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Physics > P2 - Electricity > Flashcards

P2 - Electricity Flashcards

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1
Q

What is needed for electrical charge to flow through a closed circuit?

A

The circuit must include a source of potential difference

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2
Q

What is electric current?

A

The flow of electrical charge

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3
Q

What are ammeters placed in and what do they measure?

A

Ammeters are placed in series and measure current

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4
Q

What are voltmeters placed in and what do they measure?

A

Voltmeters are placed in parallel and measure voltage/p.d.

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5
Q

What is potential difference/voltage?

A

Difference in electrical potential from one point in a circuit to another

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6
Q

What is the equation that links charge flow, current and time?

A

Q = It, where

Q is charge flow (C), I is current (A) and t is time (s)

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7
Q

What is electricity?

A

The flow of electrons between atoms

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8
Q

In a single closed loop, what value does the current have?

A

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

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9
Q

What is resistance?

A

The measure of how much a component in a circuit resists charge

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10
Q

What is the effect of greater resistance?

A

The smaller the current becomes for a given potential difference across the component

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11
Q

What is the effect of lower resistance?

A

Rate of flow of charge increases so current increases. Less power is converted to heat and lost to surroundings

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12
Q

Explain what causes resistance (3)

A

As electric current flows and electrons move through a conductor, the moving electrons can collide with other metal ions in the metal, making it difficult for current to flow

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13
Q

Describe how work is done when a charge flows through a circuit

A

Whenever a charge flows, it has to overcome the resistance of the circuit, thus requiring energy

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14
Q

What is the equation that links current, potential difference and resistance?

A

V = IR , where

V = potential difference (V)
I = current (A)
R = resistance (Ω)

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15
Q

Why does increasing the potential difference in a circuit also increase the current?

A

Current and potential difference are directly proportional

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16
Q

Why are materials with a low resistance chosen for power cables?

A

Less power is converted to heat and lost to the surrounding environment

So more of the supplied power is used efficiently.

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17
Q

RP3: Describe a method to investigate how the length of wire affects resistance

A

Attach an ammeter, a voltmeter, a ruler, a cell, crocodile clips and wires to form a circuit

Set the power supply to 6 V

Connect one crocodile clip to the zero end of the meter ruler connected to the wire

Clip the second crocodile clip to the 10 cm mark

Record the ammeter and voltmeter readings

Switch the power supply off as soon as the readings have been taken

Calculate the resistance for each measurement using R=V/I

Repeat the investigation three more times each time increasing the length of wire and repeating steps 3 to 4

Determine a mean resistance for each length and plot a graph of resistance against the length of wire, Including a line of best fit

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18
Q

What factors affect the resistance of a given length of wire?

A

The type of MATERIAL

LENGTH as longer wires have greater resistance

THICKNESS as smaller diameter wires have greater resistance

TEMPERATURE as heating a wire increases resistance

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19
Q

Suggest why it is important that ammeters have very low resistances

Suggest why voltmeters have high resistances

A

So that the ammeter affects the current as little as possible

Little current flows through the voltmeter. Most current flows through the component

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20
Q

In required practical 3, explain why the student should open the switch after each reading

A

The wire would get hot if the current was left flowing, so the resistance would increase

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21
Q

In required practical 3, suggest a reason for an anomalous result

A

Measurement of voltage was too small.

measurement of current was too small.

incorrect calculation of resistance.

Thermometer misread

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22
Q

In required practical 3, explain why there is a small amount of resistance when the length of the wire is 0cm. What type of error is this and how do you fix it?

A

There is resistance between the crocodile clip and wire

This is a zero error, which is a form of systematic error that cannot be fixed by repeats. You must subtract the zero error from all readings

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23
Q

Explain why there is a zero error in the RP3 experiment

A

It is difficult to attach the clip precisely to the zero end of the wire

So there will be some contact resistance

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24
Q

Why are measurements not taken below 20 cm in RP3?

A

The resistance is very low so the wire would heat up, which is a safety issue and will change the resistance of the wire

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25
Q

Explain why temperature must be kept constant throughout the RP3 investigation. Describe how to reduce the effect of temperature on this investigation

A

An increase in temperature causes an increase in resistance

This can be reduced by using a low potential difference, which keeps current low and reduces heating in the wire

You can also only turn on current when reading is taking place

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26
Q

What is a resistor?

A

A resistor is a component that restricts the flow of electrical current

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27
Q

What is an ohmic conductor?

A

An ohmic conductor is a conductor in which voltage is directly proportional to the current flowing through it at a constant temperature

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28
Q

Explain how temperature affects resistance

A

As electrons move through a metal some collide with other metal ions

These collisions cause resistance and generate heat

Heating the metal causes atoms to vibrate more

Which makes it more difficult for the electrons to flow, increasing resistance and decreasing current

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29
Q

What are the I-V characteristics of an ohmic conductor?

A

The current 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, so the graph will have a linear relationship.

30
Q

What is the difference between the resistance of components such as lamps, diodes, thermistors, LDRs and ohmic conductors?

A

In an ohmic conductor, resistance remains constant as current changes, whereas in the other components, the resistance is not constant as it changes with the current through the component

31
Q

Describe the IV characteristics of a filament lamp

A

As the current increases, the temperature of the lamp increases, so the resistance increases

This means less current can flow per unit potential difference so the graph gets shallower resulting in a curve

32
Q

Describe the IV characteristics of a diode

A

The current through acdiode flows in One Direction only. The diode has a very high resistance in the reverse direction

33
Q

How does the resistance of a thermistor change?

A

In hot conditions, the resistance decreases
In cool conditions, the resistance increases

34
Q

Give two uses of thermistors

A

Used for thermostats

Used for car engine temperature sensors

35
Q

How does resistance change in a light dependent resistor (LDR)?

A

In bright light, the resistance decreases
In darkness, the resistance is high

36
Q

Give two uses of light dependent resistors

A

Switching the lights on when it gets dark (automatic night lights)

Burglar detectors

37
Q

What is a series circuit and what are its properties?

A

A series circuit is a circuit with a single loop

For components connected in series:

there is the SAME CURRENT through each component.

The TOTAL P.D of the power supply is SHARED between the components

the TOTAL RESISTANCE of two components is this SUM of the resistance of each component

38
Q

What is a parallel circuit and what are its properties?

A

A parallel circuit is a circuit with more than one loop

For components connected in parallel, 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

The total resistance of two resistors is less than the resistance of the smallest, individual resistor

39
Q

Explain why adding resistors in series increases the total resistance whilst Adding resistors in parallel decreases the total resistance

A

Adding resistance in parallel forms another circuit loop so decreases the total resistance and increases the current flowing

Adding resistors in series increases resistance as it means that the current has to pass through another resistor, decreasing the rate of flow of charge

40
Q

Explain the design and use of DC series circuits for measurement and testing purposes

A
41
Q

Compare series and parallel circuits

A
42
Q

What are the two types of electricity supplies?

A

DC, direct current
AC, Alternating current.

43
Q

What type of electricity supply is Mains electricity?

A

AC

44
Q

What is direct current DC?

A

Current that always flows in the same direction used to supply cells and batteries

45
Q

What is alternating-current AC?

A

Current that constantly changes direction

46
Q

How are alternating currents produced?

A

Alternating currents are produced by alternating voltage in which the positive and negative ends keep alternating

47
Q

In the United Kingdom, the domestic electricity supply has a frequency and voltage of what?

A

The domestic electricity supply has a frequency of 50 Hz and is 230 V

48
Q

What is the difference between direct current and alternating current?

A
49
Q

Most electrical appliances are connected to the mains using three-core cable. What are they?

A

Live wire, which is brown
Neutral wire, which is blue
Earth wire, which has green and yellow stripes

50
Q

What is the function of live wire, neutral wire and Earth wire?

A

The livewire carries the alternating potential difference from the supply

The neutral wire completes the circuit

The Earth wire is a safety wire to stop the appliance becoming live

51
Q

What is the potential difference of the Earth wire and neutral wire?

A

The neutral wire is at, or close to, earth potential, which is 0 V

The Earth wire is at 0 V. It only carries a current if there is a fault

52
Q

Explain why a live wire is dangerous even when a switch in the mains circuit is open

Explain the dangers of providing any connection between the LiveWire and the earth

A
53
Q

What is power?

A

Power is the rate at which energy is transferred or work is done

54
Q

What is the equation that links power , potential difference and current?

A

P=IV , where

P = power (W)
V = p.d. (V)
I = current (A)

55
Q

What is the equation that links power, resistance and current?

A

P = I^2R, where

P = Power (W)
I = Current (A)
R = resistance (Ω)

56
Q

What does the amount of energy an appliance transfers depend on?

A

How long the appliance is switched on for and the power of the appliance

57
Q

Students should be able to describe how different domestic appliances transfer energy from batteries or ac mains to the kinetic energy of electric motors or the energy of heating devices.

A
58
Q

What is the equation that links energy transferred, power and time?

A

E=Pt

E = energy transferred (J)
P = Power (W)
T = time (S)

59
Q

What is the equation that links energy transferred, charge flow and potential difference?

A

E=Qv

E = energy transferred
Q = charge flow
V = voltage

60
Q

Students should be able to explain how the power of a circuit device is related to:
• the potential difference across it and the current through it
• the energy transferred over a given time.

Students should be able to describe, with examples, the relationship between the power ratings for domestic electrical appliances and the changes in stored energy when they are in use.

A
61
Q

What is the National Grid?

A

The National Grid is a system of cables and transformers linking power stations to consumers

62
Q

Explain why the National Grid is efficient

A

The National Grid uses a high potential difference and a low current

The low current wastes less energy as less energy is wasted to heating

It is cheaper to increase the potential difference and keep the current as low as possible

For a given power, a high potential difference is needed for a low current

63
Q

What is a transformer?

A

A transformer is a device that can change the voltage of an alternating current

64
Q

What are step up transformers used for?

A

Used to increase the potential difference from the power station to the transmission cables

65
Q

What a step down transformers used for

A

Used to decrease, to a much lower value, the potential difference for domestic use

66
Q

Describe and explain how rubbing materials against each other can get them to become charged (4)

A

When two insulating materials rub together they become electrostatically charged

Friction causes the electrons to gain energy to leave the atom

Electrons now move to the other insulating material

Material that gains electrons becomes negatively charged and conversely, the material that loses electrons becomes positively charged

67
Q

Describe why a charged rod can attract a piece of paper that is electrically neutral

A

When you bring the rod close to the paper, the changes within it are not free to move about as a current

So the negative charges repel the rod

68
Q

How does static cause a spark?

A

As electric charge builds up on an object, the potential difference between the object and the Earth increases

If the potential difference gets large enough, electrons can jump across the gap between the charged object and the Earth, causing a spark

69
Q

What is an isolated object

A

An object with no conducting path

70
Q

What does a charged object create?

A

An electric field around itself

71
Q

Where is the electric field strongest?

A

The electric field is the strongest close to the charged object. the further away from the charged object, the weaker the field

72
Q

A second charged object placed in an electric field experience what?

A

A second charged object place in electrical field experience a force. the force gets stronger as the distance between the objects decreases

Physics (8 decks)

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