SNC1
Name: _____________________
Household Electricity and Electrical Safety
1) a)Current of _______ or less can produce a sensation of shock, but do little or no damage.
b) Current of _______ can cause muscles to contract, making it impossible to release a live wire.
c) Current of _______ for only a few seconds can cause death by paralysing the breathing muscles.
d) Current of _______ can produce serious, and often fatal burns.
e) What current flows through a 100 W light bulb?
2) What is the voltage of a regular household circuit?
The current (I) that passes through an object depends on both the voltage (R) and its resistance (R).
I=V/R
The resistance of the human body depends a lot on how well hydrated you are, your salt content, bodyfat percentage, and how wet and sweaty your skin is. Your resistance for current passing through you
into the ground also depends on your shoes and the what you are standing on.
Human resistance:
standing on a dry floor in good shoes and grabbing a electrified wire: 4000 to 40,000 Ω
standing on damp concrete in bare feet and grabbing an electrified wire: 3300 Ω
3) Would a person who grabs an electrified wire be killed if... (use I=V/R, and your answers to 1 and 2)
a) they are wearing shoes on a dry floor (compare both the best and worst possible resistance)?
b) they are in bare feet on damp concrete?
4) Is it ever safe to handle unshielded electrified objects?
5) Transmission wires that carry electricity over long distances often use voltages of nearly one million
volts. Do you think that you actually need to touch these wires to receive a lethal shock?
(hint, think about sparks)
6) How are household electrical circuits wired: in parallel or in series?
7) Given what you learned in the parallel and series circuits labs and given your answer to question 6,
what will happen to the current in a household circuit as more electrical devices are switched on?
8) What could happen to the wires in your walls if too many electrical devices are switched on?
9) a) What is the function of a circuit breaker or fuse?
b) Will a circuit breaker protect us from a fatal electrical shock?
Points to consider: Most fuses and circuit breakers are designed to open the circuit only if the
current exceeds 15 amps. Will a current less than this kill us?
A short circuit is a circuit in which the electricity can flow through the circuit without passing through
a load. Thus, the resistance is very low.
10) What would happen to the current in a short circuit?
11) Most plugs should have three prongs. What are their functions?
a) the small prong (attaches to the black wire)
__________________________
b) the large prong (attaches to the white wire)
__________________________
c) the round prong (attaches to the ground wire)
__________________________
The ground wire connected to in every electrical device makes a connection between the exterior of
the electrical device and the ground.
12) a) If the exterior of the device comes in contact with the black wire, what would happen if you
touched the device and the ground wire was missing?
b) If the ground wire is present, what type of circuit will be created (hint, see question 10)?
c) What would happen to the current in the circuit in question 12b?
d) What would the circuit breaker or fuse do in question 12c?
e) So, the function of the ground wire is to create a _________ circuit if any defect occurs in a
device. This will cause the circuit breaker or fuse to _________ the circuit, before anyone touches the
device.
13) a) What wire should an electrical switch always be on: the black wire or the white wire?
Hint, would you want a device to be electrified when it is turned off?
b) What safety feature of electrical outlets ensures that the switch is always on the correct wire?
14) a) What does a GFCI outlet do?
b) Where are GFCI outlets particularly important?
15) How much current passes through a 1600 Watt hair dryer? (Power = 1600 W, Voltage=120V)
Use the P = V x I equation: re-arrange it to get I = P / V
We often use extension cords to plug devices in when the outlet is too far away. However, not all
extension cords are alike. The thinner the cord, the greater its resistance. Also, cords tend to get
greater resistance as they age since they get bent and stepped on repeatedly. If a high resistance cord
is used to plug in a device that takes a lot of current, the cord will use a large part of the voltage in the
circuit (as we saw in the series circuit lab) and it will heat up. As it heats up, it becomes more resistant
and hotter still. Eventually it may cause a fire.
16) If you use a .15 ohm extension cord to connect your hair dryer from question 15 to a distant outlet,
what amount of heat power would the extension cord produce?
a) To answer this, we need to know how much of the 120 volts that the wire is getting.
We can calculate this using our resistance equation: R=V/I re-arranged into V = R x I.
R = 0.15
I = your answer to question 15
b) Once we know the voltage, we can calculate the heating power of the extension cord
using the power equation: P = V x I
Your answer should be about the same power as small light bulb.
The moral of this question is that you should never use an extension cord that is not thicker than the
cord that comes attached to the device. Houses are supposed to have an outlet every 4 metres along
each wall so that people are not tempted to use extension cords at all!
17) A 20 ohm toaster and a 20 ohm microwave oven are both plugged in to the same outlet.
This outlet is on a circuit that has a 15 amp circuit breaker.
In another outlet on the same circuit 5 lamps with 100 Watt (144 ohm) light bulbs are plugged in.
a) What current will go through the circuit if both toaster and microwave on are at the same time?
b) Will the circuit breaker be tripped?
c) While the toaster and oven are on, some one starts turning on the 5 lamps. How many lamps does
it take to cause the circuit breaker to trip?
d) What is the best solution to this problem?