## Due before class Friday, June 29th

Physics 106, Summer 2012

Did you complete the reading assignment?

Yes
No

Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?
Yes
No

## KEY CONCEPTS:

1) Electromotive forces (emf) are external sources that produces an electric potential difference.

2) Voltage and emf are very similar, but are not the same.

3) Adding resistance to a circuit will decrease voltage, but not emf.

Electromotive force is the external work use for a unit of charge to produce an electric potential difference or a voltage across two open-circuited terminals. This means the circuit is not connected to the source of emf. The electric potential is made by separating out the positive and negative charges, which generates an electric field. This drives current flow if a circuit is attached to the source of emf. The difference between emf and voltage in a circuit can be very confusing. Here is an example to help clear up the confusion:

A battery is formed by chemically separating the positive and negative charges from each other within the battery and pushing all the positives to one end and all the negatives to the other end. This means there is an electric potential difference from one end of the battery to the other, and that if the two ends were connected to one another by an object, it would force a current to travel through that object. The current forces electrons to move through a circuit; that is why the battery is a source of electromotive force, or emf. When we now connect this battery in a circuit with a resistor, we can measure the voltage drop across that resistor which is also the voltage supplied by the battery, but this is not the battery emf for two reasons. Firstly, emf must be an external source of work per unit of charge, so it cannot be within the circuit. Secondly, the battery has internal resistance which also drops the voltage of the circuit and is not measured when the voltage of the circuit is taken.

**If the above introduction is confusing to you, please make specific mention of how it is confusing in the first box of the feedback section so I can try to clean it up and make it easier to understand**

Set up two simple circuits. Set up each circuit by putting a battery, a resistor, and a switch with wires to connect them. You get these components by dragging them from the white box on the right into the blue area. Right click on the battery for one of the circuits and choose the 'change internal resistance' option. Change this batteries internal resistance to 3 ohms. Now right click on both batteries and both resistors and click on 'show value' for each. Also, get a voltmeter from the tools selection box on the right onto the screen by clicking the box. It should look like the following image:

Measure the voltage drop across each battery while the switches are still off (switch is in up position). To measure the voltage, place the red lead of the voltmeter on one side of the battery and the black lead of the voltmeter on the other side on the battery. It will look like the following picture:

Are the voltage drops across each battery the same or different?

Same
Different

What is the emf of the battery with an internal resistance?

9.00 V
9.00 J/C

What is the voltage drop across the battery with an internal resistance?

9.00 V
9.00 J/C

Now close the switches on both circuits; this is done by clicking on the switch and dragging it until it is completely closed off. You'll know the switch is closed when the current starts to flow through the circuit. Measure the voltage drop across each battery.

Are the voltage drops across each battery the same or different?

Same
Different

What is the emf of the battery with an internal resistance? (choose the answer with units that are not misleading)

6.92 V
9.00 V
6.92 J/C
9.00 J/C

What is the voltage drop across the battery with an internal resistance? (choose the answer with units that are not misleading)

6.92 V
9.00 V
6.92 J/C
9.00 J/C

What is the difference between voltage and emf? How does adding resistance to a circuit affect each?

Battery companies sell batteries with a higher emf than the advertised voltage of the battery; in other words the emf of the battery is about 2 J/C when they advertise their battery as being 1.5 V. Why do they do this?

FEEDBACK

The comments entered in these last two boxes go into a big anonymous data file that I will use to guide the lectures of the day. That means two things:

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Was there anything that you didn't understand in the reading assignment?  What was confusing to you?