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32 Cards in this Set
- Front
- Back
State Newton's second law of motion |
- The force (acting on a body) is proportional/equal to the rate of change of momentum - And acts in the direction of the momentum change |
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Define linear momentum |
Mass x velocity |
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With an electron inside an electric field of two plates, how does force on an electron varies with the distance of the electron from the bottom plate? |
Electron experiences a constant force between the plates regardless of its distance away from the bottom plate |
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An acetate rod is rubbed with a duster. The rod becomes positively charged. Describe what happens during this process. |
- Electrons transferred from rod to duster - Same amount of charge on duster becomes negative |
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Give an example of a region in which you would expect to find a uniform electric field |
Cathode Ray Tube |
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Explain how an electron gun creates a beam of electrons. |
- Filament heated - Thermionic emission/ electrons have enough energy to leave - Escaped electrons attracted/accelerated from cathode to anode - In a vacuum |
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Explain what is meant by the term electric field |
Region/area/space in which charge experiences force |
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Define capacitance |
- Charge per unit potential difference - C = Q/V |
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In practice the time for discharge is longer than this calculated time. Suggest a reason for this. |
Time is longer because the rate of discharge decreases/ current decreases with time |
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Describe the energy changes that take place as the bob of a simple pendulum makes one complete oscillation, starting at its maximum displacement. |
- GPE to KE to GPE to KE to GPE - Energy lost to the surroundings in overcoming air resistance |
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State and explain the effect of this higher pd on the time taken for this capacitor to lose 90% of its original energy |
- Time to lost 90% energy is unchanged because time constant is unchanged (depends only on R and C) |
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State the relationship between the gravitational potential energy, Ep, and thegravitational potential, V, for a body of mass m placed in a gravitational field. |
GPE = mV |
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Explain why it is not possible to use the equation ∆Ep = mg∆h when determining thechange in the gravitational potential energy of a satellite as it moves between theseorbits. |
- G is not constant when over a large distance is involved or - G decreases as height increases - Work done per metre decreases as height increases - Field is radial not uniform |
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Outline the essential features of a step-down transformer when in operation. |
- Primary coil with more turns than secondary coil - Would around a core/ Input in ac |
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1 Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials. |
- AC current in primary and secondary coils - Energy is lost from heating of coils by currents due to coil having some resistance - Low resistance wire used for coils to reduce energy loss - Thick copper wire used for high current winding (secondary coil of a step-down transformer, because thick wire of low resistivity has a low resistance) |
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2 Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials. |
- AC current in coil continuously magnetises and demagnetises the core - Energy is required to magnetise and to demagnetise the core - This energy wasted because it heats the core - "Soft" iron can be used because it can be easily magnetised/demagnetised |
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3 Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials. |
- Magnetic flux passing through core changes continuously - Metallic core cut by flux, continuous change of flux induces emfs in the core and causes eddy currents, which heats core and energy is wasted - Reduced by laminating the core instead of having a continuous solid core |
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4 Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials. |
- As much as possible of flux created by prim must pass through the 2nd for an efficient transformer - Which will not happen if coils are widely separated from each other on the core - Better core design is needed, with two coils close together to reduced magnetic losses |
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State Newton’s law of gravitation. |
The force of attraction between two point masses is directly proportional to the product of the masses and inversely proportional to the square of the distance between them
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Switch is closed and plates are no longer connected to the battery. State and explain the effect of this on the electric field between the charged plates |
- Electric field becomes zero - Plate discharges/charge flows from one plate to another - Until pd across plate is zero/no pd across plates/plates at same potential |
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Discuss how the principles of conservation of momentum and conservation of energyapply to a system in an event of explosion |
- Momentum is conserved because no external forces acting on the overall system - During explosion, equal and opposite forces acting between the separating object - There are internal forces that act within the overall system - Momentum is a vector and has to be conserved so the separated object after an explosion must move along the original line of the movement - Energy can neither be created nor destroyed therefore total energy is always conserved - Energy may convert from one form to another - Some energy is lost in heating the surroundings after an explosion - KE after explosion is more than KE before because explosion releases some KE |
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Explain why an electron in a B-field is accelerating even though it is travelling at constant speed. |
- Direction changes meaning velocity is not constant - Acceleration involves change in velocity - Magnetic force on electron acts perpendicular to its velocity, which causes a change in direction of movement, thus accelerating it. |
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Explain why the magnitude of the emf is greatest at the θ values of 90°,270° |
- Induced emf is proportional to the rate of change of flux linkage - Flux linkage through coil changes as it rotates - Rate of change is greatest when plane of coil is parallel to B - Coil cuts flux lines perpendicularly at θ=90° or 270°, which is when induced emf is greatest |
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Reasons why a capacitor is not a suitable source for powering a cordless telephone |
- Capacitor would be impossibly large to fit in - Would need recharging quite often - Could only power the phone for a short time - Capacitor voltage would fall continuously whilst in use |
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Explain why the secondary windings of a step-down transformer should be made from thicker copper wire than the primary windings |
- To reduce heating/power loss - because secondary current > primary current |
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Discuss the principles involved in high voltage transmission systems, explaining why a.c. is used in preference to d.c. and how the energy losses are minimised |
- Voltages are changed using transformers, which work with ac but not with dc - Ac generation and transmission is therefore essential - Current in cables causes joule heating (I^2R loss) - Resistance of cables should be as low as possible - Losses are reduced if current in cables can be reduced - Current can be reduced (for same power IV) if voltage is increased - The higher the voltage, the smaller the proportion of the input power that is wasted - High voltage introduces insulation problems and raises safety issues - Voltage must be reduced as the supply reaches its consumers - This is done in stages as the supply is moved from overhead cables to underground wires - Transformers cause energy losses because they are not perfectly efficient - Features are incorporated in the design of transformers to reduced losses from them |
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Define electric potential at a point in an electric field |
- Work done per unit charge - On a small positive test charge - In moving the charge from infinity to the point (not from the point to infinity) |
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Lenz's Law |
The direction of induced current in electromagnetic induction is always such as to oppose the change that causes the current |
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Consequences of Faraday's Law |
- An emf is induced whenever there is a change in the magnetic flux passing through a conductor - The magnitude of the emf is proportional to the rate of change of magnetic flux linkage - The induced emf will cause a current to flow in any complete circuit, such as a single conducting ring |
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Explain why is is necessary for a transformer circuit to include a fuse |
Fuse prevents transformer from overheating/supplying excessive currents |
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Why is fuse placed in the primary circuit rather than in the secondary circuit? |
Transformer is disconnected from supply when fuse fails |
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Resonance |
- Frequency of applied force = natural frequency of the oscillation of the system - Total energy of system is constant (always true) - Amplitude of oscillation depends on amount of damping |