physics - electricity

what is electric current

the flow of electric charge round the circuit

current will ONLY flow…

around a complete (closed) circuit if there’s a potential difference

what is the unit for current

ampere, amps, A

what will the current be like in a single, closed loop?

the current will have the same value everywhere in the circuit

what is potential difference (or voltage)

the driving force that pushes the charge round

what is resistance

anything in the circuit which slows the flow down

what is the unit for resistance

ohms, Ω

what does the current flowing through a component depend on

the potential difference across it and the resistance of the component

how does the resistance of a component affect the current

the greater the resistance across a component, the smaller the current that flows through it

what is the size of the current also known as

the rate of flow of charge

what is the equation to find the charge

Charge = Current x Time,

what is the symbol equation to find the charge

Q = It

what is the unit for charge

coulombs (C)

what is the unit for charge

amperes (A)

what is the unit for time

seconds (s)

when will more charge pass around the circuit

when a bigger current flows

what is this

a cell

which line of the cell diagram is positive and negative

the long thin line is positive, the short thick one is negative

what is this

battery

what is this

an open switch

what is this

a closed switch

what is this

filament lamp/bulb

what is this

fuse

what is this

LED

what is this

resistor

what is this

variable resistor

what is this

ammeter

what is this

voltmeter

what is this

diode

what is this

LDR - light dependent resistor

what is this

thermistor

whats the formula to find potential difference

potential difference = current x resistance

whats the symbol equation to find potential difference

V = IR

whats the formula triangle for potential difference

whats the unit for potential difference

volts

whats the unit for current

amperes, amps, A

whats the unit for resistance

ohms, Ω

will the resistance of ohmic conductor change with current (e.g. wire/resistor)

no. at a constant temp, the current flowing through the ohmic conductor is directly proportional to the potential difference across it.

in the equation V=IR, which is constant

R (resistance)

which components will the resistance change for

filament lamps/bulbs or diodes

why does the resistance change in filament lamps/bulbs (2)

• when an electrical charge flows through a filament lamp, some energy is transferred to the thermal energy store of the filament (getting hotter)

• resistance increases with temperature, so when the current increases, the lamp heats up more and resistance increases

why does the resistance change in diodes (2)

• resistance depends on direction of the current

• will let current flow in one direction happily, but have a very high resistance if directions are reversed

what factors can affect the resistance of a circuit (2)

• are components in series or parallel?

• the length of wire used in the circuit

how to set up for practical: investigating effect of wire length on resistance

what is the ammeters role in this practical

measures the current (in amps) flowing through the test wire

whats important to remember when setting up the ammeter in an experiment

must always be placed in series with whatever you’re investigating

whats the role of the voltmeter in this practical

measures the potential difference across the test wire (in volts)

whats important to remember when setting up the voltmeter in an experiment

must always be placed in parallel around whatever you’re investigating (e.g. the ruler in this practical)

how to do the investigating resistance based on wire length practical (5)

1. attach a crocodile clip to the wire level with 0cm on the ruler

2. attach the 2nd crocodile clip to the wire (e.g. 10cm away from 1st clip). write down the length of wire between clips

3. close switch, then record current through the wire and the pd across it

4. open switch, move 2nd crocodile clip (e.g. another 10cm) along the wire. close switch again, record the new length, current and pd

5. repeat for different lengths

how to plot graph for this practical (3)

1. use measurements from practical to calculate resistance using R=V÷I

2. plot a graph against wire length and draw a line of best fit

3. your graph should be a straight line through the origin

what goes on the y axis of the graph

resistance

what goes on the x axis of the graph

length of wire

mistakes that could occur from this graph/experiment

graph doesn’t go through the origin (it could be because your 1st clip wasn’t attached exactly at 0cm so all your length readings could be off) this is called a systematic error.

hazards in this practical + 2 methods to prevent it

the wire could get hot, use a low pd to stop it getting too hot + turn off circuit between readings to allow it to cool off

what is an I-V Characteristic

a graph that shows how the current (I) flowing through a component changes as the potential difference (V) across it increases

experiment to find a component’s I-V characteristic (5)

1. set up this test circuit

2. begin to vary the variable resistor- this alters the current flowing through the circuit and the pd across the component

3. take several pairs of readings from the ammeter + voltmeter to see how the pd across the component varies as the current changes.

4. repeat each reading twice more to get an average pd at each current

5. swap over the wires connected to the battery, so the direction of the current is reversed

6. plot a graph of current against voltage for the component

i-v characteristic for ohmic conductor (e.g. resistor at a constant temp)

• current is directly proportional to pd

• straight line

i-v characteristic for filament lamp/bulb

• as current increases, temp increases, so resistance increases

• less current can flow per unit pd as a result, so the graph gets shallower

i-v characteristic for diode

• current only flows through a diode in 1 direction

• diode has very high resistance in reverse direction

what is a LDR

a resistor dependent on the intensity of light

how will the resistance vary based on the brightness the LDR is in

in bright light → resistance falls

in darkness → resistance is highest

when are LDR’s useful in products- examples (3)

1. automatic night lights

2. outdoor lighting

3. burglar detectors

what is a thermistor

a resistor that is temperature dependent

how will the resistance vary based on the thermal conditions the thermistor is in

hot conditions → resistance drops

cool conditions → resistance increases

when are thermistors useful in products - examples (2)

1. car engine temperature sensors

2. electronic thermostats

what are sensing circuits used for

turning on or increasing the power to components depending on the conditions they’re in

what are sensing circuits used for

turning on or increasing the power to components depending on the conditions that they’re in

what is this sensing circuit going to control

a fan

what two components in the circuit will have the same potential difference and why

fixed resistor and fan because they’re connected in parallel

how is the potential difference of the power supply shared out between the thermistor and the loop (fixed resistor and fan)

according to resistances- the bigger a components resistance, the more potential difference it takes

what will happen to the circuit and fan as the room gets hotter

• resistance of the thermistor decreases

• takes a smaller share of p.d from the power supply

• p.d. across the loop (fixed resistor + fan) rises

• this makes the fan go faster

how else can you create a sensing circuit?

across a variable resistor instead of a fixed resistor

what is this circuit showing

a lamp

if you connect the bulb in parallel to an LDR…

the p.d. across the LDR and bulb will be high when it’s dark. the LDR’s resistance is high

how does p.d affect energy of a component

the greater the pd across a component, the more energy it gets

as the room got darker, the bulb..

would get brighter

how are series circuits formed

all components are connected in a line, end to end

what is the exception in series circuits

voltmeters, which will always be connected in parallel (they don’t count as part of your circuit)

what will happen if you remove or disconnect 1 component in a series circuit

the circuit is broken, and they all stop

are series circuits very handy

not really, they break easily, so very few things are connected in series

how do cell p.d.’s add up in a series circuit?

all the p.d’s add up e.g. 2 batteries of voltage 1.5V will supply a total of 3V

how will the total p.d. be shared?

it will be shared between the various components. you can see this through the equation: Vtotal = V1 + V2 + …

how is the current in series circuits

the current is the same everywhere (I1 = I2 = …)

how do you calculate the size of the current in series circuits

I = V ÷ R (current = p.d ÷ resistance)

how do you find resistance in series circuits

you find the sum of all the resistances (Rtotal = R1 + R2)

why is the resistance like this

because by adding a resistor in series, the resistors will have to share the total p.d.

how will the resistance be affected when a resistor is added

the total resistance of the circuit increases

the bigger a component’s resistance…

the bigger its share of the total p.d.

how are parallel circuits formed

each component is seperately connected to the power supply

what is the exception with parallel circuits

ammeters, which will always be connected in series

what will happen if you remove one component from a parallel circuit

it will hardly affect the other components

why are parallel circuits more useful then series circuits

they don’t break or stop working as easily as series circuits

in cars and household electrics why are parallel circuits useful

you have to be able to switch everything on and off separately

everyday circuits are…

a mixture of series and parallel circuits

how is the potential difference in parallel circuits

the same across all components. (V1 = V2 = V3 = …)

why is the p.d. the same in parallel circuits everywhere

all components get the full source pd, so the voltage is the same across all components. as a result, identical bulbs connected in parallel will all be at the same brightness

how is current in a parallel circuit

current is shared between branches. (Itotal = I1+I2+…)

how is resistance in a parallel circuit

adding a resistor will reduce the total resistance. (this is because the current has more than 1 direction to go in, so will increase the total current that flows around the circuit)

how to investigate the effect of adding resistors in series? (practical)

1. need 4 identical resistors

2. build the circuit using 1 resistor. (make note of the p.d. of the battery)

3. measure the current using the ammeter

   (use to calculate resistance using R=V÷I)

4. add another resistor in series with the 1st resistor

5. measure the new current through the circuit (use this + pd of battery to calculate the overall resistance of the circuit)

6. repeat steps 4 + 5 till all resistors are added

7. plot a graph of the no. of resistors against total resistance of the circuit

what will the graph look like for this/as more resistors are added?

• adding more resistors increases the total resistance

• directly proportional, goes through origin