39 experiments found
Clear filtersIn series one current flows through everything and resistances add; in parallel each branch gets the full voltage and the total resistance falls
Wire two or three bulbs in series and in parallel, compare their brightness and the current, take one bulb out, and calculate the total resistance.
Devices transform energy from one form to another; the main useful output tells us the transformation
Find out what energy goes into and comes out of everyday devices, from an electric kettle to a micro-hydro generator, and sort them by their main energy transformation.
A spring stretches in proportion to the force on it, so its stretch can measure force
Hang standard masses on a spring to see how it stretches, then use it as a spring balance to weigh an apple, a stone and a book.
A force can change an object's speed, direction or shape
Give a rolling ball a push forwards, backwards or sideways, and see how a force can speed it up, slow it down, change its direction or change its shape.
Wait until the liquid column stops moving, then read the top of the column at eye level
Measure the temperature of ice water, tap water, warm water and boiling water, waiting for the thermometer to settle before you read it.
Two plane mirrors at an angle θ form 360° ÷ θ − 1 images
Stand a toy in front of one mirror, then between two mirrors hinged together, and count the images as you open and close the angle between them.
A convex lens forms real, inverted images or virtual, magnified ones depending on where the object is; a concave lens always forms small, upright, virtual images
Move an object along an optical bench in front of convex and concave lenses, trace the rays, find the image, and measure the focal length.
Light bends towards the normal when it slows down entering a denser medium, and away from it when it leaves
Shine a ray into a glass block and a tank of water at different angles, measure how much it bends, and find the refractive index.
A coil carrying a current is a magnet; more turns, more current and an iron core make it stronger
Wind a coil around an iron nail, pass a current through it, and count how many paper clips it can lift as you change the turns, the current and the core.
Acceleration is how fast velocity changes: v = u + at, so a steadily accelerating bus gains the same speed every second and covers more distance each second
A bus leaves the stop and speeds up steadily. Read its speedometer and the distance it has gone every 5 seconds, then make it speed up harder or start already moving.
The air around us presses on everything; its pressure falls as you go higher
Travel from Biratnagar to the top of Everest with a barometer, a sealed snack bag and a glass of water held up by a card, and see how air pressure changes with height.
A liquid pushes up on an object with a force (upthrust) equal to the weight of liquid it displaces; an object floats if it is less dense than the liquid
Lower objects on a spring balance into fresh water, salt water and oil. Measure the upthrust and find out why some float and some sink.
Pressure applied to an enclosed liquid is passed on equally in all directions, so a big piston gets a big force
Push the small piston of a hydraulic lift and see how the pressure is passed through the oil to lift a car on the big piston.
Pressure = force ÷ area: a smaller area or a bigger force gives a bigger pressure
Stand bricks on a foam block on their flat side, their long side and their end, and see how the contact area and the force change the pressure.
Shaping a material into a hollow boat lets it push aside more water, giving enough upthrust to float
Drop a ball of plasticine into water, then shape the same plasticine into boats and load them with marbles until they sink.
In a fair test we change one variable at a time; for a pendulum only the length of the string changes the time of a swing, not the mass or a small swing
Time ten swings of a pendulum. Change the length of the string, the mass of the bob and how far you pull it back, one at a time, and find out which one really changes the swing.
Levers are grouped by what is in the middle: the fulcrum (class 1), the load (class 2) or the effort (class 3); mechanical advantage = load ÷ effort = effort arm ÷ load arm
Use a crowbar, scissors, a wheelbarrow, a nutcracker, tongs and a broom. Find the fulcrum, the load and the effort on each, see which is in the middle, and work out how much each one helps.
Rubbing two materials gives them opposite charges; like charges repel, unlike charges attract, a charged object pulls uncharged bits, and earthing lets the charge flow away
Rub balloons, combs, glass rods and plastic refills, bring them near each other, and sort twelve pairs into those that attract, those that repel and those where nothing happens.
An echo is sound reflected back; the speed of sound = 2 × distance ÷ echo time, about 340 m/s in air, and a little faster in warmer air
Clap in front of a cliff and time the echo. Stand nearer or farther, try a cold and a hot day, and work out how fast sound travels.
Sound needs a medium; it travels fastest in solids, slower in liquids and slowest in gases, and not at all in a vacuum
Send a sound along 1 km of air, water, wood and steel, time how long it takes, and see what happens in a vacuum.
The planets go round the Sun at different distances: the farther a planet, the longer its year; each planet's gravity is different, so the same mass weighs differently
Visit the eight planets. Find how long a year lasts on each, how long sunlight takes to get there, and what you would weigh, from little Mercury to giant Jupiter.
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.