Transcript Chapter 7 Section2

Electric Current
Charges on the move!
What makes a charge move?
• Electric current moves due to potential
difference
• Electric potential difference is measured as
voltage
– Potential difference is measured in volts (V)
• The flow of charges due to potential
difference is current electricity
What makes charges move?
• Charges must have a pathway to move
• Pathway is called a circuit
• Circuit must have certain parts:
– Source (area of high potential)
– Path (usually wire)
– Load (something that does work using the moving
charges)
– Sink (area of low potential)
What makes charges move?
• Source of potential difference may be direct
current
• Batteries produce direct current due to a
chemical reaction
• Voltage produced by chemical reaction can
travel through wire to load
• Two kinds of battery:
– Dry cell: two elements and chemical paste
– Wet cell: two metals and liquid electrolyte
What makes charges move?
• Source of potential difference may be
generator
• Generator moves wire in magnetic field to
create alternating current (AC)
How many charges move?
• Amount of moving charges is called current
• Current measured in Amperes (amps) (A)
• Current is number of charges moving past in
one second (Coulombs/sec)
Why do charges lose potential?
• Resistance: similar to friction, resistance slows
down charges
• Resistance measured in Ohms (Ω)
• Size of wire affects resistance
– Longer wire has more resistance
– Thicker wire has less resistance
• Load in the circuit is a resistance
Ohm’s Law
• Relates current, potential difference, and
resistance
• Current = voltage / resistance
• I = V/R
• We can use our circle with this!
Ohm’s Law Example
• Calculate the voltage difference across a 25 Ω
resistor if a 0.3 A current is flowing through
it?
• Step 1: What do we know?
– Know R = 25 Ω
I = 0.3 A
V=?
Step 2: What equation will we use?
V=IR
Ohm’s Law Example Continued
• Step 3: Plug in info!
– V = (0.3 A)(25 Ω)
• Step 4: Do the math
– V = 7.5 V
• Step 5: Check for units!