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PPE 2 Flashcards for Checklist > Physics > Flashcards

Physics Flashcards

1
Q

Explain why, when certain materials are rubbed together, they end up with opposite charges

A
  • When you rub insulators together, there is a transfer of electrons that leaves one material negatively charged (as it gained the electron) and one positively charged (it lost the electron)

FOR EXAMPLE:

  • If you rub an acetate rod with a cloth, it becomes positively charged as electrons transfer from the rod to the cloth.
  • Whereas, if you rub a polythene rod with a cloth, it becomes negatively charged as electrons transfer from the cloth to rod
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2
Q

Recall the rules of attraction and repulsion between charges

A
  • Like charges will repel
  • Opposite charges will attract
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3
Q

Explain how attraction by induction occurs (use a balloon as an example)

A
  • If you rub a balloon against a jumper, the friction causes electrons to transfer from the jumper to the balloon.
  • The balloon now has a negative charge
  • When brought close to a wall, the negative charge of the balloon repels the electrons in the wall
  • The protons are brought closer to the surface of the wall as they are attracted to the balloons negative charge
  • The balloon is attracted to the positive charge on the surface of the wall causing it to stick
  • The wall has an induced charge
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4
Q

Explain how electrostatic sprayers work

A
  • The nozzle contains electrodes which negatively charge the spray droplets. This leads to two things:
  • The like charge between all droplets spread them out
  • The negative charges then charges the plant by induction to stick
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5
Q

Describe some hazards caused by charged objects discharging and how earthing can reduce these risks

A
  • Charged objects look to discharge. This is when the electrons flow in the direction that removes excess charge.

Examples of Hazards:

  • Planes can become charged when flying through the air, or even when fuel flows through a pipe. It may look to become discharged, which may cause a spark. This spark is dangerous as it can ignite the fuel, causing an explosion.

Earthing

  • Earthing connects the aircraft to the Earth, preventing a charge from building up as it provides a low resistance path into the Earth.

Note that Earthing is also used in a 3-pin plug through the Earth wire, which connects the metal part of an appliance to a metal spike going into the ground, again preventing the case from discharging through you by providing a low resistance path into the ground.

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6
Q

How do you calculate work done?

A

work done = force x distance moved in direction of the force

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7
Q

What is work done?

A

A measure of energy transferred when a force acts through a distance

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8
Q

What is power?

How do you calculate it?

A

The amount of energy transferred per second.

Power = Work Done / Time

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9
Q

Describe some situations where a force can cause rotation

A

A turning force is called a moment.

moment = force x distance perpendicular to the pivot

APPLICATIONS:

LEVER: This is when a force is applied to a bar that pivots about a point, and is used to transfer the force

GEAR: This is when the rotational effects of a force is transmitted by interlocking teeth in gears.

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10
Q

What is the principle of moments?

A

When a system involving rotational forces is in equilibrium:

the sum of clockwise moments = the sum of anti-clockwise moments

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11
Q

Explain how levers transmit rotational effects

A
  • Moments are used in levers to multiply force on a load
  • We increase the distance between the force and the pivot using a bar, a greater distance means the moment will be greater.
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12
Q

Explain how gears transmit rotational forces

A

The rotational effects of a force is transmitted by interlocking teeth in gears.

A high gear is when a large input gear turns a small output gear, leading to a high speed and a low turning effect.

A low gear is when a small input gear turns a large output gear, leading to slow speed but high turning effect

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13
Q

How do you calculate:

GPE

KE

A

GPE = mgh

gravitational potential energy = mass x gravitational field strength x height

KE = 1/2 mv2

kinetic energy = 1/2 x mass x (velocity)2

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14
Q

Explain why energy transfers always result in energy loss and where that goes

A
  • When energy is transferred from one form to the other, there will always be wasted energy, most commonly due to friction.
  • This friction produces thermal energy as a wasted energy, which then is dissipated to the surroundings, making them warmer.
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15
Q

Explain the link between resistance and current in a circuit

A
  • V = IR, current and resistance are inversely proportional
  • This is because current is the flow of electrons and resistance is the force that opposes this flow.
  • Therefore, increasing the resistance will decrease current.

HOWEVER

  • Note that they aren’t always inversely proportional.
  • Increasing current will increase resistance due to the heating effect, more electrons flowing means that there will be more collisions within the wire, therefore increasing resistance.
  • This is why, in the national grid, step up transformers are used to increase voltage but decrease current, therefore transmitting the electricity at low current, therefore low resistance.
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16
Q

Explain how and current and resistance change with potential difference in filament lamp.

Describe the IV graph for a filament lamp.

A
  • The current causes the filament to heat up and glow - the greater the p.d, the more current flows and the hotter and whiter the filament gets. However, as it heats up, the filament’s resistance increases. Therefore, the voltage and current are not in direct proportion anymore

An S shaped ‘curve’.

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17
Q

What does an IV graph look like for a regular diode and why?

A
  • A line that is nearly on the x-axis till it shoots up.
  • This is because a diode has low resistance if the potential difference is in one direction but a very high resistance if the potential difference is in the opposite direction
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18
Q

How does the resistance of a LDR vary with changing light intensity?

A
  • As we increase light intensity, resistance will decrease.
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19
Q

How does resistance in a thermistor vary with changing temperature?

A
  • As temperature increases, resistance decreases.
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20
Q

Describe uses of:

  1. Diodes
  2. LDRs
  3. Thermistors
A
  1. Protection of electrical equipment - some can be damaged if current flows in the wrong way, therefore the diode has high resistance in this direction to prevent damage
  2. Detect changes in light intensity, at night, can be used to turn on street lights
  3. They can be used to trigger fire alarms - increased temperature causes resistance to decrease, surge of current flows that triggers the alarm.
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21
Q

What are the 4 statements that are always true about field lines?

A

They:

  1. never cross
  2. show where the field is strongest (where the lines are closest together)
  3. show the direction of the force on a charge in the field
  4. start on a positive, end on a negative
22
Q

Describe how the shape of magnetic fields can be shown using plotting compasses

A
  • place the plotting compass near the magnet on a piece of paper.
  • mark the direction the compass needle points. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time.
  • join the points to show the field lines.
23
Q

How does the shape and direction of the magnetic field around a straight wire relate to the direction of the current?

A
  • We can use the Right Hand Grip Rule (see pg 171 of the textbook, diagramn C) to work out the direction of the current in the wire relative to the direction of the magnetic field.
24
Q

What are the factors that affect the strength of the magnetic field around a wire?

A
  1. Size of Current
  2. Precense of an Iron Core
  3. (if a coil) Number of Turns of Coil
25
Q

What causes the forces produced when a current flows in a magnetic field?

A
  • A wire carrying a current will experience a force if placed within another magnetic field.
  • These magnetic fields will interact, creating a force.
  • This is known as the motor effect
  • We can work out the direction of force using Fleming’s Left Hand Rule
26
Q

Describe the magnetic field inside and outside of a coil of wire carrying a current

A
  • The magnetic fields of individual coils add togethere to form a very strong field inside the solenoid
  • Outside the solenoid, the fields from one side of the coil tend to cancel out the other side to give a weaker field
27
Q

Recall Fleming’s Left Hand Rule

A
28
Q

How do you calculate the size of a force in a current-carrying wire?

A

force on conductor carrying current at right angles to the magnetic field = magnetic field strength x current x length

OR

F = B x I x L

29
Q

Describe how electromagnetic induction is used in alternators and dynamos

A
  • Both of these are generators, consisting of a coil of wire that is rotated inside a magnetic field. This induces a P.D as the coil turns

TYPES OF GENERATORS:

  • In an alternator, the two sides are connected to slip rings, so direction of the current changes. (AC CURRENT)
  • In a dynamo, a split ring commutator changes the connections of the current every half-turn, which makes it flow in one direction. (DC CURRENT)
30
Q

What are the different factors affecting the size and directions of induced current in a generator?

A
  1. Strength of Magnetic Field
  2. How fast the magnetic field moves past the coil
  3. Number of turns on the Coil of Wire
31
Q

Describe how the magnetic field produced by an induced potential difference opposes the original change

A

(I don’t really get this tbh)

  • When a p.d is induced inside a current-carrying wire, it also produces it’s own magnetic field.
  • This magnetic field will oppose the change that induced it.

For example ( PAGE 174, DIAGRAM A):

If a magnet was moved to the right into a loop of wire, then the magnetic field induced by that loop of wire will move away from the loop of wire.

However, if the magnet was then move to the left, away from the wire, the magnetic field induced will change direction, and move into the loop of wire.

32
Q

Explain a transformer works

A
  • Changing a.c current supplied
  • This produces a changing magnetic field in primary coil
  • The iron core is induced and carries this changing magnetic field
  • This creates a changing magnetic field in the secondary coil
  • This induces a changing a.c voltage in the wires connected to the secondary coil.
33
Q

What causes the changing magnetic field in the core of a transformer?

A
  • The a.c current in the primary coil causes a continuously changing magnetic field, the iron core of the transformer is able to carry this field to the secondary coil.
34
Q

What is the turn ratio equation?

A

Vp / Vs = Np / Ns

The ratio of turns is equal to the ratio of potential differences.

35
Q

How do you calculate density?

A

Mass / Volume

36
Q

How can we calculate the density of an irregular solid?

A
  1. Set an electronic balance to 0 then place the object on it and record the mass
  2. Set up a displacement can leading into an empty measuring cylinder and fill the can with water
  3. Place the object in the can. If it floats, then push it down till it is just under the surface of the water
  4. Read the value for the volume of water in the measuring cylinder (from the bottom of the meniscus)
  5. This is the volume of the object. Do mass/volume to find the density
37
Q

What is the difference between specific heat capacity and specific latent heat?

A

SPECIFIC HEAT CAPACITY

The amount of energy required to increase the temperature of 1 kg of a substance by 1°C (The diagonal lines of a heating curve)

SPECIFIC LATENT HEAT

The amount of energy required to change the state of 1kg of a substance (horizontal lines of a heating curve)

38
Q

Explain ways of reducing unwanted energy transfer through insulation

A
  • Using an insulating material such as rubber
  • Increasing the thickness of an existing material
39
Q

How do you calculate change in thermal energy?

A

change in thermal energy = mass x specific heat capacity x change in temperature

40
Q

How does density change with temperature?

A
  • Temperature causes molecules of gas to gain energy, moving further away from each other. This increases the volume
  • Density = Mass / Volume
  • Therefore, if we increase volume then density will decrease.
  • Therefore, temperature and density are inversely proportional
41
Q

What’s the difference between elastic and inelastic distortion?

A
  • Elastic distortion means that it will reutrn to its original shape when the forces are removed
  • Inelastic distortion means it will keep its new shape when forces are removed
42
Q

What’s the equation linking spring constant, force and extension?

A

force = spring constant x extension

43
Q

What is pressure and how do we calculate it?

A

Pressure is the amount of force per unit area.

Force / Area

44
Q

Explain why the pressure in a liquid depends on density and depth

A
  • Depth of the fluid: The more particles above you, the higher pressure you will experience as more particles can produce a higher force. This is why pressure is highest at sea level and reduces as you increase altitude
  • Density of fluid: The denser the fluid you are in, the more particles there are in a fixed volume, the more particles can produce a force. This is why water pressure is higher than air pressure
45
Q

How do you calculate pressure due to a column of liquid?

A

pressure due to a column of liquid = height of colum x density of liquid x gravtational field strength

46
Q

Explain how upthrust occurs

A
  • Upthrust is due to the difference in pressure above and below the object
47
Q

Explain how power transfer depends on the p.d across a device and the current through it

A

The power transfer is 1 joule per second.

Power transfer in a component is proportional to the potential difference across it and the current through it … therefore

power = current x voltage

48
Q

State the 3 equations for power

A

P = E/t

P = IV

P = I2R

49
Q

Explain the difference in function of live and neutral wire

A
  • Live wire carries 230V, bringing the voltage from the power station to the appliance whilst the neutral wire carries 0V, and returns back to the power station
50
Q

How do the following safety appliances work:

  • Earth Wire
  • Fuse
A

EARTH WIRE:

  • If a wire comes loose and touches the metal part of the appliance, the electricity will be able to flow through this.
  • When a person touches it, the electricity can pass through the person to the ground.
  • This would give them an electric shock.
  • However the earth wire provides an easier way to travel to the ground.
  • As electrcity always takes the path of least resistance, it goes this way and the person doesn’t get an electric shock

FUSE

  • A fuse is a glass tubing with a thin metal wiring that the electricity passes through.
  • A faulty appliance may draw too much current which would heat up and cause a fire.
  • However depending on the value of the fuse, it will melt once the current has reached a certain level and cause the circuit to be incomplete meaning current stops flowing through it
51
Q

Why are switches and fuses connected in the live wire?

A
  • The live wire is carrying the voltage. Fuses will only melt if the voltage is passing through it. Similarly, the switches need to be in the path of the live wire if they intend to break it.
52
Q

Explain the danger of a connection between the live wire and Earth

A
  • If a fault causes the live wire to touch a metal part, it makes a very low resistance circuit between 230V and 0V.
  • This causes a very large current to flow to the Earth, which heats up the wire and could cause a fire. If this happens, the current blows the fuse and cuts off the mains.

PPE 2 Flashcards for Checklist (3 decks)

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