Define electric current and give a symbol and unit

Electric current as the rate of flow of charge, measured in A (I)

Define potential difference and give a symbol and unit

Potential difference as work done per unit charge (V)

How do insulators work

electron attached to atom and cant move away

How do metallic conductors work

some electrons unattched so carry charge to positive terminal

Semiconductor

number of charge carriers increases with temperature therefore resistance decreases as temperature increases

Ohms law states that

I is directly proportional to the pd (V)

EMF =

electrical energy produced per unit charge passing through a point

When plotting current (x) against pd (y) for an ohmic conductor the gradient =

1/R

resistivity(p) =

resistance * cross sectional area / length

Superconductor and applications =

Superconductivity as a property of certain materials which have zero resistivity at and below a critical temperature which depends on the material. Applications (e.g. very strong electromagnets, power cables as can transfer electrical energy without wasting energy).

At any junction in a circuit….

the total current leaving the junction = the total current entering the junction

The current passing through two or more components in series is

the same throughout each component

Potential difference =

energy transfer per coulomb of charge

For 2 or more components in series, the total potential difference across all the components is

equal to the sum of the potential difference across each component

In series, V0 =

V1 + V2 + V3

In parallel, V0 =

V1 = V2 = V3

For any complete loop of a circuit, the sum of the emfs round the loop is

equal to the sum of the potential drops around the loop

In series, R0 =

R1 + R2 + R3 same current different pd

In parallel, 1/R =

1/R1 + 1/R2 same pd different current

Rate of heat transfer =

RI^2

E=IVt

P=IV

P=RI^2

Emf =

Electrical energy, E / Charge, Q

IR +Ir = terminal pd + lost pd (pd across internal resistance)

V1 - V2 / I2 - I1

Internal resistance is

the loss of potential difference per unit current

the resistance of the battery itself within the circuit (lost volts)

Circuit with emf, e and two resistors r and R, current =

I = e/R + r

When plotting Current (x) against terminal voltage (y) what can you get

y intercept = emf

gradient = negative internal resistance

Application of internal resistance

Car batteries have low internal resistance and low voltage so to provide energy needed to ignite the petrol in a short time a high current is needed

Cell current =

net emf / total circuit resistance

Pd across each resistor in series within the cell =

I * resistance of each resistor

Current through each resistor =

pd across parallel combination / resistors resistance

If cells are connected in the same direction

the net emf is the sum of the individual emfs

emf0 = emf1 + emf2 ….

If cells are connected in the opposite direction

the net emf is the difference of the individual emf’s

For a circuit with n identical cells in parallel the current through each cell is

I/n

For a circuit with n identical cells in parallel the lost potential difference in each cell is

Ir/n = V/n

Solar panels consist of

many parallel rows of identical solar cells ins series

A diode has a forward pd of ____ with a current passing through it

0.6V

A diode has a resistance of ___ in the reverse direction or when _____

infinite ohms

less than 0.6V

Potential divider =

2+ resistors in series can be used to supply a pd which is fixed at any value between 0 and the source pd

- supply a variable pd
- supply a pd that varies with a physical condition such as temperature

Current =

V/R

pd across resistors / total resistance

V0 / R1 +R2…

Potential difference =

IR

V0R/R1+R2…

Ratio of pds across each resistor =

ratio of resistance across 2 resistors

V1/V2 = R1/R2

A variable potential divider can be used

as volume or brightness control

Sensor circuits produce output pd which changes a result of a change in a physical variable such as light intensity or temperature

a change in a physical variable such as light intensity or temperature

Light sensor structure

A light sensor uses a LDR and variable resistor

Temperature sensor circuit

Temperature sensors consist of a potential divider made using a thermistor and a variable resistor

The resistance of a metal increases with temperature because

as the positive ions in the conductor vibrate more due to the increase in thermal energy it becomes harder for the conduction electrons to pass through the metal when a potential difference is applied

Metals have a ___ temperature coefficient because

Positive

Temperature causes resistance to increase

Semiconductors have a ____ temperature coefficient because and why

Negative

Temperature causes resistance to decrease because as the number of conduction electrons increases do does temperature and so as the current increases the resistance decreases

Variable resistors and potential dividers measure

the variation of current with potential difference (with potential dividers current and pd can = 0 but not with variable resistors)

Ammeter symbol and function

—–(A)—– Measure current (in series in Amps)

Voltmeter symbol and function

—–(V)—– Measure potential difference/voltage (in parallel in volts)

Cell symbol and function

—–| l—– source of energy

Bulb symbol and function

—–(X)—– light source / indicator of flowing current

Diode symbol and function

—–l>—- allows current to flow in a single direction

Light emitting diode symbol and function

—–l>—- with two arrows pointing north east from the top right - emits light when conducts

Resistor symbol and function

___

—-|___|—- Designed to have a specific resistance

Variable resistor symbol and function

___

—-|___|—- with single arrow going from bottom left to top right - measure variation of current with pd

Thermistor symbol and function

___

—-|___|—- with line from bottom left to top right that going horizontal when out of rectangle /—-

Light dependent resistor symbol and function

___

—-|___|—- with two arrows coming in from top left - as light intensity increases resistance decreases

Heater symbol and function

—-[ | | | ]—- (rectangle with 3 lines in) A transducer which converts electrical energy to heat.

Electric motor symbol and function

—–(M)—– A transducer which converts electrical energy to kinetic energy (motion).

Graph of voltage (x) against current (y) for a wire

graph of y=x, directly proportional with gradient 1/R

Graph of voltage (x) against current (y) for a lamp

stretched S shape (sinx graph for -90 < x <90)

Graph of voltage (x) against current (y) for a thermistor or LDR

y = 2x (steeper) for higher temperature/light intensity

y=x/2 (shallower) for lower temperature/light intensity

Graph of voltage (x) against current (y) for a diode

Current = 0 until voltage = 0.6V then increases almost vertically but not too far over 0.6V

State and explain one reason why it is an advantage for a rechargeable battery to have a

low internal resistance

Internal resistance limits current so low ir can provide higher current

Charges quicker as current higher and less energy wasted

State what is meant by the emf of a battery

Energy transferred to electrical energy per unit charge coulomb passing through

(pd across terminals when no current passing through cell)

(word done by the battery per unit charge)

When the switch is open the voltmeter reads 6.0 V and when it is closed it reads

5.8 V.

Explain why the readings are different

when switch is closed a current flows (through the battery)

hence a pd/lost volts develops across the internal resistance

State and explain why it is important for car batteries to have a very low internal resistance

Need large current to start a car, internal resistance limits current

Define internal resistance

Resistance of materials within battery

State and explain the effect of attempting to use a battery with a much higher internal

resistance to start the car

Car probably wont start as battery cant provide enough current

Explain why the terminal pd decreases as the current increases

emf > V

pd across internal resistance increases with current

In graph of current (x) against terminal pd what is the emf and the internal resistance

y intercept

gradient

Circuit: battery, switch, lamp, voltmeter

Why is the voltmeter reading less than the emf of the battery?

battery has internal resistance

current passes through (this resistance)

work done/voltage lost, which reduces the value of the emf

The reading on the voltmeter is less than the emf. Explain why this is so

there is a current (through the battery)

voltage ‘lost’ across the internal resistance

what will happen to the reading on the voltmeter as the temperature of the thermistor

increases.

reading will increase

resistance (of thermistor) decreases (as temperature increases)

current in circuit increase (so pd across R1

increases)

The filament of the lamp in X breaks and the lamp no longer conducts. It is observed that

the voltmeter reading decreases and lamp Y glows more brightly.

(i) Explain without calculation why the voltmeter reading decreases

(ii) Explain without calculation why the lamp Y glows more brightly.

(circuit) resistance increases

current is lower (reducing voltmeter reading

pd across/current through Y increases

hence power greater or temperature of lamp

increases

A student wishes to investigate how the resistance of a thermistor changes with

temperature.

Describe the procedure the student would follow in order to obtain accurate and

reliable measurements of the resistance of the thermistor at different temperatures

Thermistor connected with voltmeter and ammeter

Thermistor heated in beaker of water and thermometer used to measure temperature at small regular intervals

Resistance calculated at various temperatures

Repeat experiment

Plot graph of resistance agaisnt temperature

State and explain the effect on the voltmeter reading if the internal resistance of the battery

in the circuit was not negligible.

less current now flows/terminal pd lower

hence voltage across resistor will decrease

Some materials exhibit the property of superconductivity under certain conditions.

• State what is meant by superconductivity.

• Explain the required conditions for the material to become superconducting

superconductivity means a material has zero resistivity/resistance (1)

resistivity decreases with temperature or idea of cooling (1)

becomes superconducting when you reach the critical/certain/

transition temperature

State and explain what happens to the resistance of the cable when the embedded

filaments of wire are made superconducting.

the resistance decreases (to zero) (1) copper still has resistance (1) but this is in parallel with filaments (which have zero resistance) (1) hence total resistance is zero (1) current goes through filaments

describe how you

would use a voltmeter, ammeter and other standard laboratory equipment to determine a

value for the resistivity

length with a ruler

• thickness/diameter with vernier callipers/micrometer

• measure voltage

• measure current

• calculate resistance

• use of graph, eg I-V or resistance against length

• use of diameter to calculate cross-sectional area

• mention of precision, eg vernier callipers or full scale readings

for V and I

`Describe the procedure the student would follow in order to obtain an I-V curve for

the semiconductor diode.

connect circuit up (1)

measure current (I) and pd/voltage (V) (1)

vary resistance/voltage (1)

obtain a range of results (1)

reverse connections to power supply (and repeat) (1)

plot a graph (of pd against current) (1)

mention of significance of 0.6V or disconnect between readings

or change range on meters when doing reverse bias (1)

The figure below shows the I – V characteristic for a filament lamp. Explain the shape of

the characteristic.

at low V, I increases proportionally (or Ohm’s law obeyed) (1)

(as V increases) greater I heats filament/wire

(or temp of filament/wire increases) (1)

resistance increases (1)

rate of increase of I with V decreases [or ref. to gradient = 1/R] (1)

reference to same form of the curve in negative quadrant (

Internal resistance of a source of electricity is due to

the opposition to the flow of charge through the source causing produced electrical energy to dissipate when charge flows through it

Potential difference is also known as

Voltage

Emf is

Electrical energy produced per unit charge passing through a point

Internal resistance is

Work done by the battery

Opposition to flow of charge through the source causing emf to dissipate (produced electrical energy to dissipate)

Resistance is

Ratio of potential difference to current

Current is

The rate of flow of charge

Voltage is

Work done per unit charge

Potential divider is

combination of resistors in series connected across a voltage source to produce a required a potential differnce