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Physics - electricity Flashcards

1
Q

Power source for an electric circuit

A

Battery
Cell

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2
Q

Current

A

Flow of electric charge

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3
Q

A current can only flow if …

A

There is a potential difference

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4
Q

Potential distance

A

The driving force that pushes the charge around.

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5
Q

Current in a series circuit

A

The same throughout

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6
Q

Resistance (Ohms)

A

Anything that slows the flow down

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7
Q

What does the current depend on

A

The PD across it and the resistance

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8
Q

How does resistance affect the pd and current

A

The greater the resistance across a component the smaller the current that flows and the voltage

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9
Q

Relationship between current and charge

A

When current increases, charge increases

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10
Q

Resistance of a wire practical

A

Attach a crocodile clip to the wire level with 0cm on the ruler
Attach the second crocodile clip to the wire 10cm away
from the first clip
Close the switch and record the current through the wire and the pd across it
Open the switch and move the second crocodile clip another 10cm and repeat.
Repeat this for the number of different lengths
Plug the measurements into the formula V=IR
Plot a graph of resistance against wire length and draw a line of best fit

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11
Q

Limitations to resistance practical

A
  • There is a small resistance when the length of the wire is zero - zero error as some resistance is caused by the crocodile clip and the wire
  • Heating effects - If the temperature of the wire increases then the resistance of the wire increases affecting the results
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12
Q

What is zero error and what type of error is it

A

A reading on a measuring instrument when the value should be zero
Systematic error

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13
Q

How do reduce heating affects in resistance practical

A

Use a low potential difference
Only turn on current when we are taking a reading

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14
Q

What are variable resistors used for

A

To control the potential difference across the lamp, they ensure that as current increases, pd increases at the same rate

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15
Q

What happens to the resistance in an ohmic conductor

A

The resistance of an ohmic conductor doesn’t change with the current

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16
Q

What happens when to resistance in a filament lamp

A

When an electric charge flows through a filament lamp it transfers some energy to the thermal energy store of the filament lamp which is designed to heat up. Resistance increases with temperature so as the current increases the filament lamp heats up more and the resistance increases.

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17
Q

What happens to resistance in diodes

A

The resistance depends on the direction of the current. Current flows fine in one direction, but diodes have a very high resistance if it is reversed.

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18
Q

Experiment to find a component’s I-V characteristics

A
  • Set up test circuit with ammeter, voltmeter variable resistor, battery and component
  • Vary the variable resistor which alters the current flowing through the circuit and the pd across the component
  • Take several pairs of reading from the ammeter and voltmeter and find an average pd
  • Swap over the wires connected to the battery so that the current is reversed
  • Plot a graph of current against voltage for the component
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19
Q

What is an LDR

A

A resistor that is dependent on the intensity of light.

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20
Q

How does resistance in an LDR react to light

A

Resistance falls in bright light and is highest in dark light

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21
Q

What is a thermistor

A

A resistor that depends on temperature

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22
Q

How does a thermistor react to temperature

A

In hot conditions the resistance drops
In cool conditions the resistance increases

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23
Q

Uses of LDRs

A
  • Automatic night lights
  • Burglar detectors
  • Outdoor lighting
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24
Q

Uses of thermistors

A
  • Temperature detectors
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25
Q

What do sensing circuits do

A

Turn on or increase the power to components depending on the conditions that they are in

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26
Q

How are voltmeters always connected

A

In parallel

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27
Q

Problem with series circuits

A

If you remove or disconnect one component, the circuit is broken and they all stop

28
Q

Pd in a series circuit

A

The total pd of the supply is shared between the various components. So the pd around a circuit always adds up to equal the source pd

29
Q

What determines the size of the current in a series

A

The total pd of the cells and the total resistance

30
Q

Resistance in series circuit

A

The total resistance is the sum of the components resistance

31
Q

There is a bigger pd when

A

More cells are in the circuit, if they’re all connected in the same way eg. when two cells with a pd of 1.5V are connected in series they supply 3V between them

32
Q

Pd in parallel circuits

A

All components get the full source pd, so the pd is the same across all components. This means that identical bulbs connected in parallel will be at the same brightness

33
Q

Current in parallel circuits

A

The total current flowing around a circuit is equal to the total of all the currents through the separate components.
In a parallel circuit, there are junctions where the current either splits or rejoins.

34
Q

The total current going into a junction has to equal

A

The total current leaving

35
Q

Resistance in parallel circuits

A

The more components we add in parallel the lower the total resistance of the circuit will be

36
Q

Investigating resistance practical

A

Set up the circuit as shown in figure 1, turn the power supply on and close the switch.
Record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I, where R is resistance, V is potential difference and I is current.
Change the resistor and repeat step two to find the resistance of a second resistor.
Arrange the two resistors in series as shown in figure 2 and close the switch.
Record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I.
Arrange the two resistors in parallel as shown in figure 3 and close the switch.
Record the voltmeter and ammeter readings once again and calculate the total resistance of both resistors in parallel.

37
Q

How do adding resistors in series affect resistance

A

Increases resistance. The more resistors you add the larger the resistance of the whole circuit

38
Q

How does adding resistor in parallel affect resistance

A

The total current through the circuit increases so the total resistance of the circuit has decreased

39
Q

Two types of electric supply

A

Alternating and direct current

40
Q

Alternating current supplies

A

The current is constantly changing direction. They are produced by alternating voltages in which positive and negative ends keep alternating.

41
Q

What voltage is the UK mains supply and what current is it

A

The UK mains supply is an ac supply at around 230 volts

42
Q

Frequency of the ac mains supply

A

50 Hertz

43
Q

What type of current do cells and batteries supply

A

Cells and batteries supply a direct current

44
Q

Direct current

A

A current that is always flowing in the same direction. It’s created by a direct voltage

45
Q

Live wire

A
  • Colour - brown
  • Provides an alternating potential difference
  • Potential difference of around 230 V
46
Q

Neutral wire

A
  • Colour - blue
  • Completes the circuit - when the appliance is operating normally, current flows through the live and neutral wires.
  • Potential difference of around 0V
47
Q

Earth wire

A
  • Colour - yellow and green
  • Protects the wiring and for safety, it stops the appliance casing from becoming live. It doesn’t usually carry a current - only when there’s a fault
  • Potential difference of 0V
48
Q

How does a live wire give you an electric shock

A

Your body is at 0V, this means if you touch a live wire, a large potential difference is produced across your body and a current flows through you. This causes a large electric shock which could injure or even kill you.

49
Q

How do kettles work

A

They transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle.

50
Q

How do fans work

A

Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor

51
Q

What does the total energy transferred by an appliance depend on

A

How long the appliance is left on for and it’s power

52
Q

Power of an appliance

A

The energy that it transfers per second

53
Q

What does a power rating tell you

A

The maximum amount of energy transferred between stores per second when the appliance is in use

54
Q

The national grid

A

A giant system of cables and transformers that covers the UK and connects power stations to consumers.

55
Q

What does the national grid transfer

A

Electrical power from power stations anywhere on the grid to anywhere else on the grid where it’s needed

56
Q

What’s the problem with a high current in the national grid

A

Loads of energy is lost as the wires heat up and energy is transferred to the thermal energy store of the surroundings

57
Q

What pd and current does the national grid use

A

Uses a high pd and a low current making the national grid an efficient way of transferring heat

58
Q

Features of a transformer

A
  • They have 2 coils - a primary coil and a secondary coil
  • The coils are joined with an iron core
59
Q

What do step-up transformers do

A

Potential difference is increased using a step up transformer
They have more turns on the secondary coil than the primary coil
As the pd is increased by the transformer, the current decreases

60
Q

What do step-down transformers do

A

They the reduce the pd again at the local consumer end.
Step down transformers have more tunes on the primary coil than the secondary coil

61
Q

How is build-up of static caused by friction

A

When insulting materials are rubbed together, negatively charged electrons will transfer from one to the other. This will leave the materials electrically charged, with a positive static charge on one and an equal negative static charge on the other.

62
Q

How does too much static cause sparks

A

As electric charge builds 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 then earth - this is a spark.

63
Q

Features of an electric field lines

A
  • Always at a right angle to the surface
  • Always go from positive to negative
  • The closer together the lines are, the stronger the field is
64
Q

What causes the force between two electrically charged objects

A

The force is caused by the electric fields of each charged object interacting with each other. As you increase the distance between the charged objects, the strength of the field decreases and the force between them gets smaller.

65
Q

How can sparking be explained by electric fields

A

Sparks are caused when there is a high enough pd between a charged object and the earth (or an earthed object)
A high pd causes a strong electric field between the charged object and the earthed object.
The strong electric field causes electrons in the air particles to be removed.
Air is normally an insulator but when it is ionised it is much more conductive, so a current can flow through it. This is a spark.

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