What is the physics of electric cars? The physics of electric cars comes down to three ideas: a battery stores energy in a chemical store, an electric motor uses the motor effect to turn the wheels, and regenerative braking turns kinetic energy back into stored energy. Everything else, from range to acceleration, follows from those three.
This matters on your own street. According to the Society of Motor Manufacturers and Traders (SMMT), about 28% of new cars registered in the UK in September 2026 were fully electric. If you are revising GCSE energy, forces or magnetism, the physics behind electric cars is one of the best real-life examples you can use. For charging, ownership and day-to-day driving, see our general guide on how electric cars work. This article sticks to the science.
Energy stores and transfers in an electric car
Physics describes a moving car using stores and transfers of energy. Energy is never created or destroyed. It is moved from one store to another, and some of it is always dissipated, which means it spreads out into the surroundings where it is hard to use again. You can read more about the different stores in our guide to the different kinds of energy and their uses.
In an electric car, the energy chain looks like this:
- Chemical store (the battery). The battery pack holds energy in a chemical store.
- Electrical work. When the driver presses the accelerator, a current flows. The current does work as it is transferred from the battery, through the power electronics, to the motor.
- Kinetic store (the moving car). The motor turns the wheels, and the car’s kinetic store increases.
- Thermal store of the surroundings. At every stage, some energy is dissipated as heat in the battery, the motor, the electronics, the brakes, the tyres and the air, plus a little as sound.
The useful part is the energy that ends up moving the car. The rest is wasted. Efficiency compares the two:
efficiency = useful output energy ÷ total input energy
We will put real numbers on this later, when we compare electric cars with petrol cars. For now, remember that a more efficient car wastes less of the energy it is given.
Inside the battery: cells, volts and kilowatt-hours
An EV battery pack is built from many smaller lithium-ion cells. When a cell discharges, lithium ions move through the electrolyte from the negative electrode (graphite) to the positive electrode, while electrons travel round the external circuit. That flow of electrons is the current that powers the car. Charging pushes the ions back the other way.
You do not need any more chemistry than that for most exams, but if you are curious about the metal itself, see the chemical properties of lithium.
Series and parallel: building a pack
A typical lithium-ion cell has a nominal voltage of 3.6 or 3.7 V. (Lithium iron phosphate cells are about 3.2 V.) One cell is far too weak for a car, so cells are connected in groups:
- In series, the voltages add up. About 110 cells of 3.6 V in series give 3.6 × 110 ≈ 396 V, close to the roughly 400 V used by many EVs.
- In parallel, the voltage stays the same but the capacity grows, so the pack can supply current for longer.
Real packs combine both arrangements to reach the voltage and capacity the designers want.
What does kWh mean?
Battery capacity is given in kilowatt-hours (kWh). A kilowatt-hour is the energy transferred by a 1 kW device running for one hour:
1 kWh = 1,000 W × 3,600 s = 3,600,000 J = 3.6 MJ
The link between energy, power and time is E = P × t. A 7 kW charger running for 1 hour transfers 7 kWh.
The average usable battery capacity across the electric cars listed on EV Database UK is 73.4 kWh, with the smallest at 21.3 kWh and the largest at 141 kWh. The same source gives an average real-world energy use of 303 Wh per mile, which is about 3.3 miles per kWh. So a 73.4 kWh battery gives roughly 73.4 × 3.3 ≈ 240 miles on average. Individual cars vary a lot, and speed, temperature and driving style all change the real figure.
A battery stores 60 kWh. How many joules is that? (2 marks)
1 kWh = 3.6 MJ, so 60 kWh = 60 × 3.6 MJ = 216 MJ, which is 216,000,000 J.
Why not simply use petrol’s energy?
Petrol stores a huge amount of energy for its mass: about 46.4 MJ per kilogram, or roughly 12.9 kWh per kilogram. Lithium-ion batteries store about 0.36 to 0.875 MJ per kilogram, which is roughly 100 to 243 Wh per kilogram. Mass for mass, petrol holds around 50 to 130 times more energy.
That sounds like a win for petrol, and it is one reason battery packs are heavy. But the figure ignores how much of that energy actually reaches the wheels. A petrol engine wastes most of its fuel’s energy as heat, and an electric drive does not. We compare the two properly in the efficiency section below.
How an electric car motor works: the motor effect
The heart of the electric car’s physics is the motor effect. When a wire carrying a current sits in a magnetic field, the wire and the magnet exert forces on each other. Flip the current, and the force flips too. That force is what spins the motor.
For a wire at right angles to the field, the size of the force is given by an equation that is on the GCSE physics equation sheet:
F = B × I × l
Here F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A) and l is the length of wire in the field in metres (m). Double the current and you double the force. Use a stronger magnet or more wire in the field, and the force grows again.
Fleming’s left-hand rule
The direction of the force comes from Fleming’s left-hand rule. Hold your thumb and first two fingers at right angles:
- the First finger points along the magnetic Field (north to south),
- the seCond finger points along the Current (positive to negative),
- the thuMb points in the direction of Motion, which is the direction of the force.
Use your left hand for a motor. Later in this article you will meet the generator effect, which has its own right-hand rule. In the AQA GCSE specification, Fleming’s left-hand rule, the equation F = BIl and the way a coil of wire carrying a current in a magnetic field tends to rotate are Higher tier content.
A wire 0.25 m long carries a current of 40 A at right angles to a magnetic field of 0.5 T. Calculate the force on the wire. (3 marks)
F = B × I × l = 0.5 × 40 × 0.25 = 5 N.
From a simple coil to a car motor
A school motor has a single coil between two magnets. A car motor follows the same idea, but with many turns of copper wire to increase the force. In most EV motors the copper coils sit in the fixed outer part, called the stator. When current flows, each coil becomes an electromagnet. The part that turns, the rotor, sits inside.
The battery supplies direct current (DC), but the motor needs alternating current (AC), and most EVs use three-phase AC. The inverter, which is part of the car’s power electronics, switches the DC on and off very quickly to make AC. The three sets of coils are energised one after another, which makes a rotating magnetic field. The rotor is dragged round by that field, rather like a magnet following a second magnet you move past it.
Changing the frequency of the AC changes how fast the field rotates, and so how fast the motor spins. The power electronics controller manages the flow of energy from the battery, controlling both the speed of the motor and the torque it produces.
Two main types of EV motor
Most EV makers use permanent-magnet synchronous motors, but induction motors are also used. Both rely on the motor effect.
| Motor type | What is in the rotor? | Plus point |
|---|---|---|
| Permanent-magnet synchronous | Permanent magnets, usually rare-earth (neodymium) types | The most widely used type in EVs |
| Induction | No magnets: currents are induced in a cage of bars | Rugged and cheaper |
Those strong rare-earth magnets are the subject of our guide to the strongest magnets.
Why do electric cars accelerate so fast?
Many people notice that electric cars feel quick away from the lights. The physics explains why. It starts with torque, which is the turning effect of a force, just like the moment of a spanner. An EV motor can deliver its maximum torque from standstill up to what engineers call the base speed, so the car can pull hard from the moment you press the pedal. A petrol engine has to rev up before it produces its best torque.
Do electric cars have gears?
Most EVs use a single-speed reduction gear. The motor spins much faster than the wheels need to, so a fixed gear reduces the speed and, in doing so, increases the torque at the wheels. Because the motor can work over a wide range of speeds, there are no gear changes to interrupt the push. To reverse, the motor simply turns the other way.
F = ma in action
The motor turns the wheels, but the car moves because of friction. The tyres push backwards on the road, and, by Newton’s third law, the road pushes forwards on the tyres. That forward force on the tyres is the driving force. For more on this, see our guide to Newton’s laws of motion.
Newton’s second law then tells us what the car does:
a = F ÷ m
Here a is acceleration in m/s², F is the resultant force in newtons and m is the mass in kilograms. Suppose a car and its passengers have a mass of 1,800 kg and the resultant force is 5,400 N:
a = 5,400 ÷ 1,800 = 3 m/s²
At a steady 3 m/s², the car would reach 60 mph (about 26.8 m/s) in roughly 26.8 ÷ 3 ≈ 8.9 s. Real acceleration is not steady. Air resistance increases with speed, so the resultant force, and therefore the acceleration, falls as the car goes faster. A heavy battery also means more mass, so a bigger force is needed for the same acceleration.
Regenerative braking: turning motion back into energy
When you lift off the accelerator or press the brake pedal in an EV, the car can use regenerative braking. The motor is run in reverse, as a generator. Instead of the current turning the wheels, the turning wheels turn the motor.
This uses the generator effect: when a conductor moves relative to a magnetic field, a potential difference is induced. Our guide to how generators work covers it in detail, and the direction rule for a generator is Fleming’s right-hand rule. The induced current flows back through the inverter and charges the battery.
There is a second effect. The induced current produces a magnetic field that opposes the motion that caused it, so the motor pulls back on the wheels and the car slows down. In energy terms, part of the car’s kinetic store is transferred back to the battery’s chemical store, instead of being wasted.
How much kinetic energy is there?
The energy in the kinetic store of a moving object is:
KE = ½ × m × v²
Because speed is squared, speed matters a great deal. For a car and passengers of 1,800 kg travelling at 30 mph (about 13.4 m/s), the kinetic energy is about 162,000 J, or roughly 45 Wh. At 60 mph, double the speed, it is about 648,000 J, which is four times as much.
Try your own numbers below. A good experiment is to enter 30 mph, then 60 mph, and watch the energy quadruple.
The recovery share in the calculator is up to you. A 2023 analysis of a BMW i3 found that the regenerative braking system’s maximum efficiency was 60.1% under the conditions studied. Other cars and conditions give other figures, so treat 60% as an example, not a universal limit.
Friction brakes still matter
Ordinary friction brakes work differently. The pads squeeze the discs, and the car’s kinetic energy is transferred to the thermal store of the discs and pads, then dissipated into the air. This is energy wasted as heat, a good example of the friction effects described in our article on the disadvantages of friction. EVs keep friction brakes for hard stops, when regenerative braking alone cannot slow the car fast enough.
Regenerative braking helps most when you stop and start often. The same BMW i3 analysis concluded that regenerative braking suits stop-and-go urban driving best. The US fuel economy site (fueleconomy.gov) puts numbers on this for its test cycles: in city driving, net regenerative braking returns about 32% of the energy, but on the highway only about 6%, because there is little braking in highway driving.
Are electric cars more efficient than petrol cars?
Efficiency compares the useful energy output with the total energy input. For a car, the useful output is the energy that moves it down the road. The figures below come from the US Department of Energy and the Environmental Protection Agency (EPA), using their standard combined city and highway test cycle. They are US figures, so do not mix them with other sources.
| Quantity | Electric car | Petrol car |
|---|---|---|
| Share of energy that reaches the wheels | About 65–69% (87–91% when net regenerative braking is counted) | About 12–30%, depending on the drive cycle |
| Energy lost in the drive system or engine | About 15–22% | About 64–75% |
The EPA also splits the EV’s losses: about 10% is lost during charging, about 22% in the electric drive system and about 3% runs accessories such as lights and the heater. So a typical EV is about 87–91% efficient once regenerative braking is counted, compared with roughly 30% at best for a conventional petrol car.
This resolves the puzzle from earlier. Petrol stores far more energy per kilogram than a battery, but a petrol engine wastes most of it as heat. An electric drive wastes far less.
There is one more caveat. These figures start at the car’s charging point or fuel tank. They do not include energy lost at power stations, or in the grid, to make the electricity. That depends on how the electricity is generated, which our article on energy sources and their uses explains.
An EV takes 10 kWh from the charger and 8.8 kWh reaches the wheels, counting regenerative braking. Calculate its efficiency. (2 marks)
Efficiency = useful output energy ÷ total input energy = 8.8 ÷ 10 = 0.88, which is 88%.
Power, voltage and current: P = IV
The electrical power supplied to a device depends on the potential difference across it and the current through it:
P = I × V
Power P is in watts (W), current I is in amperes (A) and potential difference V is in volts (V). UK mains electricity is 230 V. A typical UK home chargepoint is rated at about 7 kW, which comes from a 32 A supply at 230 V: 230 × 32 ≈ 7,360 W, or 7.36 kW. It is sold as 7 kW or 7.4 kW.
Combine this with E = P × t and you can estimate how much energy is delivered. At 7.36 kW for 5 hours, the charger delivers about 7.36 × 5 ≈ 37 kWh. Around 10% is lost during charging, according to EPA figures, so a little less ends up stored in the battery.
Why some EVs use 800 volts
Most EVs use a battery of roughly 400 V, and a growing number use 800 V. (In practice, “400 V” covers a range of roughly 300 to 500 V.) Because P = IV, doubling the voltage gives the same power with half the current.
That matters because of heating in the wires. The power wasted as heat in a cable is:
P = I² × R
Halve the current and I² becomes a quarter of its old value, so the same cable wastes only a quarter as much power as heat. Lower current means less heating in the cables and the battery.
It is tempting to say that “800 V charges twice as fast”, but that is an oversimplification. Voltage alone does not set the charging speed. The battery’s charge curve, its temperature, its state of charge and the charger’s limits all matter too.
What limits an electric car’s range?
Range depends on how much energy is stored in the battery and how fast the car uses it. At a steady speed, any car must work against two main forces, air resistance and rolling resistance.
Air resistance (drag)
Air resistance depends on the speed, the car’s shape and its frontal area:
F = ½ × ρ × Cd × A × v²
Here ρ is the air density (about 1.2 kg/m³ near sea level), Cd is the drag coefficient, A is the frontal area and v is the speed. The key result is that drag force rises with the square of the speed. The power needed to overcome it is force × speed, so it rises with the cube of the speed.
The average modern car has a drag coefficient of about 0.25 to 0.3, SUVs are about 0.35 to 0.45, and some very streamlined recent EVs get close to 0.2. The Mercedes-Benz EQS, for example, is quoted at 0.20. An average full-size car has a drag area, CdA, of about 0.74 m².
Try the calculator with 50 mph and then 70 mph. The speed is only 40% higher, but the power needed to push through the air goes up by about 2.7 times.
At 50 mph the air resistance is about 222 N and the power about 5.0 kW. At 70 mph it is about 435 N and about 13.6 kW. That is why an EV loses range faster on the motorway than in town.
Rolling resistance
Rolling resistance comes from the tyres flexing as they roll. It is given by:
F = Crr × N
N is the normal (support) force from the road, which on level ground equals the car’s weight. For ordinary car tyres on concrete, Crr is about 0.010 to 0.015. For a 1,800 kg EV, the weight is about 17,660 N, so the rolling resistance is about 177 to 265 N. Unlike drag, it does not change much with speed. Our guide to friction in physics explains the idea behind it.
Heating, cooling and accessories
Not all of a battery’s energy goes into moving the car. Accessories use about 3% of the energy in the EPA combined cycle. The car also has a thermal management system that keeps the motor, power electronics and battery in their working temperature range.
Electric cars on UK roads in 2026
According to SMMT figures published on 5 October 2026, battery electric cars made up a record 28.3% of new registrations in September 2026. Plug-in hybrids, which combine a battery with a petrol engine and are explained in our guide to plug-in hybrid cars, took a record 17.0%. Over the year so far, the battery electric share was 26.2%, which SMMT noted is significantly below the 33% mandated for 2026 under the Zero Emission Vehicle mandate.
Check your understanding
Frequently asked questions about the physics of electric cars
What physics is involved in electric cars?
Electric cars use energy stores and transfers, electric current and potential difference (P = IV), the motor effect (F = BIl) to turn the wheels, Newton’s second law (F = ma) for acceleration, kinetic energy (½mv²), and forces such as air resistance and rolling resistance. Regenerative braking adds the generator effect.
What energy transfers happen in an electric car?
Energy is transferred from the chemical store of the battery to the kinetic store of the moving car by an electric current doing work. Some energy is dissipated as heat in the battery, motor, electronics, brakes, tyres and air. When regenerative braking is used, some kinetic energy is transferred back to the battery.
How does an electric car motor work?
A current in coils of wire inside a magnetic field experiences a force: the motor effect. The inverter turns the battery’s direct current into three-phase alternating current, which makes a rotating magnetic field in the stator. That field drags the rotor round, and the gear reduction passes the turning effect to the wheels.
Why do electric cars accelerate so quickly?
An electric motor can deliver its maximum torque from standstill up to its base speed, so the driving force is available straight away. With a single-speed reduction gear, there are no gear changes. Acceleration is the resultant force divided by mass, and it drops as air resistance grows at higher speeds.
How does regenerative braking work?
When the driver lifts off the accelerator or brakes, the motor is used as a generator. The turning wheels move the rotor, inducing a current that flows back to charge the battery. The induced current opposes the motion, so the car slows down, and kinetic energy is transferred to the battery’s chemical store.
How much energy does regenerative braking recover?
It varies with the car, speed and driving. One 2023 analysis of a BMW i3 found a maximum regenerative braking efficiency of 60.1% under the conditions studied. In US EPA tests, net regeneration returned about 32% of energy in city driving but only about 6% on the highway, because there is less braking.
Are electric cars more efficient than petrol cars?
Yes. Using US EPA test-cycle figures, a typical EV is about 87–91% efficient once regenerative braking is counted, while only about 12–30% of the energy in a conventional car’s fuel moves it down the road. These figures do not include energy lost generating the electricity at power stations.
What does kWh mean on an electric car battery?
A kilowatt-hour is a unit of energy: the energy transferred by a 1 kW device in one hour. One kWh equals 3.6 MJ (3,600,000 J). The battery’s kWh figure tells you how much energy it stores. A 60 kWh battery stores 216 MJ.
Why do electric cars have less range on the motorway?
Air resistance grows with the square of speed, and the power needed to overcome it grows with the cube of speed. In the calculator above, going from 50 to 70 mph raises the drag power from about 5.0 kW to about 13.6 kW. There is also little braking on motorways, so regeneration recovers little.
Do electric cars have gears?
Most electric cars have a single-speed reduction gear, not a gearbox with several ratios. The motor spins much faster than the wheels, so the fixed gear reduces the speed and increases the torque at the wheels. The motor works over a wide speed range, and to reverse it simply turns the other way.
