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P2 Flashcards

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1
Q

Electric current

A
  • a flow of electrical charge
  • only flow round a complete(closed) circuit if there’s p.d.
  • in a single closed loop the current has the same value everywhere.
  • p.d. pushes the charge round.
  • resistance is anything that slows the flow down.
  • current depends on p.d. across it &resistance of component.
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2
Q

Total charge (exp & equation)

A 
Charge (Q) = current (I) x time (t) 
Charge = coulombs, C 
Current = ampere, A
Time = seconds, S
^ the charge that has passed when current flows past a point in a circuit for a length of time. 
More charge = larger current.
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3
Q

P.d. Equation

A

P.d. (V) = current (A) x resistance
V = volt
A = ampere
V = I x R

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4
Q

Resistance practical

A

The Ammeter: measured the current flowing through test wire. Must be placed in series with what investigating
The Voltmeter: measures pd across wire. Must always be placed in parallel with what you’re investigating ONLY.
1. Attach croc clip to wire level with 0cm on ruler (thin wire is best).
2. Attach second croc clip eg 10cm away from first. Write down length of wire between clips.

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5
Q

Resistance practical 2

A
  1. Close switch. Record current through wire &pd.
  2. Open switch. Move second clip eg another 10cm along wire. Close switch. Record new length, current, pd.
  3. Repeat for a no. of different lengths.
  4. Use measurement to calc resistance for each length of wire.
  5. Plot graph, resistance up side, length along bottom, draw line of best fit.
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6
Q

How resistance practical graph should look

A
  • graph should be straight line through origin, so resistance is directly proportional to length:
    Longer wire, greater resistance.
  • if graph doesn’t go through origin, it may be bc first clip isn’t attached exactly at 0cm, So length readings are a bit out. Systematic error.
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7
Q

Ohmic conductors

A

Have constant resistance.

  • Resistance doesn’t change with current.
  • At a constant temp, current flowing through an ohmic conductor is directly proportional to pd across it.
  • Resistance of some resistors & components DOES change, eg diode or filament lamp.
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8
Q

Filament lamp

A

When an electrical charge flows through, it transfers some energy to the thermal energy store of the filament.
This heats it up.
Resistance increases with temp, so as current increases, lamp heats up & resistance increases.

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9
Q

Diodes

A
  • Resistance depends on direction of current.

- Current flows in one direction, there’s a high resistance if it is reversed.

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10
Q

I-V characteristic practical (wavy graphs)

A
  1. Set up test circuit.
  2. Begin to vary variable resistor. This alters current flowing through circuit, & pd across component.
  3. Take a few pairs of readings from ammeter & voltmeter to see how pd varies as current changes. Repeat reading twice to get average pd.
  4. Swap wires connected to cell, so current direction is reversed.
  5. Plot graph: current against voltage.
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11
Q

How I-V graphs should look

A

Ohmic conductor: current is directly proportional to pd, so straight line.
Filament lamp: as current increases, so does temp, so does resistance. So less current can flow per unit pd, graph gets shallower, graph curves.
Diode: current only flows in one direction. High resistance in reverse direction.

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12
Q

LDRs

A

A resistor that is dependent on intensity of light.
Bright light - resistance falls.
Darkness - resistance is highest.
Eg, automatic night lights, outdoor lighting & burglar detectors.

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13
Q

Thermistor

A

Temperature dependent resistor.
Hot - resistance drops.
Cool - resistance goes up.
Eg, temperature detectors: car engine, temp sensors & electronic thermostats.

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14
Q

Sensing circuits

A

Turn on/increase power to components, depending on what conditions in.
Operate a fan: fixed resistor & fan will always have same pd as they’re connected in parallel. Pd of power supply is shared between thermistor & loop made up of fixed resistor & fan according to their resistances - bigger resistance, takes more pd. Room gets hotter, resistance of thermistor decreases & take smaller
share of pd, so pd across fixed resistor & fan rises. Fan goes faster.

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15
Q

Sensing circuits: alternative

A

You can connect the component across the variable resistor instead.
Eg, connect bulb in parallel to LDR, pd across both will be high when it’s dark (& LDRs resistance is high).
Greater the pd across the component, more energy it gets. So bulb connected across an LDR gets brighter as room gets darker.

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16
Q

Series circuit

A

Different components are connected in a line, end to end, between + and - of the power supply. (Except voltmeters, which are connected parallel).
If you remove/disconnect one part, the circuit is broken and all stop. This is not handy, & few things are connected in series.

17
Q

Rules to design series circuit

A

-The total pd of the supply is shared between components. So pd round circuit adds up to equal source pd.
-The same current flows through all components. Size determined by total pd of cells & total resistance of circuit.
-Total resistance of 2 resistors is sum of their resistances. Bc by + a resistor, the 2 resistors share the total pd. pd across each is lower so current is lower
Total resistance of circuit increases due to total current being lower when a resistor is added (bc current is same everywhere). Bigger resistance, bigger it’s share of total pd.
-Cell pd differences add up.

18
Q

Parallel circuits

A

More practical in life.
Each component is separately connected to + & - of supply (except ammeters, they always connected in series).
Remove/disconnect one, no effect.
How most things are connected: cars. You can switch everything on & off separately. Everyday circuits often include mix of series & parallel parts.

19
Q

Rules for parallel circuits

A

-All components get full source of pd. Same across all components.
Identical bulbs at same brightness.
-Total current = all currents + together.
Junctions where current splits or rejoins. Total current going in=total leaving. 2 identical components connect in parallel have same current.
-2 resistors in parallel have total resistance less than resistance of smallest of the 2 resistors.

20
Q

Investigating resistors in series practical

A
  1. Get 4 identical resistors
  2. Build circuit with 1. Make note of pd of battery
  3. Measure current through circuit using ammeter. Use this to get resistance of circuit (R= V/I)
  4. Add another resistor, in series with 1.
  5. Again, measure & calculate.
  6. Repeat 4&5 until all resistors added
  7. Plot graph: no of identical resistors on bottom, resistance of circuit up side
21
Q

Investigating resistors in parallel practical

A
  1. Use same equipment as circuit (so it’s a fair test), build same initial circuit.
  2. Measure total current through circuit calc resistance circuit, V=pd of battery.
  3. Add another resistor. Parallel with 1.
  4. Measure total current through circuit use this & pd to calc overall resistance of circuit.
  5. Repeat 3&4 until all resistors added.
  6. Plot graph, no of resistors on bottom & total resistance up side.
22
Q

How resistor practical graphs should look

A
  • adding resistors in series increases total resistance of circuit.
  • more resistors, larger whole resistance
  • add resistors in parallel, total current through increases. So total resistance of circuit decreased.
  • more resistors added, smaller overall resistance becomes.
23
Q

Two types of electricity supplies

A

Alternating current & direct current.
In ac supplies, current always changing direction. Ac’s produced by alternating voltages where + & - ends keep alternating. UK mains supply, ac supply around 230V.
Frequency of ac mains supply is 50 cycles per second or 50Hz. However cells and batteries supply dc. Dc is a current always flowing in the same direction. Created by a direct voltage.

24
Q

Cable wires

A

Neutral wire: blue. Completed circuit, carried away current. Electricity flows in through live wire. Our through neutral. It is around 0V.
Live wire: Brown. Provides alternating pd at about 230V from mains supply.
Earth wire: green&yellow. Protect wiring & for safety. Stops appliances casing becoming live. Only carries current when there’s a fault. At 0V

25
Q

Danger of live wire

A
  • Our bodies are at 0V,if we touch live wire, a large pd is produced, a current flows through you. Cause an electric shock, can kill/injure.
  • Even if switch is off, there is a danger of shock. Current isn’t flowing but live wire still has pd. Your body would provide a link between supply & earth, current would flow through you.
  • Any connection between live&earth can be dangerous. If link created a low resistance path to earth, a huge current will flow. Could result in fire.
26
Q

Energy is transferred from cells etc.

A
  • Electrical appliances are made to transfer energy to components in circuit when a current flows.
  • Kettles transfer energy electrically from mains ac supply to thermal energy store of heating element in kettle.
  • Energy transferred electrically from battery of fan to kinetic energy stored of fans motor.
27
Q

Energy transfer equation & exp

A

ET(J) = power x Time
Total energy transferred depends on how long appliance is on for & it’s power.
Power is the energy it transfers per s.
More energy transferred, higher power.
Power ratings are labelled with max. safe power that appliance can work at. It tells you max. amount of energy transferred between stores per second when appliance in use. Lower power rating = less electricity needed so its cheaper. An appliance may be more powerful but less efficient.

28
Q

Potential difference equation 2 & exp

A

ET = charge flow x pd

  • When electrical charge goes through change in pd, then energy is transferred.
  • Energy supplied to charge at power source to ‘raise’ it through a potential.
  • Charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in circuit.
  • So battery with bigger pd supplies more energy to circuit for every coulomb of charge flowing round it, as the charge is raised ‘higher’ at start.
29
Q

Current & pd - another equation :)

A

Power = pd x current
If you don’t know the pd you can also:
P = I squared x resistance

30
Q

The National Grid 1

A
  • A giant system of cables and transformers.
  • Connects power stations to consumers - transfers from supply to demand (eg homes and industry).
  • Throughout day power stations have to produce enough electricity to meet the varying demand.
  • They can predict some bursts: people getting up in the morning, coming home, it’s dark/cold.
  • Power stations often run at well below their maximum power output - spare capacity to cope with high demand.
  • Smaller power stations in case
31
Q

The National Grid 2

A

To transmit huge amount of power, they need a high pd or high current.
High current means lots of energy lost, as wires heat up, energy transfers to thermal energy store of surroundings.
Cheaper to boost pd real high (400,000 V), keep current low as poss.
Increasing pd decreases current for given power, this decreases energy lost by heating wires. More efficient.

32
Q

The National Grid - transformers

A

To get pd to 400000V to transmit power requires transformers & big pylons with huge insulators - still cheaper.
Transformers have to step up pd at one end for efficient transmission, then bring it down to safe level at other end.
Pd increased using step up transformer
Then reduced for domestic use using a step down transformer.

33
Q

Electric current

A

Flow of electric charge.
Electric charge only flows if there’s a potential difference.
So a current only flows if there’s a source of p.d.
In a single closed loop the value is the same everywhere.

34
Q

IN THE CIRCUIT

A

Potential difference/voltage is the driving force that pushes the charge round. Unit: V
Resistance is anything slowing the flow down. Unit:ohm
The greater the resistance across a component, the smaller the current that flows (for a given p.d. across the component).

35
Q

Total Charge

A

charge flow = current x time
Size of current is the rate of flow of charge. When current flows past a point in a circuit for a length of time then the charge that has passed is given by this formula.
More charge passes around the circuit when a larger current flows.

Physics (8 decks)

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