P6

Question 1

What can variable resistors be used for?

Answer 1

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

Question 2

How can you change the resistance of a variable resistor?

Answer 2

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

Question 3

What is ohms law?

Answer 3

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.

Question 4

What is an ohmic conductor?

Answer 4

A resistor at constant temperature

Question 5

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

Answer 5

It is a straight line - the gradient shows resistance

Question 6

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

Answer 6

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

Question 7

Why does resistance increase temperature?

Answer 7

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

Question 8

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

Answer 8

Add each individual resistance

Question 9

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

Answer 9

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)

Question 10

What is a potential divider?

Answer 10

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)

Question 11

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

Answer 11

Light dependent resistor

Increased resistance in bright light, reduced resistance in dim light

Question 12

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

Answer 12

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

Question 13

What is a transistor?

Answer 13

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

Question 14

What happens when you put two transistors in a series?

Answer 14

They produce the same output as an AND gate

Question 15

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

Answer 15

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Question 16

What does increasing miniaturisation of transistors do?

Answer 16

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

Question 17

What effects how small a transistor can become?

Answer 17

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

Question 18

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

Answer 18

Concentric rings

Question 19

What does the magnetic field around a solenoid look like?

Answer 19

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

Question 20

What is the motor effect?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

What devices are electric motors found in? List 6

Answer 24

Washing machines
CD players
Food processors
Electric drills
Electric lawnmowers 
Windscreen wipers

Question 25

How can you increase the speed of the motor?

Answer 25

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

Question 26

What is a split ring commutator used for?

Answer 26

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

Question 27

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

Answer 27

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

Question 28

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

Answer 28

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

Question 29

What is generating electricity also known as?

Answer 29

The dynamo effect

Question 30

What is the concept of a DC generator?

Answer 30

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

Question 31

How is AC generated in power stations?

Answer 31

By rotating electromagnets inside coils of wire

Question 32

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

Answer 32

Slip ring commutator

Question 33

How does an AC generator change direction every half cycle?

Answer 33

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.

Question 34

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

Answer 34

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

Question 35

What voltage is mains electricity?

Answer 35

230v

Question 36

What voltage is electricity from a power station?

Answer 36

25000v

Question 37

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

Answer 37

Power loss = current2^ x resistance

Question 38

What are isolating transformers and what are they used for?

Answer 38

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)

Question 39

What is a silicon diode?

Answer 39

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

Question 40

What is a silicon diode made from?

Answer 40

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

Question 41

When and why can a current flow in a diode?

Answer 41

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.

Question 42

What happens when a diode is reverse biased and why?

Answer 42

Reversed biased is when the diode is backwards, when the diode is backwards the current can't flow because...

Question 43

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

Answer 43

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

Question 44

How does a bridge circuit work for full wave rectification?

Answer 44

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)

Question 45

What can capacitors be used for?

Answer 45

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)

Question 46

How do capacitors produce a more constant voltage?

Answer 46

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