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GCSE Physics OCR B > P6 > Flashcards

P6 Flashcards

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1
Q

What can variable resistors be used for?

A

To control current, to vary brightness of a bulb and to vary the speed of a motor

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2
Q

How can you change the resistance of a variable resistor?

A

By changing the length of the wire - more wire = higher resistance

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3
Q

What is ohms law?

A

He said that for a given ohmic conductor, the current increases as the voltage increases but the resistance remains constant (as long as the temperature stays the same). This means that in non ohmic conductors like bulbs the resistance changes.

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4
Q

What is an ohmic conductor?

A

A resistor at constant temperature

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5
Q

What does the voltage-current graph for an ohmic conductor look like?

A

It is a straight line - the gradient shows resistance

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6
Q

What does the voltage-current graph for a non-ohmic conductor look like?

A

It is a curve - the resistance (gradient) increases as the current increases because when a wire gets hot it’s resistance increases

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7
Q

Why does resistance increase temperature?

A

The electric current is the flow of charge carriers (in metals these are electrons) and when they pass through the wire they collide with atoms (ions) in the metal which makes them vibrate more so there are more collisions and the temperature increases

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8
Q

How do you find the total resistance in a circuit if the resistors are in series?

A

Add each individual resistance

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9
Q

How do you find the total resistance in a circuit if the resistors are in parallel?

A

Resistors in parallel reduce the overall resistance of a circuit so you add their reciprocal to find 1/the total resistance, so to find the total resistance you do 1/the value given (find its reciprocal)

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10
Q

What is a potential divider?

A

It can be made of fixed resistors that are arranged to produce a required voltage (pd). The lower the resistance, the lower the voltage (pd)

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11
Q

What is an LDR and when does it’s resistance change?

A

Light dependent resistor

Increased resistance in bright light, reduced resistance in dim light

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12
Q

What is a thermistor and when does it’s resistance change?

A

It is a resistor dependent on temperature
High temperature causes low resistance
Low temperature causes high resistance

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13
Q

What is a transistor?

A

An electronic switch that can work like a logic gate, used in computers and other electronic devices to speed up processing

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14
Q

What happens when you put two transistors in a series?

A

They produce the same output as an AND gate

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15
Q

How can an npn transistor be used as a switch for an LED?

A

?

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16
Q

What does increasing miniaturisation of transistors do?

A

It increases the number of transistors that can be connected in a processor and means computer processors can be made smaller

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17
Q

What effects how small a transistor can become?

A

Smaller components dissipate (lose) more heat as a current passes through them
As transistors become thinner they offer less resistance to the electrons due to quantum tunnelling

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18
Q

The magnetic field of a straight piece of wire is made up of…

A

Concentric rings

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19
Q

What does the magnetic field around a solenoid look like?

A

Like the magnetic field around a bar magnet - lots of rings/ovals that get gradually bigger

20
Q

What is the motor effect?

A

When a current carrying wire is places in a magnetic field and it moves because of the force it experiences

21
Q

How can you reverse the direction the wire is moving in?

A

By reversing the direction of the current or by reversing the direction of the magnetic field

22
Q

In Fleming’s left hand rule, which finger points to what?

A

First finger - magnetic field direction (north to south)
Second finger - direction of current
Thumb - direction of coil movement

23
Q

Explain what happens when a current carrying wire is placed in a magnetic field (the motor effect)?

A

The current flows through the wire and creates a magnetic field
The magnetic field of the magnet and of the coil interact
Each side of the coil moves in a different direction because the current flows in a different direction in the two sides
The forces combine to make the coil rotate

24
Q

What devices are electric motors found in? List 6

A 
Washing machines
CD players
Food processors
Electric drills
Electric lawnmowers 
Windscreen wipers
25
Q

How can you increase the speed of the motor?

A

Increase the current size
Increase number of coil turns
Increase strength of magnetic field

26
Q

What is a split ring commutator used for?

A

It is used to change the direction of the current in the coil every half turn so the current flows in the same direction relative to the magnet so it keeps rotating

27
Q

What is a radial field created and why are they used?

A

A radial field is created using curved pole pieces so that the maximum force is produced because the coil and the magnetic field are used to give a radial field

28
Q

What frequency is mains electricity produced at in the U.K. and what does this mean?

A

It is generated at 50Hz which means the current goes back and forth along the wire 50 times each second

29
Q

What is generating electricity also known as?

A

The dynamo effect

30
Q

What is the concept of a DC generator?

A

It is a DC motor working in reverse - instead of deeding a voltage to the coil so it moves, the coil is moved to produce a voltage

31
Q

How is AC generated in power stations?

A

By rotating electromagnets inside coils of wire

32
Q

What does an AC generator have instead of a split ring commutator?

A

Slip ring commutator

33
Q

How does an AC generator change direction every half cycle?

A

The coil turns and for the for half of the turn the current is flowing in a certain direction. When the coil is parallel to the magnetic field and begins the next half turn the current flows in the opposite direction to before because the previous downward side moves up and the previous upward side moves down which reverses the current flow in the external circuit.

34
Q

What is a transformer used for, what is it made of, and what type of electricity does it work for?

A

It changes the voltage and it is made of two coils of wire wound onto an iron core. It only works for AC because AC has a changing magnetic field which is needed to induce a voltage in the secondary coil, whereas DC only provides a steady magnetic field

35
Q

What voltage is mains electricity?

A

230v

36
Q

What voltage is electricity from a power station?

A

25000v

37
Q

In what way is power loss in transmission proportional to current?

A

Power loss = current2^ x resistance

38
Q

What are isolating transformers and what are they used for?

A

They are transformers with the same number of coils on both sides and the coils aren’t connected so when they are used in mains circuits, the user isn’t connected to the mains which makes it much safer if there is water present (less risk of contact between the live parts and the earth lead)

39
Q

What is a silicon diode?

A

A device which allows the current to flow through it in one direction only

40
Q

What is a silicon diode made from?

A

n-type and p-type silicon

n-type contains extra electrons, p-type has holes (positive charge carriers) where the electrons should be

41
Q

When and why can a current flow in a diode?

A

When the n-type is connected to the battery’s negative terminal because they the electrons can flow towards the holes and the holes can flow towards the electrons. This is when the diode is forward biased.

42
Q

What happens when a diode is reverse biased and why?

A

Reversed biased is when the diode is backwards, when the diode is backwards the current can’t flow because…

43
Q

What happens when alternating current is passed through a single diode?

A

Half wave rectification happens - the diode allows the current flowing in one direction to pass through and stops the current that flows in the other direction

44
Q

How does a bridge circuit work for full wave rectification?

A

The positive charge flows through and goes through the load and the negative charge goes a different way because the diodes that were forward biased for the positive charge are now reverse biased (and vice versa) but the current can now flow in the same direction through the load (AC to DC)

45
Q

What can capacitors be used for?

A

They store charge and discharge it later so when it has been charged and is then connected to a conductor, it discharges (behaving like a battery)

46
Q

How do capacitors produce a more constant voltage?

A

They discharge their current when the voltage falls past a certain point, then charge when it increases again so there is always a constant output

GCSE Physics OCR B (6 decks)

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