1. Electromotive Force and Current
Within a battery, a chemical reaction occurs that transfers electrons from one terminal (leaving it positively charged) to the other terminal (leaving it negatively charged.
An electric potential difference exists between the two terminals.

2. Electromotive Force and Current
The maximum potential difference is called the electromotive force (emf) of the battery.
In a car battery the potential of the positive terminal is maintained at a maximum of 12V higher than the potential of the negative terminal.

3. Electromotive Force and Current
emf = 12V for a car battery
a Volt is 1joule/coulomb
So a 1 coulomb charge leaving the battery has at most 12 joules of energy.

4. Electromotive Force and Current
The battery creates an electric field directed from the positive to the negative terminal.
The E-field exerts a force on the free electrons in the wire, and they respond by moving.

5. Electromotive Force and Current
The flow of charge is known as an electric current.
The electric current, I, is defined as the amount of charge that crosses an particular point per unit time.

6. Electromotive Force and Current
The SI unit is C/s
1 C/s = 1 A (ampere)

7. (a) how much charge flows in the circuit?
Example
A battery pack of a calculator has a voltage of 3V and delivers a current of mA. In one hour of operation
(a) how much charge flows in the circuit?
(b) how much energy does the battery deliver to the calculator circuit?
∆q = I(∆t) = (0.17x10-3A)(3600s) = 0.61C
E=QV = (0.61C)(3V) = 1.8J

8. A note about current
When we discuss circuits, and current, we are talking about conventional current.
Conventional current is based on the movement of positive charges through a circuit. (historical basis)
Today we know that it is really electrons that are moving, but we still follow historical conventions.

9. Greater pump pressure gives larger water flow rates
Ohm’s Law
The current that a battery can push through a wire is similar to the water flow that a pump can push through a pipe.
Greater pump pressure gives larger water flow rates
Greater battery voltage gives larger electrical currents
I is proportional to V

10. Different materials “resist” the flow of electrons through them
Ohm’s Law
Different materials “resist” the flow of electrons through them
The resistance, R, is defined as the ratio of the voltage applied across a material to the current, I, through the material.
R = 𝑉 𝐼
SI unit of resistance: volt/ampere = ohm
1 V/A = 1 Ω

11. Example
The filament in a light bulb is a resistor in the form of a thin piece of wire. The wire becomes hot enough to emit light because of the current in it. The batteries (1.5V each) provide a current of 0.4A in the filament. What is the resistance of the filament?
R = V/I = 3V/0.4A = 7.5Ω

12. V = ∆ 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐ℎ𝑎𝑟𝑔𝑒 = 𝐽 𝐶 ∆energy = QV V = IR
What we know so far…
V = ∆ 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐ℎ𝑎𝑟𝑔𝑒 = 𝐽 𝐶
∆energy = QV
V = IR
We can manipulate or combine these equations as needed to find what we need.

13. Assignment
Focus p. 633 #1,2
Problems p. 634 #1-8