Phya4

Front 1

State Newton's second law of motion

Back 1

- 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

Front 2

Define linear momentum

Back 2

Mass x velocity

Front 3

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?

Back 3

Electron experiences a constant force between the plates regardless of its distance away from the bottom plate

Front 4

An acetate rod is rubbed with a duster. The rod becomes positively charged. Describe what happens during this process.

Back 4

- Electrons transferred from rod to duster

- Same amount of charge on duster becomes negative

Front 5

Give an example of a region in which you would expect to find a uniform electric field

Back 5

Cathode Ray Tube

Front 6

Explain how an electron gun creates a beam of electrons.

Back 6

- Filament heated

- Thermionic emission/ electrons have enough energy to leave

- Escaped electrons attracted/accelerated from cathode to anode

- In a vacuum

Front 7

Explain what is meant by the term electric field

Back 7

Region/area/space in which charge experiences force

Front 8

Define capacitance

Back 8

- Charge per unit potential difference

- C = Q/V

Front 9

In practice the time for discharge is longer than this calculated time. Suggest a reason for this.

Back 9

Time is longer because the rate of discharge decreases/ current decreases with time

Front 10

Describe the energy changes that take place as the bob of a simple pendulum makes one complete oscillation, starting at its maximum displacement.

Back 10

- GPE to KE to GPE to KE to GPE

- Energy lost to the surroundings in overcoming air resistance

Front 11

State and explain the effect of this higher pd on the time taken for this capacitor to lose 90% of its original energy

Back 11

- Time to lost 90% energy is unchanged because time constant is unchanged (depends only on R and C)

Front 12

State the relationship between the gravitational potential energy, Ep, and thegravitational potential, V, for a body of mass m placed in a gravitational field.

Back 12

GPE = mV

Front 13

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.

Back 13

- 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

Front 14

Outline the essential features of a step-down transformer when in operation.

Back 14

- Primary coil with more turns than secondary coil

- Would around a core/ Input in ac

Front 15

1

Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials.

Back 15

- 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)

Front 16

2

Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials.

Back 16

- 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

Front 17

3

Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials.

Back 17

- 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

Front 18

4

Energy losses in a transformer and discuss how theseenergy losses may be reduced by suitable design and choice of materials.

Back 18

- 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

Front 19

State Newton’s law of gravitation.

Back 19

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

Front 20

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

Back 20

- 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

Front 21

Discuss how the principles of conservation of momentum and conservation of energyapply to a system in an event of explosion

Back 21

- 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

Front 22

Explain why an electron in a B-field is accelerating even though it is travelling at constant speed.

Back 22

- 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.

Front 23

Explain why the magnitude of the emf is greatest at the θ values of 90°,270°

Back 23

- 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

Front 24

Reasons why a capacitor is not a suitable source for powering a cordless telephone

Back 24

- 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

Front 25

Explain why the secondary windings of a step-down transformer should be made from thicker copper wire than the primary windings

Back 25

- To reduce heating/power loss

- because secondary current > primary current

Front 26

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

Back 26

- 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

Front 27

Define electric potential at a point in an electric field

Back 27

- 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)

Front 28

Lenz's Law

Back 28

The direction of induced current in electromagnetic induction is always such as to oppose the change that causes the current

Front 29

Consequences of Faraday's Law

Back 29

- 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

Front 30

Explain why is is necessary for a transformer circuit to include a fuse

Back 30

Fuse prevents transformer from overheating/supplying excessive currents

Front 31

Why is fuse placed in the primary circuit rather than in the secondary circuit?

Back 31

Transformer is disconnected from supply when fuse fails

Front 32

Resonance

Back 32

- 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