# Current Electricity

4.0 / 5

HideShow resource information

- Created by: ZinaK - Team GR
- Created on: 17-10-13 11:22

Conventional current flows around a circuit from positive to negative

However the electrons around a circuit flow in the opposite direction

1 of 61

Charge

(Q) measured in coulombs (C)

2 of 61

Charge of an electron

1.6x10^-19C

3 of 61

Current

(I) measured in A - current is the rate of flow of charge (1A = 1JC^-1)

4 of 61

Potential Difference

(V) and is measured in V - P.D. is the work done per unit of charge

5 of 61

V=W/Q

V=p.d. in volts... W= work done/energy transferred in joules(J)... Q= charge in coulombs, C

6 of 61

Resistance

The ratio of p.d.across a component to the current flowing through it and is measured in ohms

7 of 61

A resistor at a constant temperature is an

Ohmic conductor

8 of 61

What is the effect of doubling the p.d. on the current? Why?

Current is directly proportional so it doubles the current too

9 of 61

What does an IV graph look like of a resistor at a constant temperature?

It gives a straight line passing through the origin

10 of 61

Ohm's Law

"The electrical current in a conductor is proportional to the p.d. applied to it provided the temperature remains the same"

11 of 61

Ammeters measure

Current

12 of 61

Voltmeters measure

P.d.

13 of 61

IV graph of a filament lamp?

The graph curves (like an s) because as the filament heats up, the resistance goes up too (meaning the resistance of the filament is chanigng)

14 of 61

IV graph of a diode?

Because the diode only allows current to flow in one direction through it, therefore the graph shows the current going up only when the p.d. is 0.7V

15 of 61

What happens if the diode is reversed biased and the p.d. is continually increased?

The diode will eventually begin to conduct in the reverse direction (this is known as the break down voltage- about 50V)

16 of 61

Graph of a thermistor?

The resistance decreases as its temperature increases which can be seen on a graph where resistance is plotted against temperature

17 of 61

Thermistors can have...

A positive or a negative coefficient.

18 of 61

A negative coefficient of a thermistor means...

Resistance decreases with an increase in temperature, this is caused by the release of extra charge carriers in the thermistor

19 of 61

LDR

Light Dependent Resistor - graph of resistance plotted against light intensity looks similar to a thermistors (negative curve which levels off at a high light intensity) LDRs are used in circuits which automatically switch on lights as it gets dark

20 of 61

What does the resistance of a piece of wire depend on?

The length of wire (the longer the wire, the greater the resistance) and the cross-sectional area of the wire (the greater the cross sectional area, the smaller the resistance)

21 of 61

Resistivity

(p) a property of materials which takes into account their resistance (R) Length (L) and their cross-sectional area (A)

22 of 61

Unit of resistivity

Ohm meters

23 of 61

Resistivity and temperature

In metals, increases in temperature make the atoms in the structure of the metal vibrate more and this makes it more difficult for the electrons to move through the material, so the resistance of the material goes up. e.g. a filament bulb

24 of 61

In an IV graph the line curves because as the filament heats up

Its resistance also goes up

25 of 61

In semi conducting materials, the increase in temperature...

Releases more charge carriers so the resistance of the material goes down e.g. a thermistor

26 of 61

Why is a thermistor a negative coefficient thermistor?

The resistance decreases as the temperature increases

27 of 61

Uses of a thermistor?

Thermostats, in fridges and fridges (to monitor and control the temp in the room they are in

28 of 61

Superconductivity

This is when metals or alloys are at or below a critical temperature which reduces their resistance to zero

29 of 61

What is the critical temperature?

Varies depending on the material but the sorts of temperatures are such like -196 degrees celcius

30 of 61

When are superconductors used?

When very strong electromagnets are required, in MRI scanners or to reduce losses in power cables

31 of 61

Cells in series...

When cells are in series in the same direction, the total p.d. of the circuit = the p.d. of the cells added together

32 of 61

Identical cells in parallel...

When identical cells are in parallel to one another the total p.d. supplied to the circuit is the same as the p.d. of just one cell. e.g. if three 2V cells are in parallel the p.d. supplied to the circuit is 2V

33 of 61

Resistors in series...

When resistors are in series with each other, the total resistance is just their individual resistance added together

34 of 61

Resistors in parallel...

When they are in parallel, the total resistance is found using: 1/Rtotal=1/R1+1/R2=1/R3

35 of 61

Conservation of charge

"The total charge flowing into a junction of wires must equal the total charge flowing out of the junction

36 of 61

Kirchoff's first law

"The sum of the currents flowing into a junction of wires must equal the sum of the currents flowing away from the junction of wires

37 of 61

Current in series circuits

The current is the same wherever the ammeter is put

38 of 61

Current in parallel circuits

The current divides between each branch proportionally to how much resistance it has

39 of 61

Kirchhoff's second law

"The sum of the EMF's in any closed loop in a circuit must be equal to the sum of the potential differences in the closed end loop circuit"

40 of 61

P.D. in a series circuit

The total p.d. is shared out between components - if the components all have the same resistance they will have an equal p.d. across them

41 of 61

P.d. in parallel

The p.d. supplied by the cell is the same p.d. as that across each component in parallel

42 of 61

Energy

(E) Measured in joules (J)

43 of 61

Power

(P) is measured in watts (W). Power is the rate at which energy is transferred.

44 of 61

Potential divider equation

V=R/Rtotal x Vtotal

45 of 61

What is a potential divider?

A simple circuit which uses resistors (or thermistors/LDRs) to supply a variable potential difference

46 of 61

What are potential dividers used for?

Volume controls, temperature controls or monitor changes of light in a room

47 of 61

What does it do?

Divides up the pd supplied to them from a cell. The proportion of the available p.d.that the two resistors get depends on their resistance values

48 of 61

Equation to work the V(out)

V(out)=(V(out)xR1)/(R1+R2)

49 of 61

What is EMF

Electromotive force which is the energy provided by the cell or battery per coulomb of charge passing through it (measured in Volts). It is equal to the pd across the terminals of the cell when no current is passing through

50 of 61

Internal resistance

Something that all batteries and cells have, when electricity flows around a circuit the internal resistance of the cell itself resists the flow of current and so thermal energy is wasted in the cell itself

51 of 61

How can the EMF equation be rearranged?

E=V+Ir

52 of 61

DC current

Cells and batteries provide an electrical current which always flows around the circuit in the same direction

53 of 61

AC current

Alternating current is what the UK mains electricity supplies at about 230V, meaning that the current flows one way and then the other around the circuit. It constantly changes direction. In the UK the frequency of mains electricity is around 50Hz

54 of 61

What represents AC signals?

Oscilloscopes

55 of 61

What is an oscilloscope?

The oscilloscope trace can be used as a voltmeter

56 of 61

Why is an RMS value necessary?

The pd and current are varying continuously in an AC signal so we need to represent an average value and the RMS represents the effective value

57 of 61

What does it mean if we see a spot in the middle of the screen in an oscilloscope?

Input connected to ground and timebase switched off

58 of 61

... The spot is 3 squares up?

3 Volts from a DC power source and timebase off

59 of 61

...There is a horizontal line in the middle of the screen?

0 volts, DC source and timebase on

60 of 61

...There is a vertical line in the middle of the screen 6 squares long?

3V, AC source and timebase off

61 of 61

## Other cards in this set

### Card 2

#### Front

Charge

#### Back

(Q) measured in coulombs (C)

### Card 3

#### Front

Charge of an electron

#### Back

### Card 4

#### Front

Current

#### Back

### Card 5

#### Front

Potential Difference

#### Back

## Similar Physics resources:

4.5 / 5

3.0 / 5

0.0 / 5

4.0 / 5

0.0 / 5

3.0 / 5

1.0 / 5

0.0 / 5

## Comments