PHSX115 Chapter 23
What would be the maximum cost of a CFL such that the total cost (investment plus operating) would be the same for both CFL and incandescent 60-W bulbs? Assume the cost of the incandescent bulb is 25 cents and that electricity costs 10 cents/kWh. Calculate the cost for 1000 hours, as in the cost effectiveness of CFL example.
$6.25
The forces shown below are exerted on an electron as it moves through the magnetic field. In each case, what direction does the electron move? Explain your answers.
(a) left to right, (b) into the page, (c) downwards
The magnetic flux through a single wire loop changes from 3.5Wb to 1.5Wb in 2.0s. What emf is induced in the loop?
+1.0 V
An emf is induced by rotating a 1000-turn, 20.0 cm diameter coil in the Earth's 5.00×10^−5T magnetic field. What average emf is induced, given the plane of the coil is originally perpendicular to the Earth's field and is rotated to be parallel to the field in 10.0 ms?
0.157 V
A jet airplane with a 75.0 m wingspan is flying at 280 m/s. What emf is induced between wing tips if the vertical component of the Earth's field is 3.00×10^−5T? Is an emf of this magnitude likely to have any consequences?
0.630 V
The initial current in an RL circuit is 10.0 A. What is the current two time steps after it is turned off?
1.35
A large power plant generates electricity at 12.0 kV. Its old transformer once converted the voltage to 335 kV. The secondary of this transformer is being replaced so that its output can be 750 kV for more efficient cross-country transmission on upgraded transmission lines. What is the ratio of turns in the new secondary compared with the old secondary? What is the ratio of new current output to old output (at 335 kV) for the same power? If the upgraded transmission lines have the same resistance, what is the ratio of new line power loss to old?
2.24; 0.447; 0.203
The circuit shown in the image above consists of a U-shaped wire with a resistor and with the ends connected by a sliding conducting rod. The magnetic field filling the area enclosed by the circuit is constant at 0.01T. If the rod is pulled to the right at speed v=0.50m/s and the rail separation l=0.10m, what current is induced in the circuit and in what direction does the current flow?
25μA in a clockwise direction
The 335 kV AC electricity from a power transmission line is fed into the primary coil of a transformer. The ratio of the number of turns in the secondary to the number in the primary is Ns/Np=1000. What voltage is induced in the secondary? What is unreasonable about this result? Which assumption or premise is responsible?
335 MV; It is way too high, well beyond the breakdown voltage of air over reasonable distances; input voltage is too high
A 75-turn, 10.0 cm diameter coil rotates at an angular velocity of 8.00 rad/s in a 1.25 T field, starting with the plane of the coil parallel to the field. What is the peak emf? At what time is the peak emf first reached? At what time is the emf first at its most negative? What is the period of the AC voltage output?
5.89 V; At t=0 s; 0.393 s; 0.785 s
A Tethered Satellite is producing 5.00 kV, and a current of 10.0 A flows. What magnetic drag force does this produce if the system is moving at 7.80 km/s? How much kinetic energy is removed from the system in 1.00 h, neglecting any change in altitude or velocity during that time? What is the change in velocity if the mass of the system is 100,000 kg?
6.41 N; 1.8 x 10^8 J; 0.23 m/s
Consider the angle between the area vector and the magnetic field in an electric motor. At what angles is the torque on the wire loop the greatest?
90∘ and 270∘
What is a transformer?
A transformer is a device that transforms voltages from one value to another.
Which formula gives magnetic flux?
BAcosθ
Referring to the figure, what is the direction of the current induced in coil 2: If the current in coil 1 increases? If the current in coil 1 decreases? If the current in coil 1 is constant?
CCW; CW; No current induced
What type of energy change takes place in a motor?
Electrical energy to mechanical energy
A recycling center uses magnetic damping to detect and separate certain materials. Which materials will respond to this method? I. Ferromagnetic conductors II. Nonferromagnetic conductors III. Insulators
I. and II. only
Which are correct statements about an RL circuit? I. The opposition of the inductor starts at zero and quickly increases. II. The current increases exponentially. III. The characteristic time constant depends on the resistance but not on the inductance.
II. only
Explain how magnetic flux can be zero when the magnetic field is not zero.
If the angle between magnetic field and area vector is 90∘ then its product is also zero which means that there is zero flux.
What is the direction of the magnetic field at the poles of a bar magnet?
It points inward at the south pole and outward at the north pole.
Which statement correctly describes how the plastic insulation on live electrical wires is a safety feature?
It protects against shock hazards but does not prevent thermal overload.
How does the impedance of an RLC circuit change as the frequency changes from low to high?
It starts out high, and then decreases, and then increases.
What is magnetic flux?
Product of an area and magnetic field perpendicular to the area is defined as magnetic flux.
Given that (R1>R2>R3>R4), explain which resistor in the circuit below has the most current flowing through it.
R4
What is the Hall effect?
The creation of a voltage across a current-carrying conductor by a magnetic field
A counterclockwise current is induced in a wire loop when a bar magnet passes from right to left through the loop. In what direction is the induced current, if the orientation of the bar magnet is reversed and then the bar magnet is passed again from right to left through the loop?
The direction of induced current is reversed, in a clockwise direction.
The current in the wire at left is increasing. In what direction is the current induced in the wire loop?
The direction of the induced current will be counterclockwise.
When a magnet is thrust into a coil as in the figure below, what is the direction of the force exerted by the coil on the magnet? Justify your answer.
The force is in the downward direction because coil resists the change which creates induced current.
What is the relationship between the number of coils in a solenoid and the emf induced in it by a change in the magnetic flux through the solenoid?
The induced emf is directly proportional to the number of coils in a solenoid.
Why are the coils of a transformer wrapped around a loop of ferrous material?
The magnetic field from the source coil is trapped and also increased in strength.
What should be the ratio of the number of output coils to the number of input coil in a step-up transformer to increase the voltage fivefold?
The ratio is 5 times.
A simple circuit is constructed with an AC voltage source in series with an inductor. What does the graph of voltage and current versus time look like?
The sine curve of the current lags behind the sine curve of the voltage by 90∘.
A simple circuit is constructed with an AC voltage source in series with a capacitor. What does the graph of voltage and current versus time look like?
The sine curve of the voltage lags behind the sine curve of the current by 90∘.
The magnetic field lines are denser in region A compared with region B. Is the magnetic field in region A stronger or weaker than in region B? Explain.
The strength of magnetic field at the region A will be stronger than the magnetic field at the region B, because the density of the field lines is proportional to the strength of a magnetic field.
What happens to the torque provided by an electric motor if you double the number of coils in the motor?
The torque would be doubled.
Why are ordinary circuit breakers and fuses ineffective in preventing shocks?
Their reaction to current overload can occur after a shock
What would be the effect on the torque of an electric motor of doubling the width of the current loop in the motor?
Torque is doubled.
At low frequencies, which component of an RLC circuit has the greatest effect on the impedance?
capacitor
How would you place two identical flat coils in contact so that they had the greatest mutual inductance?
end-to-end on the same core
At high frequencies, which component of an RLC circuit has the greatest effect on the impedance?
inductor
Name three sources of external power that are used to generate commercial electric power.
kinetic energy of water, thermal energy using coal, and nuclear energy
What orientation of a magnetic field will cause the magnetic flux through an area to be zero even though the magnetic field is not zero?
parallel to the area
A coil is rotating at a constant angular velocity in a uniform magnetic field. What shape does the emf vs. time graph have?
sinusoidal
Given a bar magnet, how can you induce an electric current in a wire loop?
An electric current is induced if a bar magnet is moved through the wire loop.
When you insert a copper ring between the poles of two bar magnets as shown below, do the magnets exert an attractive or repulsive force on the ring? Explain your reasoning.
Magnets exert a repulsive force because magnetic field due to induced current is repulsed by the magnetic field of the magnets.
Why is a potentiometer more accurate than a standard voltmeter?
No current flows through the galvanometer in a potentiometer.
If a bar magnet moves through a wire loop as shown below, is a current induced in the loop? Explain why or why not.
No, because the net magnetic field passing through the loop is zero.
Is an emf induced in the coil shown above when it is stretched? If so, state why and give the direction of the induced current.
Yes, because area of the coil increases; the direction of the induced current is clockwise.
You want to buy a hand generator, but there are several choices. Assuming all other characteristics of the generators are the same, would you want each of the following to be high or low in order to induce a high emf? number of turns in the coil radius of the circular coil magnetic field strength
You would want all of them to be high.
By what factor does the self-inductance of a solenoid increase if you triple the number of coils and double the radius?
a factor of 36
By what factor does the energy stored in an inductor increase if both the inductance and the current increase by a factor of 2?
a factor of 8
