AS & A Level Physics - 9702 Paper 4 2011 Summer Zone 2

(a) State what is meant by a \textit{field of force}.  [1]

(b) Gravitational fields and electric fields are two examples of fields of force. State one similarity and one difference between these two fields of force.   [3]

(c) Two protons are isolated in space. Their centres are separated by a distance $R$. Each proton may be considered to be a point mass with point charge. Determine the magnitude of the ratio   [3]

$$\frac{\text{force between protons due to electric field}}{\text{force between protons due to gravitational field}}$$ 

A capacitor consists of two metal plates separated by an insulator, as shown in Fig. 3.1.


The potential difference between the plates is $V$. The variation with $V$ of the magnitude of the charge $Q$ on one plate is shown in Fig. 3.2.


(a) Explain why the capacitor stores energy but not charge.
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(b) Use Fig. 3.2 to determine
(i) the capacitance of the capacitor,

capacitance = ..................................................... $\mu$F [2]

(ii) the loss in energy stored in the capacitor when the potential difference $V$ is reduced from 10.0V to 7.5V.

energy = ............................................................ mJ [2]

(c) Three capacitors X, Y and Z, each of capacitance 10$\mu$F, are connected as shown in Fig. 3.3.


Initially, the capacitors are uncharged.
A potential difference of 12V is applied between points A and B.
Determine the magnitude of the charge on one plate of capacitor X.

charge = ............................................................ $\mu$C [3]

(a) The first law of thermodynamics may be expressed in the form $$\Delta U = q + w.$$

Explain the symbols in this expression.  [3]

• + $\Delta U$
• + $q$
• + $w$

(b) (i) State what is meant by \textit{specific latent heat}. [3]

(ii) Use the first law of thermodynamics to explain why the specific latent heat of vaporisation is greater than the specific latent heat of fusion for a particular substance. [3]

A bar magnet is suspended vertically from the free end of a helical spring, as shown in Fig. 5.1.

One pole of the magnet is situated in a coil. The coil is connected in series with a high-resistance voltmeter.

The magnet is displaced vertically and then released.

The variation with time $t$ of the reading $V$ of the voltmeter is shown in Fig. 5.2.

(a) (i) State Faraday’s law of electromagnetic induction.    [2]

(a) (ii) Use Faraday’s law to explain why

1. there is a reading on the voltmeter,    [1]
2. this reading varies in magnitude,    [1]
3. the reading has both positive and negative values.    [1]

(b) Use Fig. 5.2 to determine the frequency $f_0$ of the oscillations of the magnet.    [2]

(c) The magnet is now brought to rest and the voltmeter is replaced by a variable frequency alternating current supply that produces a constant r.m.s. current in the coil. The frequency of the supply is gradually increased from $0.7f_0$ to $1.3f_0$, where $f_0$ is the frequency calculated in (b).

On the axes of Fig. 5.3, sketch a graph to show the variation with frequency $f$ of the amplitude $A$ of the new oscillations of the bar magnet.   [2]

(d) (i) Name the phenomenon illustrated on your completed graph of Fig. 5.3.  [1]

(ii) State one situation where the phenomenon named in (i) is useful.    [1]

An alternating current supply is connected in series with a resistor R, as shown in Fig. 6.1.

The variation with time $t$ (measured in seconds) of the current $I$ (measured in amps) in the resistor is given by the expression

$I = 9.9 \sin(380t)$.

(a) For the current in the resistor $R$, determine

(i) the frequency,   [2]

(ii) the r.m.s. current.  [2]

(b) To prevent over-heating, the mean power dissipated in resistor $R$ must not exceed 400W.

Calculate the minimum resistance of $R$.  [2]

(a) State what is meant by the de Broglie wavelength.
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(b) An electron is accelerated in a vacuum from rest through a potential difference of 850V.

(i) Show that the final momentum of the electron is 1.6 \times 10^{-23}\, \text{Ns}.

[2]

(ii) Calculate the de Broglie wavelength of this electron.

wavelength = ......................................................... m [2]

(c) Describe an experiment to demonstrate the wave nature of electrons. You may draw a diagram if you wish.

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(a) State what is meant by the \textit{binding energy} of a nucleus.
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(b) Show that the energy equivalence of 1.0 u is 930 MeV.
[3]
(c) Data for the masses of some particles and nuclei are given in Fig. 8.1.

[Table_1]

Fig. 8.1
Use data from Fig. 8.1 and information from (b) to determine, in MeV,

(i) the binding energy of deuterium,
binding energy = ..................................... MeV [2]

(ii) the binding energy per nucleon of zirconium.
binding energy per nucleon = ............................................................. MeV [3]

(a) Describe the structure of a metal wire strain gauge. You may draw a diagram if you wish.
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[3]

(b) A strain gauge S is connected into the circuit of Fig. 9.1.

[Image_1: Diagram of Fig. 9.1]

V_{OUT} = \frac{R_F}{R} \times (V_2 - V_1).

(i) State the name given to the ratio \frac{R_F}{R}.
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[1]

(ii) The strain gauge S has resistance 125 \Omega when not under strain. Calculate the magnitude of V_1 such that, when the strain gauge S is not strained, the output V_{OUT} is zero.
V_1 = ................................................ V [3]

(iii) In a particular test, the resistance of S increases to 128 \Omega. V_1 is unchanged. The ratio \frac{R_F}{R} is 12. Calculate the magnitude of V_{OUT}.
V_{OUT} = ............................................ V [2]

Explain briefly the main principles of the use of magnetic resonance to obtain diagnostic information about internal body structures.

The use of ionospheric reflection of radio waves for long-distance communication has, to a great extent, been replaced by satellite communication.
(a) State and explain two reasons why this change has occurred.
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[4]
(b) The radio link between a geostationary satellite and Earth may be attenuated by as much as 190 dB.
Suggest why, as a result of this attenuation, the uplink and downlink frequencies must be different.
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[2]

(a) The signal-to-noise ratio in an optic fibre must not fall below 24 dB. The average noise power in the fibre is $5.6 \times 10^{-19}$ W.
(i) Calculate the minimum effective signal power in the optic fibre.

power = ........................................ W [3]

(ii) The fibre has an attenuation per unit length of 1.9 dB km$^{-1}$. Calculate the maximum uninterrupted length of fibre for an input signal of power 3.5 mW.

length = ........................................ km [3]

(b) Suggest why infra-red radiation, rather than ultraviolet radiation, is used for long-distance communication using optic fibres.
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