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electricity (p2) Flashcards

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1
Q

mentally picture the following diagrams:
(click space bar)

search up any of the diagrams if you’re unsure.

A
  • open switch
  • closed switch
  • cell
  • battery
  • diode
  • resistor
  • variable resistor
  • LED (light emitting diode)
  • lamp
  • fuse
  • voltmeter
  • ammeter
  • thermistor
  • LDR (light dependent resistor)
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2
Q

what is charge?

A
  • a measure of the total current that flowed within a certain period of time.
  • for current to flow through a closed circuit, the circuit must include a source of potential difference.
  • charge is represented with a Q, and is measured in coulombs (C)
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3
Q

what is the relationship between current and charge?

A

electric current is a flow of electrical charge. the size of the electrical current is the rate of flow of electrical charge.

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4
Q

what is a key fact about current?

A

a current has the same value at any point in a single closed loop.

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5
Q

what is the equation linking charge flow, current and time?

A

charge flow (Q) = current (I) x time

  • charge flow = coulombs (C)
  • current = amperes (A/amp)
  • time = seconds (s)
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6
Q

what do cells contain?

A

cells contain chemical energy that is transferred to electrical energy and is carried by the current. when this energy passes through a component, the electrical energy is transformed into other types of energy.

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7
Q

describe what is meant by resistance:

A

the current flowing through a conductor (e.g. a metal wire) can collide with atoms in the metal, and is transferred into other forms of energy, e.g. thermal. the resistance tells us the voltage required to drive a current through a component.

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8
Q

what is the correlation between current and potential difference (voltage)?

A

the current moving through a resistor is directly proportional to the potential difference. this means that the resistance is constant - it doesn’t change if we increase the current. this is an ohmic conductor.

the current through a component is dependent on both the resistance of the component and also the potential difference across the component. the greater the component’s resistance, the smaller the current for a given potential difference across the component.

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9
Q

name the equation that links current, potential difference and resistance:

A

potential difference = current x resistance

potential difference = volts (V)
current = amperes (A)
resistance = ohms

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10
Q

what’s the link between resistance and temperature?

A

the resistance will only stay constant if the temperature remains constant.

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11
Q

describe ohmic conductors:

A
  • for some resistors, their resistance remains constant but in others it can change as the current changes.
  • the current through the ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. this means that the resistance remains constant as the current changes.
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12
Q

describe current:

A

an electrical current is the flow of electrical charge around the circuit. this moves from the negative end of the cell to the positive end. however, it’s always drawn in the opposite direction, the ‘conventional current’
- the current is the same all around a series circuit

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13
Q

what is power?

A

the rate of electrical energy transfer within a circuit. e.g. if 1J of energy was transferred in 1 second, that would be 1W.

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14
Q

describe a series circuit:

A
  • same current through each component.
  • total potential difference of power supply is shared between the components.
  • the total resistance of the two components is the sum of the resistance of one component.

R (total) = R1 + R2

resistance (R) = ohms

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15
Q

what is a parallel circuit?

A
  • the potential difference across each component is the same.
  • the total current through the whole circuit is the sum of the currents through the separate components.
  • adding resistors in parallel decreases the total resistance. the current has a choice of paths and only has to pass along one branch of the circuit. it does not pass through each resistor and the total resistance of a parallel circuit is less than the resistance of the smallest individual resistor.
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16
Q

what is alternating current (a.c)?

A

the direction of the current is constantly flowing back and forth. this occurs when we use an alternating potential difference (fluctuates between pos. and neg.)
- all mains supply in the uk is a.c (50Hz, 230 V)

17
Q

what is direct current (d.c)?

A

produced by a direct potential difference (either positive or negative the entire time). the charge/current is flowing in the same direction the entire time.
- d.c is found in cells/batteries

18
Q

what are the purposes of different wires in a three core cable?

A
  • each wire is made of copper, but has an outer insulating coating of plastic

> brown: live wire. carries alternating potential difference from supply to appliance. always 230v. connected to a fuse in the plug. (extremely dangerous. fatal if touched.)

> blue: neutral wire. completes circuit with the live wire. has a potential difference of around 0v.

> green and yellow striped wire: earth wire. a safety wire to stop the appliance from becoming live.

  • potential difference between live wire and Earth wire is about 230V. the neutral wire is at, or close to, Earth potential (0v).
19
Q

what would occur if someone touched a live wire?

A

live wire on its own: even with an open switch, the live wire can still be extremely dangerous, as the section before the switch is still charged at 230V. the Earth is 0v, so touching the live wire means that a current flows through the person and into the Earth, and they’d be electrocuted.

20
Q

what happens if a person touches the live wire, but an earth wire is present?

A
  • metal-cased appliances are dangerous, as if the live wire comes loose and touches the metal case, it could make the casing live (if someone were to touch this live case, they would receive an electric shock).
  • the metal case is attached to the Earth wire (which is incredibly low resistance). if the case becomes live, a huge current flows to the Earth. the fuse melts and shuts off the current.
  • this prevents anyone from getting an electric shock from touching the case.
21
Q

describe the National Grid:

A

a system of cables and transformers linking power stations to consumers.

22
Q

describe the journey of electricity from the power station to a home:

A
  • the electricity passes from the power station, and through a step-up transformer, increasing the potential difference to several hundred thousand volts.
  • less energy is lost in the transmission cables when the potential difference is very high
  • the electricity passes through a step-down transformer, which reduces the potential difference to around 230v.
  • it then reaches the home.
23
Q

describe the static electricity that occurs between cloth and a rod:

A
  • cloth is an insulator, and is covered in electrons, which can’t move through the cloth
  • when the two are rubbed together, electrons move from the cloth onto the rod, and electrons move onto the rod, giving it a negative charge, and giving the cloth a positive charge.
24
Q

what occurs on a vandegraph, why does it make people’s hairs stand on end?

A

a moving belt passes over the top plastic roller. over time, the plastic roller transfers electrons to the belt. as the belt moves over the bottom roller, electrons pass off the belt and are transferred onto the Earth. the top roller becomes positively charged, and this builds up on a metal dome. someone who holds onto this dome now has an overall positive charge too. because of this, the hairs have the same charge, and so they repel.

25
Q

the resistance of components such as lamps and diodes is not constant; it changes with the current through the component:

describe the different shapes of graphs when investigating the relationship between current and potential difference for bulbs, diodes and resistors:

A
  • filament lamp (s-shaped curve, curved towards the ends): rises possible directly proportionally at the beginning, but as we increase voltage, we increase the current, therefore increasing the resistance, causing the filament to reach a higher temperature, therefore it’s not an ohmic conductor anymore, as the temperature is no longer constant.
  • diode: for all the negative potential difference values, there is 0 current flow in the opposite direction, as the resistance is too high. at approximately 1 volt, the current will then start to rise, and will rise in a directly proportional way - the bigger the voltage, the bigger the current.
26
Q

describe the behaviours of LDRs and thermistors as their surrounding conditions change:

A
  • resistance of a thermistor decreases as the temperature decreases (e.g. thermostats will have more current if it senses the temperature decreasing, so will trigger the release of more heat?)
  • the resistance of an LDR decreases as light intensity increases. (e.g. LDRs can be used in street lamps - they switch on when it becomes dark).
27
Q

what is ohm’s law?

A

a law stating that electric current is directly proportional to potential difference.

(V=IR)

28
Q

what is an oscilloscope?

A

a device that shows how the potential difference changes over time.

29
Q

what does a circuit breaker do?

A
  • electromagnetic device, it’s like a fuse that can be reset.
    > checks if the voltage it gives out and
    receives is the same. if it isn’t it means
    there’s been an earth leakage (the current
    has escaped the circuit), and the circuit
    breaker trips.
30
Q

what does the fuse do?

A

it has a certain amount of voltage that it can withstand, and if a voltage that’s too high passes through it, the fuse blows, and stops the current flow altogether.

31
Q

what do double insulated devices do?

A

equipment with an insulated wire and insulated case (usually plastic) doesn’t need an earth wire, as there’s no chance that the user can come into contact with the current flow.

32
Q

what is the equation linking power, potential difference and current?

A

power = potential difference x current

power = current^2 x resistance

33
Q

what equations allow you to calculate the energy transferred by electrical work?

A
  • work is done when charge flows in a circuit.
  1. energy transferred = power x time
  2. energy transferred = charge flow x potential difference
34
Q

what does a 2000 W power rating tell you about an appliance?

A

2000 W means that the kettle transfers 2000J of energy per second from one store to another.

35
Q

describe forces exerted by charged objects:

A
  • two electrically charged objects can exert a force on each other without touching (non-contact force).
  • two objects with the same type of charge will repel, whilst two oppositely charged objects will attract. this force between unlike charges is called electrostatic attraction. the force between like charges is called electrostatic repulsion.
  • electrostatic attraction causes objects to move closer to each other, electrostatic repulsion causes objects to move further away from each other.
36
Q

describe how sparks occur:

A
  • rubbing surfaces leads to static electricity, as electrons move from one surface to another. we get a build up of positive charge on one surface, and an equal static negative charge on the other surface.
  • as charge builds up on a surface, we start to create a difference in charge between the charged surface and the Earth (usually at 0V). as we continue to charge the surface, the difference becomes bigger and bigger.
  • eventually, the charge difference between the surface and the Earth becomes so big that electrons will jump from the charged surface to the Earth - this creates a visible spark.
37
Q

describe electric fields:

A

charged objects create an electric field around themself. the electric field is the strongest when closest to the charged object. the further away from the object, the weaker the field.

  • a second charged object placed in the field experiences a force. this force gets stronger as the distance between the objects decreases.
38
Q

how do you draw electric field lines?

A
  • field lines are drawn as straight lines from a charged object (like spokes of a wheel).
  • field lines always point from positive to negative.
  • the sphere itself can have either a negative (field lines pointing towards) or a positive charge (field lines pointing away).
39
Q

how can electric fields cause sparking?

A

the larger the charge difference between the two objects, the stronger the electric field. the stronger electric field means that electrons from air particles are removed. by removing electrons, the air becomes much more conductive, allowing a current to flow through it easily. this leads to the production of a spark.

physics (7 decks)

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