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P3 Flashcards

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1
Q

What is charge?

A

-Charge is a property of matter just like mass. You know that atoms are neutral overall
-They have no charge because there are equal numbers or protons and electrons
Electric charge:
Protons: positive charge
Electrons: negative charge

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2
Q

What do like charges do?

A

Repel

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3
Q

What do unlike charges do?

A

Attract

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4
Q

How do you produce static electricity?

A

-When you rub two insulators together you can transfer electrons from one insulator to the other.
-Positive charges do not move.
-One object ends up with extra electrons, and the other with not enough electrons to cancel out the positive charge

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5
Q

What is the charge on each object?

A

Static electricity or static

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6
Q

How do you discharge an object?

A

-To discharge an object you need to remove the charge
-you connect it to something that allows charge to flow- like a piece of metal, or you when touch a car door
-sparks also discharge charged objects such as van Der graaf generator, or clouds. A spark is a flow of charge, or a current, through the air

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7
Q

How can you use fields to explain static electricity?

A

-There is an electric field around a charged object or particle
-if you put another charged object in this field it will be attracted or repelled, even if the objects are not touching.
-You can work out what is happening by looking at the shape of electric fields
-Field lines are a bit like elastic bands
-If you change the shape of the field by putting another charged object t in it the field lines will “stretch”
-The force of an object will be in the direction that causes the field lines to shorten or straighten
-If the field lines are closer together it means that the field is stronger
-The direction if the field lines is the direction of the force on a positive charge

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8
Q

What is current and how do you model it?

A

-that little shock you experience when you touch a car door is the movement of electrons through your body. This is a current.
-Current is the rate of flow of charged particles, or charge. In the metal wires that you use to make circuits the charges that move are electrons. Each electron has a very small charge, but there are large numbers of them in the wires.

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9
Q

How do you make current flow?

A

-To make current flow you need a cell or battery, or a power supply, as well as a complete circuit.
-The current everywhere in a single closed loop is the same

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10
Q

Which way does current flow?

A

When you draw the direction of electric current in a circuit diagram, you draw it going in the direction of the positive terminal of the battery
-This is called conventional current
-This is opposite to electron flow

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11
Q

How do you calculate with current?

A

Charge flow(C) = current(A) x time(s)

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12
Q

What is charge measured in?

A

Coulombs (c)

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13
Q

What is current measured in?

A

Current is measured in amperes or amps(A)

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14
Q

What is time measured in?

A

Time is measured in seconds

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15
Q

What are small currents measured in?

A

Milliamps (mA)
1mA = 1x10^-3 Amps

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16
Q

How do we draw circuits?

A

You use circuits symbols to draw electric circuits.

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17
Q

What is potential difference?

A

-To make a current flow you need potential difference (p.d.) inside a cell or battery there are substances that react to separate the charges
-This makes one side of the battery positively charged and the other side negatively charged

-The positive terminal of the cell is at a higher electrical potential than the negative terminal
-The potential difference, and the current that a cell can deliver, depends on the construction of the cell, not it’s size.

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18
Q

What is potential difference measured in?

A

-Potential difference is measured in volts(V)

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19
Q

How do you know which symbol is negative or positive?

A

The longer line on the symbol for a cell is positive, and the shorter line is negative.

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20
Q

What happens when you apply a potential difference (p.d) between the ends of a piece of wire

A

-you set up an electrical field in the wire
-the field is set up very quickly(close to the speed of light in a vacuum )
-so the charged electrons start to move straight away.

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21
Q

How do you model potential difference?

A

-In the rope model you show the work done by the battery by pulling the rope.
-The work done by the charges in the components is the work done in the persons hand holding the rope
-The charges do very little work in the wires

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22
Q

What is the link between potential difference , energy and charge?

A

-When you connect a voltmeter across a component in the circuit you measure the work done or energy transferred by the charges in the component
-We call this electrical working

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23
Q

What is the equation for energy transferred(J)

A

Energy transferred(J) = potential difference(V) x charge(C)

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24
Q

How do you measure current in series circuits?

A

-You have learned that a series circuit has only one loop and the current is the same everywhere
-you measure the current in a circuit with an ammeter

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25
Q

How do you measure potential difference is series circuits?

A

A voltmeter measures a “difference” -the (electrical) potential difference
-you need to connect the voltmeter to both sides of a component

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26
Q

What does a voltmeter do?

A

-A voltmeter measures the “rise in potential” across a cell or battery
-It can also measure the “drop in potential” across a component
-if There is very little “drop” across the wires, a voltmeter will give a low reading such as “zero”

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27
Q

What do the voltmeters add up to?

A

The readings in voltmeters always add up to the reading across the cell

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28
Q

What happens in parallel circuits?

A

-It is helpful to think of a parallel circuit as having more than one loop
-In a parallel circuit each loop can be worked independently, this is what happens in your house.
-You do not have all the lights on at once
-If you measure the current at different points in a parallel circuit, the current in the loops add up to the current near the battery

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29
Q

Physics parallel circuit trigger:

A

Current at b + current at c = current at a

Current a is near the battery

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30
Q

What does current depend on?

A

The current depends on the potential difference of the cell or battery of the current.

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31
Q

What is resistance measured in?

A

Resistance in measured in ohms (Ω)

32
Q

How do you model resistance?

A

-In the rope model, resistance is modelled by the person gripping the rope
-If the person grips the rope more tightly and the other person pulls with the same force then the rope will move more slowly
-To keep the same speed the person pulling the rope would have to pull harder

33
Q

How do you calculate resistance?

A

Current(A) = potential difference (V) / resistance (Ω)

  • rearrange

Resistance(Ω) =potential difference (V) / current (I)

34
Q

What does the symbol V equate for?

A

Potential difference

35
Q

What does the symbol R equate for?

A

Resistance

36
Q

What does the symbol “I” equate for?

A

Current

37
Q

What does V = I x R mean?

A

Potential difference (Volts) = current (Amps) x resistance (Ω ohms)

38
Q

What does the resistance of some components change?

A

-A metal is made up of positively charged ions arranged in a regular pattern
-The ions are formed when electrons leave the outer shells of metal atoms
-These delocalised electrons are free to move through the structure of the metal
-Resistance is produced when electrons collide with the ions in the lattice
-This explains why the resistance of some components change with current

39
Q

What is a variable resistor?

A

A variable resistor is a circuit component you can use to change the amount of wire or other resisting material.
-you use a variable resistor every time you use a dimmer switch.

40
Q

Electrical intensity and current:

A

The letter I is used instead of C, for current, because the scientist who discovered electricity called it electrical “intensity”

41
Q

What happens with electrons in a wire?

A

-Electrons in a wire are free to move in the electrical field produced by the potential difference.

42
Q

What happens if you increase the resistance?

A

If you increase the resistance, the current decreases

43
Q

How do you get measurements for a graph of current and potential difference?

A

-A graph of current against potential difference is called a characteristic graph.
-You collect data for the graph by taking measurements of the current flowing through a component for different potential differences.
-Then you reverse the battery or power supply connections and repeat the measurements

44
Q

What can you do once you’ve recorded your measurements for a characteristic graph?

A

-when you have recorded measurements you can:
-calculate the resistance from each pair of measurements
-plot a characteristics graph with potential difference on the x-axis and current on the y-axis

45
Q

What is the characteristic graph for a resistance wire or a resistor?

A

-The wire is a linear circuit element. It’s resistance does not change as you change the potential difference
-you use resistance wire or a resistor in a circuit if you need the resistance to be constant

46
Q

What is ohms law?

A

-George ohm discovered that metal wires are linear elements
-He said that the current is proportional to the potential difference if the temperature does not change
-This is ohms law

47
Q

What is the characteristic graph for a lamp?

A

-The electrons in a wire collide with the ions in the wire.
-The ions vibrate more as the wire gets hotter
-hit wires can be used in lamps
-As that happens there are more collisions
-The wire heats up more
-The current increases as the potential difference increases, but at a slower rate.

48
Q

What is a diode?

A

-A diode is a component that only allows a current to flow one way.
-Some diodes emit light
-There are light emitting diodes (LEDS) in most of the electronic devices that you use
-The long leg of an LED should be connected to the positive terminal of a battery

-

49
Q

what does a diodes characteristic graph look like?

A

-As you apply a potential difference in the “forward” direction very little current flows.
-Then the current suddenly increases.
-If you reverse the potential difference there is no current
-You use a diode if you want the current to flow only in one direction

50
Q

What is a thermistor?

A

-At the end of many digital thermometers there is a thermistor
-The resistance of a thermistor depends on temperature
-Thermistors come in a different shapes and sizes, and detect different ranges of temperature
-a thermistor is not like a piece of wire
-It is made of a semi conducting material, such as silicon
-Electrons in the atoms of a semiconductor do not need much energy to escape from the atom to form the current

51
Q

How does the resistance of a thermistor change with temperature?

A

-You can measure the resistance of the thermistor by measuring the current through it and the potential difference across it
-If you out a thermistor in a beaker of cold water and heat the water up the resistance of the thermistor decreases
-As you heat the thermistor many electrons gain enough energy to escape from the atoms in the semiconductor

52
Q

What happens to the thermistor at different temperatures?

A

-At low temperatures, the current is small, so the resistance of the thermistor is high

-At higher temperatures the current is high, so the resistance of the thermistor is low

-If there are more electrons moving per second for a particular potential difference , the current will get bigger
-A bigger current for the same potential difference means the resistance is smaller.
-You will have thermistors all Mmmm in your house
-They are used to monitor the temperature in ovens and refrigerators, and to help to prevent your laptop or tablet from overheating.

53
Q

What is an LDR?

A

-The resistance of a light-dependent resistor (LDR) changes with light intensity.
-It is a circuit component that is also made of a semi conducting material.
-In the LDR, light causes electrons to be released into the circuit to increase the current

54
Q

How does the resistance of an LDR change with light intensity?

A

The shape of the graph for an LDR is the same as the shape of the graph for a thermistor
-As the intensity of the light increases, more electrons are released from rbe semiconductor and the resistance decreases.
-LDRs are used to automatically control lights in a building

55
Q

What is net resistance?

A

-You can calculate the resistance if something without knowing what the other components in a circuit are
-This is net resistance
-If you changed the components, or arrangements of the components, then the current could change.
-The net resistance would be different.

56
Q

What happens if you add a lamp into a series circuit?

A

-The current decreases
-the net resistance increases

57
Q

What happens if you add another lamp in a parallel circuit?

A

-The total current increases
-The net resistance decreases

58
Q

What happens to the net resistance when you add a component in a parallel or series circuit?

A

-in a series circuit the net resistance increases.
-in a parallel circuit the net resistance decreases

59
Q

What is in a series circuit?

A

You know that in a series circuit:
-The current is the same everywhere
-the potential difference across each component add up to the potential difference across the battery
-V= I x R

60
Q

How do you analyse series circuit?

A

-If you know the current and the resistance you can work out the potential difference across the lamp
V = I x R

-If you know the p.d. Across the lamp and the cell you can:
Work out the potential difference across the variable resistor

V(vr) = V(cell) -V(Lamp)

-If you know the potential difference across the variable resistor and the current through it you can:

Work out the resistance of the variable resistor:
R = V / I

61
Q

What do you know about parallel circuits?

A

-The potential difference (p.d.) is the same across each loop)
-The current in each loop adds up to the current near the battery
-V = I x R

62
Q

How do you analyse parallel circuits?

A

If you know the potential difference across the resistor and across the lamp, and if you know the resistance you can:
-work out the current through the resistor
I = V / R

If you know the current near the battery you can:
-work out the current through the lamp I(lamp) = I(battery) - current(resistor)

If you know the potential difference across the lamp and current through it you can:
Work out the resistance of the lamp

V / I = R

63
Q

How can you measure temperature with a circuit?

A

Thermistors can be used in sensing circuits to produce a potential difference that changes with temperature

-this means that you can use a sensing circuit to operate a heater or the air conditioning in a house, school or office.

-the output potential difference will depend on the potential difference of the battery and the magnitude of each resistor

-V(out) as measured by a voltmeter, will depend on the temperature

64
Q

Sensing circuits:

A

An LDR
-has a resistance that changes with light level
-you can monitor: the light level in a greenhouse

A variable resistor:
-Has a resistance that changes with how much it turns
-You could monitor: the position of a door

A pressure sensor:
-has a resistance that changes with the force applied to it
-Could monitor whether there is a burglar in your house

65
Q

what is power?

A

-You need a current to work your microwave oven or power shower.
-you select some of the appliances in your house because of their power
-you can buy a high-power hair dryer or one with low power

-you are choosing the energy transferred per second
-power is measured in watts
-Large powers are measured in kilowatts(kW) or megawatts(MW)
-You might choose the high-power appliance because you can use it for a shorter time to get a job done

66
Q

What is power measured in?

A

power is measured in watts

67
Q

How do you calculate electric power with current and resistance?

A

In an electrical circuit:
-the current is the rate of flow of charge- it tells you the amount of charge flowing through a component per second

-the potential difference tells you the energy transferred by each charge

68
Q

What is current?

A

-the current is the rate of flow of charge- it tells you the amount of charge flowing through a component per second

69
Q

What is potential difference?

A

-the potential difference tells you the energy transferred by each charge

70
Q

Equation for potential difference

A

Potential difference = energy transferred / charge

71
Q

Equation for current

A

Current = charge / time

72
Q

Equation for charge

A

Charge = current x time

73
Q

Equation for energy transferred

A

Energy transferred = potential difference x charge

74
Q

How is power related to current and resistance?

A

This means that power = potential difference x current

You can eliminate potential difference from the lower equation. This is called “substitution”

You know that potential difference = current x resistance

(Current x resistance) x current

= current^2 x resistance

You need to be able to apply

P = I^2 x R

75
Q

Equation for power:

A

P = I^2 x R

Physics Revision (5 decks)

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