Evans SPH 3U1
Physics Grade 11
Unit 5: Electricity & Magnetism
Note 2: Current Electricity & Electric Potential
Reference: Chapter 16.1 - 16.3
Current Electricity: see water analogy under confused concepts on website.
Electric current (symbol, I) is the movement (flow) of electric charge.
Current: Current is measured as the charge (Q) flowing through a cross sectional area of a conductor during a period of time (t).
SI Unit is Amperes (A). Ampere = 1 Coulomb / Second
Example 1: A 10 ampere toaster needs 900C of charge to toast bread. How long will it take? (Ans. 90 s)
Example 2: A static charge that has 1.25 x 1010 excess electrons is grounded and discharges in 0.50 seconds. Calculate the average current in the grounding wire. (Ans. 4.0 x 10 -9 A) hint: recall Q=Ne
Current Flow in Electric Circuits:
See topic under confused concepts on website (i.e. understand electron flow vs. conventional current flow)
Measured using an ammeter which must be connected in series in the circuit.
Direct Current (DC): Only in one direction and doesn't change in magnitude (i.e. batteries)
Alternating Current (AC): Direction reverses and the magnitude changes continuously (i.e. hydro)
Current will only flow if it has a complete path.
A circuit may contain many components (source, load, connector, control device, etc.)
Circuit diagrams and symbols (see Fig. 16.5 page. 547)
cell vs. battery, switch, ammeter, voltmeter, resistor, series/parallel
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Electric Potential:
A measure of the electric potential energy per unit charge.
Work is done by the power supply to separate electric charges (i.e. to increase the electrical potential energy of each charge). As the charge flows through the load, its energy decreases.
Electric Potential Difference (often called Voltage): Fig. 16.6 pg. 548
SI Unit: Volts (V) note: Volt = Joule/Coulomb
A reading of 1 volt means a source gives 1.0 J of energy to each coulomb of charge that passes through it.
Example 3: A 12 Volt
car battery supplies 1.0 x 103C of charge to a starting motor. How
much energy is used to start the car?
(Ans. 1.2 x 104J)
Example 4: IF it takes a current of 10A, 300 seconds to boil a kettle of water that requires 3.6 x 105 J of hear energy, what is the potential difference across the kettle? (hint recall electric current equation) (Ans. 1.2 x 102V)
Note: Combining equations for current and potential difference: E = VIt Fig. 16.7 pg. 550
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