# Homework Help: Can someone validate my practice exam answers

1. Apr 6, 2010

I have a good few past exam papers to practice but they don't come with solutions so I don't know if I'm answering them correctly or not. If anyone has the time can you tell me if I'm on the right path or not with these physics questions.

Note: To keep it short I'll write the scientific notation as Eexponent i.e. 5x10^7 = 5E7

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Q1.)
a.) Define the electric field E of a point charge Q.
b.) Calculate the electric field of a point charge of -2.3E-6C at a distance 5.2E-6m from the charge and show the direction of the field.
c.) Calculate the force on an electron due to that of a proton which is separated from the proton by a distance of 1.5E-10m and show the direction of the force.

A1.)
a.) A spherical field of electrical force of varying magnitude force which decreases in magnitude with distance from the charge.
b.) -9.356E-6
c.) 2.5E-30

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Q2.) A 24 V battery has an internal resistance, r, of 0.50ohms and to which is connected a 4.7ohm resistor.

a.) Calculate the current drawn from the battery.
b.) Calculate the terminal voltage of the battery.
c.) Use Kirchhoff s loop rule to determine the voltage across r.

A2.)
a.) 5.1A
b.) 21.45V
c.) 21.45V

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Q3.)

a.) Define electrical resistance.

b.) A conductor carries a current of 2.4 A with a potential difference of 12 V
across it. The same conductor carries a current of 0.10 A with a potential
difference of 0.50 V across it. State, with calculated justification, if the
conductor is ohmic or non-ohmic.

c.) A 24 V battery is connected to a pair of resistors, with resistance values
8.0ohms and 4.0ohms, which are in parallel to each other. Calculate the power
dissipated in the circuit.

A3.)
a.) The ability of a material to resist electric current.

b.)
r1 = 0.5ohms
r2 = 5ohms
The material is non ohmic because the resistance of the material because the resistance of the material does not remain constant when the voltage/current changes.

c.) 215.76W

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Q4.) 2 capacitors of capacitance values 2.4uF and 1.2uF are connected in series.

a.) Show by calculation that the effective capacitance is 0.80uF.
b.) If this arrangement is connected to a 12 V battery calculate the charge drawn from the battery. Hence calculate the voltage across each of the capacitors.
c.) Calculate the energy stored in each of the capacitors.

A4.)
a.)
1/C = 1/C1 + 1/C2 =>
1/C = 1/2.4 + 1/1.2 =>
1/C = 3/2.4 =>
C = 2.4/3 =>
C = 0.8uF

b.)
C1 = 2.4uF
C2 = 1.2uF
Total C = 0.8uF
Total Q = 15C
V1 = 6.25V
V2 = 12.5V

c.)
E1 = 46.88J
E2 = 93.75J
^^ Are joules the right unit?

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Q5.) A long straight wire carries a current of an unknown value and unknown direction.

a.) The value of the magnetic field is measured to be 0.0045T a distance of 22.1 cm from the wire. Calculate the magnitude of the current flowing in the wire.

b.) Describe how you can determine the direction of the current by measuring
the direction of the magnetic field.

c.) If the current is defined to flow in the x-direction (of a Cartesian coordinate
system) and an external magnet field is applied in the y-direction state the
direction of the force on the wire.

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Q6.) You are given a circular loop of wire and a permanent magnet.

a.) Describe two ways in which you can induce a current in the loop using only the magnet.

b.) You hold your loop near a wire attached to battery as shown below. If you disconnect the wire from the battery which direction does the induced current flow in the loop? You must explain your reasoning.

c.) A magnetic field of 0.34 T passes through a square loop (of side length 4.5 cm) at an angle of 45". If the magnetic field falls to a zero value in 0.23s calculate the average induced emf in the loop.