Section 5 - Current and Circuits Flashcards
What is current?
The flow of electric charge around the circuit
What is the charge equation?
Charge = Current x Time Coulombs = Amps x Seconds
A battery passes a current of 0.25A through a light bulb over a period of 4 hours
How much charge does the battery transfer altogether?
Q = C x T Q = 0.25 x 4(60x60) Q = 3600C
Calculate how long it takes a current of 2.5A to transfer a charge of 120C
Q = C x T T = Q / C T = 120 / 2.5 T = 48s
What is potential difference?
The energy transferred per coulomb of charge
What is the energy transferred equation?
Energy = Charge x Potential Difference Joules = Coulombs x Volts
What is the potential difference equation?
Voltage = Current x Resistance Volts = Amps x Ohms
A 4 ohm resistor in a circuit has a potential difference of 6.0 V across it
What is the current through the resistor?
V = I x R I = V / R I = 6 / 4 I = 1.5 A
What does resisitance often cause?
An increase in temperature
Why does resisitance increase heat?
When the electrons collide with the ions in the lattice that make up the resistor the ions gain energy which causes them to heat
Give one exception where resistance decreases with heat?
Thermistor
What usually happens when the resistor gets too hot?
No current will be able to flow
A current flowing through a resistor transfer 360 J of energy when 75 C of charge are passed through it
Calculate the PD across the resistor
E = Q x V V = E / Q V = 360 / 75 V = 4.8 V
What does a standard test circuit include?
Ammeter
Energy Source (eg. Battery)
Voltmeter
Component
How do you use a standard test circuit?
Change the voltage and make readings from the ammeter and voltmeter and plot graph
When testing the standard test circuit, what must you stop from happening?
The circuit getting too hot as it will affect results
What does a diode do?
Acts as a one-way switch for the current
What should you see when investigating thermistors?
As you heat the thermistor, the current through the thermistor increases as the resistance decreases
What should you see when investigating LDR’s?
As the light gets brighter, the current increases as the resistance decreases
Describe a resistor IV graph?
Directly proportional
Describe a wire IV graph?
Directly proportional
Describe a filament IV graph?
S shape
Describe a diode IV graph?
Exponential in one direction (curver above x axis only)
What are linear components?
Components that have an IV graph that’s a straight line
What are non-linear components?
Components that have an IV graph that is curved
How do LDRs affect resistance?
In bright light the resistance falls, in darkness the resistance is highest
What are LDRs used in?
Automatic night lights
Outdoor lighting
Burgular detectors
How do thermistors affect resistance?
In hot conditions the resistance drops, in cool conditions the resistance goes up
What are thermistors used in?
Temperature detectors
Car Engine sensors
Thermostats
Describe a series circuit?
A circuit of components connected in a line, end to end
What is wrong with a series circuit?
IF you remove, break or disconnect one component the circuit will stop
What are the rules behind a series circuit?
The current is the same everywhere
The voltage is shared between the componenets
The total resistance increases as you increase resistors
Describe a parallel circuit?
A circuit of components where they are separately connect
What are always connected in series?
Ammeters
What are the rules behind a parallel circuit?
The voltage is the same across all components
The current is shared between he branches
The total resistance of the circuit decreases if you add a second resisitor in parallel
A filament lamp and a resistor are connected in series
A current of 0.5A flow through the lamp
State the current flowing through the resisitor?
0.5 A
What happens in series when you add resistors?
The total resistance increases
Why does the resistance of a series circuit increase when you add resistors?
The voltage is being shared across more components, so there will be lower voltage through each resistor therefore the current will also be lower
What happens in parallel when you add resistors?
The total resistance decreases
Why does the resistance of a parallel circuit decrease when you add resistors?
Another loop will be added therefore the current will have an extra way to go which means the total current will increase therefore there will be a decrease in the total resistance
A 12V cell is connected in series with a 2ohm resistor, a 3ohm resistor and a 7ohm resistor
Calculate the current going through
V = I x R I = V / R I = 12 / (3+2+7) I = 12 / 12
What happens when an electrical charge goes through a change in potential difference?
Energy is transferred
Why is energy transferred when a charge changes voltage?
Work is done against resistance
What is the energy transferred equation?
E = I x V x t Energy = Current x Voltage x Time
What do fuses do?
Protect the circuits by melting if the current gets too high
How can heating from a circuit be good?
In toasters/bulbs/heaters, where a extremely resistant coil has electricity passed through it which gives off infrared radiation
A laptop charger is connected to a 230 V source for an hour
A current of 8.0 A flows through it
Calculate the energy transferred by the laptop charger
E = I x V x t E = 8 x 230 x (60x60) E = 6,624,000 J
What does energy transferred depend on?
How long the appliance is in for and its power
What is the power equation?
Power = Energy / Time
Which transfers more energy in the same time?
A 500W microwave
A 750W microwave
The 750W microwave
What is the voltage of a charge?
The amount of energy each unit of charge transfers
What is the power equation (IV)
P = I x V Power = Current x Voltage
A 1 kW hair dryer is connected to a 230V supply
Find the fuse needed?
P = I x V P / V = I 1000 / 230 = I 4.3 A therefore a 5 A fuse is needed
What is the power equation (IR)?
P = I² x R Power = Current² x Resistance
Calculate the difference in the amount of energy transferred by a 250 W TV and 375 W TV when they are both use for two hours?
P = E / t 250 x (2x60x60) = E1 1,800,000 = E1 375 x (2x60x60) = E2 2,700,000 = E2 E2 - E1 = 900,000
What is AC?
Movement of charge in a constantly changing direction
What produces alternating currents?
Alternating voltages
What is the voltage inside the house?
230V
What is the frequency of the house AC?
50Hz
What supply direct current?
Batteries and cells
What creates DC?
Direct Voltage
What colour is the live wire?
Brown
What is the live wire
The wire that carries the voltage
What colour is the neutral?
Blue
What is the neutral wire?
The wire that completes the circuit
What is the voltage of the blue wire?
0V
When does the blue wire have a current?
When the appliance is working normally
What colour is the earth wire?
Green and yellow
What is the earth wire?
The wire for safety and protection
It carries the current away if something goes wrong
What is the voltage of the earth wire?
0V
What is the voltage of the live wire?
230V
Explain the difference between a.c and d.c electricity supplies?
In AC the movement of the charges constantly changes whilst in a direct current supply the movement of the charges is only in one direction
What do earthing and fuses prevent?
Electrical overloads
What can current surges lead to?
Melting, fire or electric shocks
What is a fault?
Where the live wire touches the metal case
What do circuit breakers do?
Instead of melting, a large current simply shuts it off
What are the advantages of circuit breakers?
Can be reused and much quicker than fuses
What are the disadvantages of circuit breakers?
Expensive
What doesn’t need an earth wire?
A double insulated appliance
Which wire are fuses connect to?
The Live wire
Name one linear component and one non-linear component?
Resistor and diode
What happens to the resistance of a thermistor as it gets hotter?
It decreases
How does the current in each component differ?
Based off the resistance in the components
How does potential difference vary in parallel?
Total amount shared inside each branch
How does adding resistors in parallel affect the resisitance?
It decreases it
Give two disadvantages of the heating effect?
Inefficient and can break circuit
Give two advantages of the heating effect?
Can be made into useful energy (bulb or heater) and can be used in fuse
What is a power rating
The power that flows through the componenet
Why are the three wires in a three-core cable coloured?
Avoid confusion which could lead to shocks
Give the potential difference for the three wires?
Live - 230 V
Neutral - 0 V
Earthed - 0 V
Explain why touching a live wire is dangerous?
The wire has 230 V which will flow through you to reach earth
Explain how a fuse works?
If the current gets too hot, the temperature will increase and melt the fuse, stopping the circuit from working
