10) Electricity & Circuits Flashcards
What is current?
Current is flow of electric charges, measured in amperes (A)
What is the value of current in a closed loop?
Current has the same value at any point in a closed loop
How is the flow of charge (C) calculated?
Flow of charge (C) = current (A) x time (s)
How is current calculated?
current = charge / time I = Q / t
What is the symbol for current?
I is the symbol for current
What is the symbol for charge?
Q is the symbol for charge
What is the symbol for time?
t is the symbol for time
What are electrical insulators?
Electrical insulators are a material that don’t allow electric currents to pass through them
What are examples of electrical insulators?
Plastic
Glass
What are electrical conductors?
Electrical conductors are a material which allow electric current to pass through them
What are examples of electrical conductors?
All metals
Why can’t electrical insulators conduct electricity?
Electrical insulators cannot conduct electricity as there are no freely moving electrons
What is potential difference?
aka. voltage, potential difference is the difference in the amount of energy that charge carriers have between two measured points in a circuit
What is potential difference measured in?
Volts
How is voltage calculated? (in relation to energy and charge)
voltage = energy / charge V = E / Q
What is the symbol for energy?
E is the symbol for energy
How is 1 volt calculated?
1 volt = 1 joule / coloumb
What do ammeters do?
Ammeters measure the flow of current passing through them, connected in series
What do voltmeters do?
Voltmeters measured the potential difference between two points in a circuit, connected in parallel
What is resistance?
Resistance is a measure of how difficult it is for current to flow through a component
How is resistance measured?
Ohms Ω
How is the size of current affected?
Higher resistance = decreased current
Higher potential difference = increased current
How is resistance calculated?
Resistance = Voltage / Current R = V / I
What is the symbol for voltage?
V is the symbol for voltage
How do wires affect resistance?
A longer, thinner wire has more resistance than a shorter, fatter wire
How is an ohmic conductor different to normal conductors?
The resistance through an Ohmic conductor remains constant as the current changes
What is Ohm’s law?
resistance = potential difference / current R = V / I
How is wire thickness calculated?
Cross sectional area = πr^2
Therefore doubling a wire’s thickness quadruples cross sectional area
What is resistance proptional to?
Resistance is proportional to the length (m) / cross sectional area (m^2)
What does a current-voltage graph look like for a resistor?
A straight line obeys Ohm’s law. Slope = 1 / resistance
What does a current-voltage graph look like for a filament lamp?
The gradient decreases at larger potential differences creating an S shape. This is due to increasing heat increasing resistance
How does charge flow?
Charge flows from the energy source through the circuit and its components
How is energy transfer (J) calculated?
amount of energy transferred (J)= charged moved (coulombs) x potential difference (V)
How is power calculated?
P = I x V
How is power calculated with current and resistance?
P = I^2 x R
How is power calculated with energy and time?
P = E / t
What happens when a current flows through a resistor?
When a current flows through a resistor, electrons collide with particles transferring energy in the form of heat
How is heat loss in a circuit reduced?
Using low resistance wires reduces heat loss in a circuit
What are advantages of the heating effect in circuits?
Generates useful heat from electrical current (kettle)
Emits light in filament bulbs
Causes fuses to melt
What are disadvantages of the heating effect in circuits?
Energy is lost in heating transmission wires
Some components work less efficiently when hot
What is the circuit diagram symbol for a lamp?
A circle with a cross in the middle
What is the circuit diagram symbol for a heater?
Four short rectangles stacked in a line
What is the circuit diagram symbol for a magntesising coil?
A long, thin rectangle
What is the circuit diagram symbol for an electric bell?
A semicircle with two straight lines originating from the flat side
What is the circuit diagram symbol for a fixed resistor?
A rectangle
What is the circuit diagram symbol for a variable resitor?
A rectangle with a diagonal arrow pointing to the top right corner
What is the circuit diagram symbol for a galvanometer?
A circle with an arrow pointing up inside
What is a galvanometer?
A very sensitive ammeter
What is a diode?
A component which only allows current to flow through it in one direction
What is a rectifier?
A component that turns an AC current into a DC
How do diodes act as recitifers?
They dioide block the AC when it changes direction
What are thermistors and LDRs?
Devices where resistance changes with temperature or light
What is a thermistor?
A component thats resistance changes with its temperature
What can a thermistor be used for?
Turning a heater off when a house reaches certain temperatures
What are LDRs?
Devices where resistance changes with light intensity
What does LDR stand for?
Light dependent resistor
What can an LDR be used for?
Switching lights on when it gets too dark (phone camera flash)
What effect will increased light intensity have on an LDR?
Decreased resistance
What effect will increased temperature have on a thermistor?
Decreased resistance
What is true of the current in a series circuit?
The current is the same at all points of the circuit
What is true of the voltage in a series circuit?
The voltage across each component is different
How is the total resistance calculated in a series circuit?
Adding up resistances of each individual resistor
What do resistors do?
Oppose the flow of electric current
Increase resistance in a circuit
How is a battery made?
Joining cells together
How is the total potential difference calculated in a series circuit?
Adding all energy sources’ individual voltages
In a series circuit what is the total voltage of energy sources equal to?
total voltage of energy sources = total potential difference across all components
How are components connected in a series circuit?
In a row
How are components connected in a parallel circuit?
On separate branches of wires to the energy source
How is the current through an energy source in a parallel circuit calculated?
Adding the currents of each separate branch
How does resistors in parallel affect the resistance?
Lower resistance
What are the two advantages of parallel circuits?
Each branch can be switched on and off separately
If one branch breaks the current still flows through the others
What is the equation for power?
power = potential difference x current
What do power stations create?
An electric current which is an energy carrier
What voltage electricity do power stations produce?
25,000 V
What do step-up transformers do?
Increase the potential difference to 400,000 V
What are the benefits of step-up transformers?
reduced current reduces heat loss across the country which increases the National Grid efficiency
What do step-down transformers do?
Reduce the voltage to a lower value to make electricty safer (230 V)
Why can live wires still be dangerous when turned off?
They may still hold some potential difference
