Electrical quantities

1. Electrical Quantities

2. • Can describe flow of electrons in a circuit using
various measurable quantities:
– Electric Charge
– Electric Current
– Potential Difference
– Electrical Resistance
• Charging by Induction
• Methods of Discharging

3. • Electric Charge (Q): measure of charge on an
electron(s) measured in coulombs (C)
• Elementary charge on one electron = 1.6 x 10-
19 C
• Therefore approximately 6.2 x 1018 electrons
equivalent to 1 C of charge

4. • How many electrons would it take to get 2.5 C
of charge?
• Q = Ne, where N = number of electrons and e
= 1.6 x 10-19 C/e- (elementary charge)
• Therefore N= Q/e = 2.5 C/ 1.6 x 10-19 C/e- =
1.56 x 1019 electrons = 1.6 x 1019 e-

5. • Electric current (I):
measure of the rate of
electron flow (electric
charge) past a given
point in a circuit,
measured in amperes
(A)
• I = Q/t, therefore
A = C/s

6. • Calculate the amount of current flowing if it
uses 300 C of charge in 15 seconds?
• I = Q/t = 300 C/15s = 20 A

7. • Ammeter: device used to measure electric
current

8. • Potential Difference (V): the difference in
electric potential energy per unit charge
measured at two different points; measured in
volts (V)
• V = ΔE/Q, therefore V = J/C

10. • If 5 C of charge gains 20 J of energy, what is
the potential difference?
• V = ΔE/Q = 20 J/ 5 C = 4 V

11. • Voltmeter: device used to measure potential
difference

12. • Electrical Resistance (R): ability of a material
to oppose the flow of electric current;
measured in ohms (Ω)
• R = V/I (Ohm’s Law), therefore Ω = V/A
• Factors that affect resistance:
– Type of material
– Cross-sectional area
– Length
– Temperature

13. • A 3V battery sends a current of 0.10A through
a light bulb. What is the resistance of the
filament of the bulb?
• R = V/I = 3 V/ 0.10 A = 30 Ω

14. • Ohmeter: device used to measure resistance

15. • Resistor: device that converts electrical
energy to heat energy by reducing the flow of
electric current

16. • Power (P): measure of the rate at which
electrical potential energy is used up;
measured in watts (W)
• P = ΔE/t, therefore W = J/s
• P can also be expressed with respect to
potential difference and current:
– P = VI, therefore units W = J/s = J/C x C/s

17. • Calculate the power of a vacuum cleaner if the
operating voltage is 120 V and the current
flowing through it when it is used is 7.9 A.
• P = VI = 120 V x 7.9 A = 948 J/s = 950 W