Engineering Electric Cars

Once students understand basic circuits, they apply their knowledge to design an electric car that converts electrical energy into kinetic energy and begin to figure out the forces affecting its movement left to right so it can move in a straight line to its destination in this lesson. 

• Energy can be transferred in circuits through electric currents. Circuits are used in electric cars. 
• Engineers also need to know about forces and motion when designing an electric car.
• An electric car’s battery’s chemical energy is converted into electrical energy and provides the force that pushes electrons through the circuit. The electrons then travel through the conductive wires to the motor. When the electrons reach the motor, they transfer electrical energy and the motor converts it to kinetic energy, causing the motor to spin.
• Friction from tire treads on car wheels prevents cars from slipping. It does this by helping the tires grip the road surface. Car axles are smooth to decrease friction between the spinning axle and the car. Too much friction between car wheels and axles will slow the car or prevent it from moving.  
• Distribution of weight on a moving object like a vehicle can affect its movement left or right.
  
  

• How are electric cars different from more traditional vehicles?
• Why do engineers who design electric cars need to know about circuits?
• How is energy converted from one form to another in an electric car?
• How does the force of friction help a car move? How might it also make it harder to move?
• How does a car’s center of gravity affect its motion?
• What were some of the challenges you faced in designing and/or testing your prototype, and how did you overcome those challenges?  
• Why does your prototype need a battery and motor?
• How does friction help your prototype move? Where does friction cause problems with your prototype?  
  
  

Misconception: A force is necessary to keep an object moving.

Fact: An unbalanced force changes an object’s motion. An object that is already in motion will continue moving until acted on by an unbalanced force, such as friction.

Misconception: If an object is at rest, no forces are acting on it.

Fact: Forces are constantly acting on objects both at rest and in motion.

Acceleration: an increase in speed over time; measured in meters per second squared (m/s2)

Circuit:  the circular path electrons travel in a negative to positive direction

Electricity:  the flow of electrons through a conductor

Motor:  a machine that converts an input of energy into an output of kinetic energy

Powering an Electric Car

Chad owns an electric car. In 2010, Chad took a road trip. He drove from the state of Washington to California. He never had to stop for gas. Instead, he plugged his car into electrical outlets he found in campgrounds. This is similar to how you charge your phone.

Electric cars have electric motors. They don’t need gasoline or diesel to run. Instead, they use electricity to make the motors turn.

How an Electric Car Works

The electric motor is part of a circuit that includes a power source such as a rechargeable battery. The motor spins when the circuit is closed. The motor spins as the electrical energy turns into kinetic energy. This spinning provides an unbalanced force that makes the wheels move the car.

The motor is connected to an axle using gears. This motor axle is called the driveshaft. The driveshaft is connected to another axle that is connected to the wheels. The wheels spin when the axle spins.

All of the parts on the vehicle are placed to balance the car’s weight. If a vehicle’s center of gravity is too high, or too close to the front or back, it is unbalanced. This might cause it to roll over when going around sharp turns.

Designing an Electric Car

All objects have a point inside them that is their center of gravity. Imagine trying to balance a ruler on your finger. The point on the ruler where it balances is its center of gravity.

The lower your center of gravity is, the easier it is to keep your balance. Think about leaning over to pick something up. It’s easier to stay balanced if you’re sitting in a chair and you lean over, compared to if you’re standing up.

Cars are designed low to the ground so they have a low center of gravity. However, the center of gravity is not always in the middle of an object. Imagine trying to balance a hammer on your finger. Your finger will have to be closer to the heavier part of the hammer to balance the hammer’s weight.

Friction and Cars

People who design cars also have to take into account friction. Friction is the force that slows motion when two objects rub against each other. Friction makes the objects slow down because the kinetic energy that powers their movement has changed to heat.

Friction between the road and tires is called traction. Cars have tires with treads that are designed to grip the road. This traction is important. It keeps cars from sliding all over the road. This is similar to how the friction between your shoes and the cement stops you from slipping when you run down the sidewalk. When a driver pushes on the brakes, the friction between the brakes and the wheels slows it down.

Some cars use rear-wheel drive. Rear-wheel drive sends power through the rear wheels. Rear-wheel drive offers the best traction for acceleration. Acceleration is an increase in speed over time. It is measured in meters per second squared (m/s2).

When the car accelerates, the car transfers weight toward the rear wheels and away from the front wheels. This extra force pushes the rear tires into better contact with the pavement.

In this lesson, students use the engineering process to design an electric car that converts electrical energy into kinetic energy to move in a straight line to its destination. Students collect and analyze data on the distance their electric car prototypes traveled, evaluating each prototype for changes affecting its desired motion while looking for ways to improve its effectiveness.