• Electrons in motion.
• Current: The number of electrons that
pass a specific point in a circuit in one
• Current in an electrical circuit acts like the
water current of a river.
• Current is measured in Amperes (A)
Types of current
• Direct current: electrons that flow in the same
direction in a wire. (DC)
• From batteries
• Alternating current: electrons that flow in
different directions in a wire. (AC)
• From Generators
• Used in your home
• Transformers change AC to DC
• The reason electric charge flows from
one place to another is voltage.
a. Voltage is the difference between two
places where electrons are flowing.
b. Voltage is the “push” that makes
c. Measured in volts (V).
• Resistance: the force opposing the flow of
• Anything that uses electricity has resistance like
a light bulb, heater, or oven.
• Measured in ohms
• Symbol is Greek letter omega
• Thicker wire- less resistance
• Longer wire- more resistance
• Conductors- low resistance
• Insulators- high resistance
• The relationship among current (I), voltage
(V), and resistance (R).
• Ohm’s law states that the current in a circuit
is equal to the voltage divided by the
• R = V/I
Do the Math
• A car has a 12 volt system. The
headlights are on a 10 amp circuit. How
much resistance do they have?
• Your house uses 120 volts. What amount
of current would flow through a 20 ohm
• In a series circuit the
current has only one
path it can travel
• When any part of a
series circuit is
current flows through
the circuit. This is
called an open circuit.
• The amount of current is the same at all
points in a series circuit.
• Old fashioned Christmas lights were in series
so if one light went out, all the lights went
• Parallel circuits
branches for current
to move through.
• More current flows
through the paths of
• The potential
difference is the same
in each branch.
• In parallel circuits the current can take more than
• Because there are multiple branches, the current
is not the same at all points in a parallel circuit.
Advantages of parallel circuits
Parallel circuits have two big advantages
over series circuits:
1. Each device in the circuit sees the
full battery voltage.
2. Each device in the circuit may be
turned off independently without
stopping the current flowing to other
devices in the circuit.