Power and Energy

Fibrillation and death Electrical Safety Electricity can be hazardous; it must be treated with caution. The table below shows human sensitivity to electric current. Current (A, AC) Current (A, DC) Reaction 4 x10-4 Slight sensation 9 x10-3 0.05 Shock 0.02 0.07 Muscles paralyzed 0.1 0.5 Fibrillation and death

Overloaded Circuits are dangerous as they exceed the circuit’s safe limit. This can lead to a short circuit or arc. This leads to a circuit with little or no resistance; this causes a huge increase in current which can lead to overheating.

To prevent this from happening, a fuse will blow or a circuit breaker will trip.

Grounds

To ensure that electrical products are made to standard, electrical devices must pass safety standards. Products sold in Canada must be met the safety standards of CSA or UL. Insurance companies will not cover fires started by products that have not met these standards.

Electrical power and Energy Power is described as the rate of transforming energy, essentially the rate at which work is done. Power can be measured in joules per second (J/s), or watts (W).

Power Energy Time ΔE is the energy transformed in joules (J) Formulas Power Energy Time ΔE is the energy transformed in joules (J) Δt is time interval in seconds (s)

Power can also be found in other electrical systems when voltage and current is given Potential Difference Current

Formulas from the Resistance lesson Voltage Current

Power If And Than, , Than

Power If And Than, , Than Also

One watt is a small amount of power, so electrical power is often stated in kilowatts (1 kW = 103) or megawatts (1MW = 106)

Example 1: Household Power What is the maximum power that can be drawn from a standard 120 V household circuit that has a circuit breaker (or fuse) of 15 A? V = 120 V I = 15 A P = ?

P = IV P = (15 A) x (120 V) = 1800 W Therefore, the maximum power on any 15 amp breakered circuit is 1800 W.

Example 2: Power to the amp Calculate the power rating of a stereo amplifier (not the speaker power) if it is plugged into a standard 120 V outlet and has a resistance of 120 Ω. V = 120 V R = 120 Ω P = ?

= 120 W Therefore, the power rating of the stereo is 120 W.

We could have also found I using the formula, and then used the formula

Example 3: Power in the current How much power is dissipated in a circuit with a 15 Ω resistor that draws a current of 10.0 A I = 10.0 A R = 15 Ω P = ?

P = 1500 W Therefore, the power dissipated in the circuit is 1500W.

Example 4: Current in a circuit Calculate the current in a small colour TV connected to a 120 V circuit and uses 180 W P = 180 W V = 120 V I = ?

I = 1. 5 A Therefore, the TV draws a current of 1.5 A

Example 5: Power usage P = 1.2 KW, 1.2 x 103 W A 1.2 KW hair dryer is used for 5.0 min. Calculate the energy consumed in joules and megajoules. P = 1.2 KW, 1.2 x 103 W Δ t = 5.0 min = 5.0 min x 60 s =3.0 x 102 s ΔE = ?

Δ E = 3.6 x 105 J Therefore, the energy consumed is 3.6 x 105 J