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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!