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A-Level OCR Physics > Module 4- Eletricty > Flashcards

Module 4- Eletricty Flashcards

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1
Q

What is current

A

current is the rate of flow of charge

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2
Q

What is the direction of conventional current flow? And brief explanation of what it is?

A

From positive to negative regardless of the movement of electrons (because it was made before an understanding of charge carriers ect). Therefore conventional current travels in the opposite direction to electrons

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3
Q

What is the equation for current

A

I = change in Q/ change in t

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4
Q

What is a coulomb?

A

A coulomb is the unit of charge

So one coulomb is defined as the amount of charge that passes in 1 second when the current is ampere

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5
Q

What carry charge through a circuit and what charge do they have (what is the charges name)?

A

Electrons or sometimes ions. Electrons all carry the same charge (-e) where e is the elementary charge (e=1.6x10^-19)

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6
Q

What is charge known as and why

A

Charge is said to be quantized because the net charge will always be a integer multiple of e (and therefore only take certain values)

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7
Q

What charge does a Na+ ion have

A

A charge of 1.6x10-19 (protons have an charge of positive e)

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8
Q

How does current flow in metals.

A

In metals the charge carriers are free (delocalized) electrons from an atoms outer shell. These charge carriers travel through a lattice positive ions. These ions cannot move but are able to vibrate more with increasing temperatures.

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9
Q

How do ions in electrolytes (liquids that can carry a current) cause a current?

A

In an electrolyte (molten ionic crystals or ionic solutions) the positive and negative ions are the charge carriers. They will carry a charge when a positive and negative electrode are put in the electrolyte solution because these cause and ions to move (causing a flow of charge which is current).
This can also cause a flow of electrons in the metal between the electrodes because while the cathode gives out electrons the anode takes in electrons

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10
Q

State Kirchhoffs first Law what it is the conservation of

A

The total current entering and junction equals the total current leaving it
Conservation of charge

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11
Q

Conservation of charge explained and how it relates to Kirchhoffs first law

A

As charge flows through a circuit, it doesn’t get used up or lost it is conserved and therefore the amount in the universe is constant. (In a closed circuit)
So what ever flows into a junction will flow back out.
Current is rate of flow of charge it follows that whatever current flows into a junction is the same as the current flowing out.

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12
Q

Do circuit symbols sheet

A

…..

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13
Q

In a circuit diagram which is the positive terminal?

A

The longer line (+ requires more ink)

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14
Q

What is potential difference

A

The work done (energy transferred from electrical to other sources) per unit charge moved between two points in a circuit (V=W/Q)

The potential difference across a component is one vault when you do one joule of work moving one coulomb of charge through the component

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15
Q

What is the equation for potential difference

A

W=VQ

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16
Q

How should voltmeters be connected

A

In parallel

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17
Q

How does work done on a charge link to kinetic energy in terms of equations?

A

When a charged particle is accelerated by potential difference, the energy transferred is equal to the work done on the particle. Hence for an electron of charge size e W=Ve.
W also equals Kinetic energy gained by the electron.

Hence eV=1/2mv^2

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18
Q

What is electromotive force

A

The total amount of work the battery does to each coulomb of charge is called it’s electromotive force (energy transferred from other sources to electrical) e.m.f - measured in volts

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19
Q

Difference between emf and potential difference

A

One is the energy transferred to the charge carriers while the other is the energy transferred by the to charge carriers

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20
Q

What is voltmeter resistance and why

A

Very high due to so no current passes through the voltmeter itself

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21
Q

What is the equation for emf

A

e=w/q

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22
Q

What is resistance? With and equation

A

Resistance can be thought of as a measure of how difficult it is for current to flow through a component. The resistance of a component is defined as the ratio between the potential difference across a component and the current flowing through it. It is measured in ohms.

R=V/I

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23
Q

Define the ohm

A

A components has a resistance of 1 ohm if a potential difference of 1 makes a current of 1 A

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24
Q

What happens to the resistivity of a metal as it temperature increases?

A

Heating up a metal makes it harder for electrons to move about. The ions
vibrate more when heated around their positions , so the electrons collide with them more often increasing the work done of the electrons and the energy lost to other forms.
The resistivity of a metal increases as the temperature increases.

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25
Q

What is Kirchhoffs second law

A

The total emf around a series circuit = the sum of the p.d across each component.
(Conservation of energy)

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26
Q

What is it helpful to do when you need to identify amplitudes and voltage on a circuit diagram?

A

Helpful to draw the separate loops see figure 10 in note book
Think about a closed loop as a single possible path for the current

27
Q

What happen to current, resistance and voltage when kirchoffs laws are applied to a parallel circuit

A

Current is split at each junction I = I1+I2+I3
Total Potential difference at each loop with equal the EMF
1/R= 1/R1 + 1/R2 + 1/R3 (I/V=I1/V + I2/V + I3/V)

28
Q

What happen to current, resistance and voltage when kirchoffs laws are applied to a series circuit

A

Same current at all points in the circuit
EMF split between voltage so e=V1+V2+V3
R=R1+R2+R3 (because IR=IR1+IR2+IR3)

29
Q

What causes resistance

A

Often resistance is caused by electrons loosing energy (it being transferred to other stores) during collisions.

30
Q

What is internal resistance

A

The resistances caused by electron collisions with atoms inside the battery (this causes batteries to warm up as they are used)

31
Q

What are the equations you need to know involving internal resistance and E.M.F

A

E=V+Ir
E=V+v
V=E-v
E=I(R+r)

32
Q

What is the load/external resistance

A

It is the total amount of resistance of all the components in the external circuit

33
Q

what is Terminal PD

A

The potential difference across the load resistance and therefore It is the work done when one coulomb of charge flows through the load resistance (terminal PD) because V=W/Q.

34
Q

Is the terminal p.d the same as emf?

A

No because there is internal resistance causing energy to be lost.

35
Q

What is lost volts

A

The energy wasted per coulomb of charge

36
Q

Therefore how do we find total energy supplied by the source

A

Energy per coulomb used in external resistance + energy per column used wasted in internal resistance

(Proves conservation of energy)

37
Q

How can you work out the e.m.f of multiple cells in series

A

Total E = E1 + E2 + E3

38
Q

How can you work out the e.m.f of multiple cells in parallel

A

For Identical cells in parallel the total e.m.f is the same as each individual cell
E total = E1 = E2 = E3

39
Q

Give a brief explanation of how you would investigate internal resistance and e.m.f in a circuit?

A

Vary the current in the circuit by changing the external resistance with a variable resistor. Measure the p.d for several values of current. Record the data for V and I in a table.

Plot a graph of voltage against current and use the equation V=-Ir + e (e= y intercept and r=gradient)

40
Q

What is ohms law

A

Ohms law (obeyed by ohmic conductors) states that: provided the temperature is constant the current flowing through an ohmic conductor is directly proportional to the potential difference across it (V=IR)

41
Q

Why does voltage go down when current increases sometimes

A

If we apply the V=IR equation to internal resistance we see that lost volts = IR

So as current increases the voltage lost also increases

42
Q

Why does more current always go to the smaller resistance of two junctions in parallel?

A

The total PD across each branch is the same and because V=IR, Current splits in the inverse ratio to resistance (because a higher resistance must have a lower current )

43
Q

What happens when a charged particle is accelerated by a PD

A

When a charged particle is accelerated by a potential difference, the energy transferred to it is equal to the work done on the particle, W = VQ. For an electron with charge size e this can be written as W = Ve.

44
Q

Write an equation given that the energy transferred to an electron is equal to the Kenetic energy gained

A

eV=1/2mv^2

45
Q

What is the mean drift velocity of a charge carrier c

A

The mean drift velocity is just the average velocity and it’s much, much less than the electrons’ actual speed (because they move randomly in all directions, but tend to drift one way.)

46
Q

Equation for current involving mean drift velocity

A

I=Anev

A= cross sectional area
I = current
n = number density of electron (number per unit volume) m^-3
v = mean drift velocity
e= size of charge on one electron

(Note if using different charge carrier e can be replaced with charge and each carrier and n is number density of charge carriers)

47
Q

Explain the different numbers of charge carriers in semi conductors, insulators and metals

A

In a metal, the charge carriers are free electrons - they’re the ones from the outer shell of each atom. Thinking about the formula I = Anev, there are loads of charge carriers per unit volume, making n big. The drift velocity is small, even for a high current.

2) Semiconductors have fewer charge carriers, so the drift velocity needs to be higher to give the same current

3) A perfect insulator wouldn’t have any charge carriers, so n = 0 in the
formula and you’d get no current. Real insulators have a very small n.
You’ll be given this equation

48
Q

What are the things that determine resistance in a wire

A

Length (L). The longer the wire the more difficult it is to make a current flow.

2) Area (A). The wider the wire the easier it will be for the electrons to pass along it.

3) Resistivity (p). This depends on the material the wire’s made from, as the structure of the material may make it easy or difficult for charge to flow. In general, resistivity depends on environmental factors as well, like temperature.
The resistivity of a material is defined as the resistance of a 1 m length with a 1 m^2 cross-sectional area, so p= RA/L. Resistivity is measured in ohm metres.

You’ll be given equation in exam

49
Q

EXPLAIN HOW and electron gun works and what equation it helps to explain

A

A small metal filament is heated by an electric current. The electrons in this piece of wire gain kinetic energy. Some of them gain enough kinetic energy to
escape from the surface of the metal. This process is called thermionic
emission

If the heated filament is placed in a vacuum and a high p.d. applied
between the filament and an anode, the filament acts as a cathode,
and the freed electrons accelerate towards the anode, gaining kinetic energy. If the anode has a small hole in it, then electrons in line with this hole can pass through it, creating a beam of electrons with a specific kinetic energy

As the electrons accelerate towards the anode they gain kinetic energy.
From the definition for p.d, the work done on a single electron travelling
from the cathode to the anode is equal to eV, where e is the elementary
charge, the charge on each electron, and V is the accelerating p.d.

eV = 1/2 mv^2

This assumes the electrons have negligible kinetic energy at the cathode.

50
Q

Draw IV characteristic (current y against PD x) for filament lamp, ohmic conductor, thermoister, LDR, diode

A

…

51
Q

Explain the Iv characteristic of a ohmic/ metallic conductor

A

At constant temperature, the current through a metallic conductor, e.g. a wire or a resistor, is directly proportional to the potential difference.
2) The fact that the characteristic graph is a straight line through the origin tells you that the resistance doesn’t change - it’s equal to 1 / gradient.
3) Metallic conductors are ohmic - they have constant resistance provided their temperature doesn’t change (see below).

52
Q

Explain if characteristic for a filament lamp

A

The characteristic graph for a filament lamp is a curve, which starts steep but gets shallower as the potential difference rises.
3) However, current flowing through the lamp increases its temperature, so its resistance increases (see below).

53
Q

Why are semi conductors used in diodes, LDR and thermistors

A

Semiconductors have a higher resistivity than metals because there are fewer charge carriers available. However if energy is supplied to some types of semiconductor (eg. by increasing temperature, more charge carriers are released, so the resistivity decrease). this means that they make excellent sensors for detecting, changes in the environment.

54
Q

Explain the IV characteristic of a thermistor

A

A thermistor is a resistor with a resistance that depends on its temperature. You only need to know about NTC thermistors - NTC stands for ‘Negative Temperature Coefficient’. This means that the resistance decreases as the temperature goes up.
The characteristic I-V graph for an NTC thermistor curves upwards.
This is because as a thermistor warms more electrons have enough energy to escape their atoms meaning more charge carriers available and thus a lower resistance

55
Q

Explain the IV characteristic for an LDR

A

The greater the intensity of light shining on an LDR, the lower its resistance.
The explanation for this is similar to that for the thermistor. In this case, light provides the energy that releases more electrons.
More charge carriers means a lower resistance.

56
Q

Explain the IV characteristic of a diode

A

Diodes (including light emitting diodes (LEDs)) are designed to let current flow in one
Current / A direction only. You don’t need to be able to explain how they work, just what they do.

Forward bias is the direction in which the current is allowed to flow - it’s the direction the triangle points in the circuit symbols on the right.
Most diodes require a threshold voltage of about 0.6 V in the forward direction before they will conduct.
In reverse bias, the resistance of the diode is very high and the current that flows is very tiny.

57
Q

What is power defined as with and equation

A

Power (P is defined as the rate of doing work. Its measured in watts (W, where 1 watt is equivalent to Joule of work done per second.

P=W/t

58
Q

What is an equation for power involving PD and current

A

P=VI (given in formula book)

1) Potential difference (V is defined as the work done per coulomb.
2) Current (1) is defined as the number of coulombs transferred per second.
3) So p.d. x current is work done per second, i.e. power.

59
Q

Other equation invoking power

A

P=V^2/ R and P=I^2/R since P=VI (also given in formula book)

60
Q

What unit do electricity companies charge their users with?

A

Electricity companies charge their customers for units’ of electricity. Another
name for a unit is a kilowatt hour. If you know the power of an appliance and the length of time is it’s used for you can calculate the work it does in kWh.

61
Q

Last two bits of page 68 -69

A

.

62
Q

Make flash cards on potential divided

A

.

63
Q

Draw an LED and LDR

A

A-Level OCR Physics (15 decks)

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