37 experiments found
Clear filtersFor a resistor at constant temperature, current is proportional to voltage: V = I × R
Measure the current through resistors and a bulb as you add cells, plot current against voltage, and find the resistance.
Electrical energy is measured in units: 1 unit = 1 kilowatt-hour (kWh) = power in kW × hours of use; the bill counts the units, charged at a higher rate for each slab, so efficient appliances and less use save money
Choose the bulbs, the TV time, the way you cook and whether you have a fridge, then read the meter and work out the month's units and the bill.
The potential energy of water high on a hill becomes kinetic energy in the pipe, then electrical energy in the turbine and generator; the power is about 9.8 × flow (m³/s) × fall (m) kW, times the efficiency, and the rest is lost as heat and sound, never destroyed
Run a village micro-hydro plant. Change how far the water falls, how much flows, and the turbine, and see how many homes you can light, and how much energy is lost on the way.
Momentum is mass × velocity; in a collision the total momentum before equals the total after, so trolleys that stick move at (m₁v₁ + m₂v₂) ÷ (m₁ + m₂), and some kinetic energy turns into sound and heat
Send trolley A into trolley B so they stick together. Change their masses and velocities, and check that the total momentum is the same before and after, while some kinetic energy is lost.
When one object pushes on another, the second pushes back with an equal and opposite force
Launch a balloon rocket along a string. Change the amount of air, the nozzle and the load, and compare the action and reaction forces.
Objects keep moving at the same velocity unless a force acts on them (Newton's first law)
Run a trolley carrying a block into a stopper and watch the block keep going. Change the speed, the block's mass and whether it is strapped down.
Distance is the length of the path travelled; displacement is the straight-line distance from the start to the finish, in a direction; after a whole lap the distance is 400 m but the displacement is zero
Run part of a lap, a whole lap and more round a 400 m track. Compare the distance you ran with your displacement, and your average speed with your average velocity.
Heat can travel as infrared radiation; dull black surfaces absorb and emit it well, shiny surfaces poorly
Warm a matt black can and a shiny silver can of water in the sun, then let hot water cool in each, and compare how well they absorb and give off heat.
Concave mirrors can form real or magnified virtual images; convex mirrors always form small, upright, virtual images with a wide view
Move an object in front of concave, convex and plane mirrors, trace the reflected rays, and describe each image.
A magnetic field runs from N to S around a magnet; a current in a wire makes circular field lines whose direction follows the right-hand grip rule
Map the field around a bar magnet with a plotting compass, then pass a current through a straight wire and see the circular field it makes.
The right instrument depends on the quantity, its size and the accuracy needed: a tape for metres, a vernier caliper for tenths of a millimetre, a screw gauge for hundredths
Decide which instrument to use for each measurement of length, mass, time and volume, thinking about the size of the quantity and the accuracy needed.
Density = mass ÷ volume, a property that helps identify a material
Weigh blocks and a stone on a balance, find their volume by measuring their sides or by displacement, and calculate their density.
The slope of a distance-time graph is velocity; the slope of a velocity-time graph is acceleration
Follow a car moving steadily, speeding up, slowing down or parked, record its position and speed each second, and plot motion graphs.
Pressure in a liquid increases with depth and with the liquid's density (p = ρgh)
Open holes at different depths in a tall container of water and oil, and compare how hard the liquid squirts out.
Scientists repeat a measurement, leave out an odd result and take the average; the ruler falls d metres in t = √(2d ÷ g) seconds, so the catch distance gives your reaction time
A friend drops a ruler and you catch it. Catch it five times, find your average reaction time, spot the odd catch, then try again while looking at a phone.
A windlass is a wheel and axle: its velocity ratio is the handle's radius ÷ the axle's radius; friction makes the mechanical advantage smaller than the VR, and efficiency = MA ÷ VR × 100%
Lift a bucket of water from a well by turning a crank. Change the length of the handle, the thickness of the axle, the bucket and whether the axle is oiled, and work out the VR, MA and efficiency.
Stars are huge balls of hot gas that make their own light; planets and moons shine by reflected sunlight; a constellation is a pattern of stars seen from the Earth, and a galaxy is a vast collection of billions of stars
Look at thirteen things in the sky, from the Sun and Dhruvatara to Venus, Saptarishi and the Milky Way. Find out whether each makes its own light and how far away it is, then sort them.
Gravitational potential energy (mgh) turns into kinetic energy (½mv²); some becomes heat when there is friction
Release a trolley from different heights on smooth and rough ramps and watch its potential energy turn into kinetic energy.
Work done = force × distance moved in the direction of the force; power = work ÷ time
Lift boxes of different masses onto shelves at different heights, calculate the work done, and compare the power of quick and slow lifts.
A battery's capacity in mAh is the charge it holds: hours to charge = capacity ÷ charger current (plus about 20% losses), and energy in Wh = capacity (Ah) × voltage
Pick a phone battery of 3000, 4000 or 5000 mAh and a 1 A, 2 A or 3 A charger. Work out how long it takes to charge from empty, how much energy it stores and how many hours the screen can stay on.
An LED takes a fixed forward voltage (about 2 V for red, 3.2 V for blue and white); a resistor in series takes the rest, so by Ohm's law R = (supply − LED voltage) ÷ current
Light a red, green, blue or white LED from a 5 V, 9 V or 12 V supply. Work out the resistor that lets just the right current flow, and how much power the resistor turns into heat.
Energy = power × time: a 0.4 kW panel in 5 hours of sun gives about 0.4 × 5 × 0.8 = 1.6 kWh, after the losses of heat, dust and wiring
Put 2 to 10 solar panels on a roof and choose how many sunny hours a day it gets. See how many kilowatt-hours they make a day and a month, how many LED bulbs that lights for an evening, and how much CO₂ it saves against coal power.
Magnitude is a logarithmic scale: each step of 1 means 10 times the shaking on a seismograph and about 32 times (10^1.5) the energy
Pick two earthquakes by their magnitude, from a small 3 to a great 8, and see how many times more the ground shakes and how many times more energy the bigger one lets out. Find why the 7.8 Gorkha earthquake of 2015 was so much worse than a 5.
Gravity pulls a satellite towards the Earth and keeps it moving in a circle; in an orbit of radius r its speed is v = √(GM ÷ r), so higher satellites move slower and take longer to go round
Put a satellite in a low orbit like the International Space Station, in a GPS orbit, or in a geostationary orbit used for TV. See how far it is from the Earth's centre, how fast gravity makes it go and how long one trip round the Earth takes.
Light travels at about 299 792 km every second, so the time it takes is distance ÷ speed: a fraction of a second from a satellite, 8 minutes from the Sun and more than 4 years from the nearest star
Send light to us from a GPS satellite, the Moon, the Sun, Mars and the nearest star, and work out how long each journey takes. Find out why we see the Sun as it was eight minutes ago.
Light travels in straight lines, so a stick's shadow is its height ÷ tan(the sun's height): the lower the sun, the longer the shadow, and with the sun straight overhead there is no shadow at all
Stand a stick in the sun and move the sun from low in the morning sky to straight overhead. See how long the shadow is each time, and why shadows are long in the morning and evening and shortest at noon.
A scale model divides every size and distance by the same number: if the Earth were a 22 cm football, the Moon would be a 6 cm ball 6.6 m away and the Sun a 24 m ball 2.6 km away, so space is mostly empty
Shrink the Earth to a football, a cricket ball or a marble, and shrink everything else by the same scale. See how big the Moon and the Sun would be, and how far away they would sit.
Force = pressure difference × area: a fan that lowers the pressure under a car presses it onto the road
Switch on a fan that sucks the air out from under a sports car, and see how a small pressure difference over the car's floor gives a huge downforce, more grip and faster cornering. Then blow air in to make the car float, and fire thrusters to see when thrust beats weight and the car flies.
A stretched band stores elastic potential energy E = ½kx²; on release it becomes kinetic energy ½mv², so v = √(2E ÷ m), and at 45° the pellet flies about v² ÷ g: twice the stretch gives four times the energy and twice the speed
Stretch a rubber band and fire a paper pellet. Change the band, how far you stretch it and the pellet's mass, and see the energy stored in the band, the launch speed and how far the pellet flies.
A falling object speeds up until air drag equals its weight; then it falls at a steady (terminal) speed v = √(2mg ÷ (ρCA)), so a bigger canopy falls slower
Hang a 20 g, 50 g or 100 g toy under a canopy of 0.1 to 1 m², and find how fast it falls once air drag balances its weight, and how long it takes to drop 10 m.
Gear ratio = front teeth ÷ rear teeth: the back wheel turns that many times for each turn of the pedals, so a big ratio goes far and fast but is hard to push, and a small ratio is easy to push but slow
Change the front and rear gears of a bicycle and see how far one turn of the pedals takes you, and how fast you go at the same pedalling. Find the easy gear for a hill and the fast gear for a flat road.
Radioactive atoms decay at random, but in each half-life half of those left decay: after n half-lives 100% × (½)ⁿ remain, whether the half-life is hours or thousands of years
Watch a crowd of radioactive atoms decay. Choose an isotope used in hospitals or for dating old bones, let half-lives go by, and see what share of the atoms is left and how long that took.
Light arrives almost at once but sound travels only about 343 m a second, so the distance to lightning in km is the seconds from flash to thunder × 0.343
Count the seconds between a lightning flash and its thunder, and work out how far away the lightning struck. Find out why a short gap means you must go indoors at once.
Loudness is measured in decibels (dB): every 10 dB is ten times the sound energy; the level falls about 6 dB each time the distance doubles, and above 85 dB the safe listening time halves for every 3 dB more
Stand near a whisper, a conversation, busy traffic, a pressure horn or a firecracker. See the sound level in decibels, how it drops as you move away, and how long your ears can safely take it each day.
Hot things cool faster the hotter they are than the room: T = room + (start − room) × e^(−kt); a lid and a vacuum flask cut heat loss, giving a much smaller k
Pour tea at 90 °C into an open cup, a covered cup or a vacuum flask in a 25 °C room, and follow its temperature for an hour to see which keeps it hot and why.
Stopping distance = thinking distance (speed × reaction time) + braking distance (v² ÷ 2μg): doubling the speed doubles the first but four-times the second
A child steps onto the road 30 m ahead. Change the vehicle's speed, the driver's reaction time and the road, and find the thinking distance, the braking distance and whether it stops in time.
A scale shows how hard it pushes up on you: N = m(g + a), so you seem heavier when the lift speeds up upward, lighter when it speeds up downward, and weightless in free fall
Stand on a bathroom scale in a lift. See what it reads as the lift starts going up, moves steadily, starts going down, and, if the cable snapped, falls freely.