Download Lesson 13

Questions:
1. Why would it be challenging to use static electricity in electrical devices, such as your television or
stereo?
Static charges are not useful for operating electrical devices – they build up in one place and discharge,
but they do not flow continuously – to operate electrical devices, you need a continuous flow of
electrons
2. How is current electricity different from static electricity?
Static electricity – charge builds up in one place and doesn’t move
Current electricity – involves a continuous flow of electrons
3. How Do Electrons Transfer Energy in a Circuit?
When you turn on the light switch on a wall, you close the circuit and immediately the light comes on.
How do the electrons get from the switch to the light bulb so fast? You can picture electrons in a wire
like marbles in a tube. If you push a marble in at one end of the tube, the energy is transmitted through
all the marbles in the tube, and a marble comes out the other end immediately.
Electrons in a wire work in a similar way. When an energy source is connected to a circuit, electrons in
the conductor “push” or repel other electrons nearby. As soon as one electron starts to move at one end
of the wire, it pushes the next one, which pushes the next one and so on. By pushing the first electron,
you make the last electron move. That is why when you flip the switch, the light goes on instantly even
though the electrons themselves have not moved from the switch to the light bulb.
Circuit Symbols & Circuit Diagrams
Electrical Safety Devices
A fuse contains a thin piece of metal that is designed to melt when too much current passes through it.
Once the metal melts, the circuit is broken and the current stops. A blown fuse must be replaced as it can
only work once. Fuses come in various sizes and shapes depending upon the application.
A circuit breaker does the same job as a fuse except the wire does not melt. Instead, when too much
current passes through them the wire heats up and bends, which triggers or “trips” a spring mechanism
that turns off the flow of electricity. Once the breaker has cooled, it can be reset.
Some appliances and devices have an added safety feature. A ground fault circuit interrupter (GFCI) is a
device that detects a change in current and opens the circuit, stopping current flow. For example, if an
appliance gets wet while you are handling it and some current starts to flow through the water, the GFCI
opens the circuit so there is less chance of injury to you.
Electric Current & the human body
The electric potentials that cause muscle movement in the human body are produced by nerve cells and
are typically about 0.08 V. When muscles are stimulated by electrochemical impulses from the nerve
cells, the fibres in the muscle cells contract. The larger the electric current, the more strongly the
muscles contract. Even small electrical shocks can be dangerous. An electric current of about 0.002A
gives you a tingling sensation. 0.005A is the maximum level of current considered safe.
A current of 0.016A causes the muscles of the body to contract or convulse. This level is sometimes
referred to as the “let-go threshold,” because if the current is above this value, the person cannot let go
of the object giving the electric shock.
If a current of 0.050A or more passes through the chest, the heart muscles stop their regular pumping
action and merely flutter – known as ventricular fibrillation. After a few seconds, the victim will become
unconscious. Currents above 0.200A usually cause severe burns. A current of 1.0 to 4.3 A will stop your
heart.
A rough value for the internal resistance of the human body is 300-1,000 Ohms.
ELECTRIC CURRENT (I)
rate at which electrons flow past a specific point in a circuit
SI unit is ampere (A)
POTENTIAL DIFFERENCE (V)
difference in electric potential between two points in a circuit
also called voltage
SI unit is volts (V)
ELECTRIC RESISTANCE (R)
degree to which a substance resists the flow of electric current through it
SI unit is ohm (S)
any material that can slow current flow
A multimeter is a device that allows you to measure current, voltage, and resistance by changing its
settings. Multimeters are typically small, inexpensive hand held devices.
Factors that affect electrical resistance
• cross-sectional area: Thicker wires have less internal resistance than thinner ones.
• length: As you increase the length of a wire, its internal resistance increases.
• temperature:
When a wire gets warmer, the atoms that make up the wire gain energy and vibrate faster. The increased
vibration results in more collisions between the atoms and the free-flowing electrons in the current.
Since greater vibrations cause more collisions, resistance increases with temperature.
• type of material: depends on the material’s conductivity. Conductivity up, resistance down.
Ohm’s Law:
V=IR
V(v)
I(A)
R(ohm)
Electrical Power & Efficiency
ELECTRICAL POWER (P)
rate at which electrical energy is used/produced
SI unit is watt (W) L 1 W = 1 J/s
Another common unit for power is the kilowatt (kW) L 1 kW = 1000 W
P=IV
The electrical energy consumption for a household is the amount of electrical energy used, measured in
kilowatt-hours. The kilowatt-hour (kWh) is the SI unit used to measure energy usage. Electricity meters
track how much electricity is drawn from the energy grid.
1 kWh = 3.6 x 10^6 J
To calculate how much energy is used (kWh) by an electrical device, you need to know:
Îthe power rating (kW) of the device and
Ïhow long it is used (h).
Question:
A stove rated at 12 kW is used for 300 h in one year. If the cost of electricity is 0.11 $/kWh, what is the
cost of operating the stove for one year?
Answer:
cost = $396.00
Question: