Danho
Download app
PreparationInternal resistance and EMF

Physics · Related practical

Internal resistance and EMF

Use current readings at different loads to determine a source model.

Physics Paper 4, November 2012 · Question 2

Use this practical to study the skills in the linked examination question. The apparatus and samples practise the same ideas; they do not reproduce the examination setup exactly.

A Level · Form 5 to 6 · 60 minutes

Learning objectives

  • Combine external resistors with the switch open
  • Convert current readings from milliamperes to amperes
  • Determine EMF and internal resistance from a reciprocal-current graph
Practical mode

Guided gives instructions as you work. Practice lets you choose your method.

Source paper

Original examination paper

Physics · Paper 4

9188/4 · NOVEMBER · 2012 · 2 hours 30 minutes

Read the complete original paper below. The related question is identified in this practical’s introduction.

Candidates answer on the question paper.

Additional materials:
As listed in Instructions to Supervisors
Electronic calculator
Graph paper

TIME

INSTRUCTIONS TO CANDIDATES

Write your name, Centre number and candidate number in the spaces at the top of this page and on any separate answer paper used.

Answer all questions.

Write your answers in the spaces provided on the question paper.

In Questions 1 and 2, you are expected to record all your observations as soon as these observations are made, and to plan the presentation of the records so that it is not necessary to make a fair copy of them. The working of the answers is to be handed in. Marks are mainly given for a clear record of the observations actually made, for their suitability and accuracy, and for the use made of them. Routine precautions and theory are not wanted in Questions 1 and 2. You should, however, record any special precautions you have taken so as to aid accuracy. At the end of the examination, fasten any separate answer paper used securely to the question paper.

INFORMATION FOR CANDIDATES

Questions 1 and 2 carry 18 marks each and question 3 carries 14 marks.

Squared paper and Mathematical tables are available.

Additional paper and graphs should be submitted only if it becomes necessary to do so.

You are advised to spend approximately one hour on each of Questions 1 and 2 and 30 minutes on Question 3.

You are reminded of the need for good English and clear presentation in your answers.

1

In this experiment you will investigate the variation of terminal speed, V, of an air bubble (in an inclined burette filled with water) with the angle of inclination, θ.

(a)

(i)

Using a masking tape provided, fix the burette on a metre rule.

(ii)

Set up the apparatus as shown in Fig.1.1.
Apparatus arrangement of an inclined burette, with a plumb line and marked points X and Y.
Apparatus arrangement of an inclined burette, with a plumb line and marked points X and Y.

(iii)

Use a brick provided to support one end of the metre rule whilst the other end rests on a clamp.

(iv)

Tie the plumbline at the upper end of the mounted burette such that θ = 40°.

(v)

Mark the points X and Y on the upper end of the mounted burette such that they are 40 cm apart.

(b)

(i)

Tilt the system so that the bubble moves to end A. Return end A to its original position.

(ii)

Measure and record the time taken, t, by the bubble to move the distance XY.

(iii)

Change the angle of inclination and repeat steps (b) (i) and (b) (ii) such that 10° ≤ θ ≤ 40° until you have six sets of readings.

(iv)

In your table of results include values of V, V², sin² θ.

(c)

It is suggested that V and θ are related by V² = k sin²θ + C where k and C are constants.

(i)

Plot a graph of V² (y - axis) and sin²θ (x - axis)
Printed response template
Printed response template

(ii)

Comment on the validity of the relationship.

(iii)

Determine the gradient and hence the values of k and C.

(iv)

Use your graph to find the value of V when θ = 15°.
Measurements and calculations

M

R

A

G

2

In this experiment you will determine the internal resistance, r, and the e.m.f, E, of a source.

(a)

(i)

Set up the circuit as shown in Fig. 2.1 with resistance, R = 100 Ω.
Complete circuit used to measure current for six resistance combinations and determine source e.m.f. and internal resistance.
Complete circuit used to measure current for six resistance combinations and determine source e.m.f. and internal resistance.

(ii)

Close switch S and record values of current, I, and resistance, R.
Include values of 1/I in your table of results.

(iii)

Change R and repeat (a) (ii).

(iv)

Repeat (a) (iii) to obtain six sets of measurements using the three resistors provided.

[For resistors in series, R_T = R₁ + R₂ - - - and in parallel
1/R_T = 1/R₁ + 1/R₂ + - - -.]

(v)

Justify the number of significant figures you have used for 1/I.

(b)

The relationship between I and R is given by 1/I = R/E + r/E.

Plot a graph of 1/I (y - axis) against R (x - axis).

Printed response template
Printed response template

(c)

Determine the

(i)

gradient,

(ii)

e.m.f., E, of the source,

(iii)

intercept,

(iv)

internal resistance, r, of the source.
Measurements and calculations

M
R
A

G

3

The most expanding knowledge on domestic devices concerns the interaction between light and electricity.

Researches have led to the development of a semi-conducting light sensitive device, a photodiode. When light falls on the photodiode, photo-current flows. Photodiodes are usually used as disc-shaped photo-detectors.

The magnitude of the photo-current depends on the

1. light intensity, I, falling on the exposed surface area, A,

2. exposed surface area, A, of the disc, and the

3. electromagnetic component irradiated to it.

You are required to design an experiment to investigate how the magnitude of the photocurrent depends on the above factors. In your discussion, pay particular attention to :

(a)

the procedure to be followed

(b)

control of variables

(c)

how the measurements are executed

(d)

the precautions to be observed

(e)

improvements for better observations

Study notes

Overview

Change the external resistance and measure current to determine a source’s EMF and internal resistance. This source has an EMF of 3 V and internal resistance of 10 Ω, with 100, 150 and 220 Ω resistors available for different loads.

Method

1. Begin with a 100 Ω external load. Close the switch and record resistance and current, then open it.
2. Rearrange the supplied resistors with the switch open to collect six combinations.
3. Convert current to amperes and calculate reciprocal current.

Safety and measurement

Open the switch before rearranging the load and between readings. Avoid short-circuiting a physical source.

Analysis

Plot 1/I against R. For gradient m and intercept b, E = 1/m and r = b/m. Use amperes so the resulting EMF is in volts.

Practice quiz

Answer all questions, then check your answers and read the explanations.

1
Which calculation gives internal resistance from the reciprocal-current graph?
2
What should you do before rearranging the external resistors?
3
The ammeter reads 25 mA. What value should be used for current in amperes?
4
What is the reciprocal of a current of 0.025 A?
5
Which axes give the straight-line graph used in this practical?
6
A graph of 1/I in A⁻¹ against R in Ω has a gradient of 0.50 V⁻¹. What is the source EMF?
7
The same graph has gradient 0.50 V⁻¹ and vertical intercept 3.0 A⁻¹. What is the internal resistance?
8
For the fixed source model, what happens to current when external resistance increases?
9
What external resistance results from connecting the model’s 100 Ω and 150 Ω resistors in series?
10
Why should the readings cover several well-spaced external resistances?