Topic 2 - electricity

Question 1

What is current and what is it measured in ?

Answer 1

Current is a flow of electrical charge.
It is measured in Amps (A)

Question 2

What is potential difference and what is it measured in ?

Answer 2

The driving force that pushed charge around.
Measured in volts (V)

Question 3

What is resistance and what is it measured in ?

Answer 3

Opposition to the flow of current/ slows it down.
Measured in ohms

Question 4

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

Answer 4

charge flow (coulombs , c ) = Current (A) x Time (s)
Q = IT

Question 5

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

Answer 5

Potential difference (V) = current(A) x resistance(ohms)

Question 6

Describe how you can investigate the factors affecting resistance in a circuit (required practical )

Answer 6

1) set up equipment with ammeter in series and voltmeter in parallel to the crocodile clips.
2) Attach a crocodile clip to the wire, level with 0cm on the ruler.
3) attach the second clip to the wire 10cm away from the first clip and write down the length of the wire between the clips.
4) close the switch, then record the current through the wire and the pd across it using the ammeter and voltmeter.
5) open the switch then move the second crocodile clip another 10cm across the wire.
6) close the switch again and record the new length,current and pd.
7) repeat for a number of different lengths of the test wire.
8) use your measurements to calculate the resistance using r=v/i
9) plot a graph of resistance against wire length and draw a line of best fit.

Question 7

Describe a method to find a components I-V characteristic. (required practical)

Answer 7

1) set up a test circuit with an ammeter a baterry and variable resistor and a component( ohmic conductor,diode,fliament lamp) in series and a voltmeter in parallel to the component.
2) Use the voltmeter and ammeter to record the potential difference and current .
3)vary the variable resistor to alter the current flowing through the circuit and the potential difference across the component.
4) Take several pairs of readings.
5)Swap the direction of the battery so the direction of pd is reversed
6) take several readings .
7) plot a graph of current against voltage for the component.
8) repeat with a different component.

Question 8

What does an I-V graph for an ohmic conductor look like?

Answer 8

The current is directly proportional to the potential difference

Question 9

What does an I-V graph for a filament lamp look like?

Answer 9

Straight in the middle, curved on the edges
as current increases so does the resistance
this means less current can flow per unit pd so the graph gets shallower.

Question 10

What does an I-V graph for a diode look like ?

Answer 10

Current will only flow through a diode in one direction as it has very high resistance in the other.

Question 11

What is an LDR and how does it work ?
give examples of devices that use LDR’s

Answer 11

Light dependant resistor
In bright light , the resistance falls
in darkness, the resistance is highest

e.g. automatic night lights
outdoor lighting
burglar detectors

Question 12

What is a thermistor and how does it work ?
gice examples of where it can be used

Answer 12

A temperature dependant resistor
in hot conditions, the resistance drops
in cool conditions, the resistance goes up
e.g. temp detectors
electronic thermostats

Question 13

What are sensing circuits used for ?

Answer 13

Can be used to turn on or increase the power to components depending on the conditions they are in.

Question 14

Describe all the features of a series circuit.

Answer 14

If one component is removed or disconnected the whole circuit is broken/stops.
The potential difference is shared across components.
The current remains the same across the whole circuit.
the total resistance across all of the components (the ‘net resistance’) increases as more components are added.

Question 15

Describe all the features of parallel circuits.

Answer 15

If you remove one component it won’t affect the others.
Potential difference is the same across all components.
The current in a parallel circuitsplits into different branches then combines again before it goes back into the supply. When the current splits, the current in each branch after the split adds up to the same as the current just before the split.
In a parallel circuit,the net resistance decreases as more components are added, because there are more paths for the current to pass through.

Question 16

If you have two resistor in parallel , their total resistance is _____ than the resistance of the s___________ of the two resistors.

Answer 16

If you have two resistor in parallel , their total resistance is less than the resistance of the smallest of the two resistors.

Question 17

What is alternating current?

Answer 17

Current that constantly changes direction.
Produced by alternating voltages in which the positive and negative ends keep alternating.

Question 18

What is direct current?

Answer 18

Current that always flows in one direction.
it is created by a direct voltage / potential difference

Question 19

Is the uk mains supply alternating or direct current?

Answer 19

Alternating current

Question 20

What is the frequency and volts of the uk mains supply?

Answer 20

50Hz
230V

Question 21

What colour is the neutral wire in a plug and what does it do?

Answer 21

Blue
The neutral wire completes the circuit .

Question 22

What colour is the live wire in a plug and what does it do?

Answer 22

Brown
Provides the alternating potential difference (230V) from the mains supply.

Question 23

What colour is the earth wire and what does it do?

Answer 23

Green and yellow.
It is for protecting the wiring and for safety.
It stops the appliance casing from becoming live.

Question 24

Why are live wires dangerous?

Answer 24

Your body is at 0 volts, so if you touch the live wire a large potential difference is produces across your body and a current flows through you causing an electric shock.

Question 25

What is the equation that links power, energy transfered and time

Answer 25

energy transfered= power x time

Question 26

What is the power rating of an appliance?

Answer 26

Tells you the maximum amount of energy transferred between stores per second when the appliance is in use.
The lower the power rating, the less electricity an appliance uses in a given time so the cheaper it is to run.

Question 27

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

Answer 27

energy transferred (J) = Charge Flow (c) x potential difference(V)
E = QV

Question 28

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

Answer 28

Power (W) = potental difference (V) x Current (A)
P= VI

Question 29

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

Answer 29

p = I^2 x R

Question 30

What is the national grid?

Answer 30

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

Question 31

Why do we need transformers in the national grid?

Answer 31

To transmit the huge amount of power needed, you need high potential difference or current.
High current results in energy transfering to thermal store and energy lost.
it is heaper to increase pd really high and keep the curent low.

Question 32

What is a step up transformer?

Answer 32

Increases the pd and decreases the current

Question 33

What does a step down transformer do ?

Answer 33

Decreases the Pd and increases the current

Question 34

How do materials become electrically charged?

Answer 34

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

Question 35

How does too much static (electrical charge) cause a spark ?

Answer 35

As electricall charge builds, the Pd between the object and the earth (which is 0 volts) increases.
If the Pd gets large enough , electrons can jump across the gap between the charged object and the earth.
They can also jump to any earthed conductor that is nearby .
High Pd also cause a strong electric field which causes air particles to become ionised.
This means current can flow through the air.

Question 36

How are electric fields around an object drawn?

Answer 36

Electric field lines go from positive to negative.
They are always right angle to the surface.
The closer together the lines are , the stronger the field is .