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Physics > TOPIC 2 - ELECTRICITY > Flashcards

TOPIC 2 - ELECTRICITY Flashcards

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1
Q

What is current?

A

The flow of electrical charge.

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2
Q

What is needed for current to flow?

A

Potential difference.

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3
Q

What is the unit of current?

A

Ampere. A.

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4
Q

What is the current like in a closed, single loop circuit?

A

The same value everywhere in the circuit.

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5
Q

What is the potential difference?

A

The driving force that pushes the charge around.

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6
Q

What is the unit of potential difference?

A

Volt. V.

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7
Q

What is resistance?

A

Anything that slows the flow down.

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8
Q

What is resistance measured in?

A

Ohm.

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9
Q

What does the current flowing through a component depend on?

A

The potential difference across it and the resistance of the component.

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10
Q

What is the equation for charge?

A

Charge = current x time.

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11
Q

What is the size of the current?

A

The rate of flow of charge.

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12
Q

What is the equation for potential difference?

A

Potential difference = current x resistance.

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13
Q

Explain an experiment to investigate factors affecting resistance.

A

Attach a crocodile clip to the wire level with zero cm on the ruler.
Attach the second crocodile clip to the wire, eg 10 cm away from the first clip. Write down the length of the wire between the clips.
Close the switch, then record the current through the wire and the pd across it.
Open the switch then move the second crocodile clip, eg another 10 cm along the wire. Close the switch again then re odd the new length, current and pd.
Repeat this for a number of different lengths on the test wire.
Use the measurements of current and resistance to calculate the resistance for each length of wire, using R = V/I
Plot a graph of resistance against wire length and draw a line of best fit.
If the graph doesn’t pass through zero it might be because the first clip isn’t attached directly to zero. This is a systematic error.
The graph should be a straight line, meaning resistance is directly proportional to length - the longer the wire, the greater the resistance.

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14
Q

What are ohmic conductors?

A

Have a constant resistance.
At a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it.

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15
Q

What practical can be done to find a components I-V characteristics?

A

Set up a circuit with an ammeter, voltmeter, component and variable resistor.
Begin to vary the variable resistor. This alters the current flowing through the circuit and the potential difference across the component.
Take several pairs of readings from the am term and voltmeter to see how the potential difference across the component varies as the current changes. Repeat each reading twice more to get an average pd at each current.
Swap over the wires connected to the cell, so the direction of the current is reversed.
Plot a graph of current against voltage for the component.

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16
Q

Explain the graph for an ohmic conductor.

A

The current through an ohmic conductor is directly proportional to potential difference so you get a straight line.
(At a constant temperature).

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17
Q

Explain the graph for a filament lamp.

A

As the current increases, the temperature of the filament lamp increases, so the resistance increases. This means that less current can flow per unit pd, so the graph gets shallower - hence the curve.

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18
Q

Explain the graph for a diode.

A

Current will only flow in one direction through a diode. The diode has very high resistance in the revere direction. This is why the line is only on one side of the graph.

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19
Q

What is an LDR?

A

A light dependant resistor.
It is dependant on the intensity of light.
In bright light the resistance falls.
In darkness, the resistance is highest.
They have lots of applications including automatic night lights, outdoor lighting and burglar detectors.

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20
Q

Explain thermistors.

A

A thermistor is a temperature dependant resistor.
In hot conditions, the resistance drops.
In cool conditions, the resistance goes up.
Thermistors make useful temperature detectors, eg car engine temperature sensors and electronic thermostats.

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21
Q

Describe sensing circuits.

A

They can be used to turn on or increase the power to components depending on the condition they are in.

22
Q

What happens to potential difference in series?

A

Pd is shared. The potential difference round a series circuit always adds up to the equal source of pd.

23
Q

What happens to current in series?

A

The same current flows through all components. I1 = I2

The size of the current is determined by the total pd of the cells and the total resistance of the circuit.

24
Q

What happens to resistance in series?

A

Resistance adds up.
The total resistance of two components is just the sum of their resistances. This is because by adding a resistor in series, the two resistors have to share the total pd. The total current in series is reduced when a resistor is added. This means that the total resistance of the circuit increases.
The bigger the components resistance, the bigger its share of the total potential difference.

25
Q

What happens to potential difference in series?

A

There is a bigger pd when more cells are in series, if they are all connected the same way. Eg when two cells with a pd of 1.5v are connected in series they supply 3v between them.

26
Q

What happens with different components in parallel?

A

If you remove or disconnect one of them, it will hardly affect the others at all. This is how mot things are connected because you have to be able to switch things on and off separately.

27
Q

What happens to the potential difference in parallel?

A

In parallel circuits all components get the get the full source of pd. So, the pd is the same across all components. This means that identical bulbs connected in parallel will be the same brightness.

28
Q

What happens t5o current in parallel circuits?

A

Current is shared between branches. In parallel circuits the total current flowing around the circuit is equal to the total of all the currents through the separate components. If 2 identical components are connected in parallel then the same current will flow through each component.
There are junctions where the current either splits or rejoins. The total current going into a junction has to equal the total current leaving.

29
Q

What happens if you add a resistor in parallel?

A

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

30
Q

Explain why adding a resistor in parallel reduces the total resistance.

A

In parallel, both resistors have the same potential difference across them as a source. This means that the pushing force making the current flow is the same as the source pd for each resistor that you add. By adding another loop, the current has more than one direction to go in. This increases the total current that can flow around the circuit. using v=ir, an increase in current means a decrease in the total resistance of the circuit.

31
Q

What happens when you add resistors in a series circuit?

A

Increases the total resistance. (adding a resistor decreases the total current through the circuit).

32
Q

What happens when you add resistors in parallel?

A

The total current through the circuit increases, so the total resistance of the circuit has decreased. The more resistors you add, the smaller the overall resistance becomes.

33
Q

What are the two types of electricity supplies?

A

Alternating current and direct current.

34
Q

What is alternating current?

A

In ac supplies the current is constantly changing direction. alternating currents are produced by alternating voltages in which the positive and negative keep alternating. The UK mains supply 9electricity in your home) is an ac supply at around 230 volts. The frequency is 50 hertz.

35
Q

What is a direct current?

A

Cells and batteries supply direct current (dc). DDirect current is a current that is always flowing in the same direction. It is created by a direct voltage.

36
Q

What is the live wire?

A

The live wire is brown. It provides the alternating potential difference (at about 230 volts) from the mains supply.

37
Q

What is the earth wire?

A

The earth wire is green and yellow. It is for protecting the wiring and for safety - it stops the appliance casing from becoming live. It does not usually carry a current - only when there is a fault. It is at 0 V.

38
Q

What is the neutral wire?

A

The neutral wire is blue. The neutral wire completes the circuit and carrier away current - electricity normally flows in through the live wire and out through the neutral wire. It is around 0 V.

39
Q

What does the energy transferred depend on?

A

Energy transferred depends on the POWER.

40
Q

What equation links energy transferred, power and time?

A

energy transferred = power x time.

41
Q

What does the power rating of an appliance tell you?

A

The power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use.

42
Q

What does it mean if the power rating is low?

A

The lower the power rating, the less electricity an appliance uses in a given time and so the cheaper it is to run. (However, an appliance can be more powerful than another, and still less efficient).

43
Q

What is potential difference?

A

Potential difference is energy transferred per charge passed. When an electrical charge goes through a change in potential difference, the energy is transferred.

44
Q

What equation links energy transferred, charge flow and potential difference?

A

Energy transferred = charge flow x potential difference

This means that a battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows round it.

45
Q

What equation links power, potential difference and current?

A

power = potential difference x current

46
Q

What is the national grid?

A

The national grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers.

47
Q

What does the national grid use?

A

The national grid uses HIGH PD AND LOW CURRENT.
To transmit huge amounts of power you either need high PD or a high current. However, if the current is high you lose loads of energy, because energy is transferred to the thermal energy store.
It is cheaper to up the PD and keep the current low.
For a given power, increasing PD decreases the current, which decreases the energy lost. This makes the national grid an efficient way of transferring energy.

48
Q

How is potential difference changed in the national grid?

A

To get the potential difference to 400000 V to transmit power requires TRANSFORMERS as well as big pylons and huge insulators. TRANSFORMERS have to step the potential difference up at one end, for efficient transmission, and then bring it back down to safe, usable levels at the other end.
The potential difference is increased by a step-up transformer
the potential difference is reduced again by a step down transformer for domestic use.

49
Q

How does too much static cause a spark?

A

An electric charge builds on an object, the potential difference between the object and the earth (which is at 0V) increases.
If the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth - this causes a spark.
They can also jump to any earthed object nearby eg getting a static shock when you get out of the car.

50
Q

What is an electric field?

A

An electric field is created around any electrically charged object. To closer to the object you get, the stronger the field is. You can show the electric field around an object using field lines.

51
Q

How do you draw electric field lines?

A

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

52
Q

How do charged objects in an electric field feel a force?

A

When a charged object is placed in an electric field, it feels a force - either forces of attraction or repulsion (non-contact forces). The force is caused by the electric fields of each charged object interacting with each other.

Physics (25 decks)

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