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Physics GCSE > Electricity > Flashcards

Electricity Flashcards

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1
Q

Define an electric current.

A

An electric current is a flow rate of flow of electrical charge over time.
It can only flow round a closed circuit if there’a pd.
Current is measured in Ampere,A.

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2
Q

When is the current same everywhere?

A

In a single, closed loop, the current is the same everywhere in the circuit.

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3
Q

Define the potential difference?

A

PD(voltage) is the work done on the charge to move it from one point to another and the work is done by the battery and it transfers energy from electrical into other forms.

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4
Q

What is the connection between the resistance and the current?

What is resistance defined as?

A

The greater the resistance across a component, the small the current that flows for a given pd across a component.

resistance(ohms)= pd(v)/ current(amperes)

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5
Q

What is the units for power? Define Power.

The equation for power?

A

Power is 1 J of energy done in 1s and it has the units of Watts.

Power= energy transferred(J)/ time(s)

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6
Q

What is current measured in and what circuit is the specified component attached to?

What is voltage measured in and what circuit is the specified component attached to?

A

1) Current is measured by the ammeter and it is attached to a series because the current is the same everywhere in a series and the electrons will flow in one direction.
2) Voltage is measured by a voltmeter and it’s attached against a component in the parallel circuit as a voltmeter calculates a potential drop against components. The voltage is also the same everywhere in a parallel series.

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7
Q

The equation that relates charge, current and time?

A

Charge(Coloumbs,C)= I- Current (A) x Time(s)

C=IT

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8
Q

The equation that relates voltage, current and reistance?

A

V=IR

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9
Q

What is Ohm’s law?

A

Ohm’s Law is when the current through a resistor at constant temperature is directly proportional to the potential difference across a resistor. This means thay the resistance will stay the same as the current changes, provided the temperature remains the same.

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10
Q

What’s an LDR? When can it be used? (3)

A

An LDR is a resistor that is dependant on the intensity of light. In bright light, resistance falls and in darkness, resistance is the highest.
-Used in lots of appliances including automatic night lights, outdoor lightening and burglar detectors.

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11
Q

What is a thermistor?When can it be used?

A
  • A temp dependant resistor.
  • In hot conditions, resistance drops, in cool conditions, resitance goes up.
  • Thermistors make useful temperature detectors; eg car engine temperature sensors and electronic thermostats.
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12
Q

What are sensing circuits?(4)

A
  • Thermistors and LDR, when placed in a series with a fixed resistor, can be used as a sensing circuit.
  • The potential difference across one of them can be used to switch on other circuits depending on the potential diff across it.
  • Called a potential divider circuit and the pd of a supply can be split between the two resistors.
  • When one resistance of one component increases, the other component’s resistance decreases and the pd is divided between the 2 components depending on the resistance- higher the resistance across a component, more current will flow.
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13
Q

Draw a graph for a filament lamp.

Explain why it is non an ohmic conductor?

A
  • Correct graph- line going straight but curves off at the top
  • Not an ohmic conductor as when the current increases, the temp of the filament increases as it transfers some energy to the thermal energy store of the filament which heats up, so the reistance increases and thus less current can flow per unit pd.
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14
Q

Draw a graph for a diode.
What is a diode? Explain why it is non an ohmic conductor?

What’s an LED? What are they used for?(2)

A

-Correct graph- graph curving upwards and in the negative x axis curivng downwards.
Diodes allow the electric current to pass in one direction only . In the forward reaction, the line curves towards the y axis and hence current is not directly proportional to the voltage. In backward reaction, current is virtually zero so diode’s resistance in the backward reaction is much greater than in the forward reaction.

A LED is a diode that emits light when current passes through it in the forward direction. They are used in many electronic devices such as battery circuits and alarm circuits.

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15
Q

What are the rules for series circuits?
Current, PD, Resistance?

What happens when a resistor is added in a series component?

A

1) Current is the same everywhere= It= I1=I2 I3 ( An ammeter measures the current)
2) Potential difference is shared between the various components= Vtotal= V1+V2+V3+V4.
3) Resistance adds up= Rtotal= R1+R2+R3.

-When a resistor is added in a series component, resistors will have to share the total pd and therefore the pd across each resistor is lower thus the current through each resisitor is lower. The total current is reduced when a resistor is added and if the curent is reduced, then the resistance of the circuit increases. The bigger a component’s resistance, the bigger its share of the total pd.

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16
Q

What are the rules for parallel? Current, PD, Resistance?

What happens when 2 identical components are connected in parallel in regards to current?

What happens when a resistor is added in a paralle component?

A

1) Current is shared in a parallel circuit- ITotal= I1+ I2+I3 and this is because there are junctions where the current either splits or rejoins and the total current going into a junction= the total current leaving the junction.

When 2 identical components are connected in parallel, then the same current will pass through each component.

2) Voltage(PD) is same across all the components= V1=V2=V3=V4 and is measured by a voltmeter attatched agaisnt the component( to measure the potential drop)
3) If you have 2 resistors in parallel, The total resistance of components in parallel is less than the resistance of the smallest of the two resistors

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17
Q

What’s different about series and parallel?

A

-Series are diff components connected in a line, end to end, between the +ve and the -ve of the power supply thus if you disconnect one component, the circuit is broken and they all stop whereas in Parallel, each compoennt is separately connected to the +ve and -ve of the supply so if you disconnect one of them, it will hardly affect the others at all and therefore paralle circuits are more commonly used in household electrics and cars.

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18
Q

What’s the different between direct current and alternating current?

How are these currents produced and what are they mainly used for?

How many volts is the UK mains supply? What is the frequency of the main supply?

A

Direct current is where the current travels in one direction only and is created by a direct voltage.

An alternating current is constantly changing direction.
Alternating currents are produced by alternating voltages in which the positive and the negative ends keep alternating.

AC is used for main supply whereas battery supply is used for dc.

The UK mains supply is an ac supply at around 230 V. The frequency of the ac main supply is 50 Hz.

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19
Q

Most cables have three separate wires; what are their colour and purpose?

A

Mostly we use three pin plugs- the wire inside them are:
1) Live wire- Brown- it provides an alternating pd( at about 230 V) from the main supply.

2) Earth wire- green and yellow- it protects the wiring and for safety as it stops the appliance casing from becoming live and doesn’t carry a current( it only carries a current when there’s a fault) and is usually at 0V.

3) Neutral wire-blue
- it completes the circuit and carries away current- electricity normally flows through in the live wire and out through the neutral wire and it’s around 0V.

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20
Q

State the materials around the wires and why are they specefically used?(2)

A

Main cables for main appliances are made up of:

  • Each wire has a core of copper as it is a good conductor of elec, it is cheap and is malleable.
  • Thick plastic is used as it is a good insulator to prevent a person from getting an electric shock.
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21
Q

How can the live wire give us an electric shock?

A

-Body is at 0V and if you touch a live wire, a large pd is produced across your body and a current flows through you which could give an electric shock- injure or even kill you.

22
Q

When does a short circuit occur?

Define the job of the fuse. Compare it to a circuit breaker?

A
  • When a live wire comes in contact with a neutral wire, a very big current passes through the two wires at the point of contact. It causes a short circuit and could lead to a possible electrical fire. Therefore, both wires are covered with plastic to insulate themselves from each other.
    If the fuse blows, the currents cut off.

Fuse:
- From the live wire, the current flows through the fuse and inside the fuse, there is a thin wire and the fuse’s job is that if the current is too high, the fuse will melt and break the circuit which will render the appliance safe.

  • When there’s a high current flowing through the Earth wire due to a low resistant path, a circuit breaker in the fuse box then detects the problem and breaks the circuit or the excess current will cause the fuse to melt in the plug. The fuse wire takes time to melt whereas the circuit breaker’s response is immediate.
23
Q

How is electricity distributed?

A

-Electricity is distributed via the National Grid

24
Q

Describe the National Grid?

A

It is a giant system of cables and transformers that covers the UK and connects power stations to consumers. It transfers electrical power from power stations(power plants) and to where its demand is needed eg homes and industry.

25
Q

When does demand increase?

How do power stations manage this demand?

A
  • Demand for electricity alters throughout the day and demand increases when people get up in the morning, come home from school or work or when it starts to get dark or cold outside- popular events being showcased.
  • Power stations often run well below their maximum power output so there’s spare capacity to cope with a high demand and lots of power stations that can start up quickly are also kept in standby just in case.
26
Q

What does the National Grid use to transfer a huge amount of power? What’s the problem?

What’s the cost-effective way that makes National Grid efficient?

A
  • To transfer large amounts of current, they have high PD or a high current however having high current has the prob that you lose lots energy as the wires heat up and energy is transferred to the thermal energy store of te surroundings
  • It’s cheaper to increase pd and keep the current as low as possible as by increasing the pd, the current is decreased which decreases the power lost by the wires due to the resistance heating which further reduces energy lost to the surroundings and hence makes a national grid efficient.
27
Q

What is the potential difference changed by? How do they change potential difference?

A
  • Transformers
  • Step up transformers is when the pd is increased for efficient transmission from the power station to the National Grid and step down transformers are used to decrease the pd from the transmission of electricity from the National grid for domestic uses hence pd needs to be safe, usable levels.
28
Q

What is a build-up of static caused by?

A

A build-up of static is caused by friction

29
Q

How do you measure the frequency of an ac supply?

A

Measure the time period of the waves then use the equation=

frequency=1/ time period( time taken for 1 cycle)

30
Q

What is the definition of power?

What are the 3 equations for calculating power?

A

The power supplied to a device is the energy transferred each second.

  • Power= energy transferred/ time.
  • Power= VI
  • Power= current² x resistance.
31
Q

What is a charge?
What does it do?
What happens to the charge when there is a change in pd? and whats the eq?

What happens when a charge flows through a resistor?

A

-A charge is a flow of electrons.
-A charge transfers energy as it does work against the resistance of the circuit and hence energy is transferred.
-When an electrical charge goes through a change in pd, energy is transferred and the equation is
E(J)= Q(Charge,C) x V( Potential different,V)

-When a charge flows through a resistor,energy transferred to the resisitor to make it hot

32
Q

Define static electricity?

A

a stationary electric charge, typically produced by friction, which causes sparks or crackling

33
Q

How is static electricity produced?

A

When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

34
Q

What happens when an acetate rod and polytheme rod are rubbed with a cloth duster?

A

1) Rubbing a cloth on the polyethene rod transfers electrons from the cloth on to the polyethene rod which causes it to become negatively charged.
2) Rubbing a cloth on the acetate rod causes a transfer of electrons from the acetate rod on to the cloth and the loss of electrons results in it being pos charged.

35
Q

In static electricity:

What happenes when two objects are brought together?
What happens to two objects that carry the same force/ different force?

What is this force called?

A
  • They exert a force on each other.
  • Same force- electrostatic repelsion
  • Opposite forces- electrostatic attractction.

These forces get weaker when they further apart and these forces will cause the objects to move if they are able to do so.

Attraction and repulsion are non-contact forces.

36
Q

What happens when an electric charge builds on an object? What happens when the pd gets large enough?

What can a spark do?

A

When an electric charge builds on an object,t he pd between the object and the earth( which is at 0V) increases and if the pd gets large enough, electrons can jump across the gap between the charged object and the earth which results in a spark.

-A spark can jump to any earthed conductor that is nearby and this usually happens when the gap is fairly small.

37
Q

How do you get a static shock when getting out of the car?

A

A charge builds on the car’s metal frame and when you touch the car, the charge travels through you to earth.

38
Q

How is a positive charge caused?

A

It’s caused by the movement of electons when the material loses electrons. The pos charge never moves.

39
Q

What creates an electric field? Where is it created around?

A
  • Electric charges create an electric field.
  • Electric field is created around any electrcially charged object and the closer you get to it, the strong the field is.
40
Q

How is a thermistor used?

What can a thermistor be used for?(2)

A

-The resistance of a thermistor depends on its temperature. At low temperatures, the thermistor has a high resistance. As the temperature increases, the resistance decreases.
A thermistor can be used in thermostats or heat activated fire alarms.

41
Q

How is a fixed and variable resistor? What can a variable resistor be used for?

A

Fixed resistor- A resistor restricts or limits the flow of electrical current. A fixed resistor has a resistance that does not change.

Variable resistor-Moving the position of the slider on this resistor changes the resistance. A variable resistor is used in some dimmer switches and volume controls.

42
Q

How is a LDR used?

A

The resistance of a LDR depends on light intensity. At low light levels, the LDR has a high resistance. As the light intensity increases, the resistance decreases. A LDR can be used as a sensor in cameras or automatic lights that come on when it gets dark.

43
Q

What are diodes? What can they be used for?

A

A diode allows current to flow in one direction only. Current will not flow in the other direction. Diodes are used to convert an alternating current into a direct current.

44
Q

What happens when a resistor is added in a parallel component?

A

Adding a resistor in parallel reduces the total resistance. This is because in parallel, the same pd is across components and by adding another loop, the current has more than one direction to go, increasing the totoal current that can flow around the circuit. Using V=IR, an increase in current means a decrease in the total resistance of the circuit.

45
Q

What are the rules for sensing circuits?

A

1) If Vout >/ 5v- heater is switched on.

3) If Vout /< 5V- heater is switched off.

46
Q

State an equation that related energy transferred to charge?

A

energy transferred= pd x charge

47
Q

Metal cased appliances vs plastic cased appliances?

What’s the symbol for double insulated

A

-Metal casings will need an earth wire and plastic- cased appliances don’t need one because if a bare wire touches the inner side of the casing, the current won’t flow and the user is safe and plastic cased appliances are double insulated because there are 2 insulating layers one is plastic on wire and the other is case.

Symbol: one square inside another square.

48
Q

What do transformers do in regards to pd and current? How do transformers work?(3)

A

Transformers change an alternating pd( and hence the current)

1) They work through electromagnetic induction so an alternating pd is applied to primary coil, the soft iron core magnetises which acts as a solenoid and will have a changing magnetic field.
2) The soft iron core is a magnetic material that ensures the secondary coil experience is linked to the primary’s changing magnetic flux. As a consequence, an alternating pd is induced in the secondary coil.
3) If the secondary coil forms a complete circuit, an alternating current will flow in the secondary coil.

49
Q

What’s the equation that relates to transformers?

How will you know which is a step up transformer or step down transformer from this eq?

A

Ns/ Np = Vs/ Vp

Ns- number of coils in the secondary coil.
Vs- Output voltage in the secondary coil.

If Ns is greater than Np- it is a step up transformer.
If Np is greater than Ns, than our pd of vs is less hence it is a a step down transformer.

50
Q

An 18V battery could not be used as the input of a transformer. Suggest why

A
  • A battery provides a DC when we need an Ac current.
51
Q

What’s the problem with high current and so what do we as a result in the national grid?

A
  • High current results in loads of energy being lost as the wires heat up and energy is transferred to the thermal energy store of surroundings- much cheaper to boost the pd to 400 000V and keep the current as low as possible.
  • for a given power, increasing the pd decreases the current which decreases the energy lost.
52
Q

Which has more resistance in terms of wire?

A

The longer a wire is the more resistance it has due to the longer path the electrons have to flow along to get from one end to the other. The larger the cross sectional area, the lower the resistance since the electrons have a larger area to flow through.

Physics GCSE (8 decks)

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