Forces and Motion, Electricity Flashcards
What do lorries and caravans have?
Deflectors - to make them more streamlined and reduce drag.
What’s the most important factor in reducing drag in fluids?
Keep the shape of the object streamlined
What are the two types of friction and how to reduce?
Static friction (gripping surfaces) and sliding friction (sliding past each other surfaces)
Put a lubricant e.g. oil or grease between the surfaces.
What happens in a fluid if speed increases?
Friction always increases as the speed increases!
What are the different types of energy?
Thermal Kinetic Nuclear Sound Light Electrical Chemical potential Gravitational potential Elastic potential
T K N S L G C E E
Vector quantities?
Have size AND direction!
Eg force, velocity, displacement, acceleration, momentum
Scalar quantities?
only size.
Eg mass, temperature, time, length, distance
What’s air resistance?
The force that acts in the opposite direction to the motion of an object moving through the air.
(Depends on the shape and speed of the object)
What’s contact force?
The equal and opposite forces that two objects exert on each other when pushed together.
The contact force from the ground pushes up when standing still (against the weight pushing down)
What’s friction?
The force that opposes motion.
What’s gravitational force?
The force that pulls objects towards earth. (Weight) the earth pulls with a force of about 10 newtons in every kilogram of mass.
What is thinking distance and what affects it?
The distance the car travels while the driver is reacting (between the driver noticing the hazard and applying the brakes)
Affected by: speed, tiredness, drugs, alcohol, talking
What is braking distance and what affects it?
The distance the car travels from when the brakes are being applied until the vehicle stops
Affected by speed, icy roads, faulty brakes (road quality, weather conditions, tires, mass of car)
What is stopping distance?
Thinking distance + braking distance
What is Newton’s third law?
For every action there is an equal and opposite reaction.
Where does weight act from?
The weight of a body acts through its centre of gravity.
What is elastic behaviour?
The ability of a material to recover its original shape after the forces causing deformation have been removed.
What is the initial linear region of a force-extension graph associated with and what is that?
Hooke’s law!
Load is directly proportional to the extension of an object, provided you don’t surpass an object’s elastic potential
What’s electrostatic force?
Between two charged objects. (Direction depends on type of charge)
What’s tension?
In a rope or cable
What’s Newton’s first law?
When forces are balanced, an object is either stationary or moving at a constant speed.
What’s Newton’s second law?
When forces are unbalanced, an object will accelerate in that direction.
Explain safety in cars:
To reduce the size of the force acting during deceleration, must increase time taken to bring car to rest.
Eg crumple zones crumple on impact (increasing time taken for car to stop), seat belts stretch slightly (increasing time taken for wearer to stop- reduces force on chest), air bags slow person down gradually
Is gravitational field strength the same everywhere?
Gravitational field strength is how strongly something pulls an object towards it.
Earth has a higher gravitational field strenght than the moon: on earth we are pulled down so much that we can jump only for a few seconds; on the moon the time you can jump for is longer as it is pulling you back in with a weaker gravitational field. It’s also different on other planets…
The reason for this difference - earth has more mass than the moon and so has a bigger gravitational field strength. Planets with a larger mass than earth will have a higher gravitational field strength.
What does gravitational force do?
- Causes moons to orbit planets
- Causes planets to orbit the sun
- Causes artificial satellites to orbit the Earth
- Causes comets to orbit the sun
Describe the differences in the orbits of comets, moons and planets:
The orbits of planets and moons are slightly elliptical
The orbits of comets are elliptical (much longer orbital periods, come from outer edges of solar system)
Why does a comet travel faster when it’s nearer the sun/ planets nearer the sun move faster?
Because the increased pull of gravity makes it speed up.
Planets nearer the sun move faster and cover their orbit quicker because the closer they are, the stronger the force of attraction.
What’s the universe?
A large collection of billions of galaxies
What’s a galaxy?
A large collection of billions of stars
What’s our solar system?
Milky Way Galaxy
What are the hazards of electricity?
Frayed cables- live wire (which conducts electricity) exposed if insulation if damages/worn down. - electric shock
Long cables- high resistance- more at risk of over heating, likely to trip
Damaged plugs- live wire could be exposed - shock
Water around sockets - can conduct electricity at high voltages - fire, electrocution
Metal into sockets - electricity flows through metal, through person to the earth - electrocuting person
Safety features of a plug:
Fuses - contain a wire designed to melt when a specific current size is exceeded, cutting of the live supply. ‘Blows’ the fuse - needs to be replaced
Earthing - touches the outer casing and ensures outer casing is held at 0V/will not become live (provides a very low resistance path for current if the live wire was to touch the metal casing)
Insulation - covering live wire with insulating material (doesn’t conduct electricity)
Double insulation - outer casing- insulator eg plastic casing (as well as wire insulation) - no chance of getting a shock by touching casing
Circuit breakers - electrical safety device that break circuit by opening a switch when detect a surge in current. (Reset easily - flick a switch, faster and immediate- don’t have to wait for fuse to melt ➡️ more convenient than fuses)
A current in a resistor results in what?
A current in a resistor results in the electrical transfer of energy and an increase in temperature
This can be used in hair dryers or heaters.
Electrical ➡️ Thermal
How to work out to appropriate fuse to use?
P = I × V can be rearranged to find the current:
I = P ÷ V
Fuses come in standard ratings of 3 A, 5 A or 13 A.
The best fuse to use if 11.5A would be the 13A fuse. The 3A and 5A fuses would blow even when the fire was working normally.
What’s the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery?
Direct current flows in one direction only. It is supplied by cells and batteries
(It comes out as a straight line on an oscilloscope)
Alternating current changes from one direction to another rapidly. Mains electricity is alternating
(because the electricity has to go through transformers on the national grid which only work on ac current)
About series circuits:
• current depends on the applied voltage and the number and nature of other components (resistance)
V = I x R ➡️ I = V/R
- same current flows through all parts of the circuit
- resistance increases with more components
- voltage is shared between all components - useful for low powered things eg fairy lights
- if remove one component, whole circuit broken (everything is connected in one line)
About parallel circuits:
- each component is connected separately to the negative and positive of the supply
- if one component breaks- others will be unaffected and continue to be powered as the circuit is still functioning
- can switch different things on and off separately - practical to use for household electrical items eg lights in different rooms.
How are voltmeters and ammeters connectors?
Voltmeters always connected in parallel (in series circuits too)
Ammeters always connected in series! (even in parallel circuits)
How does current vary with voltage in wires, resistors, metal filament lamps, diodes? How to investigate experimentally?
This can be investigated using a voltmeter to measure the voltage. Use 5 different voltages with each component. use an ammeter to measure the current.
- metal filament lamp - as temp of metal filament increases, resistance increases (curve on an I-V graph)
- wire - current through a wire (at constant temp) is proportional to voltage (straight line through origin on I-V graph)
- different resistors - current through a resistor (at a constant temp) is proportional to voltage (but different resistors have different resistances, different straight lines all through origin)
- diode - current will only flow through a diode in one direction
Effect of changing resistance on the current?
Increasing the resistance will decrease the current. (adding more components or ones with higher resistance)
Decreasing the resistance will increase the current. (remove components or replace with lower resistance components)
Describe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperature:
An LDR is a light dependent resistor. (resistance changes with intensity of light): the BRIGHTER it is around, the LESS resistance; in DARKness, MORE resistance. Eg burglar detectors
Thermistors are temperature dependent resistors. In HOT conditions there will be LESS resistance where as in the COLD there is MORE resistance. Eg car engine temp sensors
How can lamps and LEDs can be used to indicate the presence of a current in a circuit?
For an LED to light up there must be a current in a circuit:
If a LED is in a circuit but not emitting light then there must be no current. If an LED is illuminated then it will have a current flowing through it.
What’s current?
The rate of flow of charge
What’s electric current in solid metallic conditions?
A flow of negatively charged electrons
What’s voltage?
The energy transferred per unit charge passed
A volt is a joule per coulomb
E = V x Q
What are common electrical conducting materials and insulators?
Conduct charge easily, current can flow through: metals eg copper, silver
Don’t conduct charge well, current can’t flow through: plastic, rubber
Describe experiments to investigate how insulating materials can be charged by friction:
- Get a polyethene rod and rip up some small pieces of paper, bring them together; the rod will have no effect on the paper
- rub the polyethene rod with a cloth (electrons will move from the cloth to the rod- now negatively electrically charged) now the rod will attract the pieces of paper.
OR
- gold-leaf electroscope: metal disc at top connected to metal rod that goes into the glass beaker. Metal rod is attracted to two thin piece of gold leaf
- when a rod with a known charge is brought to metal disc, electrons will either be attracted or repelled from the metal disc; inducing a charge in the metal disc, inducing a charge in the gold leaves.
- they will have the same charge and repel each other and rise. When rod taken away, they will discharge and fall.
explain how materials gain electrostatic charges:
When two insulating materials are rubbed together:
It electrons are lost, become positively charged. If electrons are gained, become negatively charged.
What attracts and what repels?
Like charges attract, unlike charges repel
Explain electrostatic phenomena
Van de Graaff generator made up of a rubber belt moving round plastic rollers underneath a metal dome.
An electrostatic charge is build up on the dome as belt goes round.
Stand on insulating chair and place hands on dome- body now has a charge
Human body conducts charge and like charges repel so the charges will spread out as much as possible throughout body-hairs repel each other and stand on end.
Explain dangers of electrostatic charges
- lightening- rain drops and ice bump together in storm clouds, knocking off electrons…leaving top of cloud positively charged and bottom of cloud negative. Creates a huge voltage and a big spark.
- fuel flows out pipe, static charges build up- spark. With flammable liquid around - explosion (solution- earth wire connected to tanker)
Explain inkjet printer
- tiny droplets of ink forced out of fine nozzle and due to friction make them electrically charged.
- Electrically charged plates deflect the path of the ink.
- voltage applied to plates, one +ve, one -ve. droplets are attracted to plate of opposite charge and repelled by plate of same charge.
- size and direction of voltage across each place on paper changes - each droplet deflected to hit different place on paper.
Explain photocopier
- photoconducting plate/drum negatively charged. Light from imagine is reflected onto plate.
- where plate absorbs light, discharges. leaving areas on the plate charged.
- toner is attracted to charge on drum
- paper heated and toner is fixed to paper
