Velocity and Calculating Speed (P4)
Velocity tells you an objects...
- direction of travel
The direction can also be positive or negative. The idea is used when describing distance;
- changes in distance in one direction are positive
- in the opposite direction they're negative.
To calculate speed, you need to know...
- the distance travelled
- the time it took to travel
The formula is...
Speed = distance travelled/time
The speed of an object at a particular point in time is called the instantaneous speed.
Distance-Time and Velocity-TIme Graphs (P4)
The slope, or gradient of a distance-time graph is a measure of the distance travelled over a certain amount of time.
A straight line shows that there is no movement.
A straight diagonal line shows there is movement at a constant speed.
The gradient of a velocity-time graph shows how quickly an object is increasing in speed.
A straight line shows that there is movement at a constant speed - unless it is 0 m/s (or which ever unit), then the object is stationary.
An upwards diagonal line shows acceleration, downwards shows decelleration.
A force occurs when two objects interact. The forces in an interaction pair are...
- equal in size
- opposite in direction.
When an object is resting on a surface...
- the object is pulled down by gravity
- and the surfaces pushes up on the object with an equal force (reaction of the surface)
When two objects slide past one another, both objects experience a force which tries to stop them moving; friction.
If more than one force acts on an object, they will add up in they are acting in the same direction. Or they will subtract if they are acting in opposite directions. This is called the resultant force.
Momentum is a measure of the motion of an object.
The formula is... momentum = mass x velocity
The extent of the change in momentum depends on...
- the size of the resultant force
- the length of time the force is acting on the object
The formula is... change in momentum = resultant force x time the force acts for
A moving object has kinetic energy. The amount depends on an object's...
Kinetic energy = 0.5 x mass x velocity²
Work is done by a force to move an object, resulting in transfer of energy, in this process the total amount of energy remains the same.
Change in Energy = Work done
Work done by a force = force x distance
When an object is lifted above the ground...
- work is done by the lifting force
- the object has potential work to do when it falls.
Change in gravitational potential energy = weight x veritcal height difference
Current and Potential Difference (P5)
Current = flow of charge measure in amperes
Direct current - flows in the same direction
Alternating current - constantly changes direction
Potential difference = voltage - measure in volts
Greater potential difference across a component - great current through the component
Adding batteries in series - increases voltage and current
Adding batter in parallel - potentional difference and current stays tge same and each battery supplies less current.
Components - resist flow of charge.
Electric current flows through component - component heats up.
Greater resistance in a circuit - smaller current.
Adding resistors in series - increases total resistance.
Adding resistors in parallel - reduces total resistance and increases current through the battery.
Current-potential difference graphs - current through resistor is directly proportional to voltage across resistor.
Thermistor - resistance depends on temperature.
LDR - resistance depends on light intensity.
Resistance = potential difference / current
Circuits and Electromagnetic Induction (P5)
In series circuits...
- current flowing through each component is the same
- potential difference across components add up to that across the battery
- potetional difference is largest across components with greatest resistance.
In parallel circuits...
- current flowing through each component depends on resistance
- current running to and from battery is equal to sum of current running through each parallel component
- current is smallest across components with greatest resistance
When a magnet is moved into a coil of wire, a voltage is induce. If ends of coil are connected, a current is induced. Moving the magnet out of the coil and moving the other pole of the magnet into the coil can induce the current in the opposite direction.
Power and Energy (P5)
Power = measure of the rate of energy transfer.
Power = potential difference x current
Energy - measured in joules. Domestic energy is measure in kilowatt hours as joules are very small amounts of energy.
Energy transferred = power x time
Efficiency = the proportion of energy that is usefully transferred by an appliance.
Efficiency (%) = energy usefully transferred / total energy supplied x 100
Waves = regular patterns of distrubance that transfer energy from one point to another. The two types are;
- Longitudal (particles vibrate to and fro - side to side)
- Transverse (particles vibrate up and down)
Amplitude = maximum distrubance caused by wave (height).
Wavelength = distance between corresponding points on two adjacent disturbances.
Frequency = number of waves produced in one second. measured in hertz.
Wave speed = frequency x wavelength
Behavoir of Waves (P6)
If a wave travels at a constant speed...
- increasing frequency, decreases wavelength
- decreasing frequency, increases wavelength.
If a wave has a constant frequency...
- increasing wave speed, increases wavelength
- decreasing wave speed, decreases wavelength.
Refraction = waves change direction when they pass between one medium to another.
Diffraction = waves spread out ferom the edges when they pass an obstacle or through a narrow gap.
Reflection = waves are reflected when a barrier is place in their path.
Light and Interference (P6)
Light is reflected when it strikes a surface.
Incident ray = light travelling towards a surface.
Reflected ray = light travelling away from a surface.
- changes direction when it passes through one medium to another
- continues straight on when it meets a boundary at a right angle.
Total internal reflection = light is reflected when the angle of refraction is greater than 90 degrees as it can't escape from the medium.
Constructive interference = two waves arrive in step and reinforce each other.
Destructive interference = two waves arrive out of step and cancel each other out.
Electromagnetic Radiation, Modulation and Signals
Electromagnetic radiations form the electromagnetic spectrum. Visible spectrum = the only part of the spectrum that can be seen.
- different frequencies
- different wave lengths
Photons = packets of energy. (intensity of a beam depends on the amount of energy carried by each photon)
Electromagnetic waves can travel through empty space but sound waves need a medium to travel through.
Modulation - makes a wave vary in amplitude or frequency.
Analogue signals = vary in exactly the way as information they carry (many different values).
Digital signals = uses two symbols (0 = off, 1 = on) which can be transmitted in a series of pulses.