3.2 Circuits
Why can you not get to 0V and 0m?
Systematic uncertainty in measuring d or V Alignment of metre stick The flame has a finite thickness so cannot get exactly to the zero point. Factors causing field to be non-uniform. A p.d. across the resistor for all readings. Poor calibration of instruments measuring V or d.
Effect of wires carrying current in opposite directions
(The current in each wire produces) a magnetic (1) field. Same direction of the magnetic fields between the (1) wires. OR interpretation of F=BIl
Switch S is opened and the iron core is removed from the inductor. Switch S is now closed. (i) Will the maximum current be bigger, smaller or the same as the maximum current calculated in (a)(ii)? 1 (ii) Explain any change in the time delay to reach the maximum current. 2 (iii) Explain why the maximum energy stored in the inductor is less than in (a)(iii).
(i) Maximum current unchanged (ii) The time delay is decreased or the time to reach maximum current is reduced (1), because the inductance is decreased (by removing the iron core).(1) OR back emf is reduced (iii) I is the same (1⁄2) but L is smaller (1⁄2).
Signal generator
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Investigate and describe the use of capacitors in ac circuits
Because capacitors pass AC but block DC signals (when charged up to the applied DC voltage), they are often used to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitive coupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactance is small at the signal frequency, is employed. Split phase induction motors: The capacitors are also used in induction motor to split a single phase supply into a two phase supply to produce a revolving magnetic field in the rotor to catch that field. This type of capacitor is mostly used in household water pumps, Fans, air conditioner and many devices which need at least two phases to work.
use of inductor in ac circuit
Crossover networks in loudspeakers: capacitor C1 allows high frequency signals to pass to loudspeaker Ls1 High frequency signals can also pass more easily through capacitor C2 than loudspeaker Ls2 Low frequency signals are blocked by C1 and C2 but pass easily through inductor L to loudspeaker LS2
The separation of the parallel plates is now increased. An alpha particle enters the electric field at the same point and with the same velocity as before. What effect does this have on the magnitude of the deflection s? You must justify your answer. 2 (8)
Deflection is less (1) E is less (1⁄2) Force/acceleration is less (1⁄2)
Switch S is closed for a short time, passing an alternating current through the coil. State the effect on the iron rod's magnetic field.
Iron rod will have a weaker magnetic field (will be demagnetised) (1) The a.c supply will set up a constantly changing magnetic field in solenoid. (1) Magnetic domains/diopoles will become randomised. (1)
Factors affecting size of the induced emf in a coil
Magnetic strength of the core in the coil of wire. (stronger --> bigger emf) Number of turns of wire in the coil (more turns --> bigger emf) The cross-sectional area of the coil (bigger area --> bigger emf). (This could also be seen as the angle you point the magnet at the coil, because flux linkage is basically the same as flux cutting.) How fast you move the magnet into/out of the coil. (Faster --> bigger emf) more turns= bigger back emf but will take longer to reach its maximum value fewer turns= smaller back emf but will take less time to reach its maximum value
Magnetic flux
Number of lines of magnetic field which pass through the coil Large flux = many lines small flux = few lines
Describe method to illustrate growth of current in a dc circuit
Set up circuit with dc supply, switch, inductor, light bulb in series and a resistor and bulb in parallel. Close switch and adjust variable resistor until lamps have same brightness. Switch power supply off. Switch power supply on again. Lamp in resistor branch lights immediately. Time delay before lamp B1 reaches its maximum brightness. EMF is induced in coil because current in coil is changing. Induced emf opposes the change in current and is called the back emf. Acts against the increase in current. Repeat experiment with inductor of more turns. Lamp B1 takes longer to light fully. Core removed light B1 will light more quickly
Describe experiments to investigate the relationship between current, frequency and inductive reactance
Set up circuit with signal generator, voltmeter across signal generator, ammeter and inductor. Vary frequency of alternating current . Keep output voltage of supply constant. Take corresponding readings of current and plot a graph of current against frequency. Inverse proportion. Reactance is proportional to frequency which is proportional to inductance which gives relationship
Explain the direction of the induced emf in terms of energy
Since the direction the magnet is moving is changing, the direction of the emf is also changing
Explain the production of the induced emf across the coil
When a current is produced by a moving magnetic field, the induced currents are produced by an emf. Electrical energy must come from somewhere. The work done by the person pushing the magnet at the coil is the source of energy. The induced current sets up a magnetic field in the coil which opposes the movement of the magnet.
Faradays laws
When the magnetic flux through a circuit is changing an emf is produced The magnitude of the induced emf is proportional to the rate of change of the magnetic flux
The inductance of an inductor is one henry if
an emf of 1V is induced the the current changes at a rate of 1A per second
Use of inductor in dc circuit
energy storage (switch mode power supplies)
As the frequency of the supply increases, the rate of change of current will
increase and the back emf will increase
Increasing the frequency of an ac signal will _____ the current in the circuit because..
increase, reactance is the opposition to current. Reactance is inversely proportional to frequency so as frequency increases reactance decreases and current increases
the work done in building up the current in an inductor is stored in the
magnetic field of the inductor
Right hand rule for
negative charges, flip answer for positive charges
Electromagnetic induction is the
production of an electromotive force across a conductor exposed to time varying magnetic fields
Describe experiments to investigate the relationship between current, frequency and capacitive reactance
set up circuit with a.c supply, ammeter and capacitor in series with an ac voltmeter across the capacitor. Vary the value of the power supply voltage and record corresponding currents on the ammeter. direct proportion. Now increase frequency and current can be shown to increase. Since a higher current means lower reactance reactance must be inversely proportional to frequency and inversely proportional to capacitance
the energy stored in the magnetic field of an inductor may be a
source of emf
An emf is produced is induced across a coil when
the current through the coil is varying
Inductive reactance is
the opposition to an alternating current
Capacitive reactance is
the opposition to current in an ac capacitive. can also be described as the ratio of voltage to current in an ac capacitive circuit
The current in an inductive circuit does not reach its maximum value instantly when switched on due to
the production of aback emf which opposes the increasing current
the back emf is
the voltage induced across an inductor due to a changing current in the inductor and changing magnetic field around the inductor
What is meant by time constant in an RC circuit?
time taken for charge charge to decay to 1/e times its initial value
Lenz's law shows that
when an induced current flows, its direction is always such that it will oppose the change which produced it. When the current decreases the effect of an inductor is to try and maintain the current. Now the induced emf acts in the same direction as the current.