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PHY 101 Lecture #7 Electrical circuits 1
PHY 101 Lecture #7:
Electric Field, Electric Potential,
and Intro to Electrical Circuits
Prof. Peter R. Saulson [email protected] http://physics.syr.edu/courses/PHY101/ Off. Hrs: Tue 9:30 –11:00, Physics 263- Prof. Schwarz’s Problem Sessions: Mon and Tues, 5:15 – 6:15, Physics 202/
PHY 101 Lecture #7 Electrical circuits 2
Outline
- Electric fields
- Electric potential energy and voltage
- A simple circuit
PHY 101 Lecture #7 Electrical circuits 3
Recall from last Thursday:
Electrical forces
Electrical phenomena recognized by electrical forces.
Force between two charges q 1 and q 2 given by Coulomb’s Law:
where charges are in coulombs, distance r in meters, and k (^) e = 8.99*10^9 N m^2 /C^2. Force comes out in newtons.
r
kq q Felec = e
PHY 101 Lecture #7 Electrical circuits 4
1. Electric fields
It is useful to represent the effect of charges on each other with the concept of the electric field. When you have more than two charges, Coulomb’s Law
would have to be applied for each pair of charges, then summed. Instead, you can write F e = q E. The electric field E represents the effect of all other charges on the charge q.
212 , r
kq q F (^) elec = e
PHY 101 Lecture #7 Electrical circuits 5
Electric field for a point charge
The electric force on a charge q due to another point charge Q is
Or, we can say that the electric field due to the charge Q is
and then recall F = q E.
2 , r E =^ kQ
r
k qQ Felec = e
PHY 101 Lecture #7 Electrical circuits 6
Electric field from
two charged metal plates
The field from a point charge is radial, and falls off with distance. Complicated We can make a uniform electric field by putting equal and opposite charges Q on two parallel metal plates. Field has strength
where A is the area of the plates, and ε 0 = 8.85 * 10 -12^ C^2 /N m 2.
0 A
Q
E
ε
PHY 101 Lecture #7 Electrical circuits 7
Motion of charge in
a uniform electric field
In a uniform field, a charge feels a constant force. Like a mass in Earth’s gravitational field. So motion is accelerated in the same way. Use this to control the motion of beams of electrons, in a cathode ray tube. Television, computer monitor, etc.
PHY 101 Lecture #7 Electrical circuits 8
2. Electrical potential energy
Electrical force means you have to do work to move an electric charge from one position to another. If you have to do work to move the charge, then you store electrical potential energy in the system. The units of electrical PE are joules.
PHY 101 Lecture #7 Electrical circuits 9
Voltage
Voltage at some location is defined as the electrical potential energy that would be stored in moving 1 C of charge from a reference point to that location.
SI unit of voltage is the joule/coulomb, also known as the volt.
PHY 101 Lecture #7 Electrical circuits 10
Voltage in real life
Voltage’s definition may sound awkward, but it is easy to measure with a voltmeter.
Voltage between two points is easy to set, using a battery, which maintains a fixed voltage between its two terminals. Many kinds of battery available, with voltages between 1 and 24 volts. Later, we will study the chemistry that makes them work.
PHY 101 Lecture #7 Electrical circuits 11
3. A simple circuit
battery (^) component
A single component completes the circuit from one battery terminal to the other.
Here, the component is a resistor. A resistor lets current flow, but not freely as a wire would.
PHY 101 Lecture #7 Electrical circuits 12
Intuition about voltage
Voltage is sometimes called “electromotive force”. It makes circuits “go.” A ball falls down because of gravitational force (or equivalently, because it can give up GPE.) Currents flow in a circuit because they are given a path from high voltage to low voltage.
