Unit 12.1 Electric Current

Current and Charge Movement Although many practical applications and devices are based on the principles of static electricity, electricity did not become an integral part of our daily lives until scientists learn to control the movement of electric charges known as current.

Current and Charge Movement Electric currents power our lights, TV’s, computers and ignite the gasoline in automobile engines.

Current and Charge Movement Electric currents are even part of the human body. This connection between physics and biology was discovered by Luigi Galvani.

Current and Charge Movement While conducting electrical experiments near a frog he had recently dissected, Galvani noticed that electrical sparks caused the frog’s legs to twitch and convulse. After further research, Galvani concluded that electricity was present in the frog.

Current and Charge Movement Today, we know that electric currents are responsible for transmitting messages between body muscles and the brain. In fact, every action involving muscles is initiated by electrical activity.

Current and Charge Movement A current exists whenever there is net movement of electric charge through a medium.

Current and Charge Movement A more precise definition is: Current is the rate at which electric charges move through a given area.

Current and Charge Movement I = ΔQ Δt electric current = charge passing thru an area time interval SI Unit for current is ampere, A

Current and Charge Movement One Ampere is equivalent to one Coulomb of charge passing through a cross-sectional area in a time interval of one second 1A = 1 C/s

Current and Charge Movement The moving charges that make up current can be positive, negative, or combination of the two. In a common conductor, it is due to motion of negatively charged electrons.

Current and Charge Movement This is because the atomic structure of solid conductors allows the electrons to be transferred easily from one atom to the next.

Current and Charge Movement In certain particle accelerators, a current exists when positively charged protons are set in motion. Positive and negative charges set in motion are sometimes called charge carriers.

Current and Charge Movement Conventional current is defined as the current consisting of positive charge that would have the same effect as the actual motion of the charge carriers.

Drift Charge Many people think that electrons flow very rapidly because when you turn on a light switch, the light comes on almost immediately. However this is not the case.

Drift Charge When you turn on the switch, an electric field is established in the wire. This field, which sets electric charges in motion, travels through the wire at nearly the speed of light. The charges themselves however, travel much more slowly.

Drift Charge When an electron moves through a conductor, collisions with the vibrating metal atoms of the conductor force the electron to change its direction constantly.

Drift Charge The average energy gained by the electrons as they are accelerated by the electric field is greater than the average loss in energy due to the collision.

Drift Charge Thus, despite the internal collision, the individual electrons moves slowly along the conductor in a direction opposite the electric field, E, with a velocity known as the drift velocity, Vdrift.

Drift Charge The magnitudes of drift velocities or drift speeds, are typically very small. In fact the drift speed is much less than the average speed between collisions.

Sources and Types of Current Free electrons in a conductor move randomly when all points in the conductor are at the same potential.

Sources and Types of Current But when a potential difference is applied across the conductor, electrons will move slowly from higher electric potential to a lower electric potential. Thus, a difference in potential maintains current in a circuit.

Sources and Types of Current Both batteries and generators maintain a potential difference across their terminals by converting other forms of energy into electrical energy.

Sources and Types of Current As charge carriers collide with the atoms of a device, their electrical potential energy is converted into kinetic energy. The battery continues to supply electrical energy to the charge carriers until its chemical energy is depleted.

Sources and Types of Current Because batteries must often be replaced or recharged, generators are sometimes preferable. Generators convert mechanical energy into electrical energy.

Sources and Types of Current Generators are the source of the potential difference across the two holes of a socket in a wall outlet in your home, which supplies the energy to operate your appliances.

Sources and Types of Current When you plug an appliance into an outlet, an average potential difference of 120V is applied to the device.

Sources and Types of Current There are two different types of current: Direct current (dc) Alternating current (ac)

Sources and Types of Current In direct current, charges move only in one direction.

Sources and Types of Current In alternating current, the motion of charges continuously changes in the forward and reverse direction.

Sources and Types of Current Since the current is changing direction, the terminals of the source of potential difference are constantly changing signs. Therefore there is no net motion of charge carriers in alternating current.

Sources and Types of Current If it were slow enough, you would notice flickering in lights and other appliances. This is why in the US, alternating current (ac) oscillates 60 times per second – frequency of 60 Hz.

Sources and Types of Current Unlike batteries, generators can produce either direct or alternating currents based on their design.

Sources and Types of Current Alternating currents is more practical in transferring electrical energy which is why power companies use this type. Alternating current is also considered safer than direct current.