Flashcards
P5
Flashcards do conjunto (51)
- 1What is an electrostatic force?A force caused by positive and negative charges
- 2What are the rules for the way charges behave?Like repel, unlike attract
- 3How are electrons held in an atom? Why can friction alter this?By electrostatic forces - the positive nucleus is attracted to the negative electrons. However this gets weaker as you get further from the nucleus.
Friction can remove the outer most electrons meaning the object that loses the electrons becomes positively charged and the object the electrons rub off on become negatively charged. - 4How does lightening occur?Friction in the air causes a charge to be built up and when this is large enough to break down the insulation of the air the charge flows between the air and the earth.
- 5Why is plastic a good insulator?It has very free electrons that can carry an electric current
- 6What makes a good conductor?A material where electrons are held loosely and can move freely
- 7What is an electric current?A flow of electrons
- 8What does the power supply do in a circuit?Provides the push that moves the electrons thus share thus current around the circuit in one direction
- 9Why is charge never used up in a circuit?Charge is conserved and energy is transferred. The energy from the electrons is transferred to the components but the electrons themselves are kept in the circuit and continue to go round the circuit due to the push of the power source.
- 10What is an ammeter? and how does it connect in the circuit?An ammeter measures amps and is connected in series with the components in the circuit so it can measure the current through the circuit
- 11What is a voltmeter and how does it connect to the circuit?Measures how much energy is transferred in a particular part of the circuit - voltage. Voltmeters are connect in parallel to the circuit.
- 12Describe the movement of the current through the circuit?The power source first does work on the electrons (which are essentially the energy carriers) these then flow round the circuit and then do work on the components. The charges then flow back to the power source where they are given energy again
- 13What is potential difference and what would one volt be in terms of P.d.?Potential difference is another term for voltage. It is the difference in energy per charge either side of a component. The P.d. between two points is one volt if one joule of electrical energy is transferred to another form of energy when one unit of charge passes between the points
- 14How is the current controlled?By the voltage supplied by the power source and the resistance within the circuit
- 15What is a variable resistor?A device that allows one to vary the amount of resistance in a circuity by moving a slide or rotating a knob.
- 16What is resistance and what is it measured in?Resistance is the me sure of how much a conductor opposes the current. Its unit is the ohm.
- 17What equation describes the relationship between voltage, current and resistance?resistance = voltage / current
- 18What is Ohm's law?Resistance = 1/gradient of the graph of voltage against current
Thus the current through a conductor is directly proportional to the voltage across its ends if the other conditions are constant. - 19Describe the features of a series circuit?The current is the same everywhere, if one bulb blows they whole circuit brakes, the voltage is shared between the components - (battery voltage = voltage across lamp + voltage across resistor - if those were the only two components)
- 20Describe the features of a parallel circuit?The components are connected in parallel and each are connected separately to the power supply. The current is shared between each branch of the circuit but each brand gets the full full voltage. If one component fails on one branch the others will continue to work
- 21What s a thermistor?A thermistor is a semiconductor device. Its resistance changes with the temperature. The resistance usually decreases as the temperature increases. They can be used as temperature sensors like in digital thermometers.
- 22What is the symbol for a thermistor?
- 23What is an LDR?Light dependent resistor. A semiconductor device which has a resistance depending on the light. In bright light its resistance is low but in darkness it has high resistance. An LDR can be used to with a street light on at night.
- 24What is the symbol for an LDR?
- 25What happens and why to the resistance in metals as they heat up?As temperature increases so does the metals resistance. This is as the positive ions in the metal structure have more energy and jiggle about meaning there are more collisions with the electrons. Meaning the speed of the electrons and thus charge goes down.
- 26Why does resistance vary in the way it does in LDR's and thermistors?When there is extra heat or light energy it provides these resistors with more free electrons. In these devices there is a greater flow of charge as there is more charge available. Why resistance goes down
- 27How would an LDR work to close open curtains automatically in the morning?When it is dawn the light from the sun will cause the resistance is the circuit to decrease. Since the p.d across the components must equal that of the power supply more p.d. will be available in the motor meaning the curtains open
- 28WHat is a magnetic field?An area in which the magnetic force acts
- 29What did Michael Faraday discover?If you moved a magnet near a wire (or vice versa) it induced a current and a voltage between the ends of the wire. This is called electromagnetic induction
- 30What else did Faraday find?the direction of the current changed if the motion of the wire was reversed,
- 31How can we increase the current/ voltage?Adding coils of wire. The more turns the stronger the voltage/current, use a stronger magnet, moving the magnet faster/ faster relative movement (or wire - whenever relative movement current made) , use an iron core
- 32How did Faraday explain electromagnetic induction?Each time the magnetic field lines were cut through the electrical conductor it induced a voltage
- 33How does a generator work?Two stationary magnets are positioned facing each other (with the poles facing) a coil then spins inducing a current. Alternatively an electromagnet spins inside a stationary coil
- 34What is an alternating current?The direction of an alternating current changes at regular intervals. The number times it alternates in a second = the number of Hz. A frequency of 50 Hz means it has changed 50 times in a second
- 35What is a uniform field?When the magnetic field is the same strength at all points between the poles. It is shown on a diagram by parallel, evenly spaced lines
- 36Why does a generator induce an alternating current?This is produced because the magnet cuts the uniform field at different points cutting different number of magnetic field lines creating different voltages. When the magnet is reversed a negative voltage is induced. The varying voltage will reduce in an alternating current when the circuit is complete. The frequency of the current will be the same as the frequency of the rotating coil/ magnet.
- 37What is the magnetic effect of a current?When an electric current flows through a wire it produces a circular magnetic field around it. When it flows through a coil it creates a similar magnetic field to a bar magnet.
- 38What is the motor effect?When a current flows through a wire in a region where there is another magnetic field, the wire experiences a force. If it is free to move - it will.
- 39How can the motor effect be made bigger?This is made biggest when the magnetic field and current are perpendicular to each other. There is no force felt if the current carrying the wire is along a magnetic field line. The force is also bigger if the current is larger or if the magnetic field is stronger.
- 40How does a motor work?A motor uses the the motor effect but is assembled so that the force gives continuos rotation. It is used like a generator but it is used in reverse. The coil is connected to a power supply and a force is produced that makes the coil turn. This can be understood by considering a rectangular coil. Each side of the coil in the field experiences and a force: on one side the force is up on the other it is down making the coil turn.
- 41What is a split ring commutator?A metal ring split into 2 halves
- 42How does it keep the motor turning?The fixed commutator swaps contacts with the coil to reverse the current through the coil. This happens each half cycle and ensures that the turning effect is always in the same direction and the coil rotates continuously.
- 43What is a transformer made of?A primary coil - which receives the current and secondary coil which is connected to the appliance
- 44What is a step up transformer?When the secondary coil is bigger than the primary coil - increases the voltage supplied to the appliance
- 45What is a step down transformer?When the primary coil has more turns than the secondary the transformer will reduce the voltage supplied to the appliance
- 46How does a transformer work even though the coils are not connected?The a.c. voltage in the primary coil creates an ever- changing magnetic field around it. THe magnetic soft iron core channels the field through the secondary coil. This repeatedly changing magnetic field cuts across through the secondary coil and an a.c. voltage is produced
- 47What is D.C?Direct current. The current flows in one direction all the time.
- 48What is alternating current?A circuit where the current keeps changing direction
- 49Why are alternating electricity generators good?They produce far more power than other forms of electricity production. Also many fuels can work them. Transformers only work with alternating current.
- 50Why is electricity transferred at high voltage?Less energy is lost
- 51What is the equation for electrical power?Power = voltage x current
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- FlashCards sobre P5, criado por blueaweiss em 03-06-2014.
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