SECTION 1 - FORCES AND MOTION
Velocity and Acceleration -
Speed - how fast you're going (m/s) Average speed = distance moved / time taken
Acceleration - how quickly velocity is changing (eg: change in speed, change in direction) (m/s squared)
Acceleration = change in velocity / time taken
Distance-time and velocity-time graphs -
Distance-time graphs - - Gradient = speed
- Flat sections = stopped
- Steeper = faster
- Downhill = other direction towards starting point
- Curves = acceleration/deceleration
- Steepening curve (increasing gradient) = speeding up
- Levelling off curve (decreasing gradient) = slowing down
Speed = gradient = vertical/horizontal
= 2/1 = 2m/s
Velocity-time graphs -
- Gradient = acceleration
- Flat sections = steady speed
- Steeper = greater acceleration/deceleration
- Uphill (/) = acceleration
- Downhill (\) = deceleration
- Area under any section of graph = distance travelled in that time interval
- Curve = changing acceleration
Acceleration = gradient = vertical/horizontal
= 2/1 = 2 m/s^2 (squared)
Speed = value from velocity axis
Distance travelled = area under graph
Mass, weight and gravity -
Gravity - force of attraction between all masses
1. Surface of a planet - all things accelerate towards ground (eg: at 10m/s^2 on Earth)
2. Gives everything a weight
3. Keeps planets, moons and satellites in orbit
Mass -
- Amount of 'stuff' in an object
- Same value anywhere in universe
- Same mass on Earth or moon/anywhere
- Measured in kilograms (kg)
- Caused by pull of gravity
- The force of gravity pulling object towards centre of Earth
- Changes in different places depending on gravitational field strength (eg: 10N on Earth, 1.6N on moon)
- Measured in newtons (N)
Weight (N) = mass (kg) x gravitational field strength (N/kg)
Forces and friction -
- Gravity/weight - straight downwards
- Reaction force - from surface, usually straight upwards
- Electrostatic force - between 2 charged objects (direction depends on type of charge, eg: repel, attract etc)
- Thrust/push/pull - eg: due to engine or rocket speeding something up
- Drag/air resistance/friction - slowing object down
- Lift - eg: due to aeroplane wing
- Tension - eg: in rope or cable
- Opposes motion (unless in space)
- To stay at a a steady speed, there needs to be a driving force to counteract friction
2. Sliding friction - between solid surfaces that are sliding past eachother (lubricant between surfaces eg: oil/grease)
3. Resistance/drag - can be reduced by making object streamlined, lowers top speed of an object (eg: parachute)
Motion experiment -
2. Mark start line
3. Measure distance between each light gate
4. Allow car to roll down from starting point (ensure light gates are linked to computer data software)
5. Repeat for average time taken for the car to reach each light gate
6. Find average speed (average time taken between gates / distance between gates)
Possible alterations -
- Mass - put weights on car to see effect
- Friction - place different materials on ramp (eg: carpet)
- Acceleration (due to gravity) - move start line higher or lower, will affect average speed between gates
- Speed - change angle of ramp
- Size/shape/weight of car - use different cars (eg: streamlined)
The 3 laws of motion -
1 - Balanced forces mean no change in velocity
- If the forces are balanced, on a non-moving object, it will just stay still
- If the object is already moving, it will continue at the same velocity
2 - A resultant force means acceleration
- Unbalanced force = object accelerates in that direction
- the bigger the force, the greater the acceleration/deceleration- the bigger the mass, the smaller the acceleration
Unbalanced force (resultant force) -
Resultant force = mass x acceleration (F = ma)
Acceleration = resultant force/mass (a = F/m)
3 - Reaction forces
- If object A exerts a force on object B, object B exerts the exact opposite force on object
- movement depends on mass etc.
- eg: swimming
Stopping distances -
Stopping distance - time between first spotting a hazard and the car coming to a complete stop Thinking distance - time between driving noticing hazard and applying brakes
Affected by:
- How fast you're going
- How aware the driver is - eg: tiredness, drugs, alcohol, age, inexperience
Braking distance - distance the car travels during deceleration while brakes are being applied
Affected by:
- How fast you're going
- Mass of vehicle (the larger the mass, the longer it takes to stop)
- Quality of brakes
- Grip - road surface, weather conditions, tyres
2 - ELECTRICITY
Safe plugs - Right coloured wire connected to each pin
- Wires screwed in
- No bare wires
- Cable grip fastened over outer layer - holds all wires in place
- Metal parts = copper or brass (very good conductors)
- Case/cable grip/cable insulation = rubber or plastic (good insulators, flexible)
- Live wire - alternates between high +ve and -ve voltage (about 230V)
- Neutral wire - 0V
- Electricity flows through live wire and out neutral wire (normally)
- Earth wire and fuse/circuit breaker - safety, work together
- Fault in live wire
- Touches metal case of an appliance - case is earthed
- Flows through case and down earth wire (not someones hand etc.)
- Surge in current melts fuse - cuts off live supply
- Isolates whole appliance - can't get electric shock from case
- No risk of fire - no heating from effect of even larger current
- Circuit breakers - electrical safety device, protect circuit from damage if too much current flows
- Detect surge in current - break circuit by opening a switch
- Can be reset easily (unlike fuses - need to be replaced)
- Faster then fuse - break current as soon as there is a current surge, don't wait to be melted (safer)
- Work for even small current changes
- eg: RCCB (residual current circuit breaker)
- Check plug isn't damaged - live parts could be exposed
- Check cable isn't frayed - live parts of wires could be exposed
- Check cables aren't too long for appliance - trip hazard
- Check there's no water nearby - good conductor
- Don't put metal in plug socket - conductors
Resistors -
- Produce heat when electric current passes through them
- Energy transfer which heats the resistor
- Heat increases the resistors resistance - less current will flow/greater voltage needed to produce same current
- Can cause components in circuit to melt - breaks circuit
- Fuses use melting to protect circuits - melt and break the circuit if the current gets too high
- Toasters contain coil of wire with very high resistance = heat
- Current passes through coil - temperature increases - glows and gives off infrared (heat) radiation
- Electrical power - rate at which an appliance transfers energy
- An appliance with a high power rating transfers a lot of energy in a short period of time
- Energy comes from current flowing through it
- High power rating appliance - draws a large current from the supply
ELECTRICAL POWER = CURRENT X VOLTAGE
- Most appliances show their power rating and voltage rating
- Fuses should be rated near but slightly higher than the normal operating current
- Working out fuse needed -
1kW = 1000W
I = P/V
= 1000/230
= 4.3A
Slightly higher = 5 amp fuse
Appliances transfer electrical energy -
- Current flows through component, energy is transferred
- eg: lightbulb - electrical energy transferred into light energy and (waste) heat energy
- Energy transferred depends on the current through it, the voltage supplied and how lond (s) it is on for
Circuits
1 - Current
- the rate of flow of charge round the circuit
- will only flow through a component if there is a voltage across that component
- unit = amp, A
- driving force that pushes the current round
- 'electrical pressure'
- unit = volt, V
- anything in the circuit that slows down the flow
- if you add more components to the circuit, there will be a higher overall resistance
- units = ohm, Ω
- voltage is trying to push the current around the circuit
- resistance is opposing voltage
- relative sizes of voltage and resistance decide how big the current will be
INCREASE RESISTANCE = LESS CURRENT FLOWS
Standard test circuit -
- Basic circuit used for testing components and getting V-I graphs
- Component, ammeter and variable resistor all in series (can be put in any order in main circuit)
- Voltmeter only placed in parallel around component being tested
- Varying variable resistor alters current flowing - can take several pairs of readings from ammeter and voltmeter
- Plot current and voltage on V-I graph
- Measures current (amps) flowing through the component
- Must be placed in series
- Anywhere in main circuit, never in parallel
- Measures voltage (volts) across component
- Must be placed in parallel
- Around the component
- a.c. (alternating current - constantly changing direction) = mains
- d.c. (direct current - current keeps flowing in same direction) = battery
- UK - mains electricity = 230 volts
- Different components connected in a continuous line
- Can't control which components current flows through - flows through all or none
- If one component is removed/broken, the circuit is broken
- eg: fairy lights
- Same current flows through all parts of the circuit
- A1 = A2
- Size of current determined by total voltage of the cells and total resistance of circuit (I = V/R)
- Total resistance depends on the number of components and the type of components
- Each component is separately connected
- If one component is removed/disconnected, hardly affects others
- Diagram - each component in its own loop
- How most things are connected (eg: household electrics - lights)
- Ammeters always connected in series
- Voltmeters always connected in parallel with a component
- Emit light when a current flows through them in a forward direction
- Numbers on digital clocks, traffic lights, remote controls
- Don't have a filament that can burn out
- Indicate the presence of current in a circuit - often used in appliances to show when they are switched on
- Type of resistor that changes its resistance based on how much light falls on it
- Bright light - resistance falls
- Darkness - resistance is highest
- Electronic circuits (eg: burglar detectors)
- Temperature-dependant resistor
- Hot - resistance drops
- Cool - resistance increases
- Temperature detectors (eg: car engine temperature sensors)
VOLTAGE = CURRENT X RESISTANCE (V = I X R)
Voltage/current graphs -
1 - Metal filament lamp
- As the temperature of the metal filament increases, the resistance increases
- (x axis = V, y axis = I)
2 - Wire
- Current through a wire (at a constant temp.) is proportional to voltage
3 - Diode
- Current will only flow through a diode in one direction
4 - Different resistors
- The current through a resistor (at a constant temp.) is proportional to voltage
- Different resistors have different resistances - different gradients
Charge, voltage, energy change
- Current = rate of flow of electrical charge (A)
- In solid metal conductors (eg: copper wire) charge is carried by negatively charged electrons
- More charge passes around a circuit when bigger current flows
- (time must be in seconds - charge is measured in coulombs, C)
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