SP10- Electricity And Circuits Flashcards
Does an atom have a charge
No - protons and electrons balance out
Why can metals carry a current
Eg sodium has one electron in the outer shell, that is only weakly attracted to the nucleus - these electrons can easily be removed so a metal wire has many “free” electrons
When a battery is attached to the wire the voltage pushes the free electrons around the circuit, electrons are negative so move toward the positive terminal of batttery
What’s the conventional direction of current in diagrams
Goes from the positive terminal to the negative terminal of the battery - left to right
Learn the circuit symbols!!
Page 141 textbook
What happens in series circuit
There is one route the current can take
Lamps can’t be switched on and off inducidually and if one lamp fails - it all switches off
What happens in a parallel circuit
There are junctions that allow the current to take different routes
Each lamp can be switched separately
What is current measured in and with
Amperes (amps) (A) and with an ammeter
How must an ammeter be connected in a circuit
In series
- to measure the current passing through a component or circuit
What happens to current in a circuit
The total amount of current stays the same on its journey around the circuit - battery is the same as the current arriving at the negative termianal
Current is conserved, - in parallel it splits but total is he same still
What do you need potential difference for
To “push” current around an electric circuit - it’s also called voltage
Electrons flow when there is a potential difference applied across a circuit
What must there be in order for a current to flow
The circuit must be closed and have a source of potential difference - such as a cell of battery
The electrons all move together when a current flows
The bigger the potential difference the ________ the current
The bigger the current
How does potential difference change in a circuit
In a parallel circuit the potential difference across each branch of the circuit is the same - when there’s more than one component in a branch - the potential difference across all the components add up to give the total voltage supplied by the cell or battery
What way is a voltmeter always connected in parallel with components
In parallel
What’s electric charge
Moving charged particles to form an electric current, electric charge is measured in coulombs (C) one coulomb is the charge that passes a point in a circuit when there is a current of 1 amp for 1 second
What is the electric current a rate of ?
Electric current is the rate of flow of charge
At any point in a circuit, the size of the current tells you how much charge flows paste that point each second
How can charge be calculated
Q = I x T
How does the charge have potential energy / how is energy transferred in a circuit
The cell/battery transfers energy to the charge, so the charge has potential to transfer energy to other components in the circuit - the charge has potential energy
What’s the potential difference of a cell
The amount of potential energy the cell transfers to each coulomb of charge flowing through it
There is a voltage of 1 V when there is 1 joule of energy to each coulomb
How is energy transferred calculated
E = Q x V
Why do some wires and components need a larger potential difference to produce a current through them than others
They have a large electrical resistance
What is resistance measured in
Ohms
How it resistance calculated
V = I x r
What happens and why to resisters connected in series
The total resistance of the circuit is increased BECUASE the pathway becomes harder for current to flow through
The PD from a cell is shared between the resistors but it may not be shared equally, greater potential difference across resistors with higher resistances
What happens and why to resistors in parallel
The total resistance of the circuit is less than the resistance of individual resistors - because there are now more paths for the current - the current splits
Potential differences are the same
What is a variable resistor used for
It’s used to change the current in the circuit , measurements of current and PD can be taken and resistance can be calculated
How does PD change across a fixed resistor
The current changes by the same percentage - the two variables are in direct proportion, the graph forms a straight line going through the origin - this happens BECUASE resistance stays the same
How does potential difference change in a diode
They have resistance that changes when pD changes
A diode has a low resistance if the potential difference is in one direction but a very high resistance if the potential difference is in the opposite direction which means current CAN FLOW ONE WAY ONLY IN A DIODE
How does pd changes in a filament lamp
Causes a current to flow through it, the current causes the filament to heat up and flow, greater the potential difference, more current and hotte and whiter it gets - resistance also increases. When pd changes, current doesn’t change at same percentage
How does resistance change with light intensity in LDR (light dependant resister)
Has a high resistance in the dark but the resistance gets smaller as the light intensity increases
What happens with thermistors and resistance and temperature
They have high resistances at low temperatures but as the temperature increases - resistance decreases
What happens when electrical work is done against resistance
The energy is transferred by heating
When a current passes through a resistor, energy is transferred from heat
When is the heating effect in electricity and what happens when it’s not
It’s useful in an electric heater or kettle
Not useful in a computer or wired BECUASE it means useful energy is spread out or dissipated
The surroundings gain thermal energy
What happens in a resistor
As electrons flow through the Lattice of vibrating ions, they collide with ions
The more collisions they make with ions, the more difficult it is for them to pass through - so higher the electrical resistance. When electrons collide with ions, they transfers energy to them
How can resistance in circuits be reduced
Using wires made from metals with low resistance / such as copper
Thicker wires have power resistance
Can also be decreased by cooling metals so that the lattice ions are not vibrating as much
How is energy transferred calculated
E = I x V x t
E = P x t
P = I x v
What is power
Energy transferred per second - often shown as power rating
1 W is 1 joule per second
How is power calculated
P = energy / time
What is power transfer in a component or appliance proportional to?
The potential difference across it and the current through it
Power = current x pd
Or power = i^2 x r
What source of electricity do large power appliances use
Mains electricity - in a power station, energy is transferred from a store of kinetic energy by electricity
Electricity is carried to our homes via cables known as the national grid
What is direct current
Where a cell has a positive and negative terminal and the movement of charge stays the same
What’s alternations current and why is it caused
When the direction of current keeps changing - mains electricity is produced using generators that rotate
The voltage also changes - causes it to decrease and peak,
Tell me about the uks mains
It’s an a.c supply
There are 50 cycles a second - frequency 50 Hz, voltage constantly changing but the average effect is the same as direct current voltage of 230 V
What’s 3kW in W
3000 W
What’s the earth wire in a plug
Connects to metal parts of appliance that is attached to a large metal spike or metal tuning that is pushed into the ground - it’s for safety and is at 0V If circuit is correct
Green/ yellow
What’s the neutral wire
The return path to the power stations
Voltage of 0V
Blue
What’s the fuse In a plug
A safety device usually marked with the current it can carry
What’s the live wire
Connects to the appliance to the generators of the power stations
Voltage is 230V
Where are switches connected in a circuit
In the live wire - when off there’s no current
What’s a fuse
A tube with a thin wire inside , the current passes through the wire and the wire gets hotter, if the wire exceeds a certain value , the wire melts and so breaks the circuit and stops the current
The fuse melts before wiring or parts of an appliance can overheat - once fault is fixed, a new fuse can be fitted
Why is a fuse necessary
If a faulty appliance draws too much current, it can cause overheating of the wiring in either the walls or in the appliance - this can cause fires
Metal parts can be at a very high voltage, could get an electric shock if you touched it - a current would flow through you to ground
If fsult causes live wire to touch a metal part - makes a very low resistance, large current causes heat and fire - current blows the fuse and cuts off the mains electricity supply
What are circuit breakers
Can be used as an alternative to fuses
They detect a change in the current and safely switch off the supply - once fault is fixed, it can switched on agasin, doesn’t need replacing
Works very quickly, a fuse takes some time to melt and won’t prevent you getting a shock eg if you touch a live wire
