surface area meets large air resistance. The air cushions its fall by pushing the paper upwards. But if we crumple this piece of paper into a tight ball, we can assume that it falls freely.

t = 0, v = 0 t = 1 s, v = 9,8 m/s

t = 2 s, v = 19,6 m/s

t = 3 s, v = 29,4 m/s

t = 4 s, v = 39,2 m/s

g = 9,8 m/s2


What to do: 1. Your teacher will clamp the ticker timer

to a high place, be it on top of a door-frame or on a retort stand on top of a table (as in the diagram). The idea is that the ticker timer is at least 1,5 m from the floor and that the object falls vertically down on the floor undisturbed.

2. Connect the power supply to the ticker timer.

3. Cut a long enough piece of ticker tape. The length of the tape must be the distance between ticker timer and floor.

4. Attach the 50 g mass piece at one end of

the tape with enough sticky tape. Thread the other end of the ticker tape through the ticker timer, under the carbon disc.

5. Choose a frequency setting for the ticker

timer. Record its value.

6. Switch on the ticker timer and let the object fall down freely.

7. Ignore the first few dots on the tape, but

make sure that you have at least six remaining dots in a row. If not, you may need to adjust the height or the frequency of the ticker timer.

8. From the frequency of your ticker timer find its period (T = 1/f). This will be the time interval, ∆t, between two successive dots on your ticker tape.

9. Use a ruler or measuring tape to measure the position, x, of each dot, relative to the dot that you have chosen as your starting point. Record these measurements in a table like below. Also record the time, t, when the object is at each position. The first three rows in the table are completed as an example. You will have different values.

power input (a.c.)

. . . . . . .

Ticker timer

Ticker tape

50 g mass-piece

Retort stand clamped at the edge of table

Carbon disc


TABLE 1 Measurement data

x (mm)

t (s)

0 0 20 0,04 54 0,08 etc. etc.

In what follows, you may use one of two methods to find the acceleration of the object.

METHOD A Introduction In this method, you will use your measurements of position and time above, to calculate the acceleration of the object.

We calculate acceleration using the equation: α = vt

∆∆

= f iv vt

−∆

The magnitudes of two velocities appear in this equation, vf and vi. These velocities are ‘instantaneous’ velocities, i.e. the velocities of the object at the start and at the end of the time interval ∆t. Hence, we need two instantaneous velocities to determine the acceleration of an object during a time interval. However, when we have gathered a set of data through measurements (which are measurements of distance and time), it is easier to determine average velocities first and then instantaneous velocities. This might sound strange considering that average velocities of an object are not real velocities, whereas instantaneous velocities are. Yet, we rely on average velocities to determine instantaneous. What to do 1. Prepare a table as shown below. When you complete the table, all values must be added in the white

cells of the table (the dark cells stay empty).

2. Add your measurements of position and time in columns 1 and 2 of the table below.


Measurement data

Calculated data: Finding average velocities, instantaneous velocities and hence acceleration

1 2 3 4 5 6 7 8 x

(mm)

t

(s)

∆x = xf − xi

(mm)

∆t = tf − ti

(s)

vav = ∆x/∆t

(mm/s)

mid of ∆t tmid = ½ (ti + tf)

(s)

v at tmid

(mm/s)

a = g = ∆v/∆t

(m/s2) 0 0

Average g = 3. Complete columns 3 and 4. In column 3, ∆x is the magnitude of the displacement of the object

between two successive dots. This is the same as the distance between two successive dots. In column 4, ∆t is the time interval between two successive dots. In other words, this is the period of the ticker timer.

4. Complete column 5, by calculating the average speed, vav, of the object during each time interval.

5. Complete column 6, by calculating the mid-point of each time interval, tmid = ½ (ti + tf).

For example the mid-point between the 4th and the 6th second is 5 s: tmid = ½ (ti + tf) = ½ (4 s + 6 s) = 5 s. Note that the values of ti and tf are values taken from column 2.

6. How does the average velocity of an object during a certain time interval compare to the value of its

instantaneous velocity at the mid-point of the same time interval? (Assume uniformly accelerated motion.)

7. Complete column 7 by adding the instantaneous speeds that correspond to the times in column 6.

8. Complete column 8 by calculating the acceleration for various time intervals using data from columns 6 and 7. Be careful, the time intervals ∆t this time refer to times from column 7.

9. Finally, using the values of acceleration from column 8, calculate an average value.

10. How does this value compare to the accepted value of 9,8 m/s2 for the acceleration of gravity? Give

reasons for any difference.


METHOD B

In this method, you are going to use your measurements of position and time from Table 1, to produce the position-time and velocity-time graphs of the motion. The gradient of the velocity-time graph is the acceleration of the object. The figure in the next page shows a summary of the graphs of motion and how we can use them to find various parameters. What to do

1. Use your data from Table 1 to plot the graph of position versus time on a piece of graph paper. When plotting a position-time graph, you first need to decide on two things: a) The direction which will be the positive direction. b) A reference level, from which we measure displacements or positions.

Once you have the position-time graph, you can use it to plot the velocity-time graph on a separate piece of graph paper. To do this, you need at least the values of three instantaneous velocities and their corresponding times.

Velocity is the gradient of the position-time graph. But because your position-time graph is a curve (a parabola), which means it does not have a constant gradient, you have to do a trick:

2. On your position-time graph, draw a secant. This is a straight line that cuts the curve of the graph in two points. See the example next page. The gradient of the secant is the average velocity of the object, during the time interval that corresponds to the two times where the secant ‘cuts’ the curve.

3. Find the instantaneous velocity of the object at the mid-point of this time interval.

4. Repeat steps 2 and 3 with a different secant, to get another pair of values for velocity and time. Repeat steps 2 and 3 as many times as you think necessary to produce a velocity-time graph.

5. Plot the velocity-time graph on a different piece of graph paper.

6. Find the gradient of the velocity-time graph.

7. How does this value compare with the accepted value of 9,8 m/s2 for the acceleration of gravity? Comment on any difference and on possible sources of error.


SUMMARY on the graphs of motion


PAGE Activity 12 P6 INTERNAL RESISTANCE:

Investigating the relationship between Vext and I in the circuit

13

Activity 12 P7 & P8

Series – Parallel Networks of Resistors 15

Activity 12 P10 Force on a Current Carrying Wire in a Magnetic Field 21

Activity 12 P11 Build a Simple Electric Motor 22

ELECTRICITY AND MAGNETISM

PPA

PPA


ACTIVITY 12 P6 INTERNAL RESISTANCE: Investigating the relationship between

Vext and I in the circuit LEARNERS’ INSTRUCTIONS You need: Microelectricity kit, two multimeters, graph paper. Prepare the set-up 1. Use components from the microelectricity kit to

set up a circuit as shown in the circuit diagram below. The photo alongside may also help you. Use five 22 Ω resistors (red-red-black) and the 3 V battery (i.e. two 1,5 V cells in the cell-holder). Insert the red lead of each multimeter in the middle socket of the instrument (as in photo).

Taking and recording readings 2. Prepare a suitable table to record the readings of the ammeter (current) and of the voltmeter

(voltage). Careful measurements are important in this practical.

3. When you are ready to take readings, select the range “200m” for the ammeter and “20” for the voltmeter. Note that your ammeter is also a switch. When it is “off” the circuit is open.

4. Start by recording the reading on the voltmeter when the circuit is open, i.e. when there is no current in the circuit. (This reading is also called the “open-circuit voltage”).

R1 R2 R3 R4 R5

V

V

A Free lead of ammeter

Note that the voltmeter is connected across the battery and that one lead of the ammeter is free (the black lead on the right of the photo).


5. You will vary the current in the circuit by changing the resistance in the circuit. You will do this by connecting the free lead from the ammeter to the spring between the pair of resistors R1- R2, then R2- R3, then R3- R4, then R4- R5 and finally beyond resistor R5. Each time, record the readings on the voltmeter and on the ammeter in your table.

The V – I graph 6. Draw the graph of the reading on the voltmeter (Vext) plotted against the current in the circuit (I).

7. The relationship between the emf of the cell, the voltage across the external resistance (Vext)

measured by the voltmeter and the potential difference across the internal resistance (Vinternal = Irinternal):

emf = Vext + Vint = Vext + I r

Vext = emf − I r (1) Equation (1) gives the relationship between the variables used in the investigation and

equation of the graph line. Using the information in your graph, deduce:

(a) the emf of the cell; (b) the internal resistance, r, of the battery (c) the short-circuit current in the battery. This is the case when there is no external

resistance in the circuit. It happens when the terminals of the battery are connected directly to each other (short-circuit). Do not connect the battery in this way – it may damage the battery.


ACTIVITY 12 P7 & P8 SERIES – PARALLEL NETWORKS OF RESISTORS

LEARNERS’ INSTRUCTIONS You need: Microelectricity kit and one multimeter PART A: Revision of series and parallel connections (OPTIONAL) Introduction – To predict 1. Consider the simple circuit diagram (a) shown alongside. Consider ideal

components in order to predict the following:

Comparing brightness What happens to the brightness of bulb B1 when you connect a second, identical bulb B2 a) in series to B1 as in diagram (b) below b) in parallel to B1 as in diagram (c) below

Comparing currents c) Compare the currents I and Iseries shown in diagrams (a) and (b). d) Compare the currents I and IT shown in diagrams (a) and (c). e) Compare the currents I and I1 and I2 shown in diagrams (a) and (c). f) Compare the currents IT and I1 and I2 shown in diagram (c). g) In which circuit, (a), (b) or (c), will the battery last the longest? Explain in terms of current in

the bulbs. Comparing potential differences h) Compare the pd across bulb B1 in circuits (a) and (b). i) Compare the pd across bulb B1 in circuits (a) and (c). j) In which circuit, (a), (b) or (c), is the battery transferring more energy per unit time? Explain

in terms of pd across bulbs.

I2

B2

V

B1

I1

IT

(b) (c)

B1 B2

V

Iseries

V

B1

I

(a)


Comparing resistance k) If the resistance of each bulb is R, what is the resistance in circuits (b) and (c)? l) Compare the resistance of circuits:

(i) (a) and (b) (ii) (a) and (c) (iii) (b) and (c) Test your predictions 2. Test your predictions on the brightness of the bulbs using a 3V battery (i.e. 2 x 1,5 V cells in cell-

holder). PART B: Series-parallel networks Predict brightness in a series-parallel network of bulbs 3. Diagram (a) below shows two identical bulbs connected in series. Diagram (b) shows the same

circuit, but this time, a third bulb B3 has been connected in parallel to bulb B2. Assume ideal components, and that cells and bulbs shown in both diagrams are identical.

In the following four questions answer using the words: Brighter, dimmer, same

brightness a) Compare the brightness of bulbs B1 and B2 in circuit (a). b) Compare the brightness of bulb B1 in circuits (a) and (b). c) Compare the brightness of bulb B2 in circuits (a) and (b). d) Compare the brightness of bulbs B2 and B3 in circuit (b).

e) Compare the resistance of the two circuits (a) and (b). f) Compare the currents Iseries and Inetwork shown in the two diagrams.

(a) (b)

B1 B2

V

Iseries

B1

B3

B2

V

Inetwork


Test your prediction 4. You may test your predictions on the brightness of the bulbs in circuits (a) and (b) using

components from the microelectricity kit. In this case use the 3 V battery (i.e. two cells in cell-holder). The photo alongside will help you to set up circuit (b). When you are ready to observe, close the circuit by touching the free end of the crocodile clip to spring marked A in the photo. You may use the same set-up for circuit (a), but in this case simply remove (or unscrew) bulb B3.

5. A group of learners used the above circuit to test their

predictions. They noticed that while bulb B1 glowed, bulbs B2 and B3 didn’t glow at all! This is shown in the small photo alongside. What might be the reason for this behaviour?

6. Another group of learners, set-up by mistake the

following circuit (c). What are they going to observe and why?

A B1 B2

B3

B3

B2

V

Inetwork

B1

(c)


7. Rank the following circuits (a), (b) and (c) in terms of: a) The total resistance in each circuit from highest to lowest. b) The brightness of bulb B1 in each circuit from brightest to dimmest.

Take measurements in a series-parallel network of resistors 8. In what follows you will prepare a series-parallel

network using resistors instead of bulbs. Resistors are ohmic devices, which is desirable when we want to take readings. The simplest circuit we can set up in this case is shown alongside.

a) Refer to the circuit diagram alongside. If each of

the resistors, R, has a resistance of 22 Ω, calculate the value of the equivalent resistance in this arrangement.

9. The following diagram shows the same circuit as in step 8, but this time ammeters have

been included at strategic points.

a) Prepare a set-up using components from the microelectricity kit, and make provision for the connections of such ammeters. The following photo might help you in this set-up. Use 22 Ω (red-red-black stripes) and the 3 V battery.

R

R

V

R

A b c

A2

A3

A1 A4

R

R

V

R a b c

B3

B2

V

Inetwork

B1

(a) (b) (c)

B1 B2

V

Iseries

B1

B3

B2

V

Inetwork


Two multimeters are shown in the photo used as ammeters, corresponding to A3 and A1. Choose the range “200 m” to measure currents in mA.

Note the metal strip connecting two springs in place of A2. It acts as a switch. When you are ready to measure this current, remove the metal strip and connect the ammeter in its place.

b) Measure the current in each resistor and in all branches as indicated in the circuit

diagram by the position of the ammeters.

c) Also, see what happens to the total current and the current in one of the branches if you remove the other branch.

d) Do your observations of current agree with your observations of the brightness of bulbs in steps 3 and 4 earlier?

10. a) Complete the circuit diagram below by drawing voltmeters at appropriate places,

positioned to measure the voltage across each resistor, external circuit and/or battery.

R

R

V

R a b c


b) Use your multimeters as voltmeters (choose the voltmeter range “20” ) to take the actual readings, as per step 7a.

11. Use your readings for voltage and current to calculate the equivalent resistance of this series-

parallel network of resistors. 12. Use a multimeter as an ohmmeter to measure the equivalent resistance directly. 13. Compare the values for the equivalent resistance that you obtained in steps 8a, 11 and 12.

Comment on these results and account for discrepancies. Which method do you think gives the most accurate result? Explain why.


ACTIVITY 12 P10 FORCE ON A CURRENT CARRYING WIRE IN A MAGNETIC FIELD

LEARNERS’ INSTRUCTIONS Electricity can produce motion. This becomes evident through the vast range of electric motors, which we find in just about every moving thing. Our task here is to understand the origins of this movement. And the origin of each movement lies on a force! We have seen that current carrying wires act like magnets. They magnetise magnetic materials or exert forces on other magnets, such as compass needles. Then why not expect the opposite – a magnet to exert a force on a current carrying wire. After all, in nature forces come in pairs, action and reaction! This is what we investigate in this activity. You need: Microelectricity kit, light aluminium foil, scissors What to do: 1. Use your electricity kit to set-up the circuit shown in the diagram below. Place the magnets so

that opposite poles face each other. Connect the thin aluminium strip loosely between springs A and B. Do not close the circuit until you are ready to begin.

2. What do you think will happen to the aluminium foil, as soon as you close the circuit? Will it

move and how?

3. Close the circuit for a very brief moment to test your prediction.

4. Make a rough drawing to record: The direction of the current in the foil, the direction of the magnetic field between the two magnets and the behaviour of the aluminium strip.

5. Repeat by reversing the current in the strip.

6. Repeat by reversing the magnetic field, i.e. by bringing the opposite poles face to face.

7. Write down your conclusions.


ACTIVITY 12 P11 BUILD A SIMPLE ELECTRIC MOTOR LEARNERS’ INSTRUCTIONS To think about The three diagrams below show current-carrying wires placed in the region of a uniform magnetic field. The arrows indicate the direction of the current, I, in each wire.

1. Find the direction of the electromagnetic force:

i) on the wire, shown in diagram (a) ii) on each of the two wires shown in diagram (b) ii) on each side of the rectangular loop shown in diagram (c).

2. Describe the forces acting on the current-carrying, rectangular loop. Think of what the net result of

these forces might be. What to do 1. Use some of the wire of the copper coil provided in the microelectricity kit. Wind this wire around

the 9 V battery, about 5 or 6 times, to make a small rectangular coil. Leave 5 cm of wire free from both ends of the coil.

2. Carefully, remove the battery and secure the windings with a bit of sticky tape. 3. Scrape the insulation from only the bottom part of the free ends of the wire (of both free ends).

5 cm Sticky tape

Scrape the insulation off from the bottom part of the wire

Leave the insulation at the top part of the wire

S

N

N

S

N

S

(a) (b) (c)


4. The diagram below shows what to do next. Connect the cells only when you are ready to test your motor.

5. Your motor should now turn. If it does not turn, it most probably means that you must remove some more insulation from the wires. Be careful however, to always leave insulation at the top.

6. When the cells are connected and the motor turns, is there always current in the coil? Explain when

and in which direction. 7. What causes this motor to turn continuously in the same direction? 8. Suggest ways to increase the torque produced by this set up. In other words, how can you make your

motor turn faster? Whenever possible, test your suggestions in practice.

Bend two metal paper-clips into this shape.

Stand the paper-clips onto some Prestik.

Attach a spring onto each paper-clip.

Two magnadur magnets under the coil

Use two 1,5 V cells in cell-holder.

Hang the coil on the paper-clips.

radmaste learner guide · 6. in this activity we do not measure speeds of trolleys. we only measure...

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