electricity Flashcards
What builds static electricity?
Friction
How does static electricity work?
When two insulating materials are rubbed together, their electrons rub off
This charges one negative and one positive because one’s electrons have been transferred
What are the charges when you rub a polythene rod with a duster?
The duster becomes positive
The rod becomes negative
What happens when you rub an acetate rod with a duster?
The duster will become negatively charged
The rod will become positive
How is a positive charge formed?
The electrons transfer to something else
How does static electricity cause things to be attracted to the object?
The object would be charged so positive ions would be attracted to negative objects
Vice versa
How does a spark occur?
If the charge on an isolated object gets too great it will get big enough to jump across the gap to the earth (as it is the opposite charge)
Why do objects attract dust?
They become charged and attract the dust as they have the opposite charge
Why do doors give you an electric shock?
If you walk on a nylon carpet, charge builds up inside you which flows into the door as it is a conductor
How can static electricity be a problem for fuel tanks?
If a charge builds up big enough to create a spark, it could ignite the fuel
How does lightning form?
Rain drops and ice bump together in the cloud, scraping electrons and making the bottom of the cloud negative and the top positive
The negative is attracted to the ground so the electrons are ripped from each other in the attraction, creating a spark
How can sparks be prevented?
Connecting a charred object to the ground (earthing) so the charges travel into the ground and don’t build up
Anti-static sprays can make the surface of a charged object conductive and provides a path for the charges to move away
How do printers work with static electricity?
The ink is negatively charged and all the places the ink needs to be are positively charged in the printer
The ink is then attracted onto the paper and heated
How do dust precipitators work with static?
When smoke reaches the chimney, it passes through a wire grid that’s negatively charged
The particles become negative
The charged smoke particles are attracted to two metal plates on the side that are positively charged
The particles stick and form big particles
These can then be knocked off and can be removed
How do defibrillators use static electricity?
The paddles are rubbed together to create a charge
The doctor holds the insulated handles and shocks the patient’s heart
The electricity goes from the paddles into the heart, potentially restarting it
What is current?
The flow of electrons round the circuit
Will only flow if there is a voltage
What is voltage?
The driving force that pushes everything round (electrical pressure)
What is resistance?
Anything that slows the flow of electrons down
What does the ammeter measure and how is it placed?
Measures current
Placed in series
What does the voltmeter measure and how is it placed?
Measures voltage
Placed in parallel around the component under test
What does a variable resistor do?
It’s a resistor that can be changed to alter the resistance
What is a diode?
A device made from a semiconductor (silicon)
It only lets current flow freely in one direction (there’s very high resistance in the other direction)
Is the UK mains ac or dc?
AC - alternating current so it can quickly changing direction
Are batteries AC or DC?
DC - direct current meaning it goes one way
What does AC look like?
A regularly repeating wave
What does an Oscilloscope show?
The voltage by time of an alternating current
What does a direct current look like on an Oscilloscope?
A straight line as the voltage stays the same
What happens if you disconnect something in a series circuit?
The circuit is broken and all components stop
If a battery is 3V and one light takes up 1V what does the other light take in a series circuit?
2V as all components share the voltage and add up to the source voltage
Does current change depending on the components in series?
No, it always stays the same
How to calculate the total resistance of a series circuit if you have three resistors?
Add up all the resistors - this will be total resistance
What is the voltage of a series circuit if you have two cells of 3V?
6V
What happens if you disconnect a component in a parallel circuit?
Only that component is disconnected
How is voltage shared in a parallel circuit?
All components have the same voltage
V = V2 = V3
How is current shared in parallel circuits?
It is split between them all - adding up to the source
How do you work out the total resistance of a parallel circuit?
R = V/I
Resistance is always less than the branch with the smallest resistance
What are safety features of a plug?
The cable grip
No bare wires
Fuse
Earth wire
Rubber wires to insulate
Why are the metal parts copper or brass on a plug?
They’re good conductors
What are the colours of the wires in a plug?
Live wire - brown
Neutral wire - blue
Earth - green/yellow
What is the voltage of the live wire?
230V
What is the voltage of the neutral wire?
0V
How does an earth wire work?
If a wire touches the metal case the current flows through the earth wire instead, as it’s easier and flows into the earth instead of the case
This then normally triggers a large surge of electricity up the live wire, blowing the fuse
How does a fuse work?
If the current exceeds a certain point, the fuse melts, breaking the circuit
Why do plastic plugs not need to be earthed?
Because there’s no metal for the wire to touch, as plastic is insulated
What is double insulation?
When a plug is plastic and doesn’t need an earth wire
What are the energy changes in lighting a light from a battery
The battery supplies electricity
Energy goes to the light and changes to light and thermal energy
What is always produced when electricity travels through a resistor?
Thermal energy
What is another way of saying 100 Watts?
100 joules/second
If a kettle is 300 W and has been left on for 10s what is the energy transferred?
3,000 joules or 3kJ
How do you know what fuse to use?
One that’s closest to the current the device is at but the next highest
What happens to charge when it goes round the circuit and meets with the components?
It starts off with lots of energy from the power source
It then loses some of this energy whenever it meets a component
How does static electricity form?
Static electricity is caused by friction, and the electrons are transferred to a different material, giving it a positive charge and the other material a negative charge
What does ‘redistribution’ mean in terms of particles?
When a negatively charged balloon touches a wall, the particles in the wall will redistribute as the electrons are repelled by the balloon. The balloon will stick as positive and negative attracts
How does a spark get created?
If there is such a great force of attraction between positive and negative that the electrons are ripped out of the air molecules, creating a current and consequently a spark
How are positively charged materials made?
From the loss of electrons
